Boiler Size

[Hoof Hearted]

I got a plumber to give me a quote for installing Central Heating. I have a large house, but I was surprised when he proposed a 140,000 BTU boiler. I've done the std calculation of measuring the room volumes and multiplying by 3,4 or 5. I only get 67,000 BTU. If I multiply all the volumes by 5 I still only get 76,000 BTUs required. I've tried a few of the on-line calculators. The worst case I found was one site that suggested a boiler 90,000 BTUs. What makes me more concerned is that the radiators he specified are only capable of giving out 86,000 BTUs.

Can anyone suggest why he might have specified such a large boiler? Are there any upsides, and what are the downsides I might see? The only thing I can think off is that i've got 18" solid stone walls. This will need a larger boiler than usual, but surely I should have radiators that can give out at least as much heat.

Ian

[spacethegas]

I need a little to go on.
how many bedrooms ?
on suites ?
how many bathrooms ?
solid walls ?
open plan ?
insulation/heat loss ?
what sort of boiler/system is proposed ?

for eg i live in a small end terrace 3 bed 1 bath. i wanted a high flow rate (13 l/min) combi.
Hence i installed a 130,000btu blr.
more than enough to run ch & will easily cope with a planned extension.

[Hoof Hearted]

It's a 3 floor 5 bedroom end terraced house. No extension possible. No Ensuites. 1 Bathroom + second toilet downstairs.
18" Solid stone Walls. No open plan. Loft insulated. Hardwood d/glazed windows. Total room volume including hall and landings 15200 cu ft.

He's proposed a Worcester Bosch 42 CDI

Ian

[spacethegas]

To be honest without me actually coming round it sounds just the job.
17ltrs/min hot water.

[Hoof Hearted]

But how about 140,000 BTU's being pumped into radiators that can only give out 86,000? Is this not wasteful?

I just measured the maximumum flow from my hot tap in the Kitchen. I got 9 Litres/min. The bath is almost never used in my house. So 9 Litres/min is plenty

[spacethegas]

its not wastefull the boiler is range rated & will run as the system demands.
is this ltr/min from cold tap ?

sorry reread it.

measure cold tap.

Your hw flow rate should improve no end. ie Superb shower

[Hoof Hearted]

The kitchen cold tap is giving about 25 Litres/min. It's near the incoming cold main. The cold bath tap is only giving 20 Litres/min

[CharterGAS]

i Hope you can range rate the cdi!!!! or you are wasting energy!!

[Hoof Hearted]

Interesting! I just took a closer look at the technical data for the boiler in question....

The Greenstar CDi Series at a glance - Worcester Homeowner area

Yes it's a 42KW boiler, but the full 42KW is only available to the hot water. The central heating gets from 9.4KW up to 30KW. This is sounding more like it. The 30KW is only 5KW above the radiator capacity.

I couldn't help but notice though, that the 30, 37 and 42CDI models all give 30KW to the central heating. So you pay your extra money just for the better hot water flow.

[fuzzy]

most modern boilers if not all are reange rated