Heat demand too low for Vaillant Heat pump?

[Nickfromwales]

35 minutes ago, Ewan said:

what’s dynamic bypass?

27 degrees shouldn’t be exceeded for what reason, flooring?

Variable resistance created by an automatic bypass ( sprung ), installed immediately prior to the manifold rails across, flow and return, which closes as the resistance downstream becomes lesser, and vice versa. 
 

General health and safety foremost, and yes, flooring manufacturers stipulate max temps before certain types of flooring get damaged / adhesives begin to reconstitute etc. 

[Vilks]

I have been going down the same rabbit hole. Trying to find the smallest unit that can satisfy at -2C and then be efficient at warmer temperatures. I can give you what I have found.

 Page 92  - min output of 2.1 kW for both 3 kW and 5 kW Vaillant units. Not the most informative table but then again, I have only seen Mitsubishi have better table than this one.

LG Therma V 3 kW unit - does not specify lowest output and jumps around with flow temperatures at different outdoor temperatures. You could guestimate around 1 kW (4.18 x min water flow of 0.25l/s). Although, according to Vaillant, you should spec heat pump for most of the year rather then couple of cold days when backup heating could be used and even though getting lower COP for those days, overall SCOP should be better when heat pump could run more efficiently rest of the year.

Research paper talks about 10 min being the minimum runtime: “…due to the need for a minimum run time to ensure good lubrication and negative impacts on compressor reliability” (Page 62).

Other users have found ways to calculate minimum system volume or buffer size from their own or other sources (I found this, but they even go as far as 20 min minimum runtimes).

So, if 2.1 kW is the minimum HP output and you suspect 1 kW is your lowest demand for warm temperatures (worst case for the heat pump) then if @JamesPa formula is used. You should get 31.58 Litres of minimum system volume to prevent Heat pump from running less than 10minutes, or 63.16litres for 20minutes (at delta5).

 

Edited December 28, 2022 by Vilks

[JohnMo]

10 hours ago, Vilks said:

found this, but they even go as far as 20 min

It actually says no more than 3 starts per hour.  So 10 mins run time, 10 on, 10 off - X3 cycles.

 

Also saying 10 to 20 could be desirable, but in a general context not specific to heat pumps.

 

So if you need 32L in your system - so yours is big enough not to need a buffer, at 69L.  Just run as a single or two zone system and make sure your open system is always big enough.

 

Questions

Do you need Trvs on the radiators?  

Could you balance the system instead?

Do you need more than one thermostat?

 

[ReedRichards]

At the moment my house is showing  cycles of power consumption which last about 20 minutes with flat-topped peaks lasting about 10 minutes and peaking at about 2750 W (give or take 100 W).  I have a 12 kW LG Therma V so seemingly it can manage a good range of modulation. But I don't know if this scales down to the lower output models.   

[SteamyTea]

5 minutes ago, ReedRichards said:

But I don't know if this scales down to the lower output models.   

Thinking about this modulation a little bit.  As an ASHPs performance is reduced when the external air gets colder, is the minimum modulation output a fixed amount i.e. 1.2 kW, or a percentage of deliverable power  i.e. 20% of what it can achieve at the temperature.

 

Quite often a supplementary heater kicks in and makes it look better than it really is.

Edited December 29, 2022 by SteamyTea

[PhilT]

7 hours ago, SteamyTea said:

Thinking about this modulation a little bit.  As an ASHPs performance is reduced when the external air gets colder, is the minimum modulation output a fixed amount i.e. 1.2 kW, or a percentage of deliverable power  i.e. 20% of what it can achieve at the temperature.

Does this shed any light? I think this says not exactly either but both within a tight range down to -7. Ecodan R32 11.2kW nominal.

 

Flow 45degC	temp	output	COP	input	output	output	input	input
					% of nom.	% of max.	% of nom.	% of max.
Min capacity	-10	5.7	2.25	2.53	56%	56%	56%	56%
	-7	3.4	2.3	1.48	30%	30%	33%	33%
	2	3.7	2.75	1.35	33%	31%	33%	28%
	7	3.5	3.5	1.00	31%	28%	33%	28%
	12	3.7	4.95	0.75	33%	28%	32%	26%
Nominal	-10	10.1	2.25	4.49
	-7	11.2	2.5	4.48
	2	11.2	2.74	4.09
	7	11.2	3.7	3.03
	12	11.2	4.85	2.31
Max capacity	-10	10.1	2.25	4.49
	-7	11.2	2.5	4.48
	2	11.9	2.49	4.78
	7	12.7	3.55	3.58
	12	13	4.5	2.89
Flow 35degC	temp	output	COP	input	output	output	input	input
					% of nom.	% of max.	% of nom.	% of max.
Min capacity	-10	6.2	2.75	2.25	61%	55%	61%	52%
	-7	3.9	2.85	1.37	35%	32%	37%	32%
	2	4.2	3.75	1.12	38%	34%	34%	28%
	7	4	4.45	0.90	36%	30%	38%	30%
	12	4	5.85	0.68	36%	29%	37%	26%
Nominal	-10	10.1	2.75	3.67
	-7	11.2	3	3.73
	2	11.2	3.44	3.26
	7	11.2	4.7	2.38
	12	11.2	6.05	1.85
Max capacity	-10	11.2	2.6	4.31
	-7	12.1	2.8	4.32
	2	12.5	3.17	3.94
	7	13.5	4.55	2.97
	12	13.7	5.25	2.61

[Vilks]

12 hours ago, JohnMo said:

It actually says no more than 3 starts per hour.  So 10 mins run time, 10 on, 10 off - X3 cycles.

 

Also saying 10 to 20 could be desirable, but in a general context not specific to heat pumps.

 

So if you need 32L in your system - so yours is big enough not to need a buffer, at 69L.  Just run as a single or two zone system and make sure your open system is always big enough.

 

Questions

Do you need Trvs on the radiators?  

Could you balance the system instead?

Do you need more than one thermostat?

 

What is a definition of a full cycle in these circumstances? Refrigerant cycle? Refrigerant cycle minus heat loss? What controls the heat pump to come on? What controls the delay between compressor start-ups? No manufacturer clearly states these things.

I made calculations for @Ewan’s numbers. Mine are closer to 12min runtimes @15C outdoors

Questions.

1.     I don't have TRVs on 6 out of 15 heating emitters in our house, but all turn to 5 (13 radiators, 3 towel radiators= water volume around 52.35 litres without pipework calculations)

2.     Originally a two zone system, but I keep both valves open. Overall heat supply is adequately balanced (some imbalance between rooms, doors opened or closed, solar gain in half of the house. To make it perfect I would have to rip open walls, floors to replace pipes to 15mm to radiators (microbore throughout). Second best option is to install pressure independent valves on all heating emitters (there might be some limitations with minimum flow – don’t always need the min specified by Danfoss one I looked at). Third just tedious individual balancing, but then emitter order is a total guesswork and with these cheap balancing valves, there isn’t much authority (min adjustment has great change and usually starts hissing at these almost closed positions).

3.     I have one downstairs, in the hallway, keeps the whole house within limits. Pointless to do much more as solar gains are unpredictable and unmeasurable. I would run Heat pump with weather comp and open widows when room gets too warm if necessary ( almost a bonus with fairly good air permeability rating of 3.6m^3/h.m^2 (to be honest, I haven't looked into how much more this impacts heat loss)).

[Vilks]

12 hours ago, SteamyTea said:

Thinking about this modulation a little bit.  As an ASHPs performance is reduced when the external air gets colder, is the minimum modulation output a fixed amount i.e. 1.2 kW, or a percentage of deliverable power  i.e. 20% of what it can achieve at the temperature.

 

Quite often a supplementary heater kicks in and makes it look better than it really is.

It is complex combination of thermodynamics (Pressure vs enthalpy) of the refrigerants and mechanics of the compressor/-s and its type/amount of them. Most of that flies well above my head currently.

In short, from what I understand so far is that at lower temperature compressor has to work harder to compress the refrigerant to achieve desired temperature at a higher pressure. That’s where COP comes from. Work (electricity in) vs energy you get out (heat in kW).

So when minimum designed compressor operation meets with amount of energy extracted from air via refrigerant at certain temperature is your minimum output.

I could be way off, so take all this with a spoonful of salt.

[Ewan]

On 28/12/2022 at 21:58, Vilks said:

I have been going down the same rabbit hole. Trying to find the smallest unit that can satisfy at -2C and then be efficient at warmer temperatures. I can give you what I have found.

 Page 92  - min output of 2.1 kW for both 3 kW and 5 kW Vaillant units. Not the most informative table but then again, I have only seen Mitsubishi have better table than this one.

LG Therma V 3 kW unit - does not specify lowest output and jumps around with flow temperatures at different outdoor temperatures. You could guestimate around 1 kW (4.18 x min water flow of 0.25l/s). Although, according to Vaillant, you should spec heat pump for most of the year rather then couple of cold days when backup heating could be used and even though getting lower COP for those days, overall SCOP should be better when heat pump could run more efficiently rest of the year.

Research paper talks about 10 min being the minimum runtime: “…due to the need for a minimum run time to ensure good lubrication and negative impacts on compressor reliability” (Page 62).

Other users have found ways to calculate minimum system volume or buffer size from their own or other sources (I found this, but they even go as far as 20 min minimum runtimes).

So, if 2.1 kW is the minimum HP output and you suspect 1 kW is your lowest demand for warm temperatures (worst case for the heat pump) then if @JamesPa formula is used. You should get 31.58 Litres of minimum system volume to prevent Heat pump from running less than 10minutes, or 63.16litres for 20minutes (at delta5).

 

 

Thanks for doing the heavy lifting for me, that's really useful. I can't make that formulae work at all, (2-1)*10/(2*4.2)-69 = -67.8095238095 ... which sounds good as a negative number means no buffer? But I also can't get the example to work where they did need a buffer:

 

Quote

if the heat pump can modulate down to 4kW, the minimum heat demand (eg at 16C) is 2kW, you allow a flow temp variation of 2C and the minimum system volume is 100l, and a min ‘on’ time of 10 minutes you need a buffer tank of 142-100=42l

 

(4-2)*10/(2*4.2)-100 = -97.619047619

Confused. Where am I going wrong? Are Pmin and Dmin in kW?

 

On 29/12/2022 at 08:40, JohnMo said:

It actually says no more than 3 starts per hour.  So 10 mins run time, 10 on, 10 off - X3 cycles.

 

Also saying 10 to 20 could be desirable, but in a general context not specific to heat pumps.

 

So if you need 32L in your system - so yours is big enough not to need a buffer, at 69L.  Just run as a single or two zone system and make sure your open system is always big enough.

 

Questions

Do you need Trvs on the radiators?  

Could you balance the system instead?

Do you need more than one thermostat?

 

 

Perhaps it's OK then, 69L is the volume of the UFH which can just be on one zone. Office and upstairs bathroom rads could also be left TRV-less.

 

Questions

 

Do you need Trvs on the radiators?  

- Fear is that upstairs will become too hot during heating season, so TRVs will be employed in the bedrooms.

 

Could you balance the system instead?

- Not sure, the heat loss in the biggest room seems very large compared to the bedrooms. I am not very knowledgable about balancing.

 

Do you need more than one thermostat?

- probably not!

 

[JohnMo]

Don't need TRV's if you balance the radiators, this limit their output without switching them on/off.  On heat Geeks there is a write up on how to balance. One thermostat can be ok, but it takes a while to get everything balanced.  If you go one thermostat, go weather compensation, so you are always on the lowest flow temp possible is best IMO.

 

If you are coming up with negative numbers you system has a large enough capacity.

 

If you do install a buffer let the buffer float on system temperature, don't install a thermostatic control on the buffer, otherwise your HP/boiler will be firing at a higher temp than required, just to heat up the buffer.  

[Ewan]

Ah yes I did see that re balancing. That would leave a lot more volume in the system. Can get more thermostats if needed? Was planning on using weather comp.

Problem is that I got a negative number on the calculation they used as an example that did require a buffer! (i.e. shouldn't have given a negative number) I must be doing it wrong? I also can't get the same results as @Vilks

 

Maths failing me 😅

 

[JohnMo]

The original post you referred to stated

 

Vb = (Pmin-Dmin)*Tmin/(Hmax*4.2)-Vs (or zero if this yields a negative number)

 

You got a negative number so buffer size is zero litres.

 

As you add thermostats your system volume decreases, the number then becomes different, if you have a small radiator on its own thermostat and the rest of the house a another zone, the small radiator could be the only part of the system on for the boiler.  So that is the volume you use in the formula.

 

The idea of WC is keeping it simple, you don't need thermostats with WC, you can add them but you set them as limit stops a couple of degrees higher than required room temperature

 

 

[Ewan]

2 hours ago, JohnMo said:

The original post you referred to stated

 

Vb = (Pmin-Dmin)*Tmin/(Hmax*4.2)-Vs (or zero if this yields a negative number)

 

You got a negative number so buffer size is zero litres.

 

As you add thermostats your system volume decreases, the number then becomes different, if you have a small radiator on its own thermostat and the rest of the house a another zone, the small radiator could be the only part of the system on for the boiler.  So that is the volume you use in the formula.

 

The idea of WC is keeping it simple, you don't need thermostats with WC, you can add them but you set them as limit stops a couple of degrees higher than required room temperature

 

 

 

 

I did the calculation twice, once for my system (negative number) and once to make sure I was doing it right with numbers that were already worked up. For the second example I quoted, that should have resulted in a positive number, but didn't! This is the example:

 

Quote

So for example

 

• if the heat pump can modulate down to 4kW, the minimum heat demand (eg at 16C) is 2kW, you allow a flow temp variation of 2C and the minimum system volume is 100l, and a min ‘on’ time of 10 minutes you need a buffer tank of 142-100=42l

 

Or: (4-2)*10/(2*4.2)-100 = -97.619047619

It should have resulted in 42L, which makes me think I'm doing MY calculation wrong also, especially as my numbers do not match the ones Vilks got for my system.

If anyone can point out what the heck is going on here, I'd be very grateful.

[JamesPa]

58 minutes ago, Ewan said:

 

 

I did the calculation twice, once for my system (negative number) and once to make sure I was doing it right with numbers that were already worked up. For the second example I quoted, that should have resulted in a positive number, but didn't! This is the example:

 

 

Or: (4-2)*10/(2*4.2)-100 = -97.619047619

It should have resulted in 42L, which makes me think I'm doing MY calculation wrong also, especially as my numbers do not match the ones Vilks got for my system.

If anyone can point out what the heck is going on here, I'd be very grateful.

Tmin needs to be expressed in seconds not minutes.  Sorry I didn't make that clear in the original post, its kind of obvious if you have a physics background, but not otherwise.

[SteamyTea]

9 minutes ago, JamesPa said:

its kind of obvious if you have a physics background, but not otherwise.

Should be obvious to everyone that has been to school.

Time = s

Distance = m

Mass = kg

[JamesPa]

6 minutes ago, SteamyTea said:

Should be obvious to everyone that has been to school.

Time = s

Distance = m

Mass = kg

True I suppose, but in fairness power = W and the formula assumes kW.  Should stick to SI consistently I suppose.

[JohnMo]

Couple of different formulas

 

Buffer sizing – from ASHP book

6[kW=kJ/s] x 60/4 [minutes/number of starts per hour] x60 [secs/mim] = 5400kJ

Mass of water required [kg] = 5400 [kJ]/(4.18 [kJ/degC kg] x 5 [degC] = 184.5kg or 184.5L

Buffer sizing – from Hydronics design book - you need to convert US imperial - not SI units

V= t(Qhsmin – Qloadmin)/500(delta T)

 

V = minimum buffer tank volume (US gallons) 

t = minimum heat source on time (minutes) 

Qhsmin = minimum stable heat output of heat source (Btu/hr) 

Qloadmin = minimum concurrent heating load when heat source is on (Btu/hr) 

ΔT = change in average tank temperature during minimum heat source on time (ºF) 

 

 

[SteamyTea]

1 hour ago, JamesPa said:

power = W

(kg.m²/s²)/s

[Gone West]

13 hours ago, SteamyTea said:

Should be obvious to everyone that has been to school.

Time = s

Distance = m

Mass = kg

Should I guess, but as with a lot of things, the less you are interested in a subject the less information you absorb. I've known several people who can't do basic maths or physics, but were taught it at school. So either the teachers weren't much good or some people are just better at some things than others.

[JamesPa]

Interesting article here about why buffer tanks and hydronic separation etc are (according to the argument) usually a bad thing.  https://renewableheatinghub.co.uk/how-to-correctly-install-heat-pumps-so-that-they-work-properly-and-efficiently

 

Basically the author is advocating a direct connection from heat pump to distribution system, applying occams razor, and claims to have evidence (which he cites) that this is best for efficiency.  I have to say that, intuitively, Im not surprised provided that the system volume is sufficiently large and the modulation depth of the pump sufficiently deep to avoid short cycling.  

[SteamyTea]

1 minute ago, Gone West said:

So either the teachers weren't much good or some people are just better at some things than others.

Really down to lack of use.

Not many of us need to do basic algebra every day, so we forget the basics, then get confused when we try and do them.

There is also the 'mental arithmetic' issue.  Some people are very good at holding information in their heads and then retrieving it quickly and accurately (I am not one of them), this does not mean they cannot do arithmetic, just that they need to write down the appropriate numbers as they go along. Spreadsheets help a lot here.  When a maths teacher puts up an equation on a board, then shows how it is calculated, they are not doing mental arithmetic, they have learnt how to do that example by rote, like an actor learning their lines.

It does get a bit more complicated when indices are used, but there are rules that are easy enough to implement and if you use them often enough, they make life easy.  If you don't do them often, you forget them.

 

So the trick is to use algebra and indices as often as you can, and do the calculation with pencil and paper, then do the final bit (the actual numbers) with a calculator.

 

You will generally, intuitively, know when something is not right, best thing to do then is start again.  It took me over 20 years to get to grips with a text book example about calculating maximum volume possible for making a container, from a single flat sheet of material, using differentiation.  I have never had to do that at work, but once I did it, it became a useful tool.  I now know that it is possible to model this before it is made, even if I cannot, right at this moment, remember how to do it.

[Dan F]

2 hours ago, JamesPa said:

Interesting article here about why buffer tanks and hydronic separation etc are (according to the argument) usually a bad thing.  https://renewableheatinghub.co.uk/how-to-correctly-install-heat-pumps-so-that-they-work-properly-and-efficiently

 

Basically the author is advocating a direct connection from heat pump to distribution system, applying occams razor, and claims to have evidence (which he cites) that this is best for efficiency.  I have to say that, intuitively, Im not surprised provided that the system volume is sufficiently large and the modulation depth of the pump sufficiently deep to avoid short cycling.  

 

This is very interesting and maybe even deservices it's own thread.

 

I understand when system 1 is bad, but the difference between system 2 and 3 I struggle to understand:

- There is clearly a slightly loss of efficient when the system starts (and I know Vaillant VF1 sensor makes this worse).

- But, once things stablize scenario 2 is using 40C flow temp and scenario 3 uses 35C flow temp for some reason.  The only reason I can think this is happening though, is because of an insufficient flow rate on the other side of the buffer tank and the delta-t being too high.  If you use a correctly size pump and tune things correctly (or use a self-balancing actuactors), I can't see why system 3 would need higher flow temperature or perform worth than system 2.

 

The author states that "the second test also showed drop in performance and high running costs due to the influence of the buffer tank and the secondary circulator working against heat pump’s controller which was maintaining steady state output and performance" but doesn't go on to explain how he beleives the buffer tank and secondary circulator working against heat pump.

 

What do others understand?

Edited January 5, 2023 by Dan F

[ReedRichards]

I understand it is a test devised by someone with a limited understanding of heat pumps.

 

Scenario 1 uses a buffer tank and no Weather Compensation (although simulating an outside temperature of 7 degrees).  A third party thermostat was used but there's no particular reason for that, I'm sure the heat pump allows you to disengage Weather Compensation on its own controller.

 

Scenario 2 eliminated the buffer tank and uses Weather Compensation.  So that's two different variables changed, which means you can never attribute any change in outcome with certainty to one or the other of the two variables.

 

Scenario 3 is the same as Scenario 2 but uses Load Compensation as well as Weather Compensation.  The average COP is a lot higher which is surprising.  Perhaps the Weather Compensation settings were not optimal?

 

  

Edited January 5, 2023 by ReedRichards

[JohnMo]

I would think Scenario 1 represents quite a lot of real life situations, where you read threads on here, and people want to zone the house with third party thermostats.  Many will have the controller isolated from the house and then on/off thermostats controlling the heating zones.  Grant for example show the installer to do that in their manual.  The OEM controller hidden away so the end user doesn't mess with it.

 

Scenario 2 although it states weather compensation the system is frigged to give a fixed flow temp, which in effect is the same as Scenario 3 based on a fixed fixed temp also.  So really the only difference becomes the buffer.

[JamesPa]

26 minutes ago, ReedRichards said:

I understand it is a test devised by someone with a limited understanding of heat pumps.

 

Scenario 1 uses a buffer tank and no Weather Compensation (although simulating an outside temperature of 7 degrees).  A third party thermostat was used but there's no particular reason for that, I'm sure the heat pump allows you to disengage Weather Compensation on its own controller.

 

Scenario 2 eliminated the buffer tank and uses Weather Compensation.  So that's two different variables changed, which means you can never attribute any change in outcome with certainty to one or the other of the two variables.

 

Scenario 3 is the same as Scenario 2 but uses Load Compensation as well as Weather Compensation.  The average COP is a lot higher which is surprising.  Perhaps the Weather Compensation settings were not optimal?

 

  

Thats not how I read it

 

Scenario 1 uses a buffer tank and no Weather Compensation and a third party thermostat which, in fairness, appears to be a fairly common setup (why it would be, other than ignorance, completely escapes me)

Scenario 2 uses a buffer tank with Weather Compensation and no third party thermostat, another common setup

Scenario 3 uses no buffer tank and does use Weather Compensation, also a common setup

 

It could be argued that there are at least two more scenarios (and several others as well).

 

Scenario 1a with a buffer tank, no Weather Compensation and a third party thermostat to control the room temperature but using the HP controller to maintain the buffer tank temperature

Scenario 1b with a buffer tank, no Weather Compensation and the HP controller maintaining the room temp

 

The three chosen are not unreasonable examples of what is done, based on what we hear on this forum.

 

I think the key is Scenario 3 vs Scenario 2.  I presume that the lower flow temp (and thus higher COP) in scenario 3 is because there are no buffer tank losses.  It is inconceivable that there is not some loss in a 4 port buffer tank due to conduction and possibly also due mixing if the flow rates don't precisely balance, and loss equals lower efficiency unless there is a compensating factor.

 

To be honest I have never understood the argument for a (4 port) buffer tank in a sufficiently simple system.  I can see the argument in a system where you need to have different flow rates/temperatures in different zones (eg when you have both radiators and UFH), and I can also see the argument for a simple volumiser (ie 2 port tank plumbed in the return) where the system volume is otherwise insufficient, but in a simple system with sufficient system volume what does a buffer tank do - other than take up space and loose heat?