I have seriously considered a combi, but when I did the maths and thought about overall system reliability a combi doesn't stack up for me...
I would like around 20 litres a minute from the shower. I want that in all weathers, so let's assume the incoming temperature is 0 celsius and the showerhead temp is 40 celsius.
The power required to raise 20 litres a minute by 40 celsius is 56kW. I've seen plenty of combis in the 30-38kW range, but that will only give me 12 litres a minute or so. That's not a bad shower, but that's 15+ minutes to fill a bath and it rules out a coliander head or bodyjets in winter.
With a 200 litre cylinder at 60 celsius, and an incoming temperature of 0 celsius I can have that 40 celsius shower at 20 litres a minute for 15 minutes. 300 litres of water in all.
Additionally, I'd like the option to add alternate heat sources in future. I'd like to be able to have that 300 litre shower with less guilt! Finally, all boilers break down, but at least with a cylinder I'll have an immersion as back-up and can employ the services of a more leisurely repairman rather than a 24 hour emergency plumber!
Why anyone would want to stay in a shower for 45 minutes with the water pouring out at 20 litres per minute is beyond me. That's 900 litres of water gone down the drain. And I thought the idea of a shower was to save water not squander it.
Don't know if there would be a big difference with the country side, but in london the incoming water never seems to go below 8C even in a week of continuous frost.
What maths have you done? Odd maths. Reliability? Who told you?
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