Boiler Size

After a couple of months of trying to get quotes from plumbers to install new boiler and about 20 radiators, (plus require) neither have actually recommended the type of boiler to install. But they have said we need 120000 BTU. But I dont think thats enough - we have four floors and if you do cubic feet x 6, (to work out BTU from what I have read) it comes to 150000. This it seems is about 44KW, looking at internet sites.

OK - I'm not a plumber, but can someone confirm which is the right BTU? And then come up with a good condensing boiler? I thought supposedly Worcester were OK, but 'Which' magazine doesn't rate them, saying Potterton are better.

And to cap it all we have pretty rubbish water pressure (lead pipes which will be changed so it should improve hopefully), and have 3 bathrooms, so should we have additional water tanks or can the combi boiler cope. The plumbers think they can, but in reality, are we going to get a trickle and able to have only one tap on at a time?

Sorry I'm asking a lot, but I'd prefer your advice on it rather than the guys that have given us quotes. Its £20k that I dont want to risk.

There are 2 ways to come up with the correct output, the 'whole house method' or sizing each room individually and adding all together. Unfortunately with 20 radiators it is a task that us on here would not think fair to work out for you without quoting for the job, which as most of us are not in your area, we can't! Search internet for rad sizing programs that are available freely and will give a fairly accurate result.

I've never heard of the method you have mentioned

Have you considered buying a smaller house ?

Forget usng a combi by itself for your house. Your fears are justified on that, unless you get something v big and v exotic!
The kW you need for the house has to be worked out considerng the insulation, your needs and so on. A likely solution would be a standard "System" boler and a 300litre storage tank, possibly unvented (mains pressure) if your mains is up to it.
The coil on a HW cylinder can absorb 25kW now.
I hope £20k wasn't just for the heating!

thanks for the replies - figured a normal combi wouldnt do the job, so its nice to confirm.

And its a big house, cos my elderly parents are moving into the basement and I need enough space between them and me.

£20k for the rewire as well by the way (which still seems excessive)

Dont forget that any heating system fitted to a property of over 1500 ft2 has to be divided into at least 2 zones plus the hw circuit.