Welcome to the forums Graham.

The idea that we need a buffer because of mismatched flows between source and load is rooted more in legacy commercial boiler design and installer habit than in good, modern low-temperature system design. The real issue here is system design… if your system is designed correctly from the start, with sufficient emitter surface area, proper modulation and sensible zoning (or better still, open-loop operation), then there’s typically no need for hydraulic separation or added volume.

Regarding Stiebel Eltron, you’re right. I think their integrated indoor units include a buffer tank by default. Aira is another one that comes to mind. This isn’t necessarily because it’s best practice for every home, but because it simplifies compatibility across a wide range of installer skill levels and property types. It’s a belt-and-braces design choice to minimise the chance of complaints about short cycling or erratic flow rates; but that doesn’t mean it’s the most efficient or elegant solution. In fact, if the heat pump is matched well to the load and the system volume is sufficient, that internal buffer becomes largely redundant. The problem is that a massive portion of heat pump installers don’t have the technical knowledge or training to design a low temperature heating system, so the buffer just tapes over their shortfallings.

The YouTube commenter’s points are technically correct in principle. Yes, a buffer can absorb micro-zoning and add volume, but the problem is that this comes at the cost of efficiency, which is sometimes significant. Stored volume isn’t the same as stored heat, and distortion (blending) across a buffer can quickly lead to lower emitter temps, longer runtimes and reduced COPs.

In terms of manufacturers mandating buffers, yes, some still do, particularly in their documentation or when using certain accessories like low-loss headers. But “mandated” and “required for good performance” aren’t always the same thing. Often it’s a lowest-common-denominator recommendation to avoid customer service issues, not necessarily to deliver the best efficiency or long-term outcome.

If one installer is pushing buffers routinely and another is using them only when needed, I’d lean toward the latter, provided they can explain what design measures they’re putting in place to ensure stable operation without it. For most domestic installations, a good installer can get it done without a buffer tank, but depending on pipe sizing and existing zoning, they might fit a volumiser. The most efficient systems we’ve seen tend to be unbuffered and open-loop.

Posted by: @grahamf

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Stiebel Eltron

If you are willing to pay rolls Royce prices for a heat pump you may as well follow their instructions, to get the warranty - but I wouldn't.

If you do one zone, you really don't need a buffer, if you zone or micro zone you have no choice really. There are different ways to plumb a buffer as well to help prevent distortion.

Just because someone has posted a video doesn't really make them an expert! 

So the question not answered is - are you zoning, do you TRVs on all radiators etc?

Posted by: @Anonymous

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If you are willing to pay rolls Royce prices for a heat pump you may as well follow their instructions, to get the warranty - but I wouldn't.

I don’t think I’d be willing to personally put it to the test, but I do think there’s a reasonable argument that you could successfully challenge a voided heat pump warranty just because a buffer tank wasn’t fitted… even if the manufacturer’s documentation recommends one.

If a system has been professionally designed, installed to recognised standards and commissioned correctly without a buffer (especially where one isn’t required by Building Regs or MCS), then it’s not cut and dry for a manufacturer to simply walk away.

That said, this is a murky area, and most homeowners understandably won’t want the stress of a legal battle or the hassle of proving a system was compliant. Manufacturers know this, which is why many lean on strict T&Cs to protect themselves. But are those conditions always legally enforceable? Not necessarily.

I’ve broken this down in more detail in a recent article if anyone’s interested: https://renewableheatinghub.co.uk/fact-or-fiction-are-buffer-tanks-really-necessary-to-protect-your-heat-pump-warranty/

One manufacturer Viessmann has to have their buffer, as it's used and only used during defrosting.

@johnmo it is a 5 bedroom detached house built in 1968 with about 350m2 spread across two floors.  It has 28 radiators.

We plan to run a single zone with TRVs in the bedrooms for cool sleepers and in one downstairs room that has a lot of solar gain.  If all 11 radiators turned off at the same time, we would still have 17 radiators running.  

We will also use weather compensation with an internal thermostat set several degrees above our target temperature, just in case the weather compensation is adjusted wrongly. 

@johnmo installer 1 uses Viessmann heat pumps and I must say I like their technology.  I have wondered about their indoor unit.  Is it really a buffer tank or a volumiser in the internal unit?  Interestingly the internal unit contains a water pump.  I'm not sure why.

@damon you've fitted a few Viessmann's – can you please elaborate a bit about the buffer/volumiser on these units? 

Posted by: @grahamf

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Is it really a buffer tank or a volumiser in the internal unit?  Interestingly the internal unit contains a water pump.  I'm not sure why.

Not a buffer or volumiser in the normal sense. The buffer only gets used during defrosting, heating the buffer up prior to defrosting to a high temperature, then using only the energy within the buffer during defrost. It is then isolated. Seen results of this arrangement and the CoP overall was better than the normal arrangement.

The water pump is the circulation pump for ASHP, just located inside not outside.

@editor it’s a defrost buffer,

it uses an Esbe 4 way valve in a three way configuration, hot water, buffer, heating.

when it senses it requires a defrost it loads the buffer to around 55/60°c and uses this to defrost. So on a direct system it doesn’t impact the house load or temp.

a caveat to this is, it only seems to use the defrost buffer on the double fanned units and not the single. Plenty of examples on oem. Not sure why it does this, and your not really saying it become a problem apart from on oem.

but when you are trying to squeeze every bit of efficiency it does seem stangez

still in my opinion the best Heatpump on the market and my choice always. 

@damon really helpful, thanks Damon.

@grahamf, is this for your home or your other project? If it’s for both, even though Damon is based out of Yorkshire, he does take on bigger projects across the country, and I would highly recommend him because he’s definitely one of the best installers in the UK. Let me know if you’d like me to connect him with you.