Start with the fixtures that run at the same time
Picture your busiest weekday morning. One person showers while another runs the kitchen faucet and the dishwasher starts. That overlap, not the total number of people, sets the flow you need.
The DOE says to add the flow rates of fixtures used together. If you do not know a fixture's flow, catch the water in a bucket for one minute and measure it. It also notes that low-flow fixtures reduce the flow the heater has to supply.
| Fixture running together | How to find its flow | Why it matters |
|---|---|---|
| Shower, adult | Time a bucket for one minute, or read the showerhead label | Usually the largest single draw |
| Second shower or bathroom sink | Same bucket test | Overlap is what sizes the unit |
| Kitchen faucet | Bucket test | Often runs while a shower is on |
| Dishwasher or washer | Check the appliance label | May draw hot water at the same time as the others |
Add the rows you expect to run together and you have your design flow. Keep two scenarios in mind, a normal morning and the worst case. The unit should cover the worst case without a sudden drop in temperature.
Temperature rise changes with the season
Temperature rise is the target output temperature minus the incoming water temperature. The DOE says that if you do not know your incoming temperature, assume 50 degrees Fahrenheit, and that for most uses a 120 degree target needs a 70 degree rise.
Local ground and pipe temperature differ by season, so winter needs more rise than summer. We size for the coldest part of the year, because a unit that is fine in August can fall short in January.
Where the numbers come from
The DOE also says tankless heaters are rated by the maximum temperature rise they can deliver at a given flow rate. It lists a 70 degree rise as commonly available at 5 GPM for gas models and 2 GPM for electric models. Higher flow or colder inlet water can lower the temperature at the farthest faucet.
Want the sizing calculation done for your house?
Tell us your fixtures and where the heater will go. We walk through flow, rise and fuel on the phone.
Why gas models need line and vent checks
Because a gas tankless unit can deliver much higher flow than an electric one, it burns a lot of fuel at once. The gas pipe from the meter has to carry that load, and an undersized line is a common reason a swap grows into a bigger job.
Venting is the other check. Exhaust routing, termination location and run length all depend on the unit. Homes of the 1964 era often have a flue built for a tank heater, not a tankless one.
Pennsauken sizing, permit and water supply notes
Plumbing permits in Pennsauken are handled by the Construction Code Department at 2400 Merchantville Ave. Gas piping and venting changes are the kind of work to ask that office about before the job starts.
Most addresses get water from the Merchantville Pennsauken Water Commission, and a few may be on New Jersey American Water. Check your bill, because the utility is the place to ask for its annual water quality report, which is useful when you plan to protect a new heater from scale.
Source type: Township of Pennsauken municipal code, Chapter 133, and Merchantville Pennsauken Water Commission public pages
Family of four sizing questions to ask before you buy
Bring these to the sizing conversation
A good sizing answer needs these inputs.
- How many showers run in the busiest hour
- Whether a dishwasher or washer runs at the same time
- Which fixtures are low-flow
- Current fuel and gas line size, if known
- Where a vent could exit the house
- Whether the same unit would serve a future bathroom
What you can measure
- Fixture flow with a bucket and a clock
- Showerhead and faucet ratings
- The busiest hour in your house
- Your current heater's fuel
What the plumber calculates
- Temperature rise for the coldest season
- Gas line capacity for the unit
- Vent route and termination
- Whether one unit or two meets the peak
What changes the cost of a tankless job
Unit capacity is only one part of the price.
We give a price by phone after we understand your fixtures and space, and we confirm it before work starts.
If you are planning with a neighbor in the next town, our Cherry Hill plumber page shows how we cover that area. For the tankless-versus-tank question itself, see our water heater installation page, which covers both setups.
Standards and rules behind this advice
Department of Energy sizing guidelines work from the combined flow rate of fixtures in use and the temperature rise, and gas tankless units are listed to ANSI Z21.10.3.
Sizing questions for four-person households
There is no single number. The DOE says to add the flow rates of the fixtures you use at the same time, and to choose a unit that meets that flow at your needed temperature rise. The DOE also says the unit's flow rate should be at least 3.25 GPM. We run the sum for your actual fixtures.
It depends on how often both run together and at what flow. If the overlap is rare, one correctly sized unit may do it. If it happens daily, a larger unit or a second unit may be better. The calculation settles it, not the room count.
Colder incoming water means the heater has to add more degrees. The DOE says to assume 50 degrees Fahrenheit if you do not know yours, but your real figure moves with the season. We size for the colder case so the shower stays steady.
Often a gas tankless unit does need more gas capacity than a tank heater. The only way to know is to check the existing pipe and the meter. Permits for gas and plumbing work go through the township Construction Code Department, and we handle that step with you. Homeowners in the Cinnaminson area ask us the same gas line question.
It matters for scale and maintenance more than size. Ask MPWC or New Jersey American Water for its water quality report, or test your tap. We do not quote a hardness number because we have not read one for your address.