ASHP - using a low loss header and modulation instead of a buffer

[ReedRichards]

40 minutes ago, gravelld said:

BTW, what are the rules regarding RHI, MCS etc when sizing radiators? Must the installer prove the radiators are adequately sized on a per room basis? I know they have to prove heat loss and the suitability of the heat pump, just unsure about emitters.

 

For example, might it be ok to allow some rooms to be slightly undersized and other, next door rooms, oversized?

 

The installer does not have to prove that the radiators are adequately sized on a per room basis.  My kitchen and a small en-suite bathroom did not have adequately sized radiators but in both cases it was difficult to replace them with larger ones.  The shortfall was brought to my attention and I had to agree to it.  Other rooms had somewhat oversized radiators so the house as a whole was fine.  Both rooms do now sometimes feel a little cool if their doors have been closed for a long period so with hindsight maybe I should have tried a little harder to find better radiators. 

[ReedRichards]

10 hours ago, pdf27 said:

One thing to note here: thermostatic radiator valves are probably not helpful with a heat pump:

• As already noted, water volume is reduced when they close increasing short-cycling problems.
• Closing a valve reduces the available radiator area, meaning the water needs to be run hotter to heat the rest of the house.
• Unheated rooms will mean that the radiant temperature of the walls from those rooms is lower, so the rest of the house needs to be slightly warmer for comfort - again increasing the flow temperature.
• Ideally a heat pump would use weather-compensation to adjust the flow temperature to the minimum possible - TRVs will make this much harder.

Essentially they work by allowing you to run some rooms cooler, reducing total heat demand. However, this is not the same as reduced **energy** - heat pumps benefit very strongly from reduced flow temperatures, and it is probable that they'll be better off running cooler with the TRVs removed or fully open.

 

This may be right but I'm not convinced.

• If you have UFH is not each room controlled by its own thermostat with the equivalent action to a TRV?
• If one room is slightly hotter than the rest of the house then other rooms will derive a small benefit from this and need less heat.  If one room is slightly cooler the the rest of the house will need slightly more heat.  But surely this is more to do with the size of the radiator relative to the need of the room, TRVs can compensate for oversized radiators so your house needs less heat than it otherwise would.
• My heat pump has a weather compensation feature but this just sets the leaving water temperature according to the outside temperature.  It knows nothing about what the TRVs are doing, just whether the room thermostat is calling for heat or not.    

 

 

[joth]

8 hours ago, SteamyTea said:

You still got that spreadsheet and are you willing to share it?

I do. I'll try and find my copy of Excel and tidy up a version and DM you

 

[gravelld]

Flow temperature 45C (∆21.5)

I used @ReedRichards' chart above (thanks!) and interpolated (in my head) a correction factor of 0.63 from ∆30.

 

So basically there are four new radiators required, and swapping about of existing radiators in five other cases (out of a total of 23 radiators).

 

The difference in RHI is £700 - I'm not sure if it's going to pay for itself, but it might be worth it anyway.

[SteamyTea]

18 minutes ago, joth said:

I do. I'll try and find my copy of Excel and tidy up a version and DM you

 

Thank you

[pdf27]

On 05/01/2022 at 20:34, ReedRichards said:

 

This may be right but I'm not convinced.

• If you have UFH is not each room controlled by its own thermostat with the equivalent action to a TRV?
• If one room is slightly hotter than the rest of the house then other rooms will derive a small benefit from this and need less heat.  If one room is slightly cooler the the rest of the house will need slightly more heat.  But surely this is more to do with the size of the radiator relative to the need of the room, TRVs can compensate for oversized radiators so your house needs less heat than it otherwise would.
• My heat pump has a weather compensation feature but this just sets the leaving water temperature according to the outside temperature.  It knows nothing about what the TRVs are doing, just whether the room thermostat is calling for heat or not.    

1. If it is you have the same problem: if you control everything on roomstats (TRV or conventional roomstat) then because they'll never quite agree the system will spent a lot of it's time heating the house only using one or two radiators (room-to-room heat transfer typically being much stronger than room-to-outside). This is masked with a boiler where you can easily oversize and accept a modest penalty in efficiency and cost, but this is much more of a problem with heat pumps. 
2. Correct, in so far as the mean internal temperature is potentially slightly reduced by running some rooms a bit cooler than otherwise. This will reduce the heat loss through the fabric implying slightly reduced total heat demand. However, with a heat pump you also have to cope with the effect that it does this by greatly reducing the surface area of radiators. This means that for the same heating power delivered to the house the water needs to be warmer in the circuit, reducing the COP (boilers also see this effect, but it is much weaker). Unless you have very big temperature differences between rooms with all the TRVs wide open (in which case they are needed for comfort), the COP effect from cooler water will outweigh the total heat demand effect from cooler rooms.
3. Indeed. The problem is that you've got two control loops if you have both a TRV and weather compensation active: the weather compensation curve is based on the assumption that the radiator area is predictable (i.e. for a given outside temperature, the TRV valve position and hence total radiator area is known).  In this situation one of three things can happen:
   1. The weather compensation is set correctly, so the correct room temperature is only achieved with the heat pump running steadily and all the room stats open (i.e. the water is at the lowest temperature at which it an achieve the desired comfort level). In this case both the thermostat and TRVs are essentially redundant.
   2. The weather compensation is set too low, so the thermostat and/or TRVs are continually calling for heat and the house is always too cold.
   3. The weather compensation is set too high, so first the TRVs will close off sections of the loop and then the thermostat will stop calling for heat. Because the volume of water in the loop will be quite low as the TRVs start to close, a buffer tank will be required to prevent short cycling.

It is also worth noting here that for very well insulated houses (such as those many people on here have built), an additional effect comes into play. Heat dissipation varies as a power law with the temperature difference between the floor and room. This has a stabilising effect - if the flow temperature is 25°C and the emitter is sized to keep the room at 21°C on a design cold day, even with no control system whatsoever then the room must be between 21 and 25°C. Underfloor heating will happily do this (from memory @jack runs his flow temperature at 25°C), radiators will struggle.

[ReedRichards]

My heat pump does not work like yours, @pdf27  It runs in 15 minute cycles,

1. The fan comes on
2. A minute of two later the compressor comes on.
3. The leaving water temperature is raised to a maximum which is either determined by a fixed setting or by the weather compensation algorithm.
4. The heat pump tries to maintain this temperature
5. If the room thermostat is satisfied (possibly also if the return water temperature gets too high) the compressor and fan are switched off.
6. The cycle repeats 15 minutes from the start of the previous one.

If the house needs more heat then the duration of the time at the maximum water temperature is increased.  But there is no change in this water temperature and so no change in the efficiency, whatever the demand.

[ReedRichards]

Thinking some more, the more generic reason why @pdf27's second point is wrong is that it assumes that the heat pump knows how hard it has to work and can change the water temperature accordingly.  It could do this by monitoring the temperature in the room with the control thermostat and modifying the water temperature according to how far the set temperature was above the actual temperature.   This would be a form of "Load Compensation", which is a feature that is becoming quite common on gas boilers.  But unfortunately it is not a common feature on ASHPs, most of them don't seem to have it.  That is a great pity because it is a means of getting more economical operation.  But I think you can hang on to your TRVs unless you have Load Compensation.

[J1mbo]

Vaillant controls have load compensation. It works by shifting the weather compensation curve at 45* in either direction in their case. It means that the flow temperature is based on three temperatures - outside, inside, and set point, which makes the system quite responsive.

[ReedRichards]

Yes, @J1mbo , I think I would have gone with Vaillant had I known this at the right time.   If more manufacturers of ASHPs also made gas boilers then maybe we would see Load Compensation more widely available.  

[Kevm]

3 hours ago, ReedRichards said:

Yes, @J1mbo , I think I would have gone with Vaillant had I known this at the right time.   If more manufacturers of ASHPs also made gas boilers then maybe we would see Load Compensation more widely available.  

 

Mitsubishi do too, via what they call Auto Adaptation.  For this to work the Ecodan controller needs to 'know' the room temperature.  It can't do this if 3rd party thermostats are used and that's how most seem to be installed.    It needs a Mitsubishi wireless controller, a wired in thermistor or the main Ecodan controller to be located in the room that's being heated. 

[J1mbo]

Same with the Vaillant @Kevin - IMHO the installation of the systems with third party controls should be specifically banned as these are the cause of the public perception problem (obviously that is just MHO).

[ReedRichards]

Or alternatively perhaps they could be built to conform to the same Opentherm standard as many gas boilers are - in which case any third party controller that supports Opentherm would be suitable.

 

My LG heat pump has the option of control based on the leaving water temperature and/or  the room temperature but what the "and" option actually does is completely undocumented.

[joth]

22 minutes ago, ReedRichards said:

Or alternatively perhaps they could be built to conform to the same Opentherm standard as many gas boilers are - in which case any third party controller that supports Opentherm would be suitable.

 

+1 to that.

 

I have Loxone control system that can modulate the flow temperatures very nicely, but trying to get the ecodan to listen to it was a job of work: HTTP to Home Assistant, experimental for of the MELcloud integration sends it to the Mitsubishi cloud and from there back to my FTC6. .

I wouldn't consider such a pukeworthy setup for long term use, but have done it for now just to test out how much more control it gives me over flow temperatures than using the FTC built in compensation curve.

(actual call for heat is still done via the relay dry contact input to the FTC, so if the cloud is unavailable it should still all still work, just with suboptimal flow temps)

[Kevm]

1 hour ago, joth said:

+1 to that.

 

I have Loxone control system that can modulate the flow temperatures very nicely, but trying to get the ecodan to listen to it was a job of work: HTTP to Home Assistant, experimental for of the MELcloud integration sends it to the Mitsubishi cloud and from there back to my FTC6. .

I wouldn't consider such a pukeworthy setup for long term use, but have done it for now just to test out how much more control it gives me over flow temperatures than using the FTC built in compensation curve.

(actual call for heat is still done via the relay dry contact input to the FTC, so if the cloud is unavailable it should still all still work, just with suboptimal flow temps)

 

that good? ?

Does it work any better?   Are you just changing the flow temp/ambient temp relationship and letting the Ecodan work to the target or are you doing anything more sophisticated?

[joth]

12 hours ago, Kevm said:

 

that good? ?

Does it work any better?   Are you just changing the flow temp/ambient temp relationship and letting the Ecodan work to the target or are you doing anything more sophisticated?

Ha. Well it was working ok but failed to come on last night: at some point yesterday I'd fat-fingered the thermostat control in Home Assistant and turned it from Flow to Room based control. Exactly the sort of error that shows how in appropriate HA is for heating controls unless really carefully done.

 

Anyway in principle I can get much finer control driving flow temperature from Loxone as it can see every room temperature, outdoor temp (Inc recent averages and forecast), UF slab temperature, current energy price, number of people in the house, etc...

My current project is maximizing heat output during the Octopus Go cheap rate, which means running it at the max safe flow temperature. So I can initially put e.g. 38°C water into the pipes and back it off as the screed starts to warm up, to stabilise around 29C flow temp for 26C screed max temp.

 

 

[J1mbo]

It’s an interesting project and credit where it’s due for getting it to work. I like Home Assistant and Home Bridge stuff.

 

I wonder if the effort is worth it financially (this is not a criticism btw and I would be and still am tinkering with my system anyway, because it’s interesting). I’ve been modelling Heat loss and heat pump performance based on hourly temperature data from literally across the road which is seeming to indicate that the traditional turn off heating at night really saves 10% of cost for gas boilers.

 

With heat pump the observed performance of my system interestingly is far more influenced by outside temperature than flow temperature. Therefore running at night on cheaper electricity (e7) is offset by the lower COP given the lower average temperature.

 

Anyway, I’d be interested in your findings too.

 

 

[SteamyTea]

7 minutes ago, J1mbo said:

With heat pump the observed performance of my system interestingly is far more influenced by outside temperature than flow temperature

Interesting.

Is it actual OAT or RH, or a combination of the two, especially in the range where heating first gets used in earnest i.e. 8°C down to 0°C.

Do you also compare the electrical energy, and the thermal energy, which is in effect the variable CoP?

[joth]

2 hours ago, J1mbo said:

With heat pump the observed performance of my system interestingly is far more influenced by outside temperature than flow temperature. Therefore running at night on cheaper electricity (e7) is offset by the lower COP given the lower average temperature.

 

Octopus Go is 4x the price in peaks vs off-peak, so it'd take a massive drop in COP overnight to offset the financial benefit.  I guess if the outside temperature subzero and so doing a lot of defrost cycles it might approach or even fall below a COP of 1, but I've not seen that in practice.

 

If this was purely about Go I wouldn't be so interested, but the same applies on sunny days (like today) when the midday sun is generating >3kW we can run heating at 1/4 of the (opportunity) cost of importing electricity, and again it makes sense to run the ASHP as hard as possible when the electricity is cheap. Indeed, I can perhaps even modulate the flow temperature based on the exact level of excess generation, but I'm a long way off that! (Eventually I'd like to see if I can make the smarts smart enough to deal with dynamic pricing like Octopus Agile; while the last 6 months has shown the version of Agile they implemented reveals too much exposure to wholesale prices, I can imagine a future variant that has a capped 24-hour average price but dynamically shifts around the peaks and troughs of pricing throughout the day based on supply/demand patterns.)

 

Altogether the project is see how far I can get with demand load shifting, before inevitably throwing in the towel and buying a house battery.

 

[J1mbo]

Haha yes I’m the same on the battery. Solar just doesn’t make sense without one with the disparity between import and export rates atm.

 

Whilst the Agile tariff has been pegged at 35p pretty much since October, it seems that we’ll  all be paying that under the new cap in April anyway.

[J1mbo]

2 hours ago, SteamyTea said:

Interesting.

Is it actual OAT or RH, or a combination of the two, especially in the range where heating first gets used in earnest i.e. 8°C down to 0°C.

Do you also compare the electrical energy, and the thermal energy, which is in effect the variable CoP?

No looked at humidity yet. This is a good idea actually as there are some days there the COP is materially different to others despite similar profiles. I have tried to model solar heat gain.

Edited January 9, 2022 by J1mbo

[SteamyTea]

11 minutes ago, J1mbo said:

No looked at humidity yet. This is a good idea actually as there are some days there the COP is materially different to others despite similar profiles. I have tried to model solar heat gain.

Taking an educated guess here, days of high humidity should cause more frosting up, but that may not correlate to a different CoP as it depends on how the HP defrosts.

Modelling solar gain is tricky.  Easy enough though windows, but not so easy though walls as they can take too long to warm up to affect the losses/gains significantly.

Worth looking at wind speed and direction though. Generally the SW wind is warm and moist, NE is cold and dry.  The SW winds tend to be faster than the NE ones.

But, SW tends to be cloudy, NE not.

I modelled my house for wind speed and direction a few years back think the charts are over at the other place.  where I am we have so few NE winds, and the speed was pretty slow anyway, that solar gain did make a difference, but the SW winds dominated so much that it was not worth the worry.

[ian192744]

On 08/01/2022 at 18:17, joth said:

+1 to that.

 

I have Loxone control system that can modulate the flow temperatures very nicely, but trying to get the ecodan to listen to it was a job of work: HTTP to Home Assistant, experimental for of the MELcloud integration sends it to the Mitsubishi cloud and from there back to my FTC6. .

I wouldn't consider such a pukeworthy setup for long term use, but have done it for now just to test out how much more control it gives me over flow temperatures than using the FTC built in compensation curve.

(actual call for heat is still done via the relay dry contact input to the FTC, so if the cloud is unavailable it should still all still work, just with suboptimal flow temps)

did either of you solve this question? I have honeywell evohome with opentherm to mod-con viessman boiler. looking for an ASHP that can consume third party stat input (large house, need programmable TRV's for reasons much discussed already, I know I'll need a fair-size buffer to maintain volume when TRV's shut). if ASHP can consume opentherm instead of on/off, so much the better.

[ReedRichards]

I've read that the Vokera AriaPRO supports OpenTherm - but their website isn't working at the time of writing this.  If it does support OpenTherm it's the only one.  

[jack]

On 07/01/2022 at 13:54, pdf27 said:

It is also worth noting here that for very well insulated houses (such as those many people on here have built), an additional effect comes into play. Heat dissipation varies as a power law with the temperature difference between the floor and room. This has a stabilising effect - if the flow temperature is 25°C and the emitter is sized to keep the room at 21°C on a design cold day, even with no control system whatsoever then the room must be between 21 and 25°C. Underfloor heating will happily do this (from memory @jack runs his flow temperature at 25°C), radiators will struggle.

 

I increased my temps slightly, and have increased temperature compensation. From memory, I now have something like 26 ºC for temps above maybe 8 ºC, with weather compensation kicking in below that to a high of 29 ºC as the temperature falls to around 0 ºC.

 

I'll likely be increasing these temps a bit further now that I'm going to try squeezing more of my heating into the Octopus Go cheap period. 

 

Also, it's worth bearing in mind that the ASHP stops heating when the return temp reaches the flow temp. That will be well before the slab temperature reaches the flow temperature. I know you're saying this approach to heating puts a cap on room temp, but unless there's a lot of solar or incidental gains, the room temp is very unlikely to reach anything like the flow temp.

 

As an example, I ran our ASHP open loop through two winters without a thermostat (just relied on flow temp control), and I don't believe we ever got the room temp above about 22 ºC.  

 

On 09/01/2022 at 12:29, SteamyTea said:

Taking an educated guess here, days of high humidity should cause more frosting up, but that may not correlate to a different CoP as it depends on how the HP defrosts.

 

True for high humidity at low temperatures, but when it's mild I assume higher humidity also means additional efficiency due to energy gained from condensation of water vapour.

 

Presumably there's also energy extracted from the phase change to ice, but that's more than balanced out by the corresponding need to defrost.