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Free heating engineer calculator
Boiler size calculator: what kW combi, system or regular boiler a house needs
Work out how much heat a house loses on a cold day and what that means for boiler size. A combi is chosen on hot water, so the calculator sizes it from the showers you want to run at once and checks the mains can feed it; a system or regular boiler is sized on the heat loss plus reheating the cylinder. It uses the same approach as TradeCalc in the Hard Hat Way app with the MCS and CIBSE method for heat loss. It is for planning and pricing: the Gas Safe engineer does the room-by-room calculation Part L requires.
What you need
Hot water
- Hot water demand
- 10 l/minone shower or bath at a time at 10 l/min each, 35 K rise: 24.4 kWkW = l/min × 35 K × 4.18 ÷ 60 (≈ 2.44 kW per l/min); HF Group 24 kW ≈ 10 l/min
- Suggested combi band
- 24–27 kW 24.4 kW needed (the larger of heating 7.6 kW and hot water 24.4 kW): gives about 9–11 l/minViessmann, Vaillant and Heatable sizing tables: 24–27 kW one bathroom, 28–34 two to three, 35–42 more
- Mains flow check
- OK 12 l/min inenough for 10 l/min of hot water; 1.5–2 bar dynamic is idealHF Group; Heating & Plumbing World flow-rate guidance
- Oversizing check
- OK 27 kW combis modulate down to roughly 2.7–3.4 kW; the house needs 7.6 kW on the coldest day
Heat loss
- Heating load
- 7.6 kW7,564 W at 21 °C inside, -3 °C outside, including a 10% marginCIBSE / BS EN 12831 method as used by MCS
- Heating load
- 25,808 BTU/hr
- Heated floor area
- 96.0 m²2 storeys of 48.0 m²; 79 W/m²
- Fabric loss: walls
- 3089 W85.8 m² of outside wall after the windows at U 1.5U-values: OpenLearn energy in buildings; Part L 2021
- Fabric loss: windows
- 972 W15 m² at U 2.7
- Fabric loss: ground floor
- 806 WU 0.7, ground floor only
- Fabric loss: roof
- 184 WU 0.16, top storey only
- Vent loss
- 1825 W0.33 × 230 m³ × 1.0 ACH × 24 K
Rules
- What Part L requires
- Room-by-room heat loss from 15 June 2022 a replacement boiler needs a room-by-room heat loss calculation, with the records kept; the whole-house CE54 method and its 20% hot-water uplift were withdrawnApproved Document L 2021; OFTEC Technical Notice 38
- Gas Safe
- Registered engineer only Gas boilers must be installed, commissioned and notified by a Gas Safe registered engineer, who will do the heat loss calculation Part L requires. This tool is for planning and pricing only and is not a system design.
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
For heating engineers: turn this into a quote
In the app, TradeCalc hands these quantities straight to ProQuote with your regional material prices, and the customer gets a branded PDF. Free to download.
Or claim your free profile link nowHow the maths works
- Heat loss through the fabric is area × U-value × temperature difference, designed at 21 °C inside against a −3 °C day (CIBSE / MCS). Wall U-values in W/m²K: solid 2.1, unfilled cavity 1.5 (most 1930–1990 houses), filled cavity 0.55, 2006-on 0.3, timber frame 0.4; glazing single 4.8, pre-2002 double 2.7, low-E 1.4, triple 1.0 (OpenLearn; Part L 2021).
- Outside wall area is the footprint perimeter × ceiling height × storeys, scaled by how many of the four walls face outside, less the windows. Roof loss is the footprint at the loft or flat-roof U-value on the top storey only; floor loss is the footprint on the ground floor only.
- Vent loss is 0.33 × the heated volume × one air change an hour × the temperature difference, a CIBSE-style whole-house average for an older house. The optional 10% margin covers warm-up after a setback; Part L 2021 says the boiler should not be significantly oversized.
- A combi is sized on hot water, not heat loss: one shower or bath needs about 10 l/min, a shower plus a basin 12, two showers 20. Lifting that through 35 K takes l/min × 35 × 4.18 ÷ 60, about 2.44 kW per l/min, so 10 l/min is 24.4 kW. The band comes from the Viessmann, Vaillant and Heatable tables: 24–27 kW for one bathroom, 28–34 for two or three, 35–42 above that.
- The mains flow check compares what you want with what the cold main brings in from a bucket test. A combi needs at least 10 l/min and about 1 bar dynamic; it cannot give more than the mains supplies.
- A system or regular boiler is the heating load plus a cylinder reheat allowance (3 kW typical for a 150–210 litre cylinder), rounded up, against the 12–18, 18–26 and 27–40 kW catalogue bands. Boilers modulate down to roughly an eighth to a tenth of their maximum, so a house that needs less than that will short-cycle.
- Part L 2021 (from 15 June 2022) needs a room-by-room heat loss for a replacement boiler; the whole-house CE54 method with its 20% hot-water uplift was withdrawn (OFTEC TN38). This page is the whole-house guide; the engineer does the rooms.
- Gas boilers must be installed, commissioned and notified by a Gas Safe registered engineer, who will do the heat loss calculation Part L requires. This tool is for planning and pricing only and is not a system design.
Worked example
An 8 m × 6 m two-storey semi with 2.4 m ceilings, unfilled cavity walls, older double glazing, 15 m² of windows, a 270 mm loft and an uninsulated solid floor has 96 m² of floor and a heating load of about 7.6 kW with the 10% margin. As a combi running one shower at a time it needs 10 l/min of hot water, which is 24.4 kW, so a 24–27 kW combi; a 12 l/min mains passes the flow check. As a system boiler with a 3 kW cylinder reheat it would be an 11 kW boiler.
Questions people ask
What size boiler do I need for a 3 bed house?
For a combi, the size is set by hot water: 24–27 kW for one bathroom, 28–34 kW for two or three, 35–42 kW for more (Viessmann, Vaillant and Heatable all publish these bands). For a system or regular boiler with a cylinder it is the heat loss plus reheat: a 1950–80s three-bed semi is around 8 kW of heating, a 2000-on house about 6 kW, so a 12–18 kW boiler. Most UK homes need under 10 kW of heating; the 30 kW on a combi is for the shower.
Why is the heating load so much lower than my combi?
Combis heat water on demand, so a 24 to 30 kW combi is sized to run a shower or fill a bath, not to heat the house. If you have a hot water cylinder, a system or regular boiler can be sized much closer to the heating load. Bigger is not better: a boiler that cannot modulate below the house load cycles on and off and runs less efficiently, and Part L says not to significantly oversize.
What is a bucket test?
Put a 10 litre bucket under the kitchen cold tap, open it fully and time the fill. 10 litres in 60 seconds is 10 l/min; in 40 seconds is 15 l/min. A combi needs at least 10 l/min and ideally 1.5–2 bar dynamic pressure; below that it cannot deliver its rated hot water however big it is, and a system boiler with a cylinder or an accumulator is the better answer.
Does this work for a heat pump?
Pick air source heat pump and it shows 100% of the heat loss with no margin, which is how MCS sizes them, but a heat pump has to be designed on a room-by-room survey to BS EN 12831 by an MCS certified installer, with every rad checked at 45–55 °C flow. Use the radiator calculator with the flow temperature set to see what that does to each room.
Can I fit a boiler myself?
No. Gas boilers must be installed, commissioned and notified by a Gas Safe registered engineer, and oil boilers by an OFTEC registered technician. The installation is notified to building control through the competent person scheme and you get a certificate you will need when you sell. Since June 2022 the engineer also has to do and keep a room-by-room heat loss calculation.
How much does a new boiler cost?
It varies with the boiler, the flue route, whether the pipework and controls need changing and where you live, so get a few quotes from Gas Safe registered engineers. Our new boiler cost guide explains what drives the price.
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