A plumber recently told me that its perfectly common to keep the old 40 gallon tank water heater in series with a new tankless water heater.
The motivation is to decrease the time it takes to get hot water to a faucet. And also to then have endless hot water once it gets there.
As I understand it, the tankless pumps hot water into the cold water inlet of the tank heater, and that in turn pumps out to the house via its hot outlet. The hot water either waits in the tank (which adds heat to keep temp constant) or passes through if a faucet is.
Heating happens in the tankless unit, and is maintained in the tank unit until use.
Does this make sense to do?
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The question is
My answer to that is, Based on how you worded your scenario, This does not make sense to me. I can not wrap my head around how it would be in parallel. What you have described is two units in series. (one after the other)
A tankless water heater is an on demand unit. It only fires up and heats water when a hot water faucet is opened.
If they were in series:
If the "tank", installed after the tankless, is not heating water then all the water in it will be cold. (if it is heating water then what is the point of having the tankless)
When you open the hot water faucet the tankless will fire up and start sending hot water to the faucet but it will have to push all of the cold water in the tank out through the faucet BEFORE any HOT water gets to the faucet.