Solar diverter - can I modify my combi boiler to work with a hot water cylinder?

[richbee]

This might be a really daft question.

I currently have an oil fired combi boiler which is fairly new (2016).
I have just ordered solar PV + battery, but currently I do not have a hot water cylinder, and as I understand you can't pre-heat water for the combi - it doesn't like it.
Could I have the boiler modified to make it work as a more conventional hot water system with cylinder, and then have the solar diverter to do a fair bit of the water heating in the summer? I could just turn off the hot water part of the boiler, but then I would be stuck with only electrical water heating, which would get expensive in the winter. I guess it would not be easy to manage an automatic transition, although I wonder if you could have a cylinder + immersion / solar diverter and a manual switch over to then turn the boiler back on to do the water heating....

[Stinsy]

I'm in the same situation! Definitely not going to buy another gas boiler. Even though mine is 20 years old, if/when it get's replaced then I'll be installing a heat pump.

In the mean time I could cap off the DHW side (it'll still need to be pressurised and have a "drain" to remove air or for testing etc.) and either use 2x 3-port valves to switch the central heating circuit between the cylinder and the rads. Or I could isolate the DHW from the gas boiler completely and have it electrically heated via cheap-rate electric and diverted solar.

Right now I pay 7.5p/kWh for gas and 5p/kWh for off-peak electric so a lament the fact I don't have a cylinder and immersion heater. However when gas was 2.5p/kWh and off-peak electric was 5p/kWh and it was the dark months I'd have been paying more.

[nowty]

I did this years ago, another nice old thread lost to elsewhere.

I found that running preheated water up to 40 degrees was ok, above this the combi would run erratically. So to avoid the legionella problem with stored warm water I typically had the tank heat up to 60 degrees and then ran the output through an automatic blending valve set to 40 degrees before going through the combi.

My new set up uses a heatpump and a solar diverter to heat a larger tank and still goes through the combi but its now unblended but the combi is typically turned off. The combi is now only a backup up in case of a heatpump failure.

[spread-tee]

There's nothing to stop you using just the heating part of the combi and fitting a diverter valve, or two two ports and heating a cylinder with it, then just don't use the hot water circuit. There is a risk though that the diverter valve in the boiler would get stuck if never used, but if it never gets used you wouldn't know it was stuck.

Desp