✦ Gas & Hydronic Sizing

Boiler Size Calculator

Size Combi, System, and Regular boilers accurately without chronic oversizing. Balance instantaneous domestic hot water (DHW) shower flow rates against space heating demands and evaluate burner modulation ratios.

⚠️ Boiler Specification Notice: Boiler kW outputs calculated here separate space heating requirements from instantaneous domestic hot water (DHW) demand under standard CIBSE guidelines. Always have an accredited Gas Safe engineer (in the UK) or licensed heating professional inspect existing pipe diameters, mains water dynamic pressure, and system volume before procuring a replacement boiler.
Boiler Type:
Fuel Type:

🔥 1. Space Heating Load

Total Space Heating Requirement: 8.8 kW

🚿 2. Bathrooms & Hot Water Demand

For combi boilers, output is sized primarily by domestic hot water flow rate, not space heating.
Recommended Boiler Output
28 – 30 kW
Catalog Boiler Rating · 95,000 – 102,000 BTU/hr
Boiler Architecture Sizing
Combi Boiler: 28 – 32 kW (DHW driven)
System Boiler (with Cylinder): 12 – 15 kW
Regular Boiler: 12 – 15 kW
💡 Anti-Cycling Guidance: Ensure your chosen boiler features a minimum 1:8 or 1:10 modulation ratio (down to ~3 kW) so it does not short-cycle during spring and autumn.
🚿 Instant Hot Water Performance

Interactive Combi Hot Water Flow Rate vs. kW Simulator

Select combi boiler kW rating to calculate instantaneous domestic hot water delivery in litres per minute.

Physics Formula: Flow Rate (L/min) = (Boiler kW × 14.33) / Temperature Rise (°C). During cold winter months when mains water enters at 5°C, hot water flow rate decreases by ~15% to maintain 45°C delivered temperature.

Delivered Hot Water Flow
13.1 Litres / min
✓ Perfect for powerful thermostatic mixer shower
Simultaneous Demands: Can support 1 shower + 1 washing tap simultaneously.
⚙️ Burner Modulation

Boiler Modulation & Anti-Cycling Visualizer

Understand why oversized boilers with poor turn-down ratios waste gas through on/off short-cycling in mild weather.

Poor 1:3 Turn-Down (Oversized)

30 kW Boiler (Min Firing: 10.0 kW)

On a mild 10°C autumn day, the house only requires 3.0 kW of heat. Because the boiler cannot fire below 10 kW, it rapidly overheats and cycles on/off 12 times per hour.

❌ Efficiency drops by 8%–15% · High component wear
Modern 1:10 Turn-Down (Right-Sized)

18 kW Boiler (Min Firing: 1.8 kW)

The burner throttles smoothly down to 1.8 kW, matching the mild 3.0 kW demand continuously. Flue gases condense at peak 94% efficiency with zero short-cycling.

✓ Peak 94% condensing efficiency · Continuous quiet running
🔄 Boiler Types

Combi vs. System vs. Regular Boiler Comparison

Match your household plumbing architecture and hot water usage patterns to the correct boiler type.

Boiler Type Hot Water Generation Storage Cylinder Needed? Best Household Fit
Combi (Combination) Instantaneous direct from mains cold pipe. No (Frees up cupboard space) Flats, apartments, 1–2 bathroom homes with single shower usage.
System Boiler Stored in unvented pressurized cylinder. Yes (Unvented hot water cylinder) Family homes with 2–3 bathrooms requiring simultaneous showers.
Regular (Conventional) Gravity-fed from cold water header tank in loft. Yes (Open-vented cylinder + loft tank) Older properties with low mains water pressure or aging radiators.
⚠️ Sizing Traps

5 Common Boiler Sizing Mistakes Homeowners Make

Avoid paying for oversized equipment that increases gas consumption.

❌

1. Replacing an Old 30 kW Boiler with Another 30 kW

Old non-condensing boilers were routinely 100% oversized. If you added double glazing or loft insulation, your home's true space heat loss may now be under 8 kW.

❌

2. Sizing a System Boiler for Instantaneous Flow

A system boiler heating a cylinder only needs space heat load + 2.5 kW. Fitting a 35 kW system boiler for an 8 kW house causes chronic short-cycling.

❌

3. Ignoring Mains Water Flow Rate

If your lead or copper incoming pipe only delivers 11 L/min, installing a 38 kW combi (capable of 15 L/min) will not increase shower pressure.

❌

4. Operating at 75°C Return Water

Setting boiler flow temperature to 80°C pushes return water above the 54°C dew point, shutting down condensing mode and wasting 10% gas. Complying with UK Boiler Plus Standards and adding weather compensation restores efficiency.

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Boiler Sizing & DHW Capacity FAQs

Technical guidance on combi instantaneous flow rates, system boiler cylinder reheat, and condensing efficiency.

Combi boilers produce domestic hot water instantaneously without a storage cylinder. Raising incoming cold mains water (at 10°C) up to a comfortable shower temperature (45°C) in real time requires tremendous thermal power. A 24 kW combi delivers ~9.8 litres/min of hot water, while a 35 kW combi delivers ~14.3 litres/min. The central heating side will modulate down to match your 8 kW space heating demand.
Boiler modulation ratio (or turn-down ratio) describes how low the burner can fire relative to its maximum capacity. A 30 kW boiler with a 1:10 turn-down ratio can modulate smoothly down to 3.0 kW. A boiler with a poor 1:4 ratio can only throttle down to 7.5 kW. On mild autumn days when the house needs only 2–3 kW of heat, a poor turn-down ratio forces the boiler into wasteful on/off short-cycling.
Combi boilers heat both radiators and hot water directly from the mains with no storage tanks. System boilers heat radiators directly and store domestic hot water in an unvented pressurized cylinder (ideal for homes with 2+ simultaneous showers). Regular (conventional) boilers work with open-vented low-pressure gravity systems with cold water storage tanks in the loft and a separate cylinder.
Condensing gas boilers only recover latent heat from water vapor in the flue gases when the return water entering the boiler is below the dew point of natural gas (~54°C). For peak 92%–94% condensing efficiency, return temperatures should ideally operate at 45°C–50°C (requiring properly balanced hydronics and adequate radiator surface areas).
A high-output 36 kW combi can produce 14.5 L/min of hot water, but it cannot create water that isn't delivered by your mains supply. Before buying a large combi, perform a weir gauge test or bucket test on your outside cold tap. If incoming cold flow is only 10 L/min, a 36 kW boiler will never deliver more than 10 L/min.
When sizing a System or Regular boiler, add 2.5 kW to 3.0 kW to your home's calculated space heating load to allow rapid cylinder reheat (within 20–30 minutes) via priority domestic hot water (PDHW) controls.