How the output is worked out
The heat load comes from a room-by-room calculation such as Manual J. A pickup allowance is then added as a percentage, to cover the extra capacity needed to raise a cold house to temperature after a setback.
Any indirect water heater load is added on top, because the boiler has to serve both. The total is the gross output the boiler must deliver.
Fifteen per cent pickup is typical for a residential system. A cast iron boiler in a house that is set back at night may need more, and a modulating condensing boiler may need none.
Worked example
A house with a 60,000 Btu/h heat load and a 15 per cent pickup needs 69,000 Btu/h, which is 69 MBH. With no water heating load, that is the gross output to look for.
Add a 30,000 Btu/h indirect water heater and the requirement rises to 99,000 Btu/h, which changes the boiler class entirely.
Gross output, net rating and pickup
Boilers are sold on gross output and sometimes quoted on a net I=B=R rating, which is the gross output divided by 1.15. The net rating already includes the pickup allowance, so comparing a net rating with a gross requirement overstates the boiler.
Check which figure the specification quotes before comparing it with the result here. The difference is about 13 per cent, which is enough to choose the wrong size.
A condensing boiler's efficiency changes with the return water temperature, so its delivered output at a low return temperature is higher than its rated output.
Limits of this calculator
It does not calculate the heat load. That needs a room-by-room assessment with the actual construction, windows and infiltration.
It does not check the chimney, the venting category, the gas supply or the near-boiler piping, any of which can rule out a boiler.
It does not size the distribution. Radiator or baseboard output at the design water temperature is a separate calculation.
For UK combi sizing in kilowatts, use the UK boiler size calculator.
Inputs for the US boiler size calculator
House heat load
Design heat loss in Btu/h from a room-by-room calculation, not from floor area.
Pickup allowance
Extra capacity to bring a cold house up to temperature. Fifteen per cent is typical.
Domestic hot water load
Indirect water heater load in Btu/h. Enter zero for a combi or a separate heater.
CALCULATOR QUESTIONS
US boiler size calculator questions
What is a pickup allowance?
Extra capacity to recover from a night setback. It is usually 10 to 20 per cent of the heat load.
What is MBH?
Thousands of BTU per hour. A 69,000 Btu/h requirement is 69 MBH.
Is gross output the same as net rating?
No. The net I=B=R rating is the gross output divided by 1.15, so it already includes the pickup.
Does a condensing boiler need pickup?
Often less, because it modulates and runs for longer at a lower temperature. Ask the manufacturer.