Choosing the right boiler - somewhere between 18kw and 42kw depending on where I look.....

[Raydeeator]

Evening all,

Our boiler hasn't been doing very well recently. Just doesn't get the radiators hot at all. It was a relic of a smaller house, which was then extended. They didn't replace the boiler.

We've decided to replace the boiler and sort out some pipework, install new valves etc, but I can't settle on the boiler size required. All radiators are already installed etc, so it's a question of finding the right sized boiler to heat it all appropriately;

- 4 bed detached house
- 1 bath, 2 showers
- 2200 sq/ft
- 15 rads, 2 towel rails

We are going for a system boiler. My calculations are as follows

- Current radiator output (Watts - ΔT = 50°) = 22620 Watts (includes towel rails and two rads in a glass conservatory)

- House heat loss = 19000 Watts (17000 if remove conservatory)

The boiler which has been suggested is the Worcester-Bosch Greenstar 30i. As far as I can tell it has 24kW CH output, and 11ish LPM flow, with a max BTU of 100,000.

If I trawl the internet I find all sorts of stuff about 3kw per radiator, or any 4 bed houses needing 42kW boilers etc. I appreciate that in the past it's been common to massively oversize boilers, but it just makes me nervous that I'm missing a trick and the boiler won't be capable. With the numbers above would the 30i suffice and are there any apparent issues? I have the numbers but don't really know how to interpret them.....

My alternative is to have the 35CDI or something, at a marginally greater cost.

Many many thanks.

 

[kris]

I would say that's for a standard copper cylinder not an unvented, As if you allow let's say 2kw

And the unvented cylinder can do a reheat time of 30 mins with 20kw, that will give you a re heat time of 5 hours from cold

So if you use all your hot water it will take 5 hours for it to be fully hot[/QUOTE

I know its different as electricity but Even an immersion heater say 2kw of electric power would heat the tank in short period