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.

What is the Boiler Replacement Sizing Calculator?

Size a replacement gas or oil hot-water boiler from a Manual J design heat loss, total EDR radiation, or fin-tube baseboard footage. Adds a pick-up factor, converts net output to gross input by AFUE, and rounds to a standard boiler size. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Boiler Replacement Sizing Calculator

Name: Boiler Replacement Sizing Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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  × 150 BTU/h
Fin-tube:        load = linear_ft × 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 × (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.

Worked example

A house with a Manual J design heat loss of 60,000 BTU/h:

Step	Calculation	Result
Design load	60,000 BTU/h (from Manual J)	60,000 BTU/h
Pick-up factor (15%)	60,000 × 1.15	69,000 BTU/h required net output
AFUE conversion (85%)	69,000 ÷ 0.85	~81,200 BTU/h gross input
Rounded to standard size	—	80,000 BTU/h output boiler

If the same house had 110 feet of fin-tube baseboard (110 × 580 = 63,800 BTU/h), the radiation estimate is within 6% of the Manual J figure — a good cross-check. When the two methods diverge sharply, trust the heat loss calculation and investigate the discrepancy before ordering equipment.

The case against matching the old boiler

Old residential boilers were routinely sized at 1.5–2.5 times the actual heat loss — partly due to over-conservative rules of thumb, partly because contractors preferred the margin. A replacement that simply matches the old nameplate carries that error into a new unit, causing:

Short cycling: The oversized boiler reaches setpoint quickly, shuts off, then restarts — cycling every few minutes instead of running steady, long burns. Frequent cycling increases wear and reduces effective AFUE below the rated value.
Poor comfort: Short cycles fail to distribute heat evenly through the radiation. Occupants experience overshoot and rapid cooldown rather than stable room temperature.
Lost condensing efficiency: Modern high-efficiency (90%+ AFUE) condensing boilers only reach their rated efficiency when return water is cool enough to condense flue gases — typically below 130°F. Oversized boilers short-cycle before the system water cools enough to sustain condensing operation.

Pick-up factor guidance

System type	Recommended pick-up factor
Well-insulated piping, modern controls	10–15%
Typical residential, some piping exposure	15–20%
Older system, long uninsulated runs	20–25%
Large system with significant heat-up mass	up to 25%

The pick-up factor accounts for heat lost in distribution piping and the extra output needed to bring the system up from a setback temperature. In a well-designed, compact system with short insulated runs, 10% is reasonable. Larger or older systems with extensive basement piping need more margin.