The three capacity numbers on a boiler
Open any boiler catalogue and each model lists three capacities, usually in this order.
Input is the fuel firing rate — how much energy goes into the burner per hour. It is the largest number and it is the one used in the model name, which is why people quote it, and why they oversize.
Gross output is what leaves the heat exchanger and enters the water. Input minus flue losses. The ratio between them is the thermal efficiency.
Net I=B=R output is gross output divided by the piping and pickup factor — 1.15 for hot water, 1.333 for steam. It is the figure you compare against your building's heat loss, and it is the smallest of the three. On an 87% boiler for hot water, the net rating is only about 76% of the input rating.
Get this backwards and the error is large. A house with a 60,000 BTU/h design heat loss needs a boiler with a net rating of 60,000. Its input rating will be near 79,000. Someone who buys a boiler with a 60,000 input rating has bought one that delivers about 45,000 net, and the house will not hold temperature on a design day.
The envelope heat loss calculator produces the design heat loss this page starts from. Do not substitute floor area for it: identical houses in the same street can differ by a factor of two on heat loss depending on windows, air-tightness and insulation.
What the pickup factor is actually for
The piping and pickup allowance covers two things that a steady-state heat loss calculation does not.
Piping loss. Distribution mains lose heat on the way to the emitters. In a heated basement most of that heat still ends up in the house, but in a crawl space, an attic or an unheated garage it does not.
Pickup. On a cold start the boiler has to raise the temperature of the water, the pipe, the emitters and the building fabric before it can begin covering the steady heat loss. Without an allowance for that, recovery from a setback or a power outage takes very much longer than the occupants will accept.
The Hydronics Institute I=B=R method sets that allowance at 1.15 for hot water and 1.333 for steam. The steam figure is much larger because steam has to fill the mains and heat the condensate before any radiator gets warm, and a steam main represents a large thermal mass sitting between the boiler and the load.
Once you have the required gross output, the input follows from efficiency:
Input = gross output ÷ thermal efficiency
Use the steady-state thermal efficiency here — the combustion efficiency from the AHRI listing — not AFUE. AFUE is a seasonal figure that also accounts for jacket losses, off-cycle draft losses and standby, so it is lower than thermal efficiency and describes fuel consumed over a year rather than firing rate at capacity. Using AFUE in this step slightly oversizes the input, which is harmless but not correct.
Domestic hot water is a separate load: Q = 500 × GPM × ΔT, where 500 is 60 min/h × 8.33 lb/gal × 1 BTU/lb·°F. The same constant drives the hydronic flow calculator. What matters is not the formula but whether you add it to the heating load or take the larger of the two.
Worked example: a 60,000 BTU/h house with an indirect tank
A heat loss calculation gives 60,000 BTU/h at the design outdoor temperature. The house has baseboard, so the system is hot water. An indirect tank recovers at 3 GPM with the water going from a 50 °F main to 120 °F. The candidate boiler lists 87% thermal efficiency.
- Domestic hot water load. 500 × 3 × 70 = 105,000 BTU/h.
- Which load governs? With priority control the tank's aquastat shuts the heating zones off while it recovers, so the boiler only has to cover the larger of the two: max(60,000, 105,000) = 105,000 BTU/h net.
- Gross output. 105,000 × 1.15 = 120,750 BTU/h.
- Input rating. 120,750 ÷ 0.87 = 138,793 BTU/h, or about 139 MBH.
- Sizing ratio. 105,000 ÷ 60,000 = 1.75.
That last number is the one to think about. A boiler 1.75 times the space heating load will short-cycle badly through the heating season, because on a mild day the house needs perhaps 20,000 BTU/h and the boiler cannot fire below its minimum. Two ways out: fit a larger indirect tank with a lower recovery rate, so the DHW load falls toward the heating load, or specify a modulating condensing boiler with a wide turndown that can fire at 20% of capacity.
Run it the other way for comparison. Without the tank, the same house needs 60,000 net, 69,000 gross and 79,310 input — a boiler roughly 43% smaller. Domestic hot water, not the building, is what sized this installation.
Reading the sizing ratio
The ratio of net output to design heat loss is the single most useful diagnostic on this page, because oversizing is the dominant boiler fault in the field.
At or near 1.0 — the boiler is matched to the building and the domestic hot water load fits underneath it. This is the target for space-heating-only installations.
Between 1.0 and 1.5 — normally driven by domestic hot water. Acceptable, particularly with a modulating boiler, but check the minimum firing rate against a mild-day load.
Above 1.5 — the domestic hot water load is now sizing the boiler and the heating season will suffer for it. Look at the tank before you look at the boiler: a 40-gallon indirect with a slower recovery covers the same household draw with far less instantaneous demand.
Oversizing costs in three ways. It short-cycles, and every cycle wastes the purge and pre-purge fuel plus the heat left in a hot heat exchanger. It swings room temperature, because a burst of heat that overshoots the thermostat is followed by a long off period. And on a condensing boiler it can prevent condensing altogether, because an oversized boiler drives return water temperature up quickly and the flue gas stops giving up its latent heat — losing the several points of efficiency you paid a premium for.
The classic defence of oversizing — "so it recovers quickly from setback" — is what the 1.15 pickup factor already covers. Adding a further safety margin on top double-counts it.
Net, gross and input for a range of design heat losses
| Design heat loss (BTU/h) | Net I=B=R output | Gross output | Input (BTU/h) | Input (MBH) |
|---|---|---|---|---|
| 40,000 | 40,000 | 46,000 | 52,874 | 52.9 |
| 50,000 | 50,000 | 57,500 | 66,092 | 66.1 |
| 60,000 | 60,000 | 69,000 | 79,310 | 79.3 |
| 75,000 | 75,000 | 86,250 | 99,138 | 99.1 |
| 100,000 | 100,000 | 115,000 | 132,184 | 132.2 |
| 125,000 | 125,000 | 143,750 | 165,230 | 165.2 |
| 150,000 | 150,000 | 172,500 | 198,276 | 198.3 |
The input column runs about 32% above the net column throughout. That gap is the pickup factor and the flue loss together, and it is the reason a boiler sized by its model number ends up too small.
Sizing mistakes that show up on service calls
- Matching the input rating to the heat loss. The single most common error, and it undersizes the installation by roughly a quarter. Match the net rating.
- Sizing from square footage. Rules of thumb in BTU per square foot span a factor of three across construction types and climates. Run the heat loss.
- Adding the domestic hot water load when the controls use priority. Priority control exists precisely so you do not have to. Adding both loads produces a boiler that is oversized for eleven months of the year.
- Sizing a replacement to the old boiler. The boiler you are replacing was probably oversized too, and the building has almost certainly had windows or insulation improved since. Sizing to the old nameplate perpetuates the error.
- Using the hot water pickup factor on a steam system. Steam needs 1.333, and a steam boiler is properly sized from connected radiation rather than from heat loss at all.
- Ignoring turndown. On a mild day the building may need a fifth of the design load. A single-stage boiler will cycle through it; a modulating boiler with 5:1 turndown will not. Turndown matters more than the top-end rating for seasonal efficiency.
Where the ratings come from
Net and gross output ratings, and the 1.15 and 1.333 piping and pickup factors, come from the I=B=R rating method administered by the Hydronics Institute Division of AHRI. Boiler thermal efficiency for commercial equipment is tested under AHRI BTS-2000. Residential boilers additionally carry an AFUE figure established under the DOE test procedure at 10 CFR Part 430, which is a seasonal number and is not interchangeable with thermal efficiency. Design heat loss should be calculated to ACCA Manual J for residential work.
After the size: what actually determines performance
Capacity is the easy part of a hydronic design. Three other decisions matter more to how the system behaves.
Water temperature. A condensing boiler only condenses when return water is below roughly 130 °F, and that depends entirely on the emitters. Cast-iron radiators sized for 180 °F water will not return water cold enough unless outdoor reset lowers the supply temperature on mild days, or the emitters are generously sized. Radiant floor systems run at 100–120 °F and condense all winter, which is why the pairing works so well — the radiant floor calculator shows the water temperature a given floor output needs.
Flow and delta-T. The boiler's output only reaches the rooms if the circulator moves enough water. Required flow is GPM = BTU/h ÷ (500 × ΔT), and the head that flow needs sets the pump. The GPM calculator and the pump head calculator cover both halves.
Emitter capacity. A correctly sized boiler feeding undersized baseboard heats the room no faster than the baseboard can release heat. Check the emitters against the room-by-room loads with the baseboard length calculator before assuming the boiler is the constraint.
If the building is heated by warm air rather than water, the equivalent exercise is in the furnace size calculator, which has no pickup factor to apply — furnaces are rated by input and output only, with none of the I=B=R structure.