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Boiler size calculator
A hot water boiler is sized from the building's design heat loss. The boiler's Net AHRI rating has to be at least that number, and ACCA Manual S caps its output at 140 percent of it. For a 56,000 BTU/h load that means a net rating of at least 56,000 BTU/h, which is a Heating Capacity of 64,400 to 78,400 BTU/h and, at 90 percent AFUE, an input of about 71,556 to 87,111 BTU/h. This page works out those ratings from your heat loss, or takes the rating of a boiler you are looking at and says what share of the load it covers. If you do not have the heat loss yet, the furnace size calculator gives a preliminary whole-house heating load from a ZIP code, and the Manual S calculator covers the other equipment types.
- Sizing turns a heat loss into the ratings to shop for. Checking takes the rating of a boiler from a manufacturer's table and compares it with the load.
- Enter the design heat loss in BTU/h and the boiler type. For sizing, add the AFUE. For checking, pick whether the table value is the Heating Capacity or the Net AHRI rating and enter it in BTU/h.
- The result shows the net rating needed, the Heating Capacity window and a ballpark input, or the percent of the load the boiler covers with a verdict and warnings.
Example result for the default inputs
Design heat loss 56,000 BTU/h, condensing boiler at 90% AFUE. Pick a model from the manufacturer's table whose figures sit inside the window.
Net AHRI rating, at least
56,000 BTU/h
Heating Capacity window
64,400 to 78,400 BTU/h
Input at this AFUE, about
71,556 to 87,111 BTU/h
| Design heat loss | 56,000 BTU/h |
|---|---|
| Net AHRI rating needed, 100% of the load | 56,000 BTU/h |
| Heating Capacity for that net rating, 1.15 x the load | 64,400 BTU/h |
| Manual S floor on output, 100% of the load | 56,000 BTU/h |
| Manual S ceiling on output, 140% of the load | 78,400 BTU/h |
| Input at the low end of the window | 71,556 BTU/h |
| Input at the high end of the window | 87,111 BTU/h |
- Published Heating Capacity and Net AHRI ratings are sea level ratings for a stated fuel. At altitude or on propane the manufacturer's table gives different figures, so read the row for your fuel and elevation.
- No domestic hot water or indirect tank load is included. Add it only as the boiler manufacturer directs.
- Ratings are for ordinary piping. The manufacturer asks to be consulted for intermittent operation or extensive piping, where a larger allowance than 1.15 can apply.
- This page is for hot water boilers. Steam boilers are selected on connected radiation, and their allowance is different.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the boiler size is calculated
A hot water boiler is rated three ways in the manufacturer's table: Input, Heating Capacity (the output by the DOE test) and the Net AHRI rating. The Net AHRI rating is what is left for the radiation after a piping and pickup allowance. The manufacturer's rule is to pick a boiler whose Net AHRI rating is at least the design heat loss. ACCA Manual S then caps the output at 140 percent of the load. This page applies the net rule to the load and the Manual S ceiling to the Heating Capacity, and shows the Manual S 100 percent floor beside them.
- Net AHRI rating
Net = Heating Capacity / 1.15The 1.15 piping and pickup allowance is printed in the ratings notes of the Dunkirk and CAC/BDP hot water boiler manuals. Dunkirk adds that it is in accordance with the AHRI standard as published by the Hydronics Institute. It is a reserve for the water in the system and the heat lost from the piping. Steam boilers print 1.333 instead, and this page does not size them.- Selection rule
Net AHRI rating >= design heat loss; so Heating Capacity >= 1.15 x design heat lossManufacturers ask for a boiler whose Net AHRI rating is greater than or equal to the calculated peak heating load. The Heating Capacity that gives a net rating equal to the load is 1.15 times the load. At 56,000 BTU/h that is 64,400 BTU/h.- Manual S ceiling
Heating Capacity <= 1.40 x design heat lossTable 1 of the ACCA Manual S summary gives 100 to 140 percent of the total heating load for boilers, at first-edition section 2-2. The article applies it to output capacity, so the ceiling is read against the Heating Capacity. At 56,000 BTU/h it is 78,400 BTU/h, the same figure the article prints for its furnace example; Table 1 gives the same 100 to 140 percent for boilers.- Manual S floor
Heating Capacity >= 1.00 x design heat lossManual S puts the floor on the output at 100 percent of the load. On a hot water boiler the net rule above is stricter, since it asks for 115 percent. The result lists both.- Input from output
Input = Heating Capacity / AFUEThe BuildSolver Manual S engine uses the same relation, but on the load itself, without the 1.15 allowance, so its minimum input is lower. AFUE is a seasonal figure, so this is a ballpark. On the CAC/BDP and PENNCO tables it lands within 2 percent of the printed input. At 90 percent AFUE the 56,000 BTU/h example gives an input of about 71,556 to 87,111 BTU/h.- Percent of the load
percent = rating / design heat loss x 100Used in the check mode for both the Net AHRI rating and the Heating Capacity. Verdicts: Heating Capacity under 100 percent is too small. Capacity at or above the load with a net rating below it is short on the manufacturer's rule. Over the Manual S ceiling is oversized.
Worked examples from published boiler ratings
The default load is the heating example in the Manual S article from ICC, a furnace there, 56,000 BTU/h, with a ceiling of 78,400 BTU/h at 140 percent. Its Net AHRI rating has to be at least 56,000 BTU/h, which needs 64,400 BTU/h of Heating Capacity. Below are three models from the Dunkirk Q90 manual. Dividing each printed Heating Capacity by 1.15 gives the printed Net AHRI rating to the table's whole-MBH rounding.
| Model | Input, MBH | Heating Capacity, MBH | Net AHRI printed, MBH | Capacity / 1.15, MBH |
|---|---|---|---|---|
| 90-50 | 50 | 45 | 39 | 39.13 |
| 90-75 | 75 | 68 | 59 | 59.13 |
| 90-100 | 100 | 90 | 78 | 78.26 |
Against the 56,000 BTU/h load, the 90-75 covers 105.6% on its net rating and 121.4% on its Heating Capacity, so it fits. Eight cases below run through the same functions the form uses. The larger house row is an example chosen for the page, with no product data behind it.
| Case | Heat loss | Setup | Result |
|---|---|---|---|
| ICC Manual S example load (a furnace in the article), sizing | 56,000 BTU/h | Sizing, condensing, 90% AFUE | Net at least 56,000; capacity 64,400 to 78,400; input about 71,556 to 87,111 BTU/h |
| Larger house, non-condensing, sizing | 100,000 BTU/h | Sizing, non-condensing, 85% AFUE | Net at least 100,000; capacity 115,000 to 140,000; input about 135,294 to 164,706 BTU/h |
| Dunkirk 90-50 on a small load | 38,000 BTU/h | Check, condensing, Heating Capacity 45,000 BTU/h | Within the limits: net 103.0%, capacity 118.4% of the load |
| Dunkirk 90-50 on a slightly larger load | 40,000 BTU/h | Check, condensing, Heating Capacity 45,000 BTU/h | Net rating below the load: net 97.8%, capacity 112.5% of the load |
| Dunkirk 90-75 on the example load | 56,000 BTU/h | Check, condensing, Heating Capacity 68,000 BTU/h | Within the limits: net 105.6%, capacity 121.4% of the load |
| Dunkirk 90-100 on the example load | 56,000 BTU/h | Check, condensing, Heating Capacity 90,000 BTU/h | Oversized: net 139.8%, capacity 160.7% of the load |
| CAC/BDP BW 140, net rating entered | 90,000 BTU/h | Check, non-condensing, Net AHRI 106,000 BTU/h | Within the limits: net 117.8%, capacity 135.4% of the load |
| Net rating exactly at the load | 60,000 BTU/h | Check, condensing, Net AHRI 60,000 BTU/h | Within the limits: net 100.0%, capacity 115.0% of the load |
The 90-50 shows why the net rule matters. On a 38,000 BTU/h load it is within the limits. At 40,000 BTU/h its Heating Capacity of 45,000 BTU/h still covers the load, but its net rating of 39,130 BTU/h does not, so the result says the net rating is below the load. The Manual S floor of 100 percent alone would have passed it.
Standards and data sources
- Dunkirk Q90 Series 2/3 condensing hot water boiler, Installation, Operation & Maintenance Manual (P/N 240009298, Rev. D, 12/2012) - the rating terms and the selection rule. Net AHRI ratings are based on a piping and pickup factor of 1.15, and the Net AHRI rating of the boiler selected should be greater than or equal to the calculated peak heating load. Table 1 prints Input, Heating Capacity and Net AHRI Rating for three models, which the tests reproduce.
- CAC/BDP BW4/BW5 oil-fired cast iron hot water boiler, Installation, Operation & Maintenance Manual IM-BW4-5-02 (Rev. B, 12/2020) - a second ratings table with the same 1.15 allowance and an AFUE column, and the return water rule for that boiler: significant condensation can form in the boiler and vent below 120°F return water. Other makers set their own minimum.
- ACCA Manual S, Table 1 as summarized by Wes Davis, ICC Building Safety Journal (Jan-Feb 2009) - the sizing limits for boilers, 100 to 140 percent of the total heating load at section 2-2, applied to output capacity, and the note that oversizing leads to higher equipment and installation cost, greater energy consumption and more wear and tear.
- PENNCO 16 Series gas-fired steam boilers, Installation, Operation & Maintenance Manual (P/N 1305004, Rev. E, 8/2016) - the steam allowance of 1.333, and the instruction to select a steam boiler on installed radiation in square feet. This is why the page covers hot water only. The table's AFUE and input columns are also used to test the input estimate.
Limitations of this calculator
- The heat loss is entered, not calculated. A boiler sized from a rule of thumb or a guess inherits its error. The furnace size calculator gives a preliminary whole-house heating load from a ZIP code, and a room-by-room Manual J is the better source for a firm number.
- Hot water boilers only. A steam boiler is selected on the installed radiation, and its piping and pickup allowance is 1.333. The page does not size steam boilers.
- The 1.15 allowance is for ordinary piping. The manufacturers ask you to consult them for intermittent system operation or extensive piping systems, where a larger allowance can apply.
- No domestic hot water, indirect tank or priority load is included. A sizing procedure for it was not available from an open source, so add it only as the boiler manufacturer directs.
- Published ratings are sea level ratings for a stated fuel. At altitude or on propane the manufacturer's table lists different figures. Read the row for your fuel and elevation.
- The input is a ballpark from the AFUE, which is a seasonal figure. The input printed in the manufacturer's table for the model you pick is the number that matters for gas piping and combustion air.
- The 1.40 ceiling is the first-edition Manual S figure. Check which edition your jurisdiction or reviewer works from.
- The page does not check venting, combustion air, gas piping, circulator, expansion tank, minimum firing rate or whether the emitters can deliver the heat at the water temperature the boiler runs.
- This is a sales-phase estimate. A licensed contractor, the boiler manufacturer's instructions and the authority having jurisdiction decide the installed size. The page makes no claim of code compliance.
Common boiler sizing mistakes
- Comparing the input rating to the heat loss. Input is the fuel going in. The heat that comes out is the Heating Capacity, and the part that reaches the radiation is the Net AHRI rating. The Dunkirk 90-75 has an input of 75 MBH, a Heating Capacity of 68 MBH and a net rating of 59 MBH. Against a 56,000 BTU/h load, the input is 133.9% of the load while the capacity is 121.4% and the net rating 105.6%. Only the last two are what the boiler delivers.
- Using the Heating Capacity as the heat that reaches the radiators. Part of the capacity is a reserve for the water in the system and the piping losses. The 90-75 gives 68 MBH of capacity and 59,130 BTU/h of net rating. The 90-50 has 45 MBH of capacity and 39,130 BTU/h net, which does not cover a 40,000 BTU/h load even though 45,000 is higher.
- Stacking a margin on the allowance. The 15 percent allowance is already in the net rule. Adding 25 percent on top of it, 56,000 x 1.15 x 1.25, asks for 80,500 BTU/h of capacity, which is 143.7% of the load and over the 140 percent Manual S ceiling.
- Picking the next size up out of habit. The Dunkirk 90-100 has a Heating Capacity of 90 MBH. On a 56,000 BTU/h load that is 160.7% of the load, over the ceiling of 78,400 BTU/h. Wes Davis's ICC article on Manual S lists higher cost, greater energy consumption and more wear and tear as the cost of oversizing.
- Mixing MBH and BTU/h. A table prints 68 under the heading MBH, which means 68,000 BTU/h. Typing 68 into a BTU/h field, or 68000 into a field that expects MBH, moves the answer by a factor of a thousand. The calculator refuses a rating that is more than ten times, or less than a tenth of, the heat loss.
- Running a non-condensing boiler on cold return water. Radiant floors and other low temperature loops return water cooler than a cast iron boiler likes. The CAC/BDP manual for its oil-fired boiler warns of significant corrosive condensation in the boiler and the vent below 120°F return water and calls for a mixing valve or bypass piping. Gas-fired non-condensing boilers often set a higher minimum, so take the figure from your boiler's manual. A correctly sized boiler still needs that piping.
Common questions about boiler sizing
What size boiler do I need?
Start from the design heat loss of the building. The boiler's Net AHRI rating should be at least that number, which means a Heating Capacity of at least 1.15 times the load. ACCA Manual S caps the capacity at 1.40 times the load. For a 56,000 BTU/h load that is a net rating of at least 56,000 BTU/h and a Heating Capacity between 64,400 and 78,400 BTU/h.
What is the difference between input, Heating Capacity and Net AHRI rating?
Input is the fuel burned per hour. Heating Capacity is the heat output measured by the DOE test procedure, so it is the input less the losses up the stack. The Net AHRI rating is the Heating Capacity less the piping and pickup allowance, which is the portion available to heat radiation or terminal units. All three are printed in the boiler's ratings table.
Why is the Net AHRI rating lower than the Heating Capacity?
The difference is a reserve for heating the volume of water in the system and offsetting heat lost from the piping. Hot water boilers are rated with an allowance of 1.15, so the net rating is the Heating Capacity divided by 1.15. The Dunkirk 90-75 shows 68 MBH of capacity and 59 MBH net, since 68 divided by 1.15 is 59.1.
How many BTU does a boiler need per square foot?
There is no figure to rely on. Heat loss depends on the climate, the insulation, the windows and the air leakage, so two houses of the same area can differ widely. Get the heat loss from a calculation, then size the boiler to it. The furnace size calculator gives a preliminary whole-house heating load from a ZIP code and a short description of the house.
Can a boiler be too big?
Yes. Manual S allows a boiler output up to 140 percent of the heating load, which is 78,400 BTU/h on a 56,000 BTU/h load. Wes Davis's ICC article on Manual S lists higher equipment and installation cost, greater energy consumption and more wear and tear for oversized equipment. In check mode, a capacity over the ceiling is flagged as oversized.
Should I size to the Heating Capacity or the Net AHRI rating?
Both, for different limits. The manufacturer's rule is that the Net AHRI rating should be at least the heat loss. Manual S compares the output capacity with the load for its 100 to 140 percent window, so the ceiling applies to the Heating Capacity. A net rating equal to the load already needs 115 percent of the load in capacity, and a net rating above 121.7 percent of the load goes over the ceiling.
How do I get the input rating I need?
Divide the Heating Capacity by the AFUE. At 90 percent AFUE, the 64,400 to 78,400 BTU/h capacity window for a 56,000 BTU/h load is an input of about 71,556 to 87,111 BTU/h. AFUE is seasonal, so use this to find the model family, then read the input printed in its table.
Does this work for steam boilers?
No. A steam boiler is selected on installed radiation in square feet, and its piping and pickup allowance is 1.333 rather than 1.15. The page covers hot water boilers only.
Does a condensing boiler change the size?
Not the ratings. The type changes the warnings. A non-condensing boiler can form corrosive condensation in the boiler and the vent when the return water is below the minimum in its manual, 120°F for the CAC/BDP oil boiler and often higher for gas-fired models. That matters on radiant floors and other low temperature loops. A condensing boiler is designed for it.
What about domestic hot water or an indirect tank?
It is not included. Adding a tank can change the boiler selection, and an open source for a sizing procedure was not available. Follow the boiler manufacturer's instructions for the tank, and treat the result here as the space heating figure only.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup.
Used on the same job
Get the heat loss behind the boiler size
The boiler size is only as good as the heat loss under it. The furnace size calculator returns a preliminary heating load from a ZIP code, and the Manual J calculator frames the full method. You can also ask the assistant to walk through the load and the boiler selection with you. A load of 56,000 BTU/h is the example on this page, not a typical house.
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