Boiler size?

I'm trying to size a combi boiler for a house with 3 bathrooms. There are 17 (single and double) rads that add up to about 62K BTU (18.1KW). Will a 100K BTU (30KW) boiler do the job? How much do you normally need for each bathroom?

The 30kW rating refers to the hot water output so the larger the better, especially if you have 3 bathrooms. A more powerful combi will give you a better hot water flowrate and reduce the effect of having two hot water outlets open at once.

For example - I went for a 37kW combi in my two bathroom 4-bed detached.

The things you have to watch out for when chosing a more powerful combi are that your cold water and gas pressures/flow rates are good enough. The gas supply pipe would have to be at least 22mm and possibly even 28mm, depending on distance and how many other gas appliances you have.

Most combis should be able to cope with your central heating demands, but this is easily checked in the combi literature.

If you have a house full of people all wanting (boiler-fed) showers at once, then you might want to also consider an unvented cylinder ?

Thanks Mike, the gas supply is good. I was thinking of the Baxi 133HE Plus Condensing Combi, which rates at

Input: CH=33.7KW, DHW=45.5KW
Output: CH=30KW, DHW=39KW

The main bathroom gets used the most but no two showers are run at the same time, yet!

Do you think this is OK?

If your mains water and gas are up to it, then it really is down to your HW demands.

We have two bathrooms and each currently has an electric shower, although I plan to change one to a thermostatic shower at some stage. Getting a 37kW combi should allow for a good shower and the result of turning another hot tap on elsewhere will hopefully not be too disasterous.

Do you have 3 boiler-fed showers in those 3 bathrooms ?

I personally wouldn't go for a Baxi because I had a (non-condensing) Baxi combi fitted at my last house (around 9 years ago) and it was a terrible boiler - it was forever locking out and cost me a lot in parts over the years.

You might also want to look into combis with built-in HW storage - my WB 37CDi has a small tank built-in, although we've not needed to use that yet. Some other manufacturers (Vaillant for example) have combis with larger tanks.