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Vaillant Aerotherm plus does not reduce flow rate despite low delta T

 
Air Source Heat Pumps (ASHPs)
Last Post by fillib 2 years ago
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 fillib
(@fillib)
Active Member Member
Joined: 2 years ago
Posts: 5
Topic starter 09/03/2024 5:51 pm   [#1418]
Hi all, I hope someone can help us with this Vaillant ASHP question:
 
Short version: Delta T is too low, but ASHP pumps at max. flow rate. Should it not reduce the flow rate to get closer to optimum delta T?
 
Our SCOP/COP is consistently around 2.3.
The reason seems to be that the delta T is very low, at only ~1- 2°C usually. 
 
Our house is nice and warm at very low flow T of usually far below 30°C (e.g. target flow 26°C outside T ~9°C, heat curve set to 0.45).
According to my understanding, low flow T should come with a very good efficiency, but not in our case!
 
I would expect, that at such low flow T and with a house at the target T, the flow rate in the system should also be very low, so that the heating has time to loose the heat into the room. I think this simulator agrees with my understanding (lower flow -> higher deltaT): https://heatpumps.co.uk/heating-simulator-for-radiators/
 
What we see, is that the ASHP, when the compressor is active, maintains a "Cuilding circuit flow rate" of~1205 l/h consistently. This is the maximum flow rate for our unit (Aerotherm plus 7kW, model name "VWL 75/6 A 230V S").
The corresponding building circuit pump power is shown as 53% and this stays very constant as well.
Sometimes the control unit shows "compressor shutdown". In this case the flow rate is consistently ~844 l/h and pump power ~35%. This is usually when the flowT==returnT 
The minimum flow rate of the ASHP is 540 l/h according to specs. 
 
I phoned the Vaillant helpline today and he was surprised, that the unit would not regulate the speed down, seeing that the delta T is so low. He suggested trying to set a lower "maximum remaining head" setting from 900 mbar (default) to 750mbar but this showed no effect. I also tried 200 mbar (minimum setting), but this had no effect on the flow rate or pump power. 
He then admitted that he didn't know the answer.
He suggested a quick test of setting flow temp to 45°C and see what happens. I tested for ~20-30 minutes during which the flow T went up to around 36°C and return T was ~29°C, so dT= 7°C. That shows that with constant flow rate, the dT increased with increased flow T. Not sure exactly what to conclude from this, other than in principle we can have the right dT, but we need to find a way to get the system to run at appropriate flow rate for each situation. In the test I basically only simulated an extreme condition, for which the apparently fixed flow rate of 1205m/l seems to be more appropriate and results in a closer to optimal dT. That confirms that the flow rate should usually be lower, right?
 
Does anyone know, why the flow rate is never regulated down?
Do I understand correctly that it should usually be lower, or does it always want to be on maximum flow for some reason, no matter the deltaT?
 
My hope is currently that there is some form of setting somewhere that solves our problem at the push of a button. 
My fear is, that our system is completely oversized and not capable of working at such low requirements as our little house. 
 
 
some general information:
Vaillant Aerotherm plus 7kW
Only radiators (all new and large) all in a single circuit
Installed by British Gas in March 2023
89sqm 3-bedroom bungalow, reasonably well insulated. Annual gas consumption before ~10,000 kWh.
EPG states ~7500 kWh demand. BG survey states >14,000 kWh demand (?!)
45l Vaillant buffer tank
Grundfos circulation pump from buffer to radiators. I tested all possible different settings for this one, but none had any effect. All work fine for keeping rooms warm. None affects the ASHP circuit pump at all (logical, I guess,  because the buffer separates )
No Temperature sensor in the buffer tank. It often comes up as a potential explanation, but this doesn't seem to be it.
Room temp. mode : "Inactive"
Heat curve setting: 0.45 (works to get room temp almost exactly to set room T, or slightly above)
Room T 19.5°C , setback T 17.5°.
Mixergy DHW cylinder. DHW SCOP 1.8 (also not great right?, at least close to what they quoted: 1.7)
 
 
 
Tommy Jones (heat geek installer) replied: "The buffer and secondary pump won't help.

Sounds like your system isn't massive and the secondary pump is probably on fixed speed which will create distortion in the buffer causing the return temp on the ashp to increase, this will prompt the hp to increase pump speed to widen the DT.
The 7kw arotherm should tick along between 850 & 1200lph depending on demand."
 
I responded: Hi Tommy, thanks! I think your explanation is the opposite of how I understood it. I am of course happy to be corrected to solve our problem: would the ASHP not LOWER its speed to widen dT? i.e. if water runs slower through system it will loose more heat?
Our current setting of the Grundfos pump (secondary pump) is "constant curve I", so slowest fixed speed. Anyway, I did try all available settings on that pump, giving it some time, but nothing changed the dT. Other users get excellent COP with the same 7kW unit, same buffer and same pump (upsidedownfork on Youtube).
 


   
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 Derek M
(@derek-m)
Illustrious Member Member
Joined: 6 years ago
Posts: 4425
09/03/2024 7:13 pm  

I assume that you have received no error messages indicating some sort of problem.

I believe that PWM controlled water pumps revert to full speed if the control signal is lost, I would therefore suggest that you check the relevant plugs and sockets on both the pump motor and the PCB.

 



   
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 HughF
(@hughf)
Honorable Member Member
Joined: 4 years ago
Posts: 489
09/03/2024 10:38 pm  

The secondary pump won’t be pwm controlled, I assume? Why do you have a buffer and secondary pump anyway?


Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.

My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.


   
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 fillib
(@fillib)
Active Member Member
Joined: 2 years ago
Posts: 5
Topic starter 10/03/2024 3:40 am  

Dear @derek-m, dear @hughf,

Thanks! No, there is no error message. The pump that I am talking about (the one producing the maximum flow rate) is the building circuit pump inside the Vaillant Aerotherm unit.

I posted this question also on facebook and from the feedback there, it seems it is the normal behaviour for Vaillant heat pumps (and Mitsubishi) to run at a constant flow rate (maximum if possible), while dT is NOT kept constant.

Interesting that you mentioned the PWM. The Grundfos UPS3 manual does go on about this, but I don't know how this would work in this context. The Vaillant controller would have to send the speed command to the unit, but I think it only switches it on and off. The speed/pressure setting of the Grundfos pump is done on the unit via a button. 

It seems our unit is simply oversized 🙁 



   
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 Derek M
(@derek-m)
Illustrious Member Member
Joined: 6 years ago
Posts: 4425
10/03/2024 7:11 pm  

On page 63 of the attached file the diagram shows the location of the water pump (item 5, labelled Heating Pump).

Page 89 shows connector X16 providing power to the Heating Pump, while X22 initiates actuation of the Heating Pump. This internal water pump may therefore have two cables connected, one to supply electrical power and the other to control the speed. If the correct water pump has been installed with the necessary cables then I would expect the Vaillant controller to be able to vary its speed. Have you looked inside the unit to confirm what is fitted? What controller are you using?

On the Mitsubishi Ecodan I think that you will find the the water pump is external to the heat pump unit, and the controller can only vary the water pump speed if the correct water pump capable of PWM control has been installed. The controller can then set the pump speed to one of five alternative settings.

Vaillant 0020297933_05 aroTHERM plus.pdf

 



   
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 fillib
(@fillib)
Active Member Member
Joined: 2 years ago
Posts: 5
Topic starter 11/01/2025 12:31 am  

I am still trying to solve the puzzle of our low efficiency. In the meantime however I found out, that the Vaillant aerotherm definitely works with a constant flow rate, which is generally fine.

I will put my remaining conundrum into a different thread, as the question of the constant flow rate I think is answered, i.e. it's constant and that's fine, so the problem must be something else. 

Thanks all!

Philipp



   
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