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Say 15kW or, even better, 20kW.

I'd like to replace my gas combi with a heat pump for space heating. Rather than find space and plumbing for a hot water cylinder, I wonder if I could just use a big (say 45kWh) battery, charged off peak, to run an electric combi boiler. Could that work? Apart from needing quite a punchy inverter, what other considerations are there?

I think you need to work backwards to a certain extent: How much HW do you need and at what temperature and how quickly (obious lmitation being the CW supply)? If you need 150L at 40C in 10 mins for a bath or example!

Heatgeek have also developed a Mini Store cylinder, which almost works like a combi boiler - it can fit under a kitchen counter, depending on size, and can allow continuous hot water, depending on cylinder size, heat pump size and presumably shower flow rate.
As far as I remember it's basically a heat exchanger cylinder, but with the heat pump side having the larger volume of water - your hot water for the shower is instantaneously heated by passing through the coil - so reverse of a 'normal' hot water cylinder.
I'm not sure how popular they are yet - I have thought about it, instead of the space and upheaval required to fit a full sized water cylinder plus all the associated pipework etc.
There are various heatgeek youtube videos on the subject if you're interested

GarethC wrote: ↑Wed Mar 26, 2025 12:52 pm Say 15kW or, even better, 20kW.

I'd like to replace my gas combi with a heat pump for space heating. Rather than find space and plumbing for a hot water cylinder, I wonder if I could just use a big (say 45kWh) battery, charged off peak, to run an electric combi boiler. Could that work? Apart from needing quite a punchy inverter, what other considerations are there?

You could. However it is MUCH cheaper and easier to just store the hot water!

Here is a list of problems with your plan:
- You'd need to get DNO approval for an inverter powerful enough to supply the 15kW boiler and the rest of your load. That'd be impossible on single phase, so you'd need a 3-phase supply.
- You'd need to buy and install the inverter, that is probably £3k at least
- The batteries would cost c. £10k on their own.
- Filling a bath from a 15kW on demand boiler takes about an hour!
- Those 15kW electric boilers take up a lot of room and make a lot of noise.

Compare that with a DHW tank that would cost maybe £800. And you'd fill the bath in 5 mins. I've seen people build insulated outdoor enclosures for their DHW tank or you could mount it in the attic...

I note the OP ran an almost identical thread less than a month ago,
https://camelot-forum.co.uk/phpBB3/view ... =14&t=2889

I am thinking that you are going to put in a big battery and big inverter anyway so maybe you are thinking can I get away with also using it for hot water.

As other have commented, Its inefficient and has practical and regulatory problems tying to heat water direct with batteries without a heatpump and tank. If you don't want a hot water tank, then an alternative would be buying a Sunamp thermal battery (uses phase change material) for your hot water and charge it on off peak leccy.

They are more expensive than hot water tanks but somewhat cheaper than an oversized battery and inverter, you just run your cold feed through it. And they are rectangular so takes up less space and a minimum of plumbing.

nowty wrote: ↑Wed Mar 26, 2025 4:26 pm I note the OP ran an almost identical thread less than a month ago,
https://camelot-forum.co.uk/phpBB3/view ... =14&t=2889

That thread was from a year ago I thought of just bumping up, but actually I realised I'd like to restrict the question a bit to just the hot water practicality. Thanks for responses. Just finishing up at work, but will consider properly this evening.

GarethC wrote: ↑Wed Mar 26, 2025 5:16 pm

nowty wrote: ↑Wed Mar 26, 2025 4:26 pm I note the OP ran an almost identical thread less than a month ago,
https://camelot-forum.co.uk/phpBB3/view ... =14&t=2889

That thread was from a year ago I thought of just bumping up, but actually I realised I'd like to restrict the question a bit to just the hot water practicality. Thanks for responses. Just finishing up at work, but will consider properly this evening.

I thought it was still 2024.

nowty wrote: ↑Wed Mar 26, 2025 6:15 pm

GarethC wrote: ↑Wed Mar 26, 2025 5:16 pm

nowty wrote: ↑Wed Mar 26, 2025 4:26 pm I note the OP ran an almost identical thread less than a month ago,
https://camelot-forum.co.uk/phpBB3/view ... =14&t=2889

That thread was from a year ago I thought of just bumping up, but actually I realised I'd like to restrict the question a bit to just the hot water practicality. Thanks for responses. Just finishing up at work, but will consider properly this evening.

I thought it was still 2024.

Wait ..... it isn't? Damn, alarm clock didn't go off.