Boiler size needed?

[grahamw01]

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

 

[grahamw01]

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

 

[oldbuffer]

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.

 

[grahamw01]

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

 

[grahamw01]

Also will be a combi boiler

 

[Harry Bloomfield]

Washers and dishwashers these days, normally take in cold and heat the water themselves - so not a factor.

 

[grahamw01]

Ok so if the main issue is feeding 14 rads at same time, what size would I need for the CH?

 

[denso13]

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?

 

[grahamw01]

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

 

[denso13]

it's what KW output I need for CH from Combi

Yes, which heats up the radiators. So you need to know the size of the radiators first. 14x1kw radiators is 14kw and 14x3kw radiators is 42kw.

Ok how?

One example https://www.logic4training.co.uk/wp-content/uploads/2014/06/ce541-Whole-House-Boiler-Sizing.pdf

Why not get your installer to recommend something?

 

[grahamw01]

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

 

[grahamw01]

Where do I find size of rads? Most were here when we moved in and are different

 

[denso13]

Where do I find size of rads? Most were here when we moved in and are different

Did you look at this https://www.logic4training.co.uk/wp-content/uploads/2014/06/ce541-Whole-House-Boiler-Sizing.pdf I posted earlier? It will give you a total house requirement rather than individual radiator sizes.

 

[grahamw01]

I tried and got site can't be reached

 

[grahamw01]

Got it now cheers