Boiler Size Calculator (Net AHRI / IBR Rating)

Background.

A house that loses 60,000 BTU an hour on a design winter day needs a hot-water boiler with a net rating somewhere between 60,000 and 84,000 BTU/h. The right boiler may well be advertised at 69,000 or 81,000, because every residential boiler carries three ratings on the same line of its spec sheet, and only one of them is meant to be compared with the heat loss.

Input is the rate at which the burner consumes fuel. Gross output, often labelled DOE heating capacity, is the heat leaving the boiler: input times its efficiency. The net AHRI or IBR rating is gross output divided by a piping and pickup allowance of 1.15 for hot water or 1.333 for residential steam. That allowance pays for two costs a furnace never carries: heat escaping from the distribution pipes, and the energy needed to bring a cold mass of water, pipe and cast iron up to temperature before any room warms. Because the manufacturer has already taken it out, the net rating goes head to head with the design load, and stacking a further margin on top counts the same contingency twice.

ACCA Manual S, Section 2-2, then sets the window: heating equipment between 100% and 140% of the total heating load, a band ACCA applies to boilers exactly as it does to furnaces. The 2021 International Residential Code points to Manual S in Section M1401.3, with the load itself coming from Manual J or another approved method.

The result is a shortlist, not a design: domestic hot water, emitters, circulators and the radiation check old steam systems need are outside it, and a licensed mechanical contractor still has to approve the boiler, piping, venting and gas supply.

What is boiler size calculator?

The net AHRI rating, also called the net IBR rating, is the heat a boiler can deliver to the rooms once distribution losses and cold-start pickup are allowed for, in BTU per hour. IBR refers to the Institute of Boiler and Radiator Manufacturers, whose method AHRI's Residential Boilers certification programme carried on; that programme covers gas- and oil-fired steam and hot-water boilers with inputs below 300 MBH. MBH means thousands of BTU per hour, so a 105 MBH boiler delivers 105,000 BTU/h. Where both labels use the same allowance, net AHRI and net IBR describe the same number.

How to use this calculator.

1. 01Choose hot water or steam. The choice sets the allowance, 1.15 or 1.333, which changes the gross output and fuel input the same load implies.
2. 02Say where the load comes from. A Manual J report, energy audit or room-by-room heat-loss calculation is the real thing; the floor-area estimate is for browsing catalogues and has no standing for a permit.
3. 03Enter the load in BTU/h, or the heated floor area with a heating factor you can justify. Leave out unheated garages and vented attics.
4. 04Enter the AFUE from the boiler's EnergyGuide label or the AHRI directory listing.
5. 05Enter the candidate boiler's net rating, from the column headed Net AHRI Water Rating, Net AHRI Steam Rating or Net IBR. Ignore the input and gross figures in the same row for this field.
6. 06Hold the gross output and input shown here against the other two columns of that spec-sheet row. If all three line up, you are reading the right model.

The formula.

Four relations run in sequence. The minimum net rating equals the design load, Net_min = L, since the allowance is already inside every published net figure. The maximum is Net_max = 1.40 × L, the Manual S ceiling. Gross output at the bottom of the band is Gross = Net_min × f, where f is 1.15 for hot water or 1.333 for steam. Fuel input is Input = Gross ÷ AFUE, with AFUE written as a fraction. In the floor-area mode the load is simply L = area × heating factor.

Manufacturers' tables run the same allowance the other way, net = gross ÷ f, rounded to the nearest MBH. A cast-iron model published at 280 MBH input and 234 MBH gross lists 203 MBH net, which is 234 ÷ 1.15. Some guides describe the net figure as gross × 0.85 instead, but that gives 199 for the same boiler and does not match the published tables, because 1 ÷ 1.15 is about 0.8696.

The input step leans on AFUE, which 10 CFR 430.2 defines as a seasonal efficiency rather than a full-fire one. For non-condensing cast iron the two sit close together: an 85% AFUE boiler published at 179 MBH gross on 210 MBH input runs 85.2% at full fire. Modulating condensing boilers earn part of their AFUE at low fire with cool return water, so a 96% AFUE unit rated 79 MBH gross on 85 MBH input manages 92.9% at full fire, and dividing by its AFUE understates the fuel it burns. From 88% AFUE upward the result carries a warning to use the manufacturer's published input instead.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Velocity Boiler Works (formerly Crown Boiler Co.), Product Selection Guide, August 2022. Read directly from the PDF on 2026-07-29. Source of the piping and pickup allowances, quoted verbatim from the guide's product notes: "The NET AHRI Steam Ratings shown are based on a Piping and Pickup Allowance of 1.333. The NET AHRI Water Ratings shown are based on a Piping and Pickup Allowance of 1.15. The Manufacturer should be consulted before selecting a boiler for installations having unusual Piping Pickup requirements, such as Intermittent System Operation, Extensive Piping Systems, etc." Its published rating tables for the Raptor-X, Aruba, Bali 2 and Bermuda series — 35 models in total — follow exactly from gross ÷ allowance, and are also the source of the condensing-versus-cast-iron gross-over-input comparison used in the AFUE warning.
2. [2]Air-Conditioning, Heating, and Refrigeration Institute, AHRI Residential Boilers (RBLR) Certification Program. The programme covers gas- and oil-fired steam and hot-water heating boilers with inputs below 300 MBH and is the source of the certified Net AHRI ratings this page is built around; the net ratings it certifies are based on piping and pickup allowances of 1.15 for hot water and 1.333 for steam. The AHRI programme page returned HTTP 403 to direct retrieval on 2026-07-29 and was not opened; the allowance values reach this page through the manufacturer's published data cited above, which states them explicitly and whose ratings for 35 certified models are consistent with them.
3. [3]Wes Davis, Air Conditioning Contractors of America, "Reviewing HVAC Designs for Compliance with ACCA Manual S", ICC Building Safety Journal, January–February 2009, Table 1 "Manual S equipment selection sizing limitations". Read directly from the PDF on 2026-07-29. Source of the sizing band used here: furnaces AND boilers are both listed at 100–140% of the total heating load, referencing Manual S Section 2-2. Written by ACCA's own technical staff about ACCA's own standard.
4. [4]International Code Council, 2021 International Residential Code, Section M1401.3 "Equipment and appliance sizing": heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S or other approved sizing methodologies based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies, with exceptions for multistage or variable-refrigerant-flow equipment whose published capacity range brackets the load and for the next-larger-standard-size case. Retrieved 2026-07-29. Adopted editions and local amendments vary by jurisdiction.
5. [5]U.S. Department of Energy, 10 CFR 430.2, definition of "Annual fuel utilization efficiency" — the efficiency descriptor for furnaces and boilers, determined using the prescribed test procedures. Retrieved 2026-07-29. This is the basis for treating AFUE as a seasonal average rather than a full-fire efficiency, which is why this page warns that the input figure it derives is optimistic for modulating condensing boilers.