The tankless brochure numbers are true. They’re just not true here. A tankless heater doesn’t store hot water — it raises the temperature of water passing through it — so everything about its performance depends on how cold the water arriving is. And in New Hampshire, the water arriving is very cold for a long stretch of the year.
A tankless unit is rated in gallons per minute at a given temperature rise. Rise is the difference between the water going in and the water you want coming out.
You want roughly 120°F at the tap. Here’s what that means in different places:
| Location | Typical winter inlet | Rise required | What that does |
|---|---|---|---|
| Gulf Coast | ~70°F | 50°F | Unit performs near its headline rating |
| Mid-Atlantic | ~55°F | 65°F | Meaningful derate |
| NH Lakes Region well | ~42°F | 78–80°F | Output roughly halved vs. headline |
A unit advertised at, say, 7 GPM is usually quoting that at a modest rise. At an 80-degree rise the same unit might deliver in the neighborhood of 3.5–4 GPM. That’s the difference between “two showers and the dishwasher” and “one shower, and don’t start the laundry.”
It’s not a defect and it’s not dishonest — it’s physics, and it’s printed in every manufacturer’s sizing chart. It just doesn’t make it into the sales conversation often enough.
| Fixture | Typical flow |
|---|---|
| Shower (modern head) | 1.8–2.5 GPM |
| Shower (older head) | 2.5–3.5 GPM |
| Bathroom faucet | 1.0–1.5 GPM |
| Kitchen faucet | 1.5–2.2 GPM |
| Dishwasher | 1.0–1.5 GPM |
| Washing machine | 1.5–3.0 GPM |
| Large soaking tub | 4.0+ GPM to fill in reasonable time |
Add up what might realistically run at once on a February morning. Two showers plus a kitchen tap is around 6 GPM — at an 80-degree rise, that’s a genuinely large tankless unit, likely with gas line and venting work to match. The sizing tool does this arithmetic for you.
Rough guide for a year-round house: 40 gallons for one to two people, 50 for three to four, 65–80 for five or more or a house with a large soaking tub. But pattern matters as much as headcount — four people showering in sequence starting at 6:30 need considerably more than four people spread across three hours.
The mistake worth avoiding is replacing like-for-like without asking whether the old one was right. If you ran out of hot water every morning for twelve years, a new 40-gallon tank will give you exactly the same twelve years.
| Type | Installed range (Lakes Region) |
|---|---|
| Electric tank, 40–50 gal | $1,400–$2,200 |
| Propane or natural gas tank | $1,700–$2,600 |
| Oil-fired / indirect off a boiler | $2,200–$3,400 |
| Gas or propane tankless | $3,800–$6,800 |
| Repair on a unit worth saving | $200–$600 |
Ranges reflect regional installed pricing including labor, code-required fittings, and haul-away; gas line upsizing or new venting adds to the tankless figures. See the full cost benchmarks for sources.
You can, but check two things first: does it have genuine freeze protection and full drain-down capability, and is it worth it at your demand level? A camp used a dozen weekends a year will never recover the cost difference in efficiency, and it adds a complication to every fall shutdown.
On energy, yes — no standby loss keeping forty gallons hot around the clock. Whether that recovers a $2,000–$4,000 price difference depends on your usage and how long you keep the house. In a big, busy, year-round household it can. In a two-person house it generally doesn’t.
A well-insulated tank heated by your boiler rather than by its own burner or element. Common in New Hampshire houses with oil or propane boilers, and often the most efficient option in a house that already has a good boiler — it uses a heat source you’re already maintaining.
Yes. Hard water scales heat exchangers, and a tankless unit’s narrow exchanger is more sensitive to it than a tank — it needs periodic descaling to keep its rated output. If you have hard water and want tankless, budget for that maintenance or address the water first.
Tell us how many people, how many baths, what fuel you have, and whether it’s year-round or seasonal. We’ll tell you honestly what fits — including when the cheaper option is the right one.
Tell us how many people, how many bathrooms, and whether it’s year-round or seasonal, and we’ll tell you what actually fits.
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