Should I match the old boiler's size?

Almost never. Old boilers were routinely oversized, so matching the nameplate carries that error into the new unit and hurts efficiency and comfort. A Manual J design heat loss is the correct basis; use connected radiation only as a sanity check against it.

What is the pick-up factor?

The pick-up or piping-and-pickup factor adds capacity to cover heat lost in the distribution piping and the extra output needed for morning recovery from setback. A 15% factor is typical for modern, well-insulated systems, with up to 25% for older systems with long uninsulated runs.

How do EDR and baseboard convert to BTU per hour?

Standard hot-water radiation emits about 150 BTU per hour per square foot of equivalent direct radiation at 170°F average water temperature. Fin-tube baseboard emits roughly 580 BTU per hour per linear foot at design conditions. Multiplying the connected radiation by these figures gives the emission capacity.

What is the difference between output and input?

Net output, also called the DOE or IBR rating, is the usable heat the boiler delivers to the system. Gross input is the fuel energy it consumes. They differ by efficiency: input equals output divided by AFUE. Size the system on output and check the input against the gas or oil line capacity.

Does this work for oil and propane boilers too?

Yes. The sizing math is identical for gas, oil, and propane hot-water boilers — only the AFUE and standard size table differ slightly by manufacturer. Enter the unit's AFUE and the tool returns the right net output and gross input.

Boiler Replacement Sizing Calculator

Replacing a boiler is the single best chance to correct decades of oversizing. The right replacement is sized to the building’s actual design heat loss plus a modest pick-up allowance, not to whatever was bolted to the floor before. This calculator accepts the proper Manual J load or, when that is not available, an estimate from the connected radiation, and returns the net output and gross input you need.

How it works

The tool first establishes the design load in BTU/h. With Manual J you enter it directly. With connected radiation it converts the emission capacity:

EDR radiation:   load = EDR_sqft  x 150 BTU/h
Fin-tube:        load = linear_ft x 580 BTU/h

It then adds the pick-up factor for piping losses and recovery, and converts the required net output into the gross fuel input the burner must consume:

Required output = load x (1 + pickup%)
Required input  = output / (AFUE / 100)

Finally the required output is rounded up to the nearest standard boiler size so you can pick a real model from a manufacturer’s range.

Example and notes

A house with a 60,000 BTU/h Manual J heat loss, a 15% pick-up factor, and an 85% AFUE replacement needs about 69,000 BTU/h of net output and roughly 81,000 BTU/h of gross input, landing on a standard 80,000 BTU/h output boiler. If the same house had 110 feet of baseboard, the radiation-based estimate would read about 64,000 BTU/h — close enough to confirm the Manual J figure rather than replace it.

When the radiation method and the heat-loss method disagree sharply, trust the heat loss and treat the radiation as a cross-check. Oversizing a modern condensing boiler hurts both efficiency and comfort because it short-cycles instead of running long, low-temperature cycles.