Should I use immersion heater or gas boiler for hot water with solar panels?

Officer_Dibble

Slightly off-topic but I'm surprised Flux makes sense for anyone at current rates. EonDrive has 7 hours of import at 6.7p every night and export 16.5p all day. No EV required. Unless you're forcing a terrific amount of export from 4-7pm (while keeping enough in the battery to see you through to 2am) I don't see how you can get near that with Flux. A shame, as I was an early adopter of the tariff, but they seem to be trying to kill it off.

Veteransaver

FreeBear said:

Veteransaver said:

thegentleway said:

Veteransaver said:

Your boiler may be claimed 85%+ efficiency, although in my experience hot water heating is generally far lower, there's lots more losses in the pipework so actually it tends to be closer to 50-60%. Also the boiler doesn't operate in condensing mode much when it is heating water due to the high return temperature.
Now obviously in winter those losses actually gomto heat the house too so it isn't totally wasted, but in summer it is.
So using the immersion, especially if you only need a small amount of hot water in the top of the cylinder for a shower for example, immersion can actually be cheaper.

Very interesting. 

Presumably when the heating and hot water is on then the return temperature is lower? and the boiler more efficient?

Summer, heating isn't on so immersion might be cheapest. 

Depends, the flow and return to the cylinder is usually 22mm and completely unrestricted so it does take most of the flow from the boiler and consequently the return temperature is very high. Many modern gas boilers also recognise that the system is calling for hot water so raise the flow temp too.
You'll find if you put the heating and hot water on at the same time the radiators will generally be much slower to heat up.

Most heating systems will have a three way valve to divert flow to the DHW and give it priority over heating.

A 3 port valve doesn't prioritise DHW, it's either heating only, hot water only or both (in the middle)
But when it's on the middle setting for both heat and Hw most of the flow goes through the cylinder in practice due to the large pipe and low resistance through the heating coil.
It's the same effect on a S plan too if both valves are open.
 You can put a gate valve on the DWH side, it's useful if you have a particularly old cylinder with a small coil and long reheat time. With small coils the hot water flowing in can't dissipate heat quickly enough, so still quite hot water flows out the cylinder coil too, so you are just circulating very hot water. So you can "balance" it with a gate valve like a radiator or turn the pump speed down. It should make it more efficient 
I've done this on my system. I almost fully close the gate valve in winter, then in summer I open it fully but turn the pump speed down to minimum.
My coil I think is only 3kW, and the boiler can output 21kW, so it simply can't take the full flow and merely diverts heat from the radiators (which can be 30/40 minutes whilst the DHW gets to temperature), but turning the flow rate down doesn't affect reheat times, but should make it all more efficient.