Heat demand too low for Vaillant Heat pump?

[ReedRichards]

3 hours ago, JamesPa said:

Thats not how I read it

 

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

 

Yes, you're right, I misread that; I don't know how.  But we don't (I think) know how the Weather Compensation parameter were set.  The use of Load Compensation in Scenario 3 could compensate for a poor choice of Weather Compensation parameters.  So I see the difference between Scenarios 2 & 3 as making two changes.  None of the scenarios compare the same control methodology with and without a buffer tank.  

[JamesPa]

30 minutes ago, ReedRichards said:

Yes, you're right, I misread that; I don't know how.  But we don't (I think) know how the Weather Compensation parameter were set.  The use of Load Compensation in Scenario 3 could compensate for a poor choice of Weather Compensation parameters.  So I see the difference between Scenarios 2 & 3 as making two changes.  None of the scenarios compare the same control methodology with and without a buffer tank.  

I think the challenge here is that there are lots of possible scenarios and the author has chosen 3 fairly common ones, rather than necessarily trying to identify the contribution of each individual change (if indeed they are strictly separable, which I doubt).   I'm not actually clear that scenario 2 does not also employ load compensation.  

[Dan F]

5 hours ago, JamesPa said:

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.

I can't see why an insulated buffer (even if it has some losses) would require 40C flow vs. 35C, there is something else going on here. not just the buffer IMO.   Potentially the post buffer pump is running too fast/slow and delta-t isn't optimal?

[JohnMo]

Report says

The buffer tank pump is set to run at 30 l/min and a delta T of 5.  The flow rate coincides with max heating demand of 10.63kW.

[JamesPa]

1 hour ago, Dan F said:

I can't see why an insulated buffer (even if it has some losses) would require 40C flow vs. 35C, there is something else going on here. not just the buffer IMO.   Potentially the post buffer pump is running too fast/slow and delta-t isn't optimal?

So far as I understand it, a 4 port buffer tank exists only because there is some mixing envisaged between flow and return.  If no mixing is required you wouldn't use a 4 port tank, you would use either one or two 2 port tanks.

 

As soon as you mix flow and return you will decrease the efficiency (even if there is zero loss) because the HP will need to deliver a higher flow temperature to get any given temp at the emitters.

[Dan F]

15 minutes ago, JohnMo said:

Report says

The buffer tank pump is set to run at 30 l/min and a delta T of 5.  The flow rate coincides with max heating demand of 10.63kW.

OK.  So, what explains why the same ASHP, with in theory the same weather compensation config, required 40C flow instead of 35C flow when a buffer is used.  This can't be buffer losses.

[JamesPa]

Just now, Dan F said:

OK.  So, what explains why the same ASHP, with in theory the same weather compensation config, required 40C flow instead of 35C flow when a buffer is used.  This can't be buffer losses.

See my comment above.  In summary, even if the buffer is lossless, there is still an explanation for poorer efficiency.

[JohnMo]

As @JamesPa says mixing. A 4 port buffer will have little or no temperature difference between top and bottom so you get a different temperature profile compared to no buffer, as seen in the test.

 

I run my 180L buffer as a two port, so only excess flow goes into the buffer, this can still lead to a 4 or 5 degree temp spike for 10 mins or so (gas boiler).  I get a bit of a sine wave +/- a few degrees either side of the weather comp set point.

[ReedRichards]

I may be mistaken but it looks to me as if the experiment is set up so that the heat pump and the buffer tank (when used) are at the same temperature, being the 7 C temperature inside the chamber.  If I am correct this means that the buffer tank in the simulation is essentially external, equivalent to a real situation where the buffer tank is outside the house.  I think the text also indicates that the pump on the "house" side runs all the time, whether or not there is a call for heat.  That's not the way my system works, if there is no call for heat the pump circulating water round the radiators turns off.  And I would consider a room thermostat with a 2 C difference between the off and on temperatures to be totally unacceptable.                    

[Dan F]

13 minutes ago, JohnMo said:

As @JamesPa says mixing. A 4 port buffer will have little or no temperature difference between top and bottom so you get a different temperature profile compared to no buffer, as seen in the test.

 

I run my 180L buffer as a two port, so only excess flow goes into the buffer, this can still lead to a 4 or 5 degree temp spike for 10 mins or so (gas boiler).  I get a bit of a sine wave +/- a few degrees either side of the weather comp set point.

 

This article is good explaining the difference between 2-port and 4-port configurations: https://www.pmmag.com/articles/100544-the-finer-points-of-applying-a-2-pipe-buffer-tank

 

If buffer mixing is an issue and how much this impacts efficiency will be implementation specific.  My ASHP for example (Vaillant), doesn't adjust flow rate in partial-load scenarios but rather maintains a constant flow rate.  I also have mixed circuits which means that the flow rate out of the 4-port buffer is actively reduced when heating load is low.  I'm not sure exactly how it behaves in practice, but I'm pretty sure it doesn't follow the scenario 3 mixing explaination.

 

The other difference between scenario 2 and 3 is that scenario 2, which may be more significant, is that system 2 is using what vaillant call "room mod: inactive" and system 3 is using what vaillant call "room mod: active".   This will effect how the weather compensation is working and in turn the flow temperature.

 

 

 

[JamesPa]

7 minutes ago, ReedRichards said:

I may be mistaken but it looks to me as if the experiment is set up so that the heat pump and the buffer tank (when used) are at the same temperature, being the 7 C temperature inside the chamber.  If I am correct this means that the buffer tank in the simulation is essentially external, equivalent to a real situation where the buffer tank is outside the house.  I think the text also indicates that the pump on the "house" side runs all the time, whether or not there is a call for heat.  That's not the way my system works, if there is no call for heat the pump circulating water round the radiators turns off.  And I would consider a room thermostat with a 2 C difference between the off and on temperatures to be totally unacceptable.                    

I see lots of pictures of buffer tanks in unheated plant rooms, so not so unrealistic.  My preferred plumber wants to put mine in the garage, ie almost at outdoor temp.  He wont be allowed to because he wont be allowed to install a buffer tank at all, but realistically there would be nowhere else to put one if I were to cave in.  Others will of course diffe,r but the scenario is not unrealistic.

 

Without some sort of feedback loop from the demand, how can the house side pump not run all, or at least most, of the time?  The only way the system can determine if more heat needs to be supplied to the house is either to sample the return temperature or get feedback from a in-room thermocouple/thermostat.  The latter is built into the HP control panel and may well turn the pump on or off in the experiment (I don't think we actually know), but of course this measures the temp only in one room.

 

This all depends on the exact config (Grant, for example, who do more or less everything else wrong from a purely thermodynamic point of view, have a sort of intelligent 'sampling' mode whereby the pump is switched off when there is no demand but switches on periodically to check the return current temp, potentially quite a good strategy).

 

A classic mechanical bi-metalic thermostat, which many houses will have, surely has (at least) a 2C hysteresis.  I agree its too much to be comfortable, but again not a totally unrealistic scenario.

 

There really are so many variants that its impossible to cover all in an experiment.  I think a certain amount of informed interpolation, combined with a bit of theory (ideally confirmed by experiment) is needed to work out where any given actual scenario lies.

 

 

[JohnMo]

The report says, they used as low a HP temp as possible to get a steady 20 deg in the thermal store. To prevent over supply of heat and reduced performance etc. So in all cases the pump would be running, test 1 cycles off for less than an hour over the 6 or hours of the test so the pump would be running for the majority of the time anyway.

[JamesPa]

13 minutes ago, Dan F said:

 

This article is good explaining the difference between 2-port and 4-port configurations: https://www.pmmag.com/articles/100544-the-finer-points-of-applying-a-2-pipe-buffer-tank

 

If buffer mixing is an issue and how much this impacts efficiency will be implementation specific.  My ASHP for example (Vaillant), doesn't adjust flow rate in partial-load scenarios but rather maintains a constant flow rate.  I also have mixed circuits which means that the flow rate out of the 4-port buffer is actively reduced when heating load is low.  I'm not sure exactly how it behaves in practice, but I'm pretty sure it doesn't follow the scenario 3 mixing explaination.

 

The other difference between scenario 2 and 3 is that scenario 2, which may be more significant, is that system 2 is using what vaillant call "room mod: inactive" and system 3 is using what vaillant call "room mod: active".   This will effect how the weather compensation is working and in turn the flow temperature.

 

 

 

Its a good article thanks.  However the key, is surely that, if you don't require mixing between flow and return (to balance flow rates or for some other concrete reason), then make it impossible by using a 2 port tank, thus eliminating the mixing loss. If you do require mixing then you will have to suffer the inevitable (so far as I can see) reduction in system efficiency.  It is (unless I have missed something) just lazy design to put in a 4 port tank when no mixing is required.

Edited January 5, 2023 by JamesPa

[Dan F]

21 minutes ago, JamesPa said:

if you don't require mixing between flow and return (to balance flow rates or for some other concrete reason), then make it impossible by using a 2 port tank, thus eliminating the mixing loss.

 

It is (unless I have missed something) just lazy design to put in a 4 port tank when no mixing is required.

If you only have a single heating zone which has large enough capacity for desfrost and is always open, the best approach is no buffer at all surely, not a 2-port buffer.

 

21 minutes ago, JamesPa said:

eliminating the mixing loss

Mixing may decrease efficiency in some scenarios, but energy doesn't get lost via mixing.

 

21 minutes ago, JamesPa said:

inevitable (so far as I can see) reduction in system efficiency

I don't see it as inevitable, I think it depends on exact scenario and flow rates and the ASHP control system.  Mixing doesn't produce energy loss, so at worst it makes the ASHP less efficient via use of a higher flow temperature (but this is turn also depends on what temp sensors and control system the ASHP is using)

 

BTW, In theory 3-port buffers are better than 2-port.

Edited January 5, 2023 by Dan F

[JamesPa]

8 minutes ago, Dan F said:

If you only have a single heating zone which has large enough capacity for desfrost and is always open, the best approach is no buffer at all surely, not a 2-port buffer

Agree entirely

 

 

8 minutes ago, Dan F said:

I don't see it as inevitable, I think it depends on exact scenario and flow rates and the ASHP control system.  Mixing doesn't produce energy loss, so at worst it makes the ASHP less efficient via use of a higher flow temperature (but this is turn also depends on what temp sensors and control system the ASHP is using)

Unless I have missed something it's almost certainly inevitable.   

 

With mixing you get hot and cold water producing warm water.  This increases entropy (disorder) and you can't restore that without inputting energy from outside the system (or violating the laws of thermodynamics).  In practice this manifests itself in a requirement for higher flow temp=lower efficiency.

 

Also mixing (unless it's purely conduction of heat in which case the 4 port buffer isn't needed) implies some circular flow (ie from hp via tank to hp, or distribution system via tank to distribution system).  That can't happen without either injecting energy from outside or creating a perpetual motion machine.  The latter violates the laws of thermodynamics.  In practice the pump(s) need to do more work to force water round the system.

 

Sadly, getting something for nothing usually violates the laws of physics (this may sound trite, but its largely true).  Sometimes the sacrifice is of no consequence (a heat pump reduces the external air temperature, but we don't care because the volume of external air is near infinite, solar panels collect energy from the sun, but we don't care because the available energy is more than we can use).   Most of the time, however, the sacrifice has to be paid for somewhere.

 

[JamesPa]

34 minutes ago, Dan F said:

BTW, In theory 3-port buffers are better than 2-port.

Intrigued by this one, what's a 3 port buffer (beyond the obvious) and do people fit them?

[Dan F]

49 minutes ago, JamesPa said:

Intrigued by this one, what's a 3 port buffer (beyond the obvious) and do people fit them?

Page 56-59 talks about pros/cons of 2,3 and 4 port buffers in heat-pump system.  https://www.caleffi.com/sites/default/files/file/idronics_27_na.pdf?fbclid=IwAR0KGbAXmvSRXNdyevldqeUhD6u7iF4RKWaI5Cxq6BNXKbYx-U19wKJ1_5E

[ReedRichards]

I've been giving the experiment and the comments some thought.  Isn't the "best" experiment to compare two scenarios where everything if the same except that two of the ports on the buffer (say the two flow ports) are bypassed and sealed so you compare the effect of a four-port buffer where mixing can occur with a two-port buffer where it can't?  Everything else, including the volume of water circulating round the system remains the same.  The control method remains the same.  That would show you exactly what effect mixing in the buffer has on performance. 

[JamesPa]

36 minutes ago, ReedRichards said:

I've been giving the experiment and the comments some thought.  Isn't the "best" experiment to compare two scenarios where everything if the same except that two of the ports on the buffer (say the two flow ports) are bypassed and sealed so you compare the effect of a four-port buffer where mixing can occur with a two-port buffer where it can't?  Everything else, including the volume of water circulating round the system remains the same.  The control method remains the same.  That would show you exactly what effect mixing in the buffer has on performance. 

That would indeed be interesting, a good experiment to do and as you say pretty much isolate the effect of buffer mixing.  There is an argument hat this should be followed by an experiment where all ports are bypassed, which reduces the system volume of course, but remains an interesting case (and an interesting comparison) if the system volume is 'sufficient'.  

[JamesPa]

10 hours ago, Dan F said:

Page 56-59 talks about pros/cons of 2,3 and 4 port buffers in heat-pump system.  https://www.caleffi.com/sites/default/files/file/idronics_27_na.pdf?fbclid=IwAR0KGbAXmvSRXNdyevldqeUhD6u7iF4RKWaI5Cxq6BNXKbYx-U19wKJ1_5E

Thanks for this, it looks like a rather good 'text' on heat pumps and related technology which perhaps ought to be better known.  Thats apart, of course, from the crazy (ie non-SI units).  However, given that Btu (have I got the capitalisation right?) means 'British thermal unit' I guess we cant complain too much. 

 

A 'ton' as a unit of power is a new one on me though, 'the average heat transfer rate associated with melting one ton of ice in a period of 24 hours' - 12000Btu/hr (3.5kW).

[JohnMo]

There Is also a difference in efficiency when the buffer is controlled by a thermostat and when not.  So you could document that difference as well?

[JamesPa]

12 hours ago, Dan F said:

BTW, In theory 3-port buffers are better than 2-port.

OK I read the Calefi article so now understand the concept of 3 port buffer tanks.  Essentially they combine the functions of a 2 port buffer tank/volumiser with that of a bypass valve.  When the load is active they are pretty much bypassed on the flow and only the return passes through, adding system volume but not much else.  When the load is inactive (because the zones are shut down) they serve as a bypass/heat store.  That seems prima facie like a reasonable idea (although Im not clear whether its better than a 2 port tank and a bypass valve).  The diagrams all show two pumps though, is there any reason, I wonder, why a 3 port tank could not be run with only 1 pump?

[Dan F]

1 hour ago, ReedRichards said:

I've been giving the experiment and the comments some thought.  Isn't the "best" experiment to compare two scenarios where everything if the same except that two of the ports on the buffer (say the two flow ports) are bypassed and sealed so you compare the effect of a four-port buffer where mixing can occur with a two-port buffer where it can't?  Everything else, including the volume of water circulating round the system remains the same.  The control method remains the same.  That would show you exactly what effect mixing in the buffer has on performance. 

Agree, there are more varibales here than just the existenace of the buffer. One of these being no use of internal temperature sensor of thermostat in system 2.

[JohnMo]

The 2 and 3 ports are using an oversized Tee, to act as hydraulic separation.

 

Was operating mine (2 port) with a thermostat, the thermostat calling the boiler to heat.  Boiler temp would go up into 40s flow temp, even though the buffer thermostat was set for 34 deg.  Rewired things so boiler just ran straight WC and buffer acted by floating at what temp it wanted.  Typical flow from boiler dropped to low 30s max.

 

Average heating efficiency with an allowance of 5kWh for DHW went from 94% to 110%.  Mostly due to the much lower firing temps and better condensing of the boiler.  So same would be true for a heat pump.

[Ewan]

This was all very interesting and took me a while to read and understand 😄 I also listened to a podcast where the guy who did those tests was interviewed, also quite interesting: https://www.buzzsprout.com/509671/11946910 - they talk about a lot of the things you all mentioned in discussion here. He sounds quite knowledgable (to my untrained ear).

Very big on pointing out that Manufacturers docs mostly show buffer tanks in use (in the UK, but not EU versions!), and why they think this is the wrong approach.

 

On 05/01/2023 at 21:23, JamesPa said:

Its a good article thanks.  However the key, is surely that, if you don't require mixing between flow and return (to balance flow rates or for some other concrete reason), then make it impossible by using a 2 port tank, thus eliminating the mixing loss. If you do require mixing then you will have to suffer the inevitable (so far as I can see) reduction in system efficiency.  It is (unless I have missed something) just lazy design to put in a 4 port tank when no mixing is required.

 

@JamesPa, from your earlier post you suggested the having UFH and radiators (5 in my case) could be a candidate for mixing to deal with flow rates etc.

One installer is suggesting we go direct with no buffer or volumiser, have the UFH & rads on one open circuit, but size the rads down slightly to avoid overheating bedrooms (at my request) and balance the whole system.

Does that sound achievable / sensible?

Edited January 18, 2023 by Ewan