I think the current requirements and installer approach to hot water cylinders creates a lot of unnecessary and avoidable disruption, and cost for homes with existing cylinders.

The standard MCS sizing formula, 45L per bedroom + 1, can result in cylinders much larger than the existing ones, which do not then fit in existing airing cupboards, with installers then wanting to relocate these into bedrooms, garages or other inconvenient spaces with all of the cost and disruption that entails. 

From a homeowner's perspective, when the existing cylinder is set to 60 deg C, and with new R290 heat pumps able to heat an equivalent cylinder to 60 deg C, it seems a bit ridiculous to incur all of that cost for a bit more stored hot water, which many households may not even need. There should be flexibility to use smaller cylinders, or cylinders of equal size to any existing cylinder, that are drop-in replacements for the existing one.

All of the major heat pump installers insist on pressurised unvented cylinders, which then add a bulky pressure vessel and need a D2 drain installing through the house, or even a D2 forwarding pumping adding. A simple vented heat pump cylinder option would be much less disruptive and costly for houses with existing vented cylinders.

The current requirements don't mention heat stores, so these are something of an novel, unusual type solution which most of the big installers don't support yet these look a brilliant idea for lots of households - compact, rapid recovery, great replacements for existing cylinders and for new installs where space is a premium. They need including specifically in the standards so they become a default, acceptable option. 

It strikes me that the UK market hasn't matured yet, this is all simple, obvious stuff which should in place and will need to be put in place if more homes are going to install heat pumps.

@jamespa, that’s a very fair summary and I completely agree that it’s important to recognise positives where they exist. I’m not against innovation or even the idea of simplifying installs. Anything that makes the process smoother for homeowners and easier for competent installers should be celebrated.

My issue, as you’ve correctly picked up, is about messaging and its knock-on effects. Heat Geek’s reach is significant and their words carry real weight across the sector. When they say “Zero Disruption”, that phrasing doesn’t just stay on their site... it seeps into how the wider market behaves. And as we’ve both seen before, less scrupulous installers will take that headline, strip out the nuance and run with it to justify cutting corners. That’s the danger.

But we have to keep holding everyone (not just Heat Geek) accountable for the clarity and honesty of the message, because the downstream consequences of overpromising are significant. Aira is another problem child. 

That’s all I’m trying to do... flag the risks early before we find ourselves back where we were five years ago, untangling the fallout from thousands of bad installs and frustrated homeowners.

Posted by: @temperature_gradient

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I think the current requirements and installer approach to hot water cylinders creates a lot of unnecessary and avoidable disruption, and cost for homes with existing cylinders.

That's a timely reminder on this note... one of the installers I spoke with this weekend suggested (though it's unverified on my end) that for a heat pump installation in the UK, the existing hot water cylinder often needs to be replaced with a 'heat pump ready' cylinder.

The reason isn't a direct building regulation mandate to replace the tank itself, but rather a performance requirement. Heat pumps run at lower temperatures than conventional boilers, so the cylinder's internal heat exchange coil must have a much larger surface area to efficiently transfer the heat and meet the minimum required hot water temperature and reheat times. Using an old cylinder can drastically reduce the heat pump's efficiency, which can violate compliance standards. 

I will look into it this week. 

Posted by: @editor

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Heat pumps run at lower temperatures than conventional boilers, so the cylinder's internal heat exchange coil must have a much larger surface area to efficiently transfer the heat and meet the minimum required hot water temperature and reheat times. Using an old cylinder can drastically reduce the heat pump's efficiency, which can violate compliance standards. 

Yes the heat pump cylinders use coils with high surface areas, but that isn't the problem, the difficulty comes from the MCS standard sizing formula requiring much larger cylinders which then makes finding a suitable location a challenge. 

There's also a bit of a contradiction in objectives, because we're fitting heat pumps to reduce emissions, but if a much bigger cylinder is required and this needs relocating to an unheated garage or attic, that results in several hundred kWh of extra heat loss, wasted each year.

Plus the lack of acceptance/support of more innovative heat-store solutions which do a better job of solving that need for compact, fast re-heating cylinder to fit in the available space.

Posted by: @editor

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Using an old cylinder can drastically reduce the heat pump's efficiency, which can violate compliance standards. 

I will look into it this week. 

Oh come on, that's a ridiculous argument by an installer who just wants to replace cylinders at any cost to the customer, hasn't got a technical argument for what he wants to do, and will happily ignore the same 'standards' when it suits them.

Depending on the circumstances there are options:

• Use the existing cylinder and add a phe and circulator pump replacing the coil.  Same or better performance, much less cost and disruption.  Potentially a bit of noise from the circulator pump depending on siting.
• Use the existing cylinder with existing coil, operate heat pump at high temp for dhw production.  Poorer performance, zero cost or disruption but very workable with many R290 heat pumps and quite possibly R32
• As above but combine with use of immersion and/or circulator pump
• Replace cylinder.  Expensive and possibly quite disruptive, but ultimately the best performance with the exception of the first option.  But what's the (carbon/gbp) payback time relative to any of the above?

The trade offs depend on household usage, funds availability, heat pump capacity, nature of any existing cylinder and householder attitude to any noise which a circulator pump may cause.  There are indeed many cases where replacing an existing dhw cylinder is necessary or desirable, but overall this is another example of the homeowner being denied choice for the convenience or profit of installers, unnecessarily pushing up the price of heat pump conversions at the expense of the taxpayer, the customer and the climate.

If you already have a reasonably recent UVC of sufficient size, I think I would argue that replacing it just to get a larger coil is probably the last option to be considered.  Unless you consume unusually large amounts of DHW the payback time is just too long, particularly if you have, or intend to get, a ToU tarrif.

As it happens I did replace my small vented cylinder.  One installer was prepared to reuse it (I suspect another would also have done so), but after discussion I was happy that the usability advantages to me outweighed the disadvantages.  Had it been a recent UVC (say < 10-15 years old), there was no way this would have been the case and thus no way it would have been replaced.

This view is going to be unpopular with many installers, but not with all.  Let those with whom it is unpopular defend their position, with reference if necessary to the specific regulations that they claim are mandating a particular approach.  If they can prove the above wrong I will happily eat my hat.  I'm pretty convinced my hat is safe.