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Heat Pump Manufacturers Recommending Buffer Tanks

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(@grahamf)
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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?

This topic was modified 7 hours ago by Mars
This topic was modified 6 hours ago 2 times by Mars

   
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Mars
 Mars
(@editor)
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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.

This post was modified 6 hours ago 2 times by Mars

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(@johnmo)
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Posted by: @grahamf

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?

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Mars
 Mars
(@editor)
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Joined: 4 years ago
Posts: 3190
 

Posted by: @johnmo

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/

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(@johnmo)
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Joined: 2 years ago
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One manufacturer Viessmann has to have their buffer, as it's used and only used during defrosting.

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