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

 
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Air Source Heat Pumps (ASHPs)
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Mars
 Mars
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Topic starter 23/01/2024 7:00 am   [#1318]
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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


This topic was modified 12 months ago 2 times by Mars
This topic was modified 2 months ago 5 times by Mars

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JamesPa
 JamesPa
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23/01/2024 10:13 am  

Posted by: @editor
↑

Remember, you're the homeowner,

Well said!

I (and I suspect many others) have met a fair few installers who don't seem to accept 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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Toodles
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23/01/2024 12:00 pm  

Thank you Mars, I accept that a LLH may affect the efficiency of the overall efficiency - but how can the average individual who has a system installed with a LLH know whether they would be better off without it? Ripping apart a system that has one such and a secondary pump is a little radical - and is ther any guarantee that the efficiency would improve without these components. Presumably, LLH was a concept with a purpose - is it just to protect the designer and installers? Regards, Toodles.


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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 HughF
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23/01/2024 12:02 pm  

@toodles It is purely there to allow the heat pump to operate without any flow errors... Primary side just sits there running into the header, microbore or TRVs closing don't have any effect on it.


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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Toodles
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23/01/2024 12:18 pm  

@hughf If a system doesn't have any variable restrictions like TRV’s, would such a system be better able to perform to the optimum without an LLH then? My system has carefully adjusted lockshield valves (constant restrictions in other words) but does have one TRV in a bedroom. There is an LLH and secondary pump - I was told this means achieving a Delta T of 5 degrees across the radiators is not crucial as the LLH is acting as an intermediary. Regards, Toodles.


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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JamesPa
 JamesPa
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23/01/2024 12:23 pm  

Posted by: @toodles
↑

Thank you Mars, I accept that a LLH may affect the efficiency of the overall efficiency - but how can the average individual who has a system installed with a LLH know whether they would be better off without it? Ripping apart a system that has one such and a secondary pump is a little radical - and is ther any guarantee that the efficiency would improve without these components. Presumably, LLH was a concept with a purpose - is it just to protect the designer and installers? Regards, Toodles.

It's a fair question

There are two approaches to answering it I can think of.

1.  Measure the flow-flow temperature drop across the llh.  For each degree you are losing 2-3% system efficiency.

2.  Plumb in  bypasses for the llh on the flow side and for the secondary pump with a couple of manual diverter valves so you can switch them in and out.  Perform the experiment 

Posted by: @toodles
↑

Presumably, LLH was a concept with a purpose - is it just to protect the designer and installers?

In most cases it seems, yes.

Posted by: @hughf
↑

@toodles It is purely there to allow the heat pump to operate without any flow errors... Primary side just sits there running into the header, microbore or TRVs closing don't have any effect on it

One might reasonably ask, why is the heat pump designed to register them as errors.  Answer - because the situation shouldn't occur. 

Putting sticking plaster over a red warning light doesn't make the problem go away.  However it does reduce the liklihood of call outs.  Now who does that help?


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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Marzipan71
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23/01/2024 12:34 pm  

Hi @toodles don't know if you've seen them before but there a couple of articles which may be of interest from Protons for Breakfast regarding his system performance before and after removal of the LLH (I have a LLH on my Daikin ASHP) - links here and here



   
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 prjohn
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23/01/2024 12:45 pm  

Are all Low Loss Headers bad? Mine was fitted as per Samsung schematics. This I would imagine would remain part of the warranty agreement. The effect on my system seems to be negligible. I did discusses this at a resent service and the engineer comment was "if its not broken don't fix it". As my efficiency appears to be high, 2288kw/h energy use over a year as opposed to 1700litres (17000kw/h) of oil it seems my system is working OK. Another point is if by removing the LLH increases efficiency could it have an effect on the size of HP fitted? One other point, can a LLH act as a debris trap for older heating system thereby protecting the HP?



   
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Toodles
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23/01/2024 12:57 pm  

@marzipan71 Thanks very much Marzipan, very interesting indeed. I think that that data excuses my uncertainty of LLH - Good or Bad? Regards, Toodles.


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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Toodles
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23/01/2024 1:02 pm  

@prjohn Ooooh! Mars! Did you consider you might be opening a massive can of worms with this topic?! ;-)))


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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 HughF
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23/01/2024 1:10 pm  

 

Posted by: @hughf
↑

@toodles It is purely there to allow the heat pump to operate without any flow errors... Primary side just sits there running into the header, microbore or TRVs closing don't have any effect on it
 
One might reasonably ask, why is the heat pump designed to register them as errors.  Answer - because the situation shouldn't occur. 
 
Putting sticking plaster over a red warning light doesn't make the problem go away.  However it does reduce the liklihood of call outs.  Now who does that help?

On commissioning, I was having a nightmare getting rid of flow errors, I suspected a faulty flow switch so just jumpered it out in the terminal strip. It has been running like that since september.
Turned out to be air in the system. Haven't had a flow error since 😀 
 


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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Mars
 Mars
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Topic starter 23/01/2024 1:14 pm  

Posted by: @toodles
↑

@prjohn Ooooh! Mars! Did you consider you might be opening a massive can of worms with this topic?! ;-)))

Always good to have discussions about topics like this.

 


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