Reading a North American Gas Appliance Rating in Kilowatts
A US or Canadian furnace data plate says 80 000 BTU/h. A British or European boiler brochure says 24 kW. Both describe how fast the appliance burns gas or delivers heat, and neither market ever prints the other's unit — so anyone shopping across the Atlantic, quoting on an imported appliance, or reading a manufacturer's global catalogue has to move between the two before any comparison means anything.
What the Number on the Plate Is Describing
The firing rate on a data plate
The kW headline on a UK boiler
Warm air against wet systems
Hot water on demand
Working Through a Boiler or Furnace Data Plate
Two minutes with the plate and this converter puts an imported appliance on the same footing as anything in a local merchant's catalogue.
Find which rating the plate is quoting
Look for the words next to the figure. "Input", "gas input" or "heating capacity input" is what the burner consumes; "output", "heating capacity" or "net output" is what leaves the appliance. Comparing one against the other is the classic mistake.
Type the BTU/h figure straight in
Enter 80 000, 100 000, 199 000 — whatever the plate says — and the kilowatt value appears as you type. Spaces are ignored and a comma is accepted as the decimal separator, so a pasted figure normally works untouched.
Apply the efficiency yourself
Multiply the kW you just got by the AFUE or seasonal efficiency to reach usable output. A 100 000 BTU/h appliance is 29.31 kW of gas: about 23.4 kW of heat at 80 % AFUE, or 27.8 kW from a 95 % condensing model.
Send a European figure back the other way
Quoting a UK appliance for a North American reader? The swap button (↔) runs kW → BTU/h, so a 30 kW combi becomes 102 364 BTU/h in language a US installer recognises.
Domestic Boiler and Furnace Ratings Side by Side
Typical domestic appliance sizes with the gas input as the plate states it, the same input in kilowatts, and the output a stated efficiency leaves you with.
| Appliance class | Gas input (BTU/h) | Input (kW) | Output (kW) at stated efficiency |
|---|---|---|---|
| Small warm-air furnace | 40 000 BTU/h | 11.72 kW | 11.1 kW at 95 % |
| Mid warm-air furnace, non-condensing | 60 000 BTU/h | 17.58 kW | 14.1 kW at 80 % |
| Compact condensing combi | 80 000 BTU/h | 23.45 kW | 22.3 kW at 95 % |
| Large warm-air furnace, non-condensing | 100 000 BTU/h | 29.31 kW | 23.4 kW at 80 % |
| Wall-hung condensing combi, mid range | 120 000 BTU/h | 35.17 kW | 33.4 kW at 95 % |
| Hydronic system boiler, cast iron | 140 000 BTU/h | 41.03 kW | 39.4 kW at 96 % |
| High-output combi, hot-water led | 199 000 BTU/h | 58.32 kW | 55.4 kW at 95 % |
Two things fall out of that table. First, the household sizes overlap almost exactly once you convert — the markets are describing the same appliances in different words. Second, the 199 000 BTU/h line is not a bigger house, it is a bigger bathroom: US domestic ratings end just under 200 000 BTU/h before an appliance is treated as commercial, and manufacturers use that headroom for hot-water flow.
What Helps When Comparing Two Markets' Catalogues
Both fields track the data plate as you type
Enter figures on either side and the opposite box keeps up, so you can run through a whole brochure page of model sizes in one sitting.
Reverse to kW → BTU/h for a US quotation
One press of the swap button turns a European specification into the input figure an American merchant, gas fitter or permit form expects to see.
Every rating unit a heating catalogue uses
Searchable dropdowns hold BTU/h, kW, W and the rest, which covers appliances quoted in older or regional units without leaving the page.
Plain figures for the appliance comparison sheet
The copy button hands over the number alone, so a column of converted ratings pastes into a spreadsheet without unit text to clean up afterwards.
Boiler and Furnace Rating Questions
Is the BTU/h number on a US furnace the input or the output?
Almost always the input — the gas the burner consumes. The model number itself usually encodes it, so an "080" in the code means 80 000 BTU/h of input, or 23.45 kW. The output appears lower down the plate, sometimes labelled heating capacity or bonnet capacity, and it is always the smaller figure. British boilers do the opposite and lead with output, which is exactly why a straight comparison of headline numbers flatters the American appliance.
How does AFUE change the kilowatt figure I should be comparing?
AFUE is a seasonal average of how much of the fuel energy becomes useful heat, so it converts input into output. Take 100 000 BTU/h, which is 29.31 kW of gas: an 80 % AFUE furnace delivers about 23.4 kW while a 95 % condensing model delivers about 27.8 kW from the same burner rating. If you are matching an appliance to a calculated heat loss, the heat loss is an output figure — compare it against output, never against input.
Why do British boilers get quoted in kW while American furnaces use BTU/h?
Habit reinforced by regulation. The UK went metric for gas appliance ratings decades ago and kW became the language of building regulations, benchmark commissioning records and installer training. North America kept the British thermal unit in its gas codes, its appliance labelling and its load calculation methods, so every part of the supply chain quotes BTU/h. Neither is more correct; they are two units for one quantity, separated by a factor of 3 412.14.
What does a combi's kW rating tell me about hot water flow?
Roughly 2 kW gets you one litre a minute at a 35 °C temperature rise, which is about what a shower needs going from cold mains to comfortable. So a 24 kW combi manages around 10 l/min, a 35 kW model around 14 l/min. That is why a 199 000 BTU/h appliance exists at all in a domestic range: 58.32 kW is far more than any house needs for heating, and it is there so two outlets can run at once.
Do gross and net calorific values change the kilowatts I calculate?
The conversion itself never changes — BTU/h and kW are both plain rates of energy. What changes is which fuel energy the rating counted. Gross (higher heating value) includes the latent heat in the water vapour the flame produces; net (lower heating value) does not, and is about 10 % smaller for natural gas. European efficiency figures are traditionally quoted against net, which is how a condensing boiler is advertised above 100 %, while AFUE is a gross-based figure. Compare like with like before deciding one appliance beats another.
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