How to size a boiler

[Lower]

We have a 1920's semi that i want to change the boiler to a combi.

How can i calculate what size of boiler we need?

Its a 3 bedroom house with upstairs bathroom (radiator in each room), 3 reception rooms downstairs (each with rad), large flat roofed kitchen at the rear and also a downstairs bathroom.

Solid wall construction (no cavity), suspended floors in the reception rooms, partially converted loft (insulated, plastered ceilings and walls but no proper stairs)

 

[mickyg]

Pretty much all combis will satisfy your heating demand. You really only need to know what you want from your hot water performance if thats what you want.
Are you likely to need more than 1 hot water outlet operating at any one time?
Do you want high flow rate for showers?
Whats the current flow rate into your house on the cold water supply? (Time a known volume bucket to work out your litres/min from kitchen tap)

 

[totalshell]

ther are some loverly calculations you can do , with which you can appear very knowledgable.

however at the end of the day probably the most important piece of information is missing.. how many people in the house, kids etc

a single person is unlikely to want all the rooms at max temp and all the hot taps on at the same time so would be v. happy with something between 22 /24 kw

fill the house with two adults and 4 teenagers and you'd want 30kw plus

 

[Bahco]

Read here

 

[Deleted member 105431]

personally I'd say 30kw should be adequate and the 2 boilers I'd recommend are the glow worm ultracom 30cxi or the (more expensive better quality) Vaillant ecotec plus 831. The heating only requires roughly two thirds of the boiler power-the hw is what uses the kw's.

 

[Lower]

Kitchen tap is fed by quite a small bore flexible hose so probably not the best tap to measure from. The outside tap would probably be more representative as it come straight off the mains feed into the house.

There's currently only two of us in the house, but i need to allow for a child and we would want to run a shower off it too.

Would a 30kw combi do the job for that?

The other question i have is i'd want to fit the combi boiler to the existing pipework. Our current boiler is a Baxi Solo 60PF. Would the central heating system run at a higher pressure than it currently does with the Baxi and with a combi do i still need a header tank in the same way?

 

[Corgigrouch]

You need to know what your maximum water flow is. and match the boiler to what your supply can provide... An exaggerated example would be if you had a maximum flow to your home of 5 litres per minute, then there would be no point in fitting a boiler that would provide 20 litres per minute as you would have far more capacity than you would need. Similarly if your supply was 20l/m and your boiler could only heat 5l/m then that would be silly too. Generally speaking I find that a 30kW boiler does very nicely in an average sized 3 bed semi in my area though if up against price and the water supply is particularly good, then I would price for what I would expect others to price for and then give a separate quote for the ideal installation.. Although this is a generalisation I find that almost any combi is man enough to heat an average 3 bed semi.... It is more about water flow

 

[mickyg]

Kitchen tap is fed by quite a small bore flexible hose so probably not the best tap to measure from. The outside tap would probably be more representative as it come straight off the mains feed into the house.

There's currently only two of us in the house, but i need to allow for a child and we would want to run a shower off it too.

Would a 30kw combi do the job for that?

The other question i have is i'd want to fit the combi boiler to the existing pipework. Our current boiler is a Baxi Solo 60PF. Would the central heating system run at a higher pressure than it currently does with the Baxi and with a combi do i still need a header tank in the same way?

30kw would normally be fine in this instance.
You can use your existing rads and pipework but be aware with a combi it will become pressurised(no header tank). So any weak joints on the system may be exposed. The only time this might be a major issue is if you have any pipes that are inaccessible(concrete floors etc.)

 

[Lower]

That's really useful info all, thank you.

Does a combi come with its own expansion vessel or will i have a 'football' type (layman's term ) expansion vessel somewhere else in the house?

 

[mickyg]

its own one