Boiler size needed?

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I have a 3 story semi with 11 rads feeding 4 bedrooms, a large family bathroom. The largest bedroom has an ensuite. We're having an extension that will house a small WC, a laundry room and small office so # rads will increase to 14. What size boiler would suffice? Thanks, Graham
 
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It's actually 11 rads currently, increasing to 14. On the ground floor are 5 rads, 1 in hallway, 2 in living room, 1 in playroom and 1 in kitchen. The other 6 are upstairs
 
1. Size depends on what type of boiler you are having:
1a. Combi - size for hot water requirements.
1b. System or Heat Only / Regular - size for heating requirements and add 2 to 3 kW for hot water.
2. Rule of thumb = about 1.5kW per radiator, so allowing for hot water ((14 x 1.5) +2) = 23kW, so probably a 24 or 25 kW boiler if not combi.
3. If combi, work out maximum simultaneous hot water usage, and get a boiler which gets as close to supplying that as you can. Probably a 30 to 35 kW boiler. If going this route, make sure it can modulate down to around 21 kW - which shouldn't be a problem. Almost certainly won't get two simultaneous showers.
4. If you want to do it properly, work out heating requirements for each room and sum them, then add for hot water.
5. Any new boiler will be condensing. To maximise gas economy you need a lower return temperature, thus a lower "delta T" for the radiators, which may mean bigger radiators.
 
Thanks for response, hot water isn't as much an issue to me as feeding the rads. There's only myself, wife, 8 year old daughter and a new born. We'd always be in a situation where only one shower on at a time etc, mostly the dishwasher and washing machine on at same time as running bath or shower
 
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Washers and dishwashers these days, normally take in cold and heat the water themselves - so not a factor.
 
Work out the heat loss of the building/rooms and size the radiators accordingly. This will then give you the required boiler output.

Not being difficult but you can't expect a proper answer to..

Ok so if the main issue is feeding 14 rads at same time, what size would I need for the CH?
 
Work out the heat loss of the building/rooms and size the radiators accordingly. This will then give you the required boiler output.

Not being difficult but you can't expect a proper answer to..

Ok how? And size of rads isn't what I'm after, it's what KW output I need for CH from Combi
 
Yeah I plan on asking whoever I get to replace but I wanted some level on knowledge and confidence in what they're telling me so I know they're not BS'ing me
 

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