Step one: add up your peak flow
Walk the house and list the fixtures that might run together. Federal limits cap a showerhead at 2.5 gallons per minute, and many fixtures run lower. A kitchen faucet commonly runs around 1.5 to 2.2 gallons per minute. A dishwasher and washing machine draw less but run for long stretches.
Then ask the honest question: which two or three will run at the same moment on your busiest morning? Add those flows together. That total is your target peak flow, and a unit must deliver it at the temperature rise you need.
Typical flow by fixture
Use the numbers as a starting point and check your own fixtures for flow ratings stamped on them.
| Fixture | Typical flow (gpm) | Notes |
|---|---|---|
| Showerhead | 1.5 to 2.5 | Federal maximum is 2.5. |
| Kitchen faucet | 1.5 to 2.2 | Often mixed with cold water. |
| Bathroom sink | 0.5 to 1.5 | Short draws. |
| Dishwasher | 1.0 to 2.0 | Draws during fill cycles only. |
| Clothes washer (hot) | 1.5 to 3.0 | Depends on the cycle. |
| Soaking tub fill | 4.0 or more | Check the tub's spout rating. |
Step two: find your temperature rise
Temperature rise is the number of degrees the heater must add. If your hot water setting is 120 degrees Fahrenheit and the cold water entering the heater is 45 degrees in January, the rise is 75 degrees. In July, with incoming water in the 60s, the rise is closer to 55.
A unit rated for a flow at a 77 degree rise delivers less in a cold winter, because it has to add more heat to each gallon. This is why a heater that feels fine in August can struggle in February. Size for the coldest month, not the warmest one.
Why gas line capacity changes the answer
A tank heater commonly burns 30,000 to 50,000 BTU per hour. A whole-house tankless unit often needs 150,000 to 199,000 BTU per hour. If the existing gas pipe and meter were sized for a small tank heater, the supply can drop when the larger burner starts, and the heater may fault.
Upgrading the gas piping is licensed work under the Massachusetts fuel gas code, 248 CMR 4.00. Permit rules vary, so ask the Westborough Building Department. Our tankless water heater page explains how we check pipe size, meter capacity, and the vent path together before a unit is ordered.
Checklist before you buy
Bring these answers to your plumber and the quote becomes much more accurate.
- How many bathrooms and how many people shower in the same hour.
- Any soaking tub, body-spray shower, or large kitchen in the house.
- The coldest incoming water temperature you can measure in winter.
- Gas or electric, and the size of the existing gas pipe or electric panel.
- Where the exhaust can terminate on an outside wall.
- Your water hardness, because scale shortens heat exchanger life.
Condensate, venting, and maintenance
High-efficiency condensing tankless units produce acidic condensate that needs a drain, often with a neutralizer. They also vent through plastic pipe, which changes where the exhaust can leave the house. These details affect the install as much as the capacity.
Plan for a yearly flush. Descaling clears mineral buildup from the heat exchanger and keeps the unit near its rated output. Our water heater installation page lists what a planned upgrade covers.
A worked example in plain words
Picture a family of four with two bathrooms. On a winter morning one shower runs at 2.0 gallons per minute and a second at 1.8, while the kitchen faucet draws 1.5 for a minute. The combined peak is 5.3 gallons per minute.
Now take a winter incoming temperature of 45 degrees and a set point of 120. That is a 75 degree rise. The plumber looks up the unit's rated flow at a 75 degree rise, not the headline number on the box. If the unit delivers only 4.5 at that rise, it falls short of the 5.3 target, and a larger unit or a pair of smaller ones is the answer.
Some households solve the shortfall by habit. Staggering the second shower by a few minutes lowers the peak. A low-flow showerhead can trim the total as well. Treat these as options to discuss, not as a substitute for sizing the unit correctly.
Westborough, MA note: cold water, treatment, and scale
Westborough's public supply is the town's wells and the Westboro Reservoir. MassDEP's older source water assessment lists fluoride and corrosion-control treatment with potassium hydroxide and zinc orthophosphate. It states no hardness figure here, and we do not repeat one.
Practically, scale control is a design input. Test your tap for hardness, ask the Westborough Water Department for its current annual report, and decide whether a flush schedule or a scale filter belongs in the plan. Reservoir-fed water also tends to run colder in winter than a deep well, so measure rather than assume.
The best-sized tankless heater is the one that fills your busiest hour at winter temperatures without taxing your gas line. We serve Northborough and Grafton as well as Westborough if you want an on-site sizing check.
Bring your fixture list, your winter water temperature, and a photo of your gas meter to the first visit. With those three items a plumber can size the unit on the spot and tell you if the gas line needs work.
Tankless water heater sizing: questions answered
Often yes, if the unit's flow at your winter temperature rise covers both showers. Add the two flow rates and compare against the unit's rated output at your rise.
No. Oversizing raises the install cost and may demand a larger gas line without improving daily use. Size to peak flow at winter rise.
Sometimes. A whole-house unit draws far more gas than a tank heater, so the pipe and meter are checked first.
Many last 15 to 20 years with yearly flushing. Scale and neglect shorten that.
Sizing method is identical. Northborough buys 100 percent of its water from the MWRA, so a homeowner there can read that supply's report, while a Westborough homeowner reads the town's own.
Want a licensed plumber to look at it? Call (508) 389-4045
Plumbing problem in Westborough?
Call dispatch any hour and get a clear price before work starts.