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Why Buffer Tanks & Low-Loss Headers Can Sabotage Your Heat Pump's Efficiency

Mars · 2024-01-23T06:00:00Z

Low-loss headers and buffer tanks are routinely specified in domestic heat pump systems across the UK. In many cases they are presented as best practice, a way to protect the appliance, improve stability or simplify installation. The problem is that they are often installed as a default rather than as a solution to a defined hydraulic issue. When used unnecessarily, they undermine the two conditions that determine heat pump efficiency: low flow temperatures and adequate, stable system flow. Heat pumps are most efficient when they operate at the lowest possible flow temperature consistent with maintaining comfort. Every unnecessary increase in flow temperature increases compressor lift and reduces coefficient of performance. At the same time, heat pumps depend on steady, unrestricted water flow through the emitters. The system should be designed so that the heat pump can “see” the full load of the property and modulate smoothly against it. When hydraulic separation is introduced without a clear engineering need, that relationship is disrupted. A low-loss header creates hydraulic separation between the heat pump circuit and the distribution circuit. In theory this protects minimum flow through the primary side and prevents interference between pumps. In practice, it frequently introduces mixing. If the secondary circuit flow rate differs from the primary, warmer flow water blends with cooler return water inside the header. The result is elevated return temperatures back to the heat pump and a reduced effective temperature difference across the system. Higher return temperatures reduce efficiency and can impair defrost performance. The heat pump works harder, not smarter. Buffers are often justified as a way to increase system volume, prevent short cycling or accommodate zoning. There are circumstances where additional water volume is required, particularly in small radiator circuits with low thermal mass. However, most modern inverter-driven heat pumps are designed to modulate output over a wide range. If a system is correctly sized and designed for continuous operation, short cycling should not be a routine problem. Adding a buffer as a precaution frequently introduces standing heat losses, additional pumps, more controls and more commissioning variables. It also creates the potential for thermal stratification and control conflicts between the heat pump and external thermostats. In many poorly performing systems we see the same pattern: a buffer installed to compensate for undersized emitters, aggressive zoning or reliance on on-off room thermostats. The buffer masks the underlying design issue rather than resolving it. Instead of increasing emitter surface area and lowering design temperatures, the system is made more complex. Complexity in hydronic design rarely improves seasonal performance. It increases the number of failure points and makes diagnosis more difficult when things go wrong. The alternative is straightforward but requires discipline. Design emitters for low flow temperatures at design conditions. Ensure pipework and pumps can deliver the required flow rate without excessive pressure drop. Allow the heat pump’s own controls, particularly weather compensation, to regulate output continuously. Minimise zoning and avoid large numbers of motorised valves that fragment the load. Where thermostatic radiator valves are used, they should not routinely throttle down the majority of the system and force the heat pump into artificial low-load conditions. There are legitimate use cases for hydraulic separation and buffering, such as integrating multiple heat sources, managing very small secondary circuits or dealing with legacy systems that cannot be economically upgraded. But these are specific engineering decisions, not defaults. A buffer or low-loss header should be the outcome of a calculation, not a line item copied from a standard schematic. Heat pump performance is determined by fundamentals: low temperature operation, continuous modulation and adequate flow. Anything that interferes with those fundamentals should be justified with evidence. If a proposed design includes hydraulic separation, ask what problem it is solving, how it affects return temperatures and what impact it will have on seasonal efficiency. In many domestic installations, the most efficient system is also the simplest one

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Air Source Heat Pumps (ASHPs)

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Tim441

(@tim441)

Honorable Member Contributor

Joined: 4 years ago

Posts: 460

17/02/2025 3:07 pm

@jamespa measuring with clamp-on heating pipe thermometer? I see ebay/amazon has them around £6 or £7 .. Good enough?

Listed Grade 2 building with large modern extension.
LG Therma V 16kw ASHP
Underfloor heating + Rads
8.7kw pv solar
3 x 8.2kw GivEnergy batteries
1 x GivEnergy Gen1 hybrid 5.0kw inverter
Manual changeover EPS
MG4 EV

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5269

17/02/2025 3:15 pm

Posted by: @tim441

↑

@jamespa measuring with clamp-on heating pipe thermometer? I see ebay/amazon has them around £6 or £7 .. Good enough?

I believe so, we are looking for difference so as long as you can clamp consistently then it doesn't matter if the figure isn't perfect.

I stress there is absolutely no need to do this unless you want to.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

👍

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Tim441

(@tim441)

Honorable Member Contributor

Joined: 4 years ago

Posts: 460

20/02/2025 3:32 pm

Borrowed a clamp on thermometer.

Buffer tank: PAW-BTANK50L-2 buffer tank
Only 48l
According to ashp controls
leaving temp 33deg
Return 31 deg

Arr from ashp BT 34.4flow
Leave BT 34.1 flow

Arr from house BT 33.2 return
Leave 33.8 BT return

I suspect not great test right now... as mild... so not working hard
Will be better to check on cold morning.. perhaps.

Will report again once more results

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Mars

(@editor)

Illustrious Member Admin

Joined: 6 years ago

Posts: 4809

Topic starter 20/02/2025 10:59 pm

@tim441 the other thing to “check” is where the respective flow and returns are to see if your buffer has minimal distortion. Ideally, it’s plumbed like this:

New to heat pumps? Take the Heat Pump Fundamentals course and use INTRO50 at checkout for 50% off.
Rewritten and updated for 2027
📘 From Zero to Heat Pump Hero
🛠️ Bodge Buster

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Tim441

(@tim441)

Honorable Member Contributor

Joined: 4 years ago

Posts: 460

21/02/2025 9:32 am

@editor

Logically the electronic motorised valve.. I think... stops buffer being used when heating DHW/hot water tank. So will be on flow to BT?

With the pump on right on flow from BT?

Tap at bottom right ... return to BT?

So same as your diagram. but I'll try the thermometer a few times in different scenarios

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JamesPa

(@jamespa)

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Joined: 3 years ago

Posts: 5269

21/02/2025 9:53 am

Posted by: @tim441

↑

Logically the electronic motorised valve.. I think... stops buffer being used when heating DHW/hot water tank. So will be on flow to BT?

Is it a 2 way valve or a 3 way diverter valve. Normally the (3 way) diverter valve is the only motorised valve needed.

As you say taking some temperature measurements when its working harder would be good, but your initial ones are encouraging.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

👍

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Tim441

(@tim441)

Honorable Member Contributor

Joined: 4 years ago

Posts: 460

21/02/2025 10:00 am

@jamespa it's 3 way

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5269

21/02/2025 10:02 am

Posted by: @tim441

↑

@jamespa it's 3 way

I presume the diverter valve then, which would make sense. As you say you would expect this to be on the inlet to the BT

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

👍

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Potatoman

(@potatoman)

Trusted Member Member

Joined: 2 years ago

Posts: 57

29/03/2025 8:59 pm

Hi, so if I wanted to make my system more efficient, It's open loop 28/22/15mm ,all rads,run via a Homely is it possible to remove the buffer tank but still comply with 7-year LG warranty requiring hydraulic separation. Thanks

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Mars

(@editor)

Illustrious Member Admin

Joined: 6 years ago

Posts: 4809

Topic starter 29/03/2025 11:32 pm

@potatoman can you share LG's official documentation that explicitly requires hydraulic separation to maintain the 7-year warranty?

If you've got that document, it would be extremely valuable to see it.

New to heat pumps? Take the Heat Pump Fundamentals course and use INTRO50 at checkout for 50% off.
Rewritten and updated for 2027
📘 From Zero to Heat Pump Hero
🛠️ Bodge Buster

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Potatoman

(@potatoman)

Trusted Member Member

Joined: 2 years ago

Posts: 57

30/03/2025 11:09 am

@editor

487221638 10163211903663552 6665514382284125271 n

Regards

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5269

30/03/2025 2:14 pm

To me that reads as 'Hydraulic separation optional but if present covered for 7 years if you take out the extended warranty', as opposed to 'Hydraulic separation mandatory'. The very last line in the table (mag filter) would seem to confirm this.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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