Connected load, and why nobody discounts it
Connected load is the sum of the input ratings of every gas appliance a system serves, assuming all of them fire at once. Nobody actually runs the furnace, the range, the dryer, the fireplace and the water heater simultaneously - but NFPA 54 sizes residential piping as though they did, and for a good reason: the consequence of being wrong is a starved burner, incomplete combustion and carbon monoxide, and the cost of being conservative is one pipe size.
Input rating is the gas an appliance consumes. It is not the output, and the difference is not small. A furnace advertised at 80,000 BTU/hr of heating output at 80% AFUE consumes 100,000 BTU/hr of gas. Size the pipe from the output figure and the system is undersized by a quarter. The rating plate always gives input, usually labelled Input or Gas input, and it is what you use.
The total is used in two separate checks. The pipe has to carry it - that is the pipe sizing problem, which also depends on developed length. And the meter and service regulator have to pass it, which is a straight capacity comparison and is what the utility cares about. A system can be perfectly piped and still be starved by a meter that was sized for the house before the tankless heater and the generator went in.
Once the load is in BTU per hour, converting to volume is a single division by the heating value of the gas. Natural gas is around 1,030 BTU per cubic foot, so a 237,000 BTU/hr house needs about 230 cubic feet an hour. Propane packs 2,516 BTU into a cubic foot, so the same house on propane needs only 94 CFH - one reason propane lines look small for the load they carry.
Reading the meter badge and the regulator rating
Every gas meter carries a badge with a capacity in cubic feet per hour at a stated pressure. Residential diaphragm meters in the common domestic size are typically rated 250 CFH, with larger ratings available for houses with heavy loads. The service regulator that sits ahead of the meter has its own capacity, and either can be the binding constraint.
Compare your calculated CFH against that rating. Below about 80% of it, you have real headroom. Between 80 and 100% the meter is technically adequate but nothing further can be added, and a utility may decline to approve a new appliance on it. Above 100% the meter must be changed before the appliances are connected, and no amount of pipe upsizing helps - a meter that cannot pass the flow will hold the downstream pressure down no matter how generous the pipe is.
Meter changes are the utility's work, not the installer's, and they usually take weeks rather than days. Doing this calculation before ordering equipment is the difference between a scheduled meter swap and a house with a new tankless heater that cannot be commissioned.
Watch the loads that move the total most. A tankless water heater at 199,000 BTU/hr is typically five times the tank heater it replaces and is the single most common cause of a meter suddenly being too small. A standby generator can add 150,000 to 250,000 BTU/hr. A pool heater can be 400,000. Each of these on its own can outweigh the entire rest of a house.
Worked example: a typical house, then the same house with a tankless heater
Start with the default house: an 80,000 BTU/hr furnace, a 40,000 BTU/hr tank water heater, a 65,000 BTU/hr range, a 22,000 BTU/hr dryer and a 30,000 BTU/hr direct-vent fireplace, on natural gas at 1,030 BTU per cubic foot through a 250 CFH meter.
- Total connected load. 80,000 + 40,000 + 65,000 + 22,000 + 30,000 = 237,000 BTU/hr.
- Convert to flow. 237,000 ÷ 1,030 = 230.1 CFH.
- Meter check. 230.1 ÷ 250 = 92.0% of the rated capacity.
- Spare. 250 − 230.1 = 19.9 CFH, which at 1,030 BTU per cubic foot is 20,500 BTU/hr - not enough for another dryer, and barely enough for a small log set.
Now replace the 40,000 BTU/hr tank heater with a 199,000 BTU/hr tankless unit. The connected load becomes 80,000 + 199,000 + 65,000 + 22,000 + 30,000 = 396,000 BTU/hr, which is 384.5 CFH. That is 154% of the 250 CFH meter. The pipe from the meter almost certainly needs upsizing as well, but the meter is the first problem and it is the utility's to solve.
The same arithmetic in reverse tells you what will fit. With 19.9 CFH spare on the original house, the largest appliance that could be added is 19.9 × 1,030 = 20,500 BTU/hr - a patio heater, perhaps, and nothing more.
What to do with the number
Take the CFH figure to two places. First to the pipe sizing calculation, where it becomes the flow the main from the meter has to carry over the developed length to the most remote appliance. Each downstream section is then sized on the load beyond it, so the branch to the range carries only the range.
Second, to the utility, if the meter check is tight. Ask for the meter and regulator ratings in writing rather than reading the badge through a decade of paint, and tell them the total connected load you have calculated. Utilities size their service on the same figure you have just produced.
If you want to know what the system is really drawing rather than what it could draw, clock the meter. With everything else off, time a known dial revolution while one appliance runs at full fire and convert to CFH - the method the gas meter clocking calculator sets out. That measures actual consumption and is the standard way to verify that an appliance is firing at its rated input, which is a different question from whether the system can carry it.
Finally, plan for the load you will have rather than the one you have. Adding a gas dryer, a generator, an outdoor kitchen or a pool heater later is straightforward if the meter and the main were sized with room, and an expensive retrofit if they were not. Enter the future appliances in the Other field now and see what it costs to allow for them.
Table-basis CFH and metered CFH are not the same number
This calculator's meter check uses the CFH implied by the heating value you selected - 1,030 BTU/ft³ for typical pipeline natural gas, or 2,516 for propane - because that is the actual volume the meter has to pass. NFPA 54's pipe-sizing capacity tables are built on a different assumption: the option in the heating-value list labelled ‘table basis, 1,000 BTU/ft³’ exists because that round number is what the published pipe-sizing tables are keyed to, not the true heating value of the gas itself.
The two figures diverge in a fixed direction. Dividing the same BTU/hr demand by 1,030 always gives a smaller CFH than dividing it by 1,000, because the divisor is larger. So a load run through this calculator at the actual heating value produces a CFH that sits a few percent below what the pipe-sizing tables expect for that same appliance load - the gap is the fixed ratio 1,000/1,030, regardless of how large the load is.
That difference is too small to change a meter decision, which is a straightforward capacity comparison at the real heating value. It matters when the same CFH number is carried over to look up a pipe size in NFPA 54's capacity tables: plugging in a CFH computed at 1,030 where the table's own columns assume 1,000 is one small step toward reading the table as though the load were lighter than it is. Recompute CFH at the table-basis value before the pipe-sizing lookup, and keep the actual-heating-value CFH for the meter check - the two answer different questions and neither substitutes for the other.
Typical input ratings for common gas appliances
| Appliance | Typical input (BTU/hr) | Flow at 1,030 BTU/ft3 (CFH) |
|---|---|---|
| Furnace, mid-size residential | 60,000 - 100,000 | 58 - 97 |
| Boiler, residential | 80,000 - 150,000 | 78 - 146 |
| Storage water heater, 40-50 gal | 32,000 - 50,000 | 31 - 49 |
| Tankless water heater | 140,000 - 199,000 | 136 - 193 |
| Range with oven | 50,000 - 80,000 | 49 - 78 |
| Clothes dryer | 20,000 - 25,000 | 19 - 24 |
| Direct-vent fireplace | 20,000 - 40,000 | 19 - 39 |
| Standby generator, 12-22 kW | 150,000 - 250,000 | 146 - 243 |
| Pool or spa heater | 200,000 - 400,000 | 194 - 388 |
| Outdoor grill or patio heater | 30,000 - 60,000 | 29 - 58 |
The CFH column is the BTU column divided by 1,030. On propane, divide by 2,516 instead, which gives about 41% of these figures.
Where load totals go wrong
- Using output instead of input. The most common error and always in the unsafe direction. A furnace's input exceeds its output by the inverse of its efficiency.
- Applying a diversity factor on residential work. NFPA 54 does not permit it. Multi-family and commercial systems sometimes do under specific utility rules, but a residential plan review will expect the full connected load.
- Forgetting the appliance nobody thinks of. The fireplace in the basement, the garage heater, the outdoor grill on a quick-connect. Every one of them is connected load.
- Checking pipe but not the meter. Pipe sizing and meter capacity are separate constraints. A correctly sized pipe fed by an overloaded meter still starves the appliances.
- Mixing natural gas and propane figures. The heating values differ by a factor of about 2.4 and the specific gravities by 2.5. A propane system sized from natural gas tables will be wrong in both directions at once.
- Assuming the utility's service is unlimited. The service regulator, the service line and sometimes the street main all have limits. A very large added load can require work well beyond the meter.
The code basis
NFPA 54, the National Fuel Gas Code (co-published as ANSI Z223.1 and reproduced in the International Fuel Gas Code), requires piping to be sized for the total connected load of all appliances served, using the input ratings marked on the appliances. Where an appliance is not yet selected, the code allows a load to be assumed and the piping sized for it. Demand factors and diversity are a feature of commercial and multi-family design under specific engineered methods, not of ordinary residential sizing. Work from the edition your jurisdiction has adopted, and remember that gas piping is licensed work almost everywhere.
Key terms
- Connected load
- The sum of the input ratings of every appliance served, with no allowance for the fact that they will not all run at once.
- Input rating
- The gas an appliance consumes at full fire, in BTU per hour, as marked on its rating plate. Always larger than the appliance's heat output.
- CFH
- Cubic feet of gas per hour. Connected load divided by the heating value of the gas.
- Heating value
- Heat released by burning one cubic foot of the gas. About 1,030 BTU/ft3 for pipeline natural gas and 2,516 BTU/ft3 for propane vapour.
- Meter capacity
- The maximum flow a meter can pass without excessive pressure loss, marked on the meter badge in CFH.