Has anyone run wet central heating from a hot water cylinder?

So, I’m in the throes of retrofit right now: EWI is going on next month, we’re getting quotes for windows and doors, the loft has been insulated to within an inch of its life and I’ll air tightening like a madman when our MVHR goes in. The project is… pricey, but going ahead.

What I’m wondering is if anyone has used an EAHP like the Tesy Aquathermica 200 to provide space heating as well as hot water?

This is possible because there’s a heat exchange coil in the unit, and the pump claims to be able to hit 65C. Obviously the heat demand is a massive factor here as the EAHP only supplies 1.5kw of heat, but I’ve been toying with the idea that once all the work is complete, we’d be able to avoid ripping the old wet heating system out and instead drive it at the cylinder temperature.

Working out how this is done without compromising the hot water temperature is hard, though. Aiming for 40C to the tap gives 25C of wiggle room, so 4.18kj per kg of water (200L) times 25 makes about 20Mj to play with.

The output of the radiators in the old CH is supposedly around 2.6kw, and assuming the heat exchanger can supply it, that might do the trick. I need to re-run the heat loss calcs, but with EWI alone we got to 6kw for heating requirements. New windows and doors, plus best recovery, will, I think, cut two thirds off this, but I need to do the maths.

What I do know, though, is that removing a net 1.1kj/s from the cylinder via the hx (that’s radiator output minus heat pump supply) the cylinder when it is at 65C will run for just over 5 hours before the temperature hits 40C.

Skipping to the end, it is still fairly borderline as to whether or not this is a goer, with the main questions relating to recovery time and how long the CH and DHW are in use each time.

So, I’m wondering: has anybody done this? And… does it sound insane? I feel like it might be.

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Thermal stores are designed for this kind of use but then your heat pump would probably be only one of several heat sources.

Using a DHW cylinder would work, provided you don’t actually want to use it for hot water!

Also remember that just because you can achieve 65°C doesn’t mean that it will achieve a decent COP.

Have you trialled running your old system at a lower temperature?

I have been asking myself this very same question? :roll_eyes:
What are the possibilities here?
Is there a hot water tank capable of storing hot water, which can be used with a wet heating system and is compatible with a heat pump further down the line??. :thinking:

From our forum here and my own research online I have gotten as far as understanding how a thermal store might be the best choice for our home energy system - we have solar, but no battery - therefore need to use the solar energy as it comes in - rather than sending it off to the grid.

Retrofit to reduce carbon can be so frustrating - trying to navigate the gauntlet of knowledge and market competition to find the most energy efficient systems for our homes.
I can’t help but wonder how many of us are making wrong installation choices, only to replace it further down the line with a more compatible system/tank - these ‘bad’ choices will certainly drive up carbon consumption.
Education for householders is key to driving down carbon consumption!! :+1:t3:
With so much new technology, (plus the very fortunate addition of solar to our home) I already know my retrofit survey advisories would look very different now. :100:
It is clear how hot water tanks can be a key part in many efficient home energy systems and an increased demand for householders to understand these systems.
@cc_staff - In the interests of reducing carbon as householders scramble to change DHW and heating systems, is it possible you might facilitate a webinar or workshop to helps members better understand the options available to us? :pleading_face:
There must be examples amongst CC members for DHW and heating which can be shared in a more visual way for members struggling to make best choices here - maybe a webinar Q&A?

In the meantime, I shared some links which have enabled me to improve my understanding of our home energy system and the possibilities - hope they are helpful.


There isn’t an old system to speak of, alas - we moved in and the old boiler was immediately condemned. Should’ve been caught by surveys but such is life. I’m trying to work out how much I can usefully squeeze out of this system for now - hence my other posts about preheating air going into the ASHP unit, and storing excess ducted heat to be released later!

I suspect I will end up using this as supplementary heating or just DHW, but it is tempting to try to prove (albeit with naive models) either way.

I’m feeling very very similar: the time and cost involved is pretty forbidding at times. It doesn’t help that there are so many half solutions, really.

It has occurred to me to… get another cylinder and use that for heating.

Insulation feels straightforward enough, even if the motto is “see what ecomerchant sell” that works as a decent minimum. But if you’re trying to heat your house and can’t have a great big ASHP unit outside, there are seemingly not many easy options.

Edit: affordable easy options.

You were correct first time!

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Short answer is no.

Eventually we will. But for numerous reasons it’s not viable yet. This is reflected by the fact there are very few systems that do this.

New multifamily projects with large CO2 heat pumps, specially designed thermal stores, smart variable tariffs and controllers, will be the first.

It’s hard to do it in retrofit at unifamiliar scales. I suspect we will need low temperature heat networks with feed in tariffs to do it, and you’ll likely need about 1m3 worth of storage.

That’s a pity. I suppose we’ll get there in the end. Mulling it over, I’m considering using the heat exchange on the existing cylinder to link to a second cylinder, itself with more heat exchangers. Dual cylinder setups don’t seem common but also don’t seem so obscure that nobody would touch them. I’ll look at running the heat pump pretty much all the time, and using the secondary cylinder as a thermal store for the wet heating - bonus points if I can manage some solar thermal on the roof, all the more so if I can manage the “aerovoltaic” panels people like noreus do to boost the intake temperature - see https://www.noreus.co.uk/wp-content/uploads/2016/08/Noreus-aerovoltaic-brochure-download.pdf

But like I say, still thinking. I’ve not been able to plan this as well as I’d have liked (old system boiler was condemned a week or so after we moved in) so it is both fun and frustrating to get creative without going too mad. I’ll do an update if I run the numbers and find it is sufficient with enough insulation.

Buffer tanks are common for solar thermal. You can often find a use for them.

However, lower temperature heat isn’t that valuable so the mechanical space and additional complexity isn’t worth it compared to PV. It’s why we don’t use much solar thermal any more.

This is too late for you, but if you wanted to use a lower temperature water source then underfloor heating would have been ideal. It has to designed from the very start.

True, I had considered that. Also ThermaSkirt.

For the time being I’m cracking on with insulation, ventilation, and air tightening. I’ll recalculate the heating requirements when I have those in order and see if a trickle from the boiler is enough to make the system responsive etc.

Wasn’t aware of this - I just assumed people love the new and shiny!

One thing I wonder is whether or not a single solar thermal panel in a loop with a water-to-air preheater might achieve what I’m setting out to do, namely supplying warm enough air that the COP of the heat pump is increased.

Still, I’ve concluded I need to focus elsewhere for now!

:slight_smile: this sounds fascinating, and I am tempted by loads of shiny kit including another hot water tank, heat store but must ask shouldn’t we be moving the kit out of individual homes to modern district heating supplied by mega heat pumps powered by renewables?

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Solar thermal can then become ubiquitous because it can be used to top up the district

No arguments here, but I can’t really wait for it to happen. Winter is coming, and if I can get away with using my little DHW setup to drive enough heat through the radiators to keep warm once I’ve insulated the house, I’d like to. It doesn’t seem like that’s feasible just yet though.

In parallel to this I decided to try bothering my local council about district heating schemes, suggesting an ambient closed loop network that people could plug ground source heat pumps into. It seems like an economical option from their perspective, much more so than buying expensive centralised plant. Just… pipes in the ground, absorbing ambient heat.

It was a non-starter, the councillors described it as a “fascinating option for the future”. They weren’t willing to meet, or ask council officers to meet, any representatives of companies who build this kind of infrastructure. Not even a sniff of a pilot. Still lots of work to do to get any mainstream politicians into cheap, sane options.

Bristol Energy Co-op helpfully pointed me in the direction of Community Energy England who support this kind of thing: Who we are & what we do | Community Energy England

I’ve not yet adequately engaged with CEE (busy with renovating, and also planning a campaign with cycling UK…!) but I’m hoping to get there… eventually. It’d be nice to be able to get a local group together and put together a community benefit society to build out an ambient loop network, at some point.

Alas, life is so busy.

:slight_smile: I assume your council has declared a climate emergency and is fully aware of and about to implement best practice! Swaffham Prior Heat Network - Cambridgeshire County Council

Is anyone designing systems to join up with the district heating networks when they become necessities like broadband?

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Alas, my council is all elderly men who like cars. District is Labour and shows some green shoots (a fellow cyling UK rep is a councillor in the labour group) but the County is absolutely dire. It is so hard to get this stuff off the ground outside of the cities. People just don’t want change unless it is comically simple. It seems heat pump installers are tuned into this fact, which is why they’ll size the pump to an uninsulated house and walk away without even suggesting how people might benefit from reducing their heat loss.

Nice link on Rotterdam, though!

To update on where I’m at with this… I’m probably going to do it anyway. I’m fitting a gratuitous amount of EWI (150mm of cork or wood fibre if I can get a decent price, 100mm otherwise) so heat demand should be pretty low. Still haven’t managed to run the numbers on air loss and MVHR, but I’m not convinced in either direction. Which is to say, I’m not inclined to believe the answer is “no” just yet :slight_smile:


Some bigger cities are doing this.

Community initiatives would be interesting. Any community near a water source could start with sharing the environmental heat exchanger to run their water to water heat pumps.

This could then be joined together at some stage to a local heat network.

Fascinating how people say something that exists is a fascinating future option! Duck billed platypus syndrome, people said they were fake https://twitter.com/patrickwillcock/status/1591738840984821763?s=46&t=Gjxi8MMtXwmiPQes3aLCow

There are several clear steps that must be taken. Full forensic neighbourhood energy and mobility audits, cooperative mutual energy and mobility societies, understanding full range of options, working out best fit for a neighbourhood, heat networks, full insulation and storage