Combi boiler size

GDB2222

I’ve just received a quote for fitting a 24kw combi. Is that big enough to give a reasonable shower, or should I ask for a bigger one?

Just to add, there’s just one shower in the flat, and I’m sure the boiler is adequate for the heating. 

FreeBear

Combi boilers should be sized for the cold water flow and the temperature you want hot water at.

Measure the flow rate from the bath tap (kitchen taps are usually restricted). For 10l/min, a 24kW combi will be fine. 14l/min would want a 30kW for a 35°C temperature rise.

For heating, you are unlikely to need more than 6kW, so look very carefully at the minimum output. Even although I have a 30kW boiler in a 3 bed semi, heating consumes 4-6kW and only ever peaks at ~10kW at start up on a very cold day. So I would suggest a combi that can modulate down to ~3kW (Viessmann 050 or WB Greenstar 4000/8000). Also well worth increasing radiator sizes by 30-50% so that you can take advantage of low flow temperatures.

ThisIsWeird

As FreeBear says, your mains cold water flow rate is your starting point, so that needs checking.
Did your plumber do this?
What boiler do you currently have, and how do you find the shower?
In general, tho', I'd be looking at a 28 or 30kW jobbie for a really enjoyable shower.

FreeBear

ThisIsWeird said: In general, tho', I'd be looking at a 28 or 30kW jobbie for a really enjoyable shower.

Down side to going large is boiler efficiency drops off at low outputs. With a 24kW boiler ticking over at ~3kW for heating, you may be getting 96% efficiency. With a 30kW, efficiency could drop to below 90%.

Currently collecting data on my boiler performance, and efficiency seems to swing from a low of 85% up to 96%. Longer periods of the boiler running appears to return higher efficiency.

GDB2222

I'm a bit concerned about the cold water feed being big enough, so it sounds like 24kw is a sensible size.

Thanks, all, for your input.

ThisIsWeird

GDB2222 said:

I'm a bit concerned about the cold water feed being big enough, so it sounds like 24kw is a sensible size.
Thanks, all, for your input.

Most likely a good call.

And the shower should still be decent - around three times as a powerful as the largest leccy shower.