Guides
Guides
One page per question, with the numbers already worked out for the common cases. Every table is produced by the same formulas as the tools, so you can open the tool and change any figure.
AC BTU calculator
Open the tool- How many BTU do I need per square foot? (room size chart)The ENERGY STAR room size to BTU chart with its adjustments, heating BTU and tons by home size and climate zone, and m² to kW at three insulation levels.AC BTU calculator
- How many BTU do I need for a bedroom? (12x12 and other room sizes)A 12 x 12 bedroom is 5,000 BTU/h on the ENERGY STAR chart. Eight bedroom sizes with the sun, people and ceiling adjustments, m² to kW, and the unit class for each.AC BTU calculator
- How many BTU do I need for 1,000 sq ft? (200 to 1,200 sq ft, cooling and heating)1,000 sq ft is 21,000 BTU/h on the ENERGY STAR chart (1.75 tons) and 32,500-55,000 BTU/h to heat by zone. 200 to 1,200 sq ft, with the adjustments.AC BTU calculator
- What size air conditioner do I need for a 1,500 sq ft house? (tons by home size)The chart reads 1,500 sq ft as 30,000 BTU/h (2.5 tons); room by room the units add to 44,000. Both readings, 1,000-3,000 sq ft, and a seven-room example.AC BTU calculator
- How many BTU do I need for a garage? (heater, AC and mini split, 20x20 to 30x40)A 24x24 garage with an 8 ft ceiling needs 24,900-30,400 BTU/h of heat well insulated, 41,500-52,500 uninsulated (60°F rise, a heater maker's formula). AC, mini split.AC BTU calculator
- What size mini split do I need? (300 to 1,500 sq ft, BTU and tons)500 sq ft is 12,000 BTU/h on the ENERGY STAR chart, a 1-ton (12,000 BTU/h) unit; 800 sq ft is 18,000, a 1.5-ton unit. 300 to 1,500 sq ft, zones, heating.AC BTU calculator
Boiler size calculator
Open the tool- What size boiler do I need? UK combi boiler size calculatorCombi boiler size calculator (UK): 24-27 kW for 1-2 bedrooms, 28-34 kW for 3 (4 with 1 bath), 35-42 kW for 5+ (4 with 2+ baths). Hot water L/min too.Boiler size calculator
- What size radiator do I need? (BTU and watts by room size and type, UK)BTU/h and watts for UK room sizes by type from the merchants' ft³ x factor rule, each adjustment, and the flat x4 rule. A 4 x 4 m living room: 7,500 BTU/h.Boiler size calculator
- What size boiler for a 3-bedroom house? (UK: 28-34 kW combi, 18-26 kW system)A 3-bedroom house with one or two bathrooms needs 28-34 kW from a combi or 18-26 kW from a system boiler on the UK tables; heat loss by floor area.Boiler size calculator
- What size boiler for a 4-bedroom house? (UK: 28-34 kW combi with 1 bathroom, 35-42 kW with 2 or more)A 4-bedroom house is 28-34 kW combi or 18-26 kW system with one bathroom, 35-42 kW combi or 27-40 kW system with two or more. Combi or system, heat loss, radiators.Boiler size calculator
- 28kW boiler: how many radiators can it run? (10 to 15, UK)The manufacturer's guide puts a 28 kW combi at 10 to 15 radiators. Radiators and bathrooms to kW, 24 and 32 kW for comparison, and heat loss.Boiler size calculator
- Boiler output calculator: the kW a UK house needs by floor area, in kW and BTU/hBoiler output calculator (UK): 90 m² needs 7.2-11.7 kW to heat; 24-27, 28-34 or 35-42 kW for a combi. kW to BTU/h and the floor area each output covers.Boiler size calculator
- What size boiler do I need for 8 to 14 radiators? (UK table)8 or 9 radiators, one bathroom: 24-27 kW combi or 12-18 kW system. 10 to 14, or two bathrooms: 28-34 kW or 18-26 kW. Three bathrooms: 35-42 kW. Hot water L/min too.Boiler size calculator
- System boiler vs combi boiler (UK): which suits your homeA combi heats mains water as you use it; a system boiler heats a cylinder. Which suits your home by showers, bathrooms and mains flow; efficiency, costs, rules.Boiler size calculator
- Smart or wireless thermostat for a combi boiler (UK): what fits, and what Boiler Plus requiresCombi: single-channel programmer or thermostat. Cylinder: two channels (S-Plan or Y-Plan). Underfloor: thermostat per zone. Boiler Plus (Approved Document L), England.Boiler size calculator