What size tankless water heater for a tub and a shower in the southern US?
| Heat needed6 GPM × 500 × 40°F | 120,000BTU/hr |
|---|---|
| Unit sizenameplate input, at about 95% combustion efficiency | 150k BTU |
| Simultaneous flowthe worst realistic case in most homes | 6GPM |
| Temperature rise105°F at the tap from 65°F incoming | 40°F |
| Same unit in the southwarmer incoming water, so more flow from the same box | 7.1GPM |
| Same unit in the northcolder incoming water — the same unit delivers less | 4.8GPM |
Notes
- **A tankless heater has no capacity, only a curve.** a tub filling and a shower means 6 GPM, and lifting that from 65°F to 105°F takes **120,000 BTU/hr** delivered to the water. Ask for GPM at a stated rise, never for "how many bathrooms".
- **Geography changes the answer more than the house does.** The same unit gives **7.1 GPM** in the south and **4.8 GPM** in the north. The box did not change — the incoming water did. Marketing that promises "3 bathrooms" is quoting the southern number.
- **The cold-water sandwich is real and it is not a fault.** A tankless fires when flow starts, so there is a slug of cooled water between one draw and the next. Low-flow trickles below the minimum activation rate — a bathroom tap barely open — may not fire it at all.
- **Gas line and venting are usually the real cost.** A 150k BTU unit often needs a larger gas line than the old tank had, plus dedicated stainless or PVC venting. On a retrofit those two lines regularly exceed the price of the heater itself, and they are what turns a cheap quote into an expensive one.
- **Hard water shortens tankless life more than anything else.** The heat exchanger scales, and scale is what kills them. Annual descaling, or a softener ahead of it, is not optional above about 7 gpg.
a tub and a shower in the southern US
6 GPM across a 40°F rise — that is 120,000 BTU/hr delivered to the water.
A tankless heater has no capacity, only a curve
A tank is chosen by gallons. A tankless is chosen by GPM at a stated temperature rise, and the same box gives very different answers in different places.
Ask a supplier for GPM at a rise. Never for "how many bathrooms".
Geography changes it more than the house does
The same unit delivers 7.1 GPM in the south and 4.8 GPM in the north.
The box did not change — the incoming water did. Marketing that promises three bathrooms is quoting the southern number.
The cold-water sandwich is not a fault
A tankless fires when flow starts, so there is a slug of cooled water between one draw and the next.
And a trickle below the minimum activation rate — a bathroom tap barely open — may not fire it at all.
Gas line and venting are usually the real cost
These units often need a larger gas line than the old tank had, plus dedicated stainless or PVC venting.
On a retrofit those two lines regularly cost more than the heater. They are what turns a cheap quote into an expensive one.
Hard water is what kills them
The heat exchanger scales, and scale is what ends its life. Annual descaling, or a softener ahead of it, stops being optional above about 7 gpg.