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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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Mars

(@editor)

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Topic starter 02/05/2025 7:38 am

In light of the recent buffer tank/warranty comments in this topic, I’ve written this piece below. It’s a 10 minute read, but I think it warrants a dedicated video/podcast, because there’s a lot of ambiguity and grey areas here that need clarification.

https://renewableheatinghub.co.uk/fact-or-fiction-are-buffer-tanks-really-necessary-to-protect-your-heat-pump-warranty/

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Heatgeek

(@heatgeek)

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Joined: 2 years ago

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02/05/2025 8:55 am

@potatoman As no one seems to be obliging, I attach a diagram. The idea is that only bypass flow goes through the tank when necessary so it is undisturbed by main flow. Connection to tank must be closely coupled.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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ASHP-BOBBA

(@ashp-bobba)

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Posts: 503

02/05/2025 9:02 am

@heatgeek is it that this design satisfies the manufacturers? its still (although balance by bypass) a double hydronic volumiser/buffer with 2 pumps

AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.

👍

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Heatgeek

(@heatgeek)

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02/05/2025 9:14 am

@ashp-bobba I am afraid that you are talking to the wrong person about manufacturers preferences. However, as far as I can see, it functions the same as a 4 port buffer except that flow doesn't go through the top of the tank causing mixing, therefore I would have thought that it is a perfectly acceptable alternative to a 4-port.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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ASHP-BOBBA

(@ashp-bobba)

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02/05/2025 9:57 am

@heatgeek Oh agreed, its a slightly better and slightly less resistive solution, perhaps even the middle ground of no buffer V buffer.

AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.

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Potatoman

(@potatoman)

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02/05/2025 11:53 am

@heatgeek Thank-you for your reply, can you clarify the circulating pump situation, would you have to have 3 circulating pumps in operation, one in the heat pump and two in the design. Thanks

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ASHP-BOBBA

(@ashp-bobba)

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02/05/2025 11:59 am

@potatoman hi, the scheme detailed is 2 pumps, one for the ASHP (primary) and 1 for the secondary circuits.

AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.

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Heatgeek

(@heatgeek)

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Posts: 117

02/05/2025 12:04 pm

@potatoman No, the installation is exactly the same as a 4 port buffer with primary and secondary pumps except that the output flow bypasses the top of tank so as not to disturb the stratification of the tank. This removes some of the inefficiency problems associated with 4 port buffer tanks, like output temperature being reduced by mixing.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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Heatgeek

(@heatgeek)

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02/05/2025 12:35 pm

Note a key point here though. The primary pump is connected in the return pipe while the secondary pump is connected in the flow pipe after the buffer tank to provide good hydronic separation between the primary and secondary pumps.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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Heatgeek

(@heatgeek)

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02/05/2025 12:48 pm

Only the difference in flow between the heat pump and the emitters passes through the tank. This lower flow helps to preserve the stratification of the tank so that dilution of temperature to the emitters is minimised.

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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Heatgeek

(@heatgeek)

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Joined: 2 years ago

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02/05/2025 12:59 pm

Apologies. Poor choice of words. Delete "dilution of temperature to emitters is minimised".

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.

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JamesPa

(@jamespa)

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02/05/2025 1:56 pm

Posted by: @heatgeek

↑

Note a key point here though. The primary pump is connected in the return pipe while the secondary pump is connected in the flow pipe after the buffer tank to provide good hydronic separation between the primary and secondary pumps.

That all sounds very believable. Presumably mixing of return water with flow water to the emitters (and thus a reduction of ft to emitters) will however still occur if the primary pump throughput is less than the secondary, so the requirement to set up properly remains albeit that it is less sensitive than a 4 port.

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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