Why the GPM rating on the box misleads
This is the single most useful thing to understand before buying a tankless water heater, and it explains almost every complaint about them.
The rating is a pair of numbers pretending to be one
A tankless heater has a fixed energy output. It can use that to heat a lot of water a little, or a little water a lot. So its capacity is not a flow rate at all: it is a flow rate at a stated temperature rise, and the two only mean something together.
Manufacturers quote the flow at a small rise, conventionally around 35 degrees, because that produces the largest number. It is not dishonest, and the full table is usually in the specification sheet. It is simply the number that ends up on the box and in the listing title.
The same unit, across real conditions
| Temperature rise | Flow delivered | What that covers |
|---|---|---|
| 30 F | 12.3 GPM | 5 showers at once |
| 40 F | 9.2 GPM | 4 showers at once |
| 50 F | 7.4 GPM | 3 showers at once |
| 60 F | 6.2 GPM | 2 showers at once |
| 70 F | 5.3 GPM | 2 showers at once |
| 80 F | 4.6 GPM | 2 showers at once |
That is one unit. The advertised figure is 10.6 GPM. A household in Minnesota wanting 120 F water from 40 F supply faces an 80 degree rise and gets about 4.6 GPM from it, which is one shower with nothing to spare.
How to read a specification properly
- Find your temperature rise. Target temperature minus incoming water temperature, using a winter figure.
- Find the unit's flow at that rise, not at the headline rise. Every manufacturer publishes the full curve, usually as a graph in the manual.
- Compare that with your simultaneous demand, not with your total fixture count.
If the specification only gives one number and no rise, that is worth noticing.
Why this is not a fault
Nothing is wrong with a unit that behaves this way; it is conservation of energy. A heater that delivered its headline flow at any temperature rise would be producing energy from nothing. The problem is only ever that the unit was chosen against the wrong rise.
It is the same shape of error as buying a generator on its surge rating or an air conditioner on its nameplate: a number measured under favourable conditions, applied to unfavourable ones.