Hot Water Cylinder Fed By PV Powered Immersion (and gas)

[Lewis Carr]

First post, so thanks in advance for help and opinions.

I have recently bought a four bed, just partner and I but kids in future etc.

Currently has conventional / system boiler with 20+ Yr old boiler, small poorly insulated cylinder and unsealed header tank.

I was about to convert to combi (and accept the dual hot water consumption limitations). But I have recently thought about making better use of the approx 2.5kw solar array on the roof. For reasons I can explain if people want, it makes strong financial sense for me to usefully consume as much of the solar energy as I can as opposed to sell to grid.

So now I am thinking to have a new conventional boiler fitted with a new pressurised cylinder and use a solar diverter to send all spare power to the cylinder via an immersion.

Has anyone done this or got experience or ideas? What I want to happen is to heat water in this order of preference
1. Surplus electricity (free)
2. Gas (8.5p / kwh)
3. (never) grid electricity (30p/kwh)

Having the solar diverter takes care of the third point above. But would am I right in thinking I can set different trigger temps for immersion heater and boiler heater (to fire the boiler)?

Say;
1.Immersion heater to come on (when surplus power only, dealt with by diverter) at any temp and switch off at the max safe temp of the tank (say 65 deg c?).
2. Boiler to fire and send heat to cylinder when temp drops to 50deg and cut off when tank at 55 deg.

Basically I'm Scottish and tight and want the boiler to fire when it needs to, but not unnecessarily!

Any thoughts on feasibility to diffent temps for each thermostat?

Have I over looked somthing?

I know about top and bottom immersion heaters and the logic of that, but I think in this instance I'd want them both low down...?

Cheers

 

[Lewis Carr]

Here is a solar diverter if people haven't seen them.

https://www.deegesolar.co.uk/heating_your_water_with_solar_pv/

Might go crazy and get a quooker tap (having a new kitchen fitted) and have that as the second load from the diverter, but do need to think about that as I'll need a timed override if there is no surplus or otherwise I can only drink tea on sunny days :/

 

[JohnD]

If you look at the cost of HW with a gas boiler (the summer gas bill for usage will show you) you will find it is very low. Standing charge will be unaffected, of course.

If you have a modern boiler, and properly insulated cylinder and pipes, I think you will find that savings (on sunny days only, in summer only) do not repay the cost of the Immersion controller (£250 or so?) in a worthwhile period.

If you live in a sunny country or have no gas, the payback will be better.

We save a bit by doing washing when sun is forecast, and using the tumble drier on sunny days when possible. Maybe half the time. A washing line would save even more. We have no other power hungry appliance that is used in summer

The washer and dishwasher have heating cycles of about 10 minutes so not significant.

 

[JohnD]

Here are my figures

I calculated it earlier


https://www.diynot.com/diy/threads/socket-immersion-heater.574197/#post-5005942

 

[JohnD]

The advert in your link specifies that the customer has a boiler and cylinder

But claims savings by ignoring the very low cost of heating a cylinder by gas

So it misleadingly suggests a false saving.

 

[Lewis Carr]

Cheers for response, may be more trouble than it's worth. Here are my calcs

125 litres l/day HW (average according to Google).

Say 10 deg in to 60 deg out = 7.27 kwh. *8.5p (my gas, extortionate I know) = 62p/day = £226/yr.

But....thats assuming it can all be heated by free electric and no gas required. I've actually just checked the solar statement from previous owner and generated 6.7kwh/day over a 12 month period. So perhaps £150-£200 a Yr saving on gas noting how we may not require that volume of HW and some solar goes to eletrical load such as fridge etc.

.... And all of that is based on 8.5p/kwh, god knows where we're going with gas rates longer term but feels like its about £1000-£2000 over a 10 Yr period.

 

[oldbutnotdead]

Good for you for actually doing the sums...I'm fairly certain that energy prices are not going to go down (or not by much) so you'll probably start to see payback at or before the 10 year mark- depends if you plan to be there that long.
Yes you can have different thermostats doing different jobs. You might want to use a relay in your setup to inhibit the gas dhw thermostat when PV power is being delivered to the immersion heater (otherwise the gas will kick in with the PV if the cylinder temp drops far enough in the daytime, highly possible after the morning shower surge)- or set a timer on the gas dhw such that it doesn't fire in daylight hours.

 

[lloyda]

That's exactly what I do with mine - Immersion stat higher than tank stat, and boiler only timed to come on late afternoon. Very occasionally boost on a cloudy day when there is a lot of hot water being consumed, usually as a result of a day long baking session. The economics didn't really come into it for me. I designed and built my own diverter just to see if I could.