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Heat Pump Manufacturers Recommending Buffer Tanks

GrahamF · 2025-05-09T07:16:50Z

I have read and viewed lots of information about buffer tanks in heat pump systems. Renewable Heating Hub and Heat Geek are generally against them and this appears to be backed up by the excellent research paper that Brendon Uys commissioned. I am surprised to find buffer tanks are recommended for ASHPs, because none of my family has a buffer system in their gas central heating system. I am getting quotes for an ASHP. One of the installers routinely installs buffer tanks. Another installs them, "when needed".This second installer installs Midea and Stiebel Eltron heat pumps. He says Midea are mid-market and Stiebel Eltron are high-end. I researched the Stiebel Eltron and discovered that their indoor unit includes a buffer tank and secondary pump. I queried this on a Stiebel Eltron YouTube video. Here is the discussion. GF: "Why do you need the buffer tank? The reason I ask is that typically the flow rates will be different between the primary and secondary water circuits. This leads to mixing of water in the buffer tank. This results in distortion - the secondary circuit runs at a lower flow temperature than the primary circuit. Therefore, the heat pump has to run at a higher flow temperature to achieve a given flow temperature in the emitters. This reduces efficiency." @tanklesswaterheaters5863: "The buffer tank is needed for hydraulic separation of the source and load sides of the system. Both the source and load sides will have different flow characteristics and need to be separated. In addition, the buffer tank adds system volume allowing longer run times and provides stored energy for quick response. Any micro zones in the system will be absorbed into the buffer tank and may not require a call for heat to the heat pump. A hydraulic separator could also be used if the system zones are a match to the minimum input of the heat pump. The buffer tank provides extra cushion to ensure proper operation and longevity of the equipment." GF: "Very helpful thanks. If you have a system with just radiators, could you manage without the buffer tank?" @tanklesswaterheaters5863: "If the radiators are zoned individually, using thermostatic heads, this could create a micro zone situation with very low flow and low load requirements. Low flow and load would cause the HP to short cycle. If there are several radiators on one zone, the load is higher, and less potential for short cycling. But either way, we need to separate the HP flow from the radiator flow because they will be different, and a buffer tank works perfectly for this. The Stiebel Eltron installation manual says that a buffer tank is required, and that’s mostly to cover all installation situations. Typically the buffer tank can be small, 20-40 gallons." Therefore, it seems that if you buy a Stiebel Eltron, you have no choice but to install their buffer tank. I wonder how many manufacturers are effectively mandating them?

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Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

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Anonymous 5011

(@Anonymous 5011)

Joined: 3 years ago

Posts: 562

12/05/2025 11:44 am

Posted by: @heatgeek

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As an example for a low energy house with UFH and grossly over-sized heat pump, a 300l independently heated (by HP) low temperature thermal store with 8C minimum DT low flow rate to thermostatically controlled rooms will achieve much better energy saving than any form of open-loop working,

Are you sure about that. I have done almost every configuration of heating possible including what you describe,it isn't the cheapest or most comfortable.

By far the lowest cost is batch charging direct to floor in cheap period, if without battery, so around 7 hours overnight. That is open loop direct to floor (you a single low hysterisis thermostat for that). Heat pump should run continuously for whole period and ASHP generally be well oversized.

If you have a battery run elevated flow temperature for 7 hours then normal WC (in a low energy house the ASHP should stay off for nearly the whole day, when really cold may start again early evening - again all open loop.

Neither of the above get any advantage from the thermal store. Especially if using a dT of 8.

If you have battery and PV, just run heat pump at elevated temperature on excess PV once battery is nearly charged, then it's free heating. Most the shoulder seasons can be done this way. Then back to straight WC. When we did this in Feb and March, night temps were dropping to zero and the floor remained hot enough to keep heat pump off.

Running 8 dT, means heating your thermal store hotter than needed, compared to dT of 4, for the same mean flow temperature in to floor, so isn't the correct way to go. And thermostat in all rooms, been there also, not the cheapest for low energy house

Are you sure you have a low energy house?

Posted by: @heatgeek

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Large solar gain

Are you sure you get large solar gains, we can get temperature close to 30 in the living room, if we let the solar gain run uncontrolled.

SCoP is pretty meaningless cost is all important. I am currently making my scope worse each day as I am now in cooling mode getting a negative CoP.

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4634

12/05/2025 12:48 pm

Posted by: @damon

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id use a buffer when the job would dictate.for example on a 13/16 Viessmann vitocal you don’t get much head left on the pump, so a buffer would allow separation and additional pump.

id usually go with a 3 pipe buffer on the flow, upsize the tee and entry point init the buffer.

...

you also need inside your buffer correctly, generally you find issues with smaller buffers and header where low volume affects the temps more severely and also the set up, the little buffers under cylinders are useless.

Thanks for that thoughtful reply.

Unfortunately we get far too many reports here where 50l 4 port buffers have been installed 'because that's what we do' often, so far as we collectively can tell (remotely), contributing significantly to poor performance which is why many on here will advise against them unless there is an identifiable reason.

It sounds, in contrast, like you

Only use a buffer when there is an identifiable reason
If you do use a buffer, take several careful steps to reduce system distortion

Is this a correct reading of your approach.

Posted by: @damon

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you could also use a buffer where you have different circuits and the customer wants control over rooms.

Is there any reason in not to use trvs (or zone valves) without a buffer in the rooms which need (and can sustain) control (usually a minority). That's how my system is set up and it seems to work well.

As ever I'm thinking about ordinary houses (typically 100sq. m in the UK, but my comment is intended to apply to houses up to perhaps 250-300sq m which are not very long and thin). I can see that with a very extensive and complex system multiple pumps and a buffer might be useful.

This post was modified 11 months ago by JamesPa

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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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4634

13/05/2025 10:15 pm

Posted by: @damon

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Posted by: @damon

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scop is all well and good but we should also be looking at energy usage, you could be getting a better scop but using more energy.

Well said, Sir! I have been trying to get this point across to reluctant ears.

I'm curious who you think are the reluctant ears. Certainly not me, and if you do think i am one of the people you refer to, then you are misunderstanding my comments!

Nor have I seen many others suggesting what you say, with the very important exception of those who go to the next step and look for lowest cost using tou tarrifs, often as a result consuming more energy than would otherwise be the case.

This post was modified 11 months ago 2 times by JamesPa

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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