sizing a combi boiler

[jhauxwell]

I'm suffering from some confusion regarding the selection of a combi boiler for my house. I have used an online utility to help me size each radiator for the individual rooms (www.radcalcs.com) but the problem seems to be that it doesn't add up to much! Our house is quite small, having 2 bedrooms, a bathroom, a living room and a kitchen. i've figured out the total required BTU's to be about 21200.

I have a few issues with this - firstly does this figure seem a little low? Secondly, at the beginning of my renovation project I bought a Biasi Combi boiler at a really good price - the only problem seems to be that its far too big for the job - 86000 btus maximum output, 37000 BTU minimum output. I presume that if the adjustment for the CH side of things is wound down, then the boilers hot water capacity will be altered wich is potentially bad cos ive got a BIG bath which will take long enough to fill as it is!

To add further complication, i hope to build an extension starting next summer. This would add 2 bedrooms and a dining room to the house so id like to have the capacity to be able to extend the central heating into the new rooms.

Can anyone please advise me on what i can do?!!

Looking forward to a reply........

J Hauxwell

 

[COG]

21000 does seem a little low, have you alloed for the pipework etc ?. Not familier with your boiler but you will find that most are either range rated ,in that you manually adjust the gas output to your CH demand whan commissioning the boiler or fully modulateing which means it basically will find its own leval, your min output seems high but alot of manafacturers raised the min levals in order to meet sedbunk rateings

 

[Gees]

In my opinion ( NOT a heating engineer ) you will be better off going for a 80,000 btu/hr. or above. I fell into this trap at a previus house and had to uprate the boiler. You can underrun a big one but once its capacity is reached thats all you will get. If you are demanding more than the boiler will produce it will just run all the time and get no hotter.

 

[corgiman]

most combis these days are range rated on heating side from 30 to 80,000 btu so the heating side wont be a problem

the hot water may be before going for it check you have enough flow rate and pressure at the mains. if you dont and you install a combi you will curse it

 

[ChrisR]

". I presume that if the adjustment for the CH side of things is wound down, then the boilers hot water capacity will be altered "

Nope, it stays at 86kBtu/hr.

Range rating to 37000 or whatever means the flames turn down. Below that demand they turn on and off. The Biasi is a decent boiler but you must do what it says and turn down the heating o/p otherwise it WILL overheat and lock out. IIRC that means testing gas pressure which I urge you not to do - get someone corgi. In fact get some advice at the start, before you fit the thing. There are lots of things you can get wrong, the flue is a typical area on this boiler.

21000 is a bit low perhaps but it depends - terraced house? There's another heatloss calc on the MYSON site which you can download. Most calculators leave out things like air changes which are important.

Note that your clearances are odd - 25mm left and 50mm right, which are vital.
Found one of these recently with the case siliconed to thewall and fitted tight in a corner as well. I knew the installer was unregistered so I capped the meter off and put warning labels everywhere. Charged the cheapskate landlord £62.40. (£2.40 for the labels)