Boiler Sizing for Large 3 Bed House

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Hi All,

Hoping to get some sound advice regarding combi boiler sizing for a large 3 Bed House.

We are refurbishing the house and plan to strip out the old boiler and pipework from the loft to relocate to the kitchen / diner whilst also renewing all copper pipework and installing new radiators.

We are also keen to future proof for a potential loft extension to come in the next 5-10 years which could potentially add 2 more bedrooms and a small en-suite shower room.

I've used 3 different BTU calculators online which based on my room sizes recommend the following BTU requirements per room

Lounge & Dining Room (we've knocked through): 14691
Hallway Downstairs: TBC - hard to calculate due to varying ceiling height and will also heat upstairs hallway - the neighbors have a radiator that gives out approx 6000 BTU
Kitchen: 10958
Our Bedroom: 5614
Bedroom2: 4105
Bedroom3: 2878
Bathroom: 2900

This equates to a total of 47.5 KW. Surely this doesn't mean that we would need a 47.5KW combi boiler? It seems excessive but as stated the BTU requirements are taken as an average from 3 different websites.

We do have a 5KW wood burner stove going in the Lounge / diner which could reduce requirements accordingly?

Plumbers that have quoted for the work are recommending between 33-38KW, Valiant Eco Tech or similar.

I've attached the floor plan to give you an idea of the scale of the house,

any advice you can give would be much appreciated,

Thank you
 

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You are calculating heat loss in btu's/hr.
You need to divide the total by 3412 to give kw. about 14kw by my calculation
 
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Hi All,

Hoping to get some sound advice regarding combi boiler sizing for a large 3 Bed House.

We are refurbishing the house and plan to strip out the old boiler and pipework from the loft to relocate to the kitchen / diner whilst also renewing all copper pipework and installing new radiators.

We are also keen to future proof for a potential loft extension to come in the next 5-10 years which could potentially add 2 more bedrooms and a small en-suite shower room.

I've used 3 different BTU calculators online which based on my room sizes recommend the following BTU requirements per room

Lounge & Dining Room (we've knocked through): 14691
Hallway Downstairs: TBC - hard to calculate due to varying ceiling height and will also heat upstairs hallway - the neighbors have a radiator that gives out approx 6000 BTU
Kitchen: 10958
Our Bedroom: 5614
Bedroom2: 4105
Bedroom3: 2878
Bathroom: 2900

This equates to a total of 47.5 KW. Surely this doesn't mean that we would need a 47.5KW combi boiler? It seems excessive but as stated the BTU requirements are taken as an average from 3 different websites.

We do have a 5KW wood burner stove going in the Lounge / diner which could reduce requirements accordingly?

Plumbers that have quoted for the work are recommending between 33-38KW, Valiant Eco Tech or similar.

I've attached the floor plan to give you an idea of the scale of the house,

any advice you can give would be much appreciated,

Thank you
I have a 4-bed semi, 2-storey, a little less total area than yours, but not by much. Not particularly well insulated, it's mansard style and the spaces behind the sloping bits are hard to insulate, nobody will quote. But I have loft and cavity wall insulation, and double glazing. Heat-only boiler with HW cylinder. Boiler 50000 BTU (14.7 kW) nominal, actual rather less as the burner pressure is lower than design for historical reasons. Copes fine. I leave CH and HW on 24/7, the roomstat at 20°C, all year round. When the weather gets cool the heating comes on.
 
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You are calculating heat loss in btu's/hr.
You need to divide the total by 3412 to give kw. about 14kw by my calculation
Yes you're right, I hadn't checked the sums, assumed his 47.5 kW was correct. 14 kW compares nicely with my place!
 
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As the others have said, you've confused yourself by switching between BTU and kW. Stick to kW and all will be much clearer. www.heat-engineer.com is the most accurate heat loss calculator I've found, well worth the £12 for the room by room survey.

You need to know your incoming mains pressure & flow rate before specifying a mains-driven system, be it a combi or an unvented cylinder. Generally speaking a combi would be unsuitable for running more than one bathroom at once so your extension plans would suggest that a system boiler and unvented cylinder would be more appropriate, assuming the incoming mains is up to the job of supplying it
 
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While you're doing your total refit, you could (if the house layout suits and subject to mains pressure/flow) have a combi and an unvented cylinder- leave the kitchen on the combi hot water, have the unvented near the bathroom(s).
 

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