Under sink water heaters

[dangermouse]

So now I've got my panels and batteries sorted out (3.2kWp of PV and 8.3kWh of storage) I'm wondering what I can do to replace some of my gas usage with free (or cheap rate) electricity.

My house and water is heated by a (new and hopefully efficient) gas combi boiler, and I don't really have the space to fit a large hot water tank. But I bet 90% of my hot water use is either showers or washing the dishes.

So for the kitchen I was considering something like this undersink heater. It holds 15L of water at up to 60C, enough to fill the kitchen sink. A rough calculation suggests that heating 15L of water to 60C will take approx 1kWh, which matches my idle observations of the smart meter gas usage when doing the dishes (is that sad?).

As a rough cost/benefit analysis - We can assume that if it takes this device 1kWh to heat 15L, then doing it on gas will take a bit more as the boiler isn't 100% efficient. But gas is a bit cheaper so lets say it costs 12p in either gas or cheap rate electricity to heat 15L. If I wash the dishes 5 times a week, and can heat the water for free with solar power, that's 60p a week saved, or £31 / year. In reality I think the savings would be higher because washing dishes is not the only use of the kitchen tap, of course. Cost of the heater, plus an expansion chamber and misc fittings would be about £200.

I was also thinking that I might connect the new heater to the kitchen tap via a T to the existing combi boiler hot water supply, with a ball valve on each hot feed, so I can effectively switch my hot tap between gas and electricity. So over winter I could switch back to gas heated water if the batteries are looking low.

I have seen mention of those "boiling hot water taps" on these forums, but I think for general usage this type of heater might be more practical, as it has a larger reservoir of hot water (albeit at a lower temp), it will connect to my existing tap, and is quite significantly cheaper to buy. But I'd be interested if anyone has experience of this or any comments on my thought process

I'm also idly thinking about doing something similar to my shower, but that would be more complicated as electric showers use more than 2kW and so it'd involve new wiring in the bathroom, whereas the under sink unit can be run from an existing socket.

[Mr Gus]

DM OGB maybe? (not on since xmas) as to fitting problem potential & general thoughts, he certainly enjoys his "boiler"

Please super insulate the blighter though I doubt the standard insulation will be anything special, you may as well make an energysavings project out of it by reducing the heat loss & re-heat cycles. an exra 100mm of celotex / similar boxed in & internal voids filled will doubtless get you a decent return against the factory standard.

[ducabi]

Have you considered a hybrid, ASHP x gas? That's what I've just started thinking about. I'm a bit worried that my house is not suitable for ASHP but perhaps I could do UFH run by ASHP and also use ASHP only during warmer days (when it's around 10C I can drop boiler temp to around 40 and it's good enough to heat house).

[Thebeeman]

Please be aware that the heater will require an outlet to a drain or the outside. As it's an unvented heater there will be a pressure reducer inline and a pressure safety valve which needs a safe exit if it "blows". Through the outside wall if you can but any "spill" will be at 60 deg so think about water that hot re kiddies etc passing by outside, maybe route it down into a drain if you can.
We had one 20 years ago and it did the job well, hot supply in a shed remote from the house. We ceased needing it and it died of serious minus temperatures without being drained.

[Adokforme]

It's certainly an option @dangermouse but just wonder if you intend heating up once per day or leaving it on for daylight hours. I didn't know if something like the unit below would allow you to heat precisely what you need, when you need it accepting that the flow rate might be somewhat less!

https://www.plumbnation.co.uk/site/trit ... er-heater/

[Thebeeman]

I'm also idly thinking about doing something similar to my shower, but that would be more complicated as electric showers use more than 2kW and so it'd involve new wiring in the bathroom, whereas the under sink unit can be run from an existing socket.

The "instant" heaters above will require bespoke, larger, cable to them.

[AGT]

Have you considered a hybrid, ASHP x gas? That's what I've just started thinking about. I'm a bit worried that my house is not suitable for ASHP but perhaps I could do UFH run by ASHP and also use ASHP only during warmer days (when it's around 10C I can drop boiler temp to around 40 and it's good enough to heat house).

My long term plan is ASHP for heating but keeping the gas boiler for DHW.
Added storage heater on smart plug as per Joe/Nowty install.

A2A is to heat the ground floor hall/upstairs hall and anything off the hall with doors open on the cheaper night time tariff, then the ASHP heating the whole house.
Wood burners to assist as and when, before we get to old to bother with the hassle
Long term plan of course

[dangermouse]

Thanks for the replies. Good point about the overflow vent required, I'd noticed that in the specs, I'll have to see what can be done there.

That heater I initially linked actually has a load of bad reviews, apparently the vessel is plain steel, and relies on a sacrificial anode to stop corrosion. Need to replace anode every year! A bit more searching turns up this model which has a copper vessel and so should last longer. It is more expensive though.

My thinking is that I'd run one of these storage heaters (as pointed out, the instant heaters will take loads more power, I think about 6kW) on a timer switch, turn it on in the morning and let it stay hot all day. If insulated well I can see it using a couple of kWh a day - in the winter I doubt my batteries would like that so I'd switch back to gas but in the summer no problem.

[dangermouse]

Have you considered a hybrid, ASHP x gas? That's what I've just started thinking about. I'm a bit worried that my house is not suitable for ASHP but perhaps I could do UFH run by ASHP and also use ASHP only during warmer days (when it's around 10C I can drop boiler temp to around 40 and it's good enough to heat house).

I've been thinking about this too - I want to fit a new floor downstairs so that'd be the ideal time to get UFH. But to make it worthwhile I think I'd need the extra efficiency of a heat pump, rather than plain electric UFH. And to supplement the gas boiler, not replace it, as you say.

I've got a friend who's FGAS qualified, I'll quiz him about it when I get chance.

[ducabi]

I've been thinking about this too - I want to fit a new floor downstairs so that'd be the ideal time to get UFH. But to make it worthwhile I think I'd need the extra efficiency of a heat pump, rather than plain electric UFH. And to supplement the gas boiler, not replace it, as you say.

I've got a friend who's FGAS qualified, I'll quiz him about it when I get chance.

I think for me UFH is no option. Based on what I read it would require around 100-150mm reduction in height to increase concrete insulation. Perhaps I could just increase size of radiators.