What rules apply to two water heaters plumbed together. As in cold to one and hot out to the other unit and then the second unit, hot out to home.
There was an old thread in the archives about this subject last year. You might be able to pull it out.
I have seen this occasionally, but only the **second **heater in line is heating.
The first one is used as a “tempering tank”. The reasoning is that the water coming in from outside is very cold and when it is allowed to store in a “Pre-tank” it warms a bit from its’ own bulk. The second unit doesn’t require as much heat to raise the temperature.
Or so they say. It sounds reasonable, but I’m a little skeptical.
I’m here to alleviate your skepticism. That is the theory, and it does work.
That’s also why I have no problem with someone calling it a “hot water heater.” After all, the only time that the “water heater” will be heating cold water is when it is totally drained. Most of the time it is heating hot water. If the thermostat is set at 120°F, then when the water temperature falls to 115°F or so, the water heater will come on and start heating that 115°F water (which is hot, by the way) up to 120°F again. My water heater comes on after running about 3 gallons of hot water at a faucet. I’m changing to tankless water heaters this summer, hopefull, if I can find the money–might have to steal it from my employees margarita fund.