Boiler Sizing Calculator (Hot Water and Steam)
Estimate the heat loss for your ZIP code and house, then see the boiler output and input it implies, with an optional allowance for an indirect water heater and the radiation it equals in square feet of EDR. Estimate only. For steam, the radiation decides the size, and a room-by-room Manual J is still the basis for hot-water design.
Boiler sizing calculator
Northeast US (default)Zone 5A · 5°F heating / 88°F cooling design
Estimate only. Not a substitute for a room-by-room Manual J.
Estimated heating load
36,700 BTU/h
31,200 BTU/h – 42,200 BTU/h at 5°F outdoor
Recommended output
40,400 BTU/h
110% of heat loss
Input at AFUE
46,400 BTU/h
output ÷ 87% AFUE
EDR equivalent of the load
245 sq ft
at 150 BTU/h per sq ft EDR
Hot-water boilers are sized to the heat loss; confirm the radiators can deliver it at your supply temperature.
Compare the EDR equivalent with the radiation actually in the house. If the house has much more radiation than the load needs, you can run lower water temperatures, which helps a condensing boiler or a future air-to-water heat pump.
Assumptions behind this estimate
- Climate
- Northeast US (default) (zone 5A)
- Design temperature
- 5°F outdoor / 70°F indoor
- Insulation level
- average
- Pickup allowance
- 10% over heat loss (not the old 33% rule)
- Indirect DHW
- None
- EDR conversion
- 150 BTU/h per sq ft (hot water, typical rating)
- Piping and jacket losses
- Not included
Do this room by room in Tradewright
Real equipment matches, a 15-year cost comparison and a signed proposal, all from one visit.
Why old boilers are so often oversized
Many boilers being replaced today were sized by matching the old nameplate, then adding a generous pickup factor on top. Repeat that over a couple of replacements and the boiler can end up several times larger than the heat loss, especially after the house has had new windows and insulation.
An oversized hot-water boiler short cycles, wastes fuel heating its own mass and, if it is a condensing boiler, can spend most of its time above the return temperature where it actually condenses. A heat-loss based size with a modest allowance is the better starting point. This calculator adds 10% over the heat loss, plus any indirect water heater load you choose.
Steam is different. A steam boiler has to fill every radiator and the piping with steam, so it is sized to the connected radiation (EDR) plus a piping allowance from the manufacturer, not to the heat loss. Count and measure the radiators. The heat loss figure is still useful: if the radiation is far larger than the load, the house is overradiated and a conversion to a lower-temperature system may be practical.
How the estimate works
Step 1: Design temperature
Your ZIP sets an approximate 99% heating design temperature for the region.
Step 2: Heat loss
Walls, windows, ceiling, floor and air leakage from your size, year and insulation, at 70°F indoors.
Step 3: Output and input
Recommended output is heat loss × 1.1 plus any indirect water heater allowance. Input is output divided by AFUE.
Step 4: EDR check
The heat loss is converted to square feet of EDR (240 BTU/h per sq ft for steam, 150 for hot water) so you can compare it with the radiators in the house.
Questions
What size boiler do I need?
For hot water, enough output to cover the heat loss at your design temperature with a modest allowance, plus any indirect water heater load if the boiler does not use priority control. For steam, size to the connected radiation and piping, not the heat loss.
How do I size a steam boiler?
Measure every radiator, look up or calculate its square feet of EDR, add them up and apply the boiler manufacturer's piping allowance. Steam boilers that are sized to the heat loss instead of the radiation often fail to heat the far radiators.
What is EDR?
Equivalent Direct Radiation is the way radiator output is rated. For steam, one square foot of EDR is 240 BTU/h. For hot water the output per square foot depends on the water temperature.
Do I need to add capacity for an indirect water heater?
It depends on the controls. With domestic hot water priority, the boiler pauses space heating while it heats the tank, so many designers do not add the full indirect load. Without priority, add it. The calculator lets you choose none, 20,000 or 40,000 BTU/h.
Should I add a pickup factor?
Old rules added a large pickup factor for warming up a cold system. Modern practice keeps the allowance small, because a tight, continuously heated house rarely starts cold and oversizing has real costs. This calculator uses 10%.
Can a heat pump replace my boiler?
Often, at least for most of the heating hours. It depends on the heat loss, the radiation and the water temperature it needs. An air-to-water heat pump or ducted or ductless air-source heat pumps are the usual options.
Is this a Manual J?
No. It is a whole-house estimate with generalized assumptions. Use a room-by-room Manual J to check each radiator against each room's load.
Related calculators and guides
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