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Can I start an argument about buffer tanks please? 🙂

Air Source Heat Pumps (ASHPs)

Last Post by Mars 42 minutes ago

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AndyTheMinion

(@andytheminion)

New Member Member

Joined: 6 hours ago

Posts: 1

Topic starter 04/08/2026 7:29 pm [#3125]

Hello All,

I'm not really after starting an argument, but I am about to take a first step into heat pump ownership 🙂 I come from a boiler R&D background so I have 'some previous' in heating design, appliances and controls but I noticed something strange being stated when I started looking for a suitable heat pump.

The background, we have just bought a ±200 year old cottage with 75cm solid walls and probably a real thirst for energy as winter get down to -15°C. There is no gas in the village so it currently has a three phase 12kW electric boiler. We have thrown some decent controls at the system, corrected some mistakes, balanced the system so it is running well turned down to a lower maximum rate and at lower flow temperatures, but clearly the boiler needs to go.

The heat loss (as always) will be a guess because of the fabric, but I can see the current radiators are delivering 21°C at around 20% of their 75/65/20 output with an outside temperature 5°C, so from some weather curves (if you believe them) they should meet design heat loss at a flow temp of 55°C but I will upgrade them and lower the temperatures further. The biggest issue I currently see is the secondary plumbing, I absolutely cannot get to the pipework to achieve the flow rates needed for a DeltaT of 5°C. This seemed to be a no-brainer, fit a buffer tank! - it adds essential system volume and was always the correct solution to achieve a neutral point in mixed hydronic systems, but then I saw in the UK (I am English BTW) they seem to be viewed as the work of the Devil based on 'distortion'.

From what I can see the argument is based on an assumption that radiators are sized to a 5°C DeltaT and an efficient flow temperature, then a buffer is fitted afterwards, the DeltaT increased back with a boiler flow rate and the room gets cold. A higher DeltaT is obviously going to reduce the average radiator temperature. A lower temperature will reduce radiator output, and this in turn means the flow temperature needs to be higher. No issue with that what-so-ever, it is correct, but that is emitter under-sizing issue, not an evil buffer changing/mixing flows and temperatures in some spooky way. It would be exactly the same if the under-sized radiator are in the primary circuit. A long time ago but I was taught that buffers and low loss headers are used to prevent flow distortion in systems, when did this change and more to the point why?

I'm happy to accept the argument that buffers increase radiator sizes, but that should be on the heat pump list already. When I have spoken to R&D departments they welcomed system designs with buffers as it prevent system and commissioning issues affecting appliance reliability and lifetime.

Mind you, poor internet.... buffering, I don't like that. Okay I agree, buffers are bad sometimes.

Regards AtM

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4796

04/08/2026 8:45 pm

@AndyTheMinion, welcome to the forums. You have certainly chosen one of our less contentious subjects for your first post. 🙂

My view is that buffers have acquired such a bad reputation because they are routinely fitted to straightforward domestic systems whether they are needed or not. They add another pump, more controls and another opportunity for the heat pump to produce hotter water than the radiators actually receive. In most ordinary homes, a direct system with the required water volume and flow seems the cleaner option.

Where I think your case may be different is that you have identified a specific problem the buffer would be there to solve. If the existing distribution pipework genuinely cannot carry the flow required by the heat pump, and cannot realistically be replaced, then separating the heat pump circuit from the radiator circuit may be a reasonable compromise rather than an automatic design habit.

I don’t think the concern is solely that the radiators were sized at the wrong delta T, though. If the flow on the heat pump side and the flow on the radiator side do not match, water moves through the buffer in ways that can alter the temperatures each circuit sees. You can end up mixing cooler return water into the radiator flow (distortion) or sending some hot flow directly back towards the heat pump. Neither is mysterious, but both can cost efficiency unless the design has properly allowed for them.

That doesn’t make the buffer inherently wrong. It means the designer needs to know what flow each side will actually run at, what temperature will reach the radiators and what return temperature the heat pump will see. If the emitters are then sized for those real conditions, rather than for an idealised direct system, the case for it sounds much stronger. Problem is, designers and installers can't be bothered to work any of this out.

The existing electric boiler may also give you better evidence than a conventional heat loss guess. Do you have any cold weather electricity data, particularly during a steady period close to the local design temperature? A 12kW boiler maintaining the house at a known indoor temperature could tell you quite a lot about the actual demand, although hot water use would need separating out.

How large is the cottage, what pipe sizes can you see, and what flow rate or flow/return difference are you achieving with the present system? It would also be useful to know which heat pump and output you are also considering.

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