aroTHERM Plus DHW Flow Temperature

[Dan F]

Our 7kw aroTHERM is working great, but I have one issue I can't resolve; no matter how low I configure the DHW flow temperature and DHW flow temperature offset, the ASHP ignores what is configured and uses 70C

 

I currently have the following configured:

- DHW Temperature: 55C

- Cylinder charging offset: 0C

- DHW Mode: Normal 

- Max cyl charger time: 120min

 

The ASHP Hydraulic Unit and sensoComfort controller both show a target temperature of 55C, but the actual flow temperature ignores this.

 

 

I do have an abnormal setup, but I'd hoped I could still constrain the flow temperature

- UVC is Mixergy and I'm using primary coil and not the external PHE.

- Mixergy controls DHW demand via a resister switch that simulates demand by faking the DHW temperature sensor.  Initially it was using 26C(on)/60C(off), but I've now adjusted to 50C(on)/60C(off) and see no real difference in behaviour, not in "normal" mode anyway.

 

Using DHW Mode "Eco" seems to help somewhat and it sticks around 62C for the first 50min and then climbs to 67C, but it's still ignoring the target of 55C.  

 

COP I'm getting at 70C is around 1.9 wheras a flow temperature of 55C should give me around 3 if it's 7C+ outside.

 

@J1mbo@Bruno  What do you see with yours? Any ideas?

Edited April 30, 2022 by Dan F

[SteamyTea]

15 hours ago, Dan F said:

no matter how low I configure the DHW flow temperature and DHW flow temperature offset, the ASHP ignores what is configured and uses 70C

Is 70°C a function of the refrigerant gas?  May the be upper limit to make the compressor more efficient.  Maybe the lower limit is varied i.e. between -30°C and 0°C.

Does changing the water flow rate make any difference?

Edited May 1, 2022 by SteamyTea

[Dan F]

3 hours ago, SteamyTea said:

Is 70°C a function of the refrigerant gas?  May the be upper limit to make the compressor more efficient. 

 

The refigerant means it can heat up to 70/75C, but as with all heat-pumps efficiency is impacted at such high temperatures.  This heat-pump at 7C external is supposed to give COP of 3 at 55C and 2.4 at 65C.  I get around 1.9 at 70C!

I don't think there is any way to change the flow rate, not so that it impacts DHW only, anyway.  The ASHP should modulate as required.

 

My only theories are:

- The DHW mode, by design, doesn't try to constrain flow temperature and instead assumes that the flow temperature follows the whole tank temperatue and that demand will be satisfied (via temp probe) when this temperature is reached.

- The ASHP doesn't see the UVC temperature increase over time (it's fixed due to resistor switch), so it inceases the flow temperature in an attempt to make it increase.

- The (shorter) Indirect coil at top of the tank means return temperatures are higher than with a standard HP coil lower down in the UVC.  The higher return temperature pushes up flow temperature (which is not constrained) as the ASHP attempts to maintain a certain delta-t.

(it could also be a combination of these)

 

Edited May 1, 2022 by Dan F
spelling

[Dan F]

For reference the way Mixergy works in my setup (using indiect coil) is this:

1) Mixergy signals DHW demand using it's own controls.  It does this using a "resistor switch" which simulates UVC geing below target temperature.

2) The ASHP flow heats the top of the tank using the indirect coil to 50C.

3) Once the top of tank has reached 50C demand is not satisified, but rather a pump turns on which sprays cold water (from the bottom of the tank) into the top of the tank.

4) The ASHP flow continues to maintain the top of the UVC at 50C and pushes the termocline.

5) When the Mixergy is at a desired %, the resistor switch is flipped to simulate the UVC being at 60C and ASHP DHW turns off.

 

I don't expect ASHP to behave in the same way as with a standard HP coil, where flow temperature gradually increases over time (with higher efficiencies to start with) and only hits 55C at the end of this period.  But, I do expect a fairly constant 55C flow (once top of the tank is at 50C), rather than 70C.

[markocosic]

Interesting pairing of equipment.

 

 

I think these two issues:

 

(1) The (shorter) Indirect coil at top of the tank means return temperatures are higher than with a standard HP coil lower down in the UVC.  The higher return temperature pushes up flow temperature (which is not constrained) as the ASHP attempts to maintain a [flow temperature that tracks the gradually increasing cylinder temperature]

 

(2) The ASHP doesn't see the UVC temperature increase over time (it's fixed due to resistor switch), so the ASHP [ratchets up the flow temperature to try heat this cylinder that is apparently being drained so fast that it cannot be reheated]

 

 

You will achieve the highest COP by heating the entire cylinder from cold to hot "in one go" at as low an average temperature as possible, vs incrementally heating a cylinder from full cold to full hot "a bit at a time" at the maximum temperature.

 

In a "standard" cylinder for heat pumps (with a ruddy great coil from top to bottom) the heat pump sees cool water that warms up as the cylinder warms. If it is DUMB them it runs at a fixed temperature. If it is a little smarter the heat pump can set flow temp = 10C above return temp (or some other number that achieves a reasonable power input / reheat time) and up the temperature gradient it runs until the cylinder is satisfied. If the heat pump is bit smarter again it can, if tsenses that he cylinder is already part heated, always run at a minimum of say 50C output in order to avoid cooling the top part (that's ready to use) whilst it's heating the bottom part. If it is REALLY clever it will have a desuperheater on it to top up the cylinder whilst providing space heating.

 

The "mixergy" coil is configured for heating a cylinder "a bit at a time" from a heat source that doesn't much care about temperature. It's the total opposite of what you want for a heat pump. If you do everything that you can to minimise this stratification and heating in parts - i.e. defeat most of the purpose of the mixergy tank except for the inlet diffuser - then you will maximise heat pump performance. I don't think these are remotely appropriate for use with heat pumps myself.

 

 

Your heat pump is going to do "whatever it wants to" in order to achieve a tank at the target setpoint - in your case 55C.

 

It has two ways to see what the temperature is.

 

(1) flow and return temps from cylinder primary

(2) temperature probe on cylinder

 

I would guess that is is kinda smart. If given a totally cold tank it would ramp the temperature up at return + X degC until setpoint achieved to maximise efficiency. If given a part warm tank it would ramp up at return + X degC, subject to a minimum of say 50 degC so that it doesn't discharge what's already at the top of the cylinder. It will use (2) to decide when to stop and (1) to decide what temperature to use.

 

If the heat pump were DUMB then you can set it to X and rely on an external contact. X is 50C for slow reheat to 45 or 65C for faster reheat to 55C etc.

 

The Arotherm is probably too smart for this application. Try heating a stone cold tank in ECO mode and watching the flow tempeatures? If that works then you know the issue.

 

The Mixergy tank, unless the smarts are defeated, is going to cause problems. You're trying heat the cylinder from full cold to full hot "a bit at a time" as it were. This will result in fairly high return temperatures to the heat pump and the heat pump thinking that the cylinder is almost fully heated. It is logical for it to increase the setpoint in this scenario. You don't have any way to recreate the effect of a "top to bottom" coil. Running a destratification pump to try mimic the effect of a "top top bottom" coil will blend the tank temperature; improving heat pump performance; but killing the "hot top" that you get with a heat pump tank. Running the water through a plate is more flexible but ultimately has the same drawback. Low flow through the plate means "heating a bit of the tank from cold to hot in one go" whilst high flow through the plate mixes up the entire cylinder and loses your "hot top" again.

 

Perhaps set to space heating mode to make the setpoint dumber and allow mixergy to control it?

 

Swap the mixergy tank for one that's more suitable?

[JohnMo]

I have a gas boiler and I am using the indirect coil at very low temperature 30 degs.

 

I see exactly the same as you.  My flow temp increases well past the set point and eventually trips at 55 degC.

 

So what is happening.  In my case and most possibly yours, is the coil surface area is not large enough to transfer the heat to the cylinder contents.  The lower the heating medium temperature the larger the coil required, to transfer the heat to the cylinder.

 

Two ways to fix, can you heat the cylinder directly instead of indirectly (this will increase volume of glycol required).  If not, a pumped circuit could be used through a large plate exchanger.  So HP is used on hot side of the plate exchanger, the cold side connected to you cylinder.

 

In my case my boiler should be generating 7kW, with boiler output of 30 degC, and return of 25degC.  Have sized for a 2 deg approach temp, so that 28 degC is coming out on the cold side to the cylinder.  PHE is a 40 plate and sized at 430x81mm.  So quite big.

 

Handy calc tool

https://www.heat-exchangers.uk/online-calculator/

[JohnMo]

What I forgot to mention, is the temperature ramps up, because the control system is trying to maintain a delta T between supply and return, not enough heat is being extracted, so it sees higher than expected return temperatures so ramps up supply temperature.

[Dan F]

5 hours ago, JohnMo said:

I see exactly the same as you.  My flow temp increases well past the set point and eventually trips at 55 degC.

Are you using Mixergy + Vaillant?  So, even with with a gas boiler same can happen to a lesser extent.. interesting.

 

5 hours ago, JohnMo said:

What I forgot to mention, is the temperature ramps up, because the control system is trying to maintain a delta T between supply and return, not enough heat is being extracted, so it sees higher than expected return temperatures so ramps up supply temperature.

Yes, this is what I was starting to conclude.  I had wronly assumed (when Mixergy is configured for 55C) I would get flow=60C, return=55C and top of UVC=55C.   But, the actual observed steady state (now I've renabled eco mode) I observe is flow=67C, return=62C, top of UVC=55C.  

This must be because there isn't enough heat-transfer at just a 5C teperature differential with the top tank and 12C is needed.  I don't understand the mechanism, but the ASHP is clearly choosing to increase the flow temp, beyond it's own target temperature,  rather that do anything else  (reduce flow rate?) to deal with this situation.

 

A PHE is the solution if I want to heat the tank more efiiciently at a lower flow temperature, and Mixergy sell one which can be retrofitted.   The advantage of using the primary coil (which I knew was a bit of a risk) was the battery-like "top up" functionality, which works suprisingly well and means you aren't always heating the whole tank,

 

[Dan F]

6 hours ago, markocosic said:

Interesting pairing of equipment.

Yes, I know this was unconvential but I also knew that it would work with the aroTHERM plus (Mixergy have tested this setup) and it would come down to efficiencies, so decided to give it a go.

 

6 hours ago, markocosic said:

You will achieve the highest COP by heating the entire cylinder from cold to hot "in one go" at as low an average temperature as possible, vs incrementally heating a cylinder from full cold to full hot "a bit at a time" at the maximum temperature.

Yes, but you have to heat the full cyclinder, so while the COP is better, you're using more electricity to heat the tank and have increased losses, so it's not quite so simple. 

 

6 hours ago, markocosic said:

I would guess that is is kinda smart. If given a totally cold tank it would ramp the temperature up at return + X degC until setpoint achieved to maximise efficiency. If given a part warm tank it would ramp up at return + X degC, subject to a minimum of say 50 degC so that it doesn't discharge what's already at the top of the cylinder. It will use (2) to decide when to stop and (1) to decide what temperature to use.

The Vailant controls have three modes: eco, normal and balanced.  I'm not exactly sure that the different is, but these use different strategies that trade-off efficiency with reheat time.

 

6 hours ago, markocosic said:

The Arotherm is probably too smart for this application. Try heating a stone cold tank in ECO mode and watching the flow tempeatures? If that works then you know the issue.

I haven't done this, but I'm pretty sure I know what will happen. The controller use a flow temperature of around 8C above the the UVC temperature, but once the top of the tank is already hot this delta will increase to around 13C in an attempt to maintain delta-t of 5k.

 

6 hours ago, markocosic said:

(2) temperature probe on cylinder

I've played around with the UVC temperatue that the Mixergy "resistor switch" simulates and tried initial 26C that installer configured and also 45C. This was in case the controller had some smarts that increase flow temperatue further when it through the tank was expecially cold, but this doesn't alter the ASHP behaviour in this scenario.

 

6 hours ago, markocosic said:

The Mixergy tank, unless the smarts are defeated, is going to cause problems. You're trying heat the cylinder from full cold to full hot "a bit at a time" as it were. This will result in fairly high return temperatures to the heat pump and the heat pump thinking that the cylinder is almost fully heated. It is logical for it to increase the setpoint in this scenario. You don't have any way to recreate the effect of a "top to bottom" coil. Running a destratification pump to try mimic the effect of a "top top bottom" coil will blend the tank temperature; improving heat pump performance; but killing the "hot top" that you get with a heat pump tank. Running the water through a plate is more flexible but ultimately has the same drawback. Low flow through the plate means "heating a bit of the tank from cold to hot in one go" whilst high flow through the plate mixes up the entire cylinder and loses your "hot top" again.

It's not the Mixergy smarts that's the issue I don't think, but rather than small top coil. Even if I turned off the smarts I'd have the same issue I thnk.   The solution if I want lower flow-temps and higher COP I think is the PHE, and yes the disadvantages us loosing the hot-top and partial charging.

 

6 hours ago, markocosic said:

Perhaps set to space heating mode to make the setpoint dumber and allow mixergy to control it?

Agreed, a heating circuit might modulate differently and do a better job at respecting configured flow temperatures, but if it does this then reheat time would probably be impacted.  I'd give this a go if it was easy, but it would need some replumbing..

 

6 hours ago, markocosic said:

Swap the mixergy tank for one that's more suitable?

No need to swap the tank.  They come "heat pump" ready and allow a PHE assembly to be fitted externally.  This was always the backup plan.  I think I'll reduce Mixergy setpoint to 50C and see what efficiency I get at 63C flow temperature first though.

[markocosic]

The ASHP can control:

 

- Compressor speed

- Circulator speed

- Fan speed (ignore for now)

 

 

 

It can control compressor speed and circulator speed to achieve the target operating temperatures. If these are infinitely variable then happy days the AHSP can do what it likes.

 

 

Perhaps the heat pump attempts to reheat the cylinder at a set power (essentially a set compressor speed) but even on ECO mode (minimum compressor speed / longest reheat time) the available turndown is limited.

 

 

Tech specs on Mixergy are difficult to find on their website. 

 

 

Via Google here's their external plate kit to make the cylinder compatible with heat pumps. With the plate heat exchanger it can support 4-7 kW input with an incoming flow temperature of 60C and the cylinder at 55C. The returns from will be somewhere between 55 and 60C. 

 

https://www.mixergy.co.uk/wp-content/uploads/2021/11/Mixergy-Heat-Pump-Kit-Installation-Guide.pdf

 

 

I couldn't find an equivalent specification for the coil in a standard cylinder; but I would guess that it is materially lower than this. The effect of this would be that even at minimum turndown / minimum modulation you're seeing a deltaT (from coil:water) of 10 degC and this is in fact what's dictating flow and returns of 67/62C in ECO mode; not the flow temperatures that the heat pump would like to operate at.

 

 

Bigger coil fixes this BUT in order to get that bigger coil you're probably reheating the entire cylinder at once. (or fitting the PHE that is effectively the same thing and supports a worst case scenario of 5 degC average primary water to potable water deltaT)

Edited May 4, 2022 by markocosic

[markocosic]

Is heating the full cylinder a problem?

 

Winter? Space heat leaking out of cylinder = meh useful anyway

 

Summer? Increased volume heated come at a cost; but also at a discount (higher COP); at a time of year where there's ample electricity (PV)

 

The diffuser is worth having in all scenarios in order to extend the runtime of a full cylinder / allow a smaller cylinder.

 

 

Jury out on the balance in that there are simpler ways to achieve much the same effect when you're running heat pumps with F-Gas or Propane. CO2 heat pumps would do better plate loading though! (these perform well with a high deltaT from flow:return - slowly charging a stratified tank bit by bit)

[JohnMo]

1 hour ago, Dan F said:

Are you using Mixergy + Vaillant?  So, even with with a gas boiler same can happen to a lesser extent.. interesting.

 

Yes, this is what I was starting to conclude.  I had wronly assumed (when Mixergy is configured for 55C) I would get flow=60C, return=55C and top of UVC=55C.   But, the actual observed steady state (now I've renabled eco mode) I observe is flow=67C, return=62C, top of UVC=55C.  

This must be because there isn't enough heat-transfer at just a 5C teperature differential with the top tank and 12C is needed.  I don't understand the mechanism, but the ASHP is clearly choosing to increase the flow temp, beyond it's own target temperature,  rather that do anything else  (reduce flow rate?) to deal with this situation.

 

A PHE is the solution if I want to heat the tank more efiiciently at a lower flow temperature, and Mixergy sell one which can be retrofitted.   The advantage of using the primary coil (which I knew was a bit of a risk) was the battery-like "top up" functionality, which works suprisingly well and means you aren't always heating the whole tank,

 

Not using mixergy or Vaillant, but same thermodynamics apply.

 

You will have a max temperature settings within the control scheme, the system is slowly working upwards that limit, hole trying to maintain the correct deltaT.

[Dan F]

59 minutes ago, JohnMo said:

You will have a max temperature settings within the control scheme, the system is slowly working upwards that limit, hole trying to maintain the correct deltaT.

Yes, just that it goes 20C beyond the max temperature configured in this case.

[Dan F]

1 hour ago, markocosic said:

It can control compressor speed and circulator speed to achieve the target operating temperatures. If these are infinitely variable then happy days the AHSP can do what it likes.

The configurable variables are:

- pump speed (50-100%)

- DHW mode   (eco, balanced, normal)

- DHW target temperature

- DHW offest (flow temp = target+ offset)

- Max reheat time. (was 2hrs, but i've turned this off now)

 

1 hour ago, markocosic said:

Perhaps the heat pump attempts to reheat the cylinder at a set power (essentially a set compressor speed) but even on ECO mode (minimum compressor speed / longest reheat time) the available turndown is limited.

It is outputting just over 5kW. Compressor speed is 50%.  (minumum output at A12W65 is 2.3kW at 25%)

 

1 hour ago, markocosic said:

Tech specs on Mixergy are difficult to find on their website. 

The primary coil is at the top of the tank and has around 1m2 surface area if the guy on the phone remembed correclty.

 

1 hour ago, markocosic said:

Via Google here's their external plate kit to make the cylinder compatible with heat pumps. With the plate heat exchanger it can support 4-7 kW input with an incoming flow temperature of 60C and the cylinder at 55C. The returns from will be somewhere between 55 and 60C. 

The PHE is equivilant to a 3m2 coil, which is what most HP coils are. 

 

1 hour ago, markocosic said:

The effect of this would be that even at minimum turndown / minimum modulation you're seeing a deltaT (from coil:water) of 10 degC and this is in fact what's dictating flow and returns of 67/62C in ECO mode; not the flow temperatures that the heat pump would like to operate at.

 

Bigger coil fixes this BUT in order to get that bigger coil you're probably reheating the entire cylinder at once. (or fitting the PHE that is effectively the same thing and supports a worst case scenario of 5 degC average primary water to potable water deltaT)

DeltaT (from coil:water) is actually 13C.   Yes, using PHE (equivilant to larger coil) means rehating cyclinder at once, albeit more efficiencly because i) lower max flow temp ii) flow temp ramps up rather than almost constantly being at 65C.

 

1 hour ago, markocosic said:

Is heating the full cylinder a problem?

No, not at all, just I liked the "top-up" approach Mixergy provides give we don't need 300L of water most of the time.

 

1 hour ago, markocosic said:

The diffuser is worth having in all scenarios in order to extend the runtime of a full cylinder / allow a smaller cylinder.

Agree.

 

I could try to model both scenarios and see which makes most sense.  There is an additional complication in the model in our case though; WWHRS.   The WWHRS units (at least i the configuration we are using) are a fair bit more efficient when the UVC temperature is higher. With a UVC temperature of only 48C they are ineffective simply because the cold flow rate through the heat exchanger is very low.

 

Edited May 4, 2022 by Dan F

[markocosic]

8 hours ago, Dan F said:

The configurable variables are:

- pump speed (50-100%)

- DHW mode   (eco, balanced, normal)

- DHW target temperature

- DHW offest (flow temp = target+ offset)

- Max reheat time. (was 2hrs, but i've turned this off now)

 

So you have:

 

100%? (to minimise the deltaT between flow/return; which for a given coil:water deltaT will minimise the flow temperature and maximise the COP at the expense of increased pump energy)

ECO (I would guess that ECO reheats at minimum output; normal at maximum output; and balanced between the two)

Target temperature (I did wonder why you were nailing this by running to 55C rather than [better] 48C but we'll come to the WWHRS later)

DHW offset (0...could there be a bug here I wonder...where 0 is effectively maximum rather than 0...perhaps try 1C so that it's not an implausible number...or even a more realistic number such as 5C?)

Max reheat time (off...I would guess this only exists to prevent the house cooling too much during DHW production)

 

 

8 hours ago, Dan F said:

 

It is outputting just over 5kW. Compressor speed is 50%.  (minumum output at A12W65 is 2.3kW at 25%)

 

It's definitely choosing to run at that then; rather than being limited by turndown

 

 

8 hours ago, Dan F said:

 

The primary coil is at the top of the tank and has around 1m2 surface area if the guy on the phone remembed correclty.

 

The PHE is equivilant to a 3m2 coil, which is what most HP coils are. 

 

Ok. But as above you're not turndown limited by the primary coil as it stands.

 

 

8 hours ago, Dan F said:

 

DeltaT (from coil:water) is actually 13C.

 

 

How so?

 

[simplistically] the Delta T for a heat exchanger is (mean temperature) - (mean temperature).

 

So for a radiator at 80/60C flow/return and a room at 20C the deltaT is 50C. (70C-20C)

 

That's what drive the heat transfer.

 

Flow 67C / Return 62C / Water 55C gives deltaT of just under 10C. (64.5C-55C)

 

Flow 65C / Return 64C would give you a similar deltaT and power transfer to water at 55C.

 

[calculating properly would involve logarithmic mean temperature differences; but for our purposes this is fine]

 

 

8 hours ago, Dan F said:

Yes, using PHE (equivilant to larger coil) means rehating cyclinder at once, albeit more efficiencly because i) lower max flow temp ii) flow temp ramps up rather than almost constantly being at 65C.

 

I disagree - that is assumption is NOT a given.

 

 

Dumbest option? You try to achieve max (e.g. 65C) flow temperature the entire time.

 

In practice even the dumbest unit trying to achieve 65C the entire time by maxxing out the compressor will probably end up "ramping" to a degree anyway because even at max refrigerant output a cold cylinder will draw more power from the coil than it is able to deliver; thus dragging the flow temperature down

 

 

Standard option? You run at a fixed compressor speed / pump speed.

 

This will result in ramping. You put approximately "X kW" into the coil and it reheats the cylinder. A cylinder at 10C will draw loads from the coil. Say a 3m2 coil gives a 10C deltaT at 5 kW. Coil runs at 22C flow / 18C return. A cylinder at 50C runs 62C flow / 58C return.

 

A half-full cylinder is trickier. If the coil is top to bottom (minimise convection currents) then initially the coil will cool the top of that cylinder and move the heat to the bottom. It's reasonable to assume that a half full cylinder is the same as a cylinder at 35C so runs at 47/43C. If the coil is only at the bottom the behaviour will depend on the convection currents being setup in the cylinder; iI would guess that it probably stirs it up more than a top to bottom coil but Mixergy will have modelled this better I'm sure.

 

Either way it's definitely ramping.

 

Here's a nice example of output-constrained ramping: for a "half full" cylinder (top at 50C but bottom has dropped to 40C

https://heatpumps.co.uk/2015/09/09/heat-pump-performance-monitoring-examples/

 

 

Any sales pitch that "only a plate can ramp" is absolutely bunk. 

 

Big coils are dirt simple and work adequately well.

 

 

Better option? (I think) You try to avoid mixing to less than say 45C.

 

If you're feeding a top to bottom coil and your minimum flow temp is 45C then you will never cool the cylinder below a "useful" delivery temperature. Speed up the compressor a bit / reduce the pump speed a bit so that the water into your coil is never below 45C and you avoid losing the hot top. 

 

Again works fine with a coil and will ramp.

 

 

Best option? Desuperheat during space heating.

 

There are three stages of cooling for the refrigerant

 

1) Desuperheat. This is taking "hot gas" and making it into "cool gas" (if it was water, then imagine steam at 130C cooled to steam at 101C)

2) Condense. This is taking "cool gas" and condensing it into "hot liquid" (if it was water, then imagine steam at 101C turns into water at 99C) - most of your heat is given off at this stage

3) Subcool. This is taking "hot liquid" and making it into "cool liquid" (if it was water then imagine water at 99C into water at 60C)

 

You will get superheat even when you are running in space heating mode. You only want say 35C flow but there will still be some superheat available at 60C+. Heat pumps with desuperheaters will divert the heat from this "hot gas" into the DHW cylinder whilst in space heating mode (subject to an upper limit - usually 95C unless regulations or scaling dictates otherwise). The advantage? You get the same COP as if you were taking all the heat off at 35C; but you get to have 10-20% of it at 60C+

 

 

These heat pumps will initially "bulk heat" to the minimum usable temperature (say 45C) then constantly "top off" during space heating to a much higher temperature.

 

I've only seen it implemented with ground source units. Never with monobloc air source units. Not really worth it for low energy houses.

 

Again this works fine...with a coil...and will ramp.

 

 

 

An aide? Sillier games with subcooling and hardware cooling. 

 

 

Very big stuff might have subcoolers on it (where you're returning at a particulary low temperature) and oil / electrical cooling of components. Electrical stuff can run hot. Hotter than the condenser. So use that for "top up" heating. Oil can run stupidly hot. So use that for "final" heating. I think the RED / Octopus units implement some of this. (compressor heat / inverter losses as "finishing heat" rather than lost to atmosphere.

 

I'm the kind of saddo that went to see this for tourism. It's an absolute work of art of staged heat transfer to achieve COP 3 from 90C flow / 65C return with a source temperature in the single digits. Plenty of plate heat exchange here because it isn't working to batch heat. 🙂

 

https://en.wikipedia.org/wiki/Drammen_Heat_Pump

 

 

Biggest advantage of plates? Cleaning.

 

Single biggest advantage of plates is cleanliness. If you have naff water (well water poorly filtered; hard city water) then it's easier to unbung a plate than it is to unbung a coil. Minimising the cylinder temperature to avoid scaling and filtering your water should probably be the first port of call though!

 

 

 

8 hours ago, Dan F said:

 

I liked the "top-up" approach Mixergy provides give we don't need 300L of water most of the time.

 

That's fair.

 

It's a definite advantage in a fossil fuelled scenario, or an electric immersion scenario, where there is no penalty for heating at a high deltaT and some benefit in reducing heat losses during the summer.

 

It would be an advantage in a district heating scenario too; only if the mixergy unit were to control the primary flow properly. (it doesn't on a district heating application - you're better buying from thermal integration who understand this use case)

 

And it would be an advantage in a CO2 heat pump scenario; again only if the mixergy unit were to control the primary flow properly.

 

With F-Gas / Propane heat pumps...the performance loss from "top up" heating mostly likely outweighs any benefit from reduced heat losses during the summer. It's a quality tank though and it has neat graphs. 😉

 

 

8 hours ago, Dan F said:

I could try to model both scenarios and see which makes most sense.

 

Well volunteered! 😉

 

(and I'd be interested)

 

If (average) COP = 3 for water heating to 48C the traditional way; vs (average) COP = 2 for water heating to 55C the mixergy way; I'd guess that the standard way wins if your usage is > 2 kWh/day.

 

Don't forget the primary losses. These will be similar regardless of how much you're heating and will not be trivial compared with cylinder losses.

 

 

 

8 hours ago, Dan F said:

There is an additional complication in the model in our case though; WWHRS.   The WWHRS units (at least i the configuration we are using) are a fair bit more efficient when the UVC temperature is higher.

 

That'll be plumbed "wrong" for this application - should be feeding both the UVC and the cold feed to the shower; with the UVC set to 48C. Is it too far from the UVC to make this viable? Does it have too high a pressure drop? More than one shower making it impossible?

 

Inclusion of this will make the performance conclusions very shades of grey. Any potential to monitor it?

 

 

8 hours ago, Dan F said:

 

With a UVC temperature of only 48C they are ineffective simply because the cold flow rate through the heat exchanger is very low.

 

 

If plumbed "wrong" yes. (quotation marks used because the ideal plumbing might not be a practical option)

 

Work just fine for preheating the feed to a cylinder. What is your COP for heating water from 10C to 25C though? (likely lots)

[Dan F]

4 hours ago, markocosic said:

100%? (to minimise the deltaT between flow/return; which for a given coil:water deltaT will minimise the flow temperature and maximise the COP at the expense of increased pump energy).  Runs at about 1050l/h (88% of spec sheet). Maybe slightly limited by pressure drop.

ECO (I would guess that ECO reheats at minimum output; normal at maximum output; and balanced between the two). Currently Eco. See below for mode details.

Target temperature (I did wonder why you were nailing this by running to 55C rather than [better] 48C but we'll come to the WWHRS later). Mixergy minimum when using top coil is also 50C

DHW offset (0...could there be a bug here I wonder...where 0 is effectively maximum rather than 0...perhaps try 1C so that it's not an implausible number...or even a more realistic number such as 5C?). It was 8C, i lowered in when trying to constraint t he flow temperature.

Max reheat time (off...I would guess this only exists to prevent the house cooling too much during DHW production). Yes. Unless it's used in conjunction DHW sensor gradient to ajust flow temp, I don't think so though.

 

4 hours ago, markocosic said:

low 67C / Return 62C / Water 55C gives deltaT of just under 10C. (64.5C-55C)

Right.  I wasn't using average.

 

4 hours ago, markocosic said:

13 hours ago, Dan F said:

Yes, using PHE (equivilant to larger coil) means rehating cyclinder at once, albeit more efficiencly because i) lower max flow temp ii) flow temp ramps up rather than almost constantly being at 65C.

I disagree - that is assumption is NOT a given.

Fair enough, but without modelling it, I do think it's fairly obvious that it would be more efficent as the flow temperature wouldn't be 65C for the whole time.

 

4 hours ago, markocosic said:

Either way it's definitely ramping.

I has an initial 15min ramp as ASHP kicks-in and the top of the tank is brough up to 55C, but after this there is 1hr+ at 67C almost fixed.  This is a direct result of the Mixergy design and the way it keeps the hot top by spraying in cold water from the bottom of the tank.

 

4 hours ago, markocosic said:

With F-Gas / Propane heat pumps...the performance loss from "top up" heating mostly likely outweighs any benefit from reduced heat losses during the summer. It's a quality tank though and it has neat graphs. 😉

Yeah!  At a flow temp of 55C I was fairly convinced the top-up would outweigh the improved COP.  But at flow temp of 67C this starts to become very questionable.  If i reduce the mixergy temp to 50 (and therefore flow temp to 62), it might start to make a little bit more sense though as COP be more like 2.5+, rather than 1.9.

 

4 hours ago, markocosic said:

If (average) COP = 3 for water heating to 48C the traditional way; vs (average) COP = 2 for water heating to 55C the mixergy way; I'd guess that the standard way wins if your usage is > 2 kWh/day.

Heating to 55C the mixergy way is around 1.9.  Heating to 50C the mixergy way should be closer to 2.5.  Hard to know what the traditional way would be, but I'd expect closer to 3.5+ given A12W45 at

 

4 hours ago, markocosic said:

That'll be plumbed "wrong" for this application - should be feeding both the UVC and the cold feed to the shower; with the UVC set to 48C. Is it too far from the UVC to make this viable? Does it have too high a pressure drop? More than one shower making it impossible?

 

Inclusion of this will make the performance conclusions very shades of grey. Any potential to monitor it?

50% compressor is COP 4.

 

4 hours ago, markocosic said:

That'll be plumbed "wrong" for this application - should be feeding both the UVC and the cold feed to the shower; with the UVC set to 48C. Is it too far from the UVC to make this viable? Does it have too high a pressure drop? More than one shower making it impossible?

 

Inclusion of this will make the performance conclusions very shades of grey. Any potential to monitor it?

Showers are not close to UVC so wasn't really practical. Also at the time I was assuming I'd be using "top-up" (didn't realise it would require 67C flow temp) and other approachs for WWHRS would have messed up the stratification I think.  The set up I'm using still provides significant amount of recovery, just not if DHW is at 45C of course.   Yes I can monitor this easily as I have heat meter on UVC and flow meter on cold water, so it's a case of looking at the ratio of cold/hot by substracting these flows rates.  The other approah is to look at the instantaneous kW output from UVC required for 41C shower at different UVC temperatures.   I'd need to sit down and write up some test scenarios though.

 

DHW Modes

Normal: Max. compressor speed 120 rps possible.

Eco: The max. compressor output is reduced to 50 rps (S+M)/40 rps (L). The speed limit is lifted at air inlet temperatures below -7 °C.

Balance: If return temperature in the cylinder charging circuit, is equal to or below 45 °C, the full max. compressor output (Eco) is enabled, while at temperatures above that, the reduced max. compressor output is enabled.

Edited May 5, 2022 by Dan F

[markocosic]

So:

 

Normal: no limit on compressor

Balanced: limits max compressor to 50% for finish-heating

Eco: limits max compressor to 50%

 

The unit is choosing to run up to that 50% limit though; irrespective of cylinder temperature and the target differential. The latter is confusing. Perhaps it will behave differently charging a "25C" cylinder and a "40C" cylinder and a "50C" sylinder in ECO mode? Are you sure that it's reading the fake cylinder temperature correctly? Perhaps it assumes use of a 3 m² coil (or equivalent) and it only really controls to the target differential when in normal mode?

[BenY]

 

 

This is my first post! Apologies if I'm in the wrong place or should have started my own thread.

 

I have a 5Kw AroTherm Plus and a 200L UVC .  The UVC is designed for a heat pump and I heat my tank each morning to 50C.  I am trying to figure out the correct flow temperature to use to be most efficient?   

The DHW offset setting on my system seems to work and yesterday I lowered the offset from +20 to + 10 and saw the flow temp correspondingly drop from 70C to 60C. I confess I only watched it for a minute or so as I don't as yet have monitoring in place.

 

Id like to be able to reproduce the graph from the first post in order to measure everything, @Dan F  Would you mind providing some info on how you got the data and what software you are using?  I'm presuming you are using ebus to get the info.  I also use eeBus with Loxone but it wont provide that level of detail which is a real shame.  Specifically wondered what hardware adapter you used to talk to ebus?  I have reserved an eBUS Adapter 3 but it's unlikely to arrive for 6 months so would like as simple an alternative as possible.  My soldering skills are non existant.

 

 

 

[Dan F]

On 24/09/2022 at 12:38, BenY said:

Id like to be able to reproduce the graph from the first post in order to measure everything, @Dan F  Would you mind providing some info on how you got the data and what software you are using? 

This graph is from the PassivLiving MMSP package which was installed as an extra as part of install.

 

On 24/09/2022 at 12:38, BenY said:

I also use eeBus with Loxone but it wont provide that level of detail which is a real shame.

I use eeBus for controlling heating in the winter based on slab/room temperatures and tariffs, really annoyed that I can't control cooling via EEBus though.

 

On 24/09/2022 at 12:38, BenY said:

Specifically wondered what hardware adapter you used to talk to ebus?  I have reserved an eBUS Adapter 3 but it's unlikely to arrive for 6 months so would like as simple an alternative as possible.  My soldering skills are non existant.

I don't have any ebus hardward and haven't tried this yet.  It's on my TODO list though, as I really do want to find a way to control cooling, if possible.  Vaillant promised a Modbus interface, but nothing exists yet 😞. My only concern with ebus is I haven't seen a coprehenisve list of registers/commands for arothem plus anywhere.

 

[BenY]

On 24/10/2022 at 00:35, Dan F said:

This graph is from the PassivLiving MMSP package which was installed as an extra as part of install.

 

Oh I see, bad assumption on my part that you had used ebus.  I really want to get some detailed info out of the vaillant but there seems to be no way to do that at the moment.  All the ebus interfaces are out of stock. 

 

On 24/10/2022 at 00:35, Dan F said:

I use eeBus for controlling heating in the winter based on slab/room temperatures and tariffs, really annoyed that I can't control cooling via EEBus though.

 

I'm currently doing the same for the heating.  I think the vaillant eeBus implementation is terrible.  So little data is provided it's almost pointless.  It also occasionally ignores commands from my loxone system so I've had to configure an alerting system for when it gets out of sync!

 

On 24/10/2022 at 00:35, Dan F said:

 My only concern with ebus is I haven't seen a coprehenisve list of registers/commands for arothem plus anywhere.

I've looked through the github repo for the ebusd templates and someone appears to have found the cooling commands necessary (https://github.com/john30/ebusd-configuration/pull/191) although they are not on the main code branch yet.  (https://github.com/john30/ebusd-configuration/blob/master/ebusd-2.1.x/en/vaillant/15.700.csv) Given I might have to wait for another year to get ebus to work I've not looked into it any further but it does give me hope for an all singing all dancing ebus system in the future.

 

 p.s.  New to the forum, I forgot to follow this topic so I didn't see your reply until just now.   

 

 

[Dan F]

36 minutes ago, BenY said:

Oh I see, bad assumption on my part that you had used ebus.  I really want to get some detailed info out of the vaillant but there seems to be no way to do that at the moment.  All the ebus interfaces are out of stock. 

 

I'm currently doing the same for the heating.  I think the vaillant eeBus implementation is terrible.  So little data is provided it's almost pointless.  It also occasionally ignores commands from my loxone system so I've had to configure an alerting system for when it gets out of sync!

 

I've looked through the github repo for the ebusd templates and someone appears to have found the cooling commands necessary (https://github.com/john30/ebusd-configuration/pull/191) although they are not on the main code branch yet.  (https://github.com/john30/ebusd-configuration/blob/master/ebusd-2.1.x/en/vaillant/15.700.csv) Given I might have to wait for another year to get ebus to work I've not looked into it any further but it does give me hope for an all singing all dancing ebus system in the future.

 

 p.s.  New to the forum, I forgot to follow this topic so I didn't see your reply until just now.   

 

Might be worth badgering Vaillant about alternate approach to external control as apparently there is a modbus solution that they have using (or experimenting with) with some customers.

 

[BenY]

10 hours ago, Dan F said:

Might be worth badgering Vaillant about alternate approach to external control as apparently there is a modbus solution that they have using (or experimenting with) with some customers.

 

I've got both my ashp installer and my loxone installer to try but really there is little to no progress on either an update to their eebus integration or using modbus.  They are probably talking to the wrong people but I also got feedback that there is a disconnect between Germany and the UK anyway so it's hard to make progress from the uk.  I'm mailing jens.wichtermann@vaillant-group.com who's email is listed on the eebus announcment from a few years ago but I doubt it will help lol.

 

[BenY]

A little update on cooling.  I now have cooling working with external control on my vaillant.  I'm using ebusd, loxberry ebus plugin and loxone to do this.    Cooling can be controlled and the cooling target temperature set.   I have some more experimenting to do with the settings but basically all good.

 

 

[Dan F]

4 hours ago, BenY said:

A little update on cooling.  I now have cooling working with external control on my vaillant.  I'm using ebusd, loxberry ebus plugin and loxone to do this.    Cooling can be controlled and the cooling target temperature set.   I have some more experimenting to do with the settings but basically all good.

 

 

Nice!  I'm still waiting on an ebusd adapter still (I never ordered one early on as I was holding out for a Vaillant solution).  Once I get it though, I'll be very keen to get this set up and may give you a shout.

 

My main issue is the current Vaillant-based controls is that the response time from cooling the slab is so slow, that the room thermostat isn't really suitable.  Instead, it'd be much better to calculate a desired slab temperature based on room temperature + weather forecast and use this. via loxone+ebusd to control ASHP.

 

What is your control strategy?

Edited June 28, 2023 by Dan F

[BenY]

16 minutes ago, Dan F said:

What is your control strategy?

 

Not too complex at the moment.  I'm just cooling a few rooms using the standard loxone intelligent room controller.  I've set the temp it's trying to maintain at 24 degrees and loxone does the rest.  I have fan coils for some rooms and i'm just using them.  I haven't started cooling the slab yet.