Gas on-demand w/ electric w/h

I inspected a fairly high-end home ($1.8M) yesterday that had two separate water heater systems that each had a 16k BTU/hr on-demand, gas fired Noritz heater in series with a 6 gallon, electric water heater. Theory apparently being that it is cheap to electrically heat 6 gallons of water but when that ran out then the Noritz would come on. This seems a little self-defeating to me and I wonder what others think of this setup.

Excerpt from City Energy Code:

My example seems to make the 6 gallon electric w/h the primary thereby violating the City’s Energy Code.

I agree Mike, it’s an unnecessary redundancy. The tankless heater, however, will likely always fire-up when there is a demand for hot water, unless the EWH is set at a higher temperature than the tankless.

Technically, the primary source would be dependant on which system is in-line first.

Yes Jeff, I think I need to bone up a little more on this configuration, it didn’t quite compute with me. It does seem like the tankless would fire up immediately to re-supply the electric w/h when even any hot water was turned on. Maybe the electric was upstream of the tankless thereby only supplying hot water to the tankless, at least the 1st 6 gallons, then if more was needed the tankless would come on. And yes, the electric w/h temp would have to be set higher than the tankless in order for it to operate properly. I guess that does make more sense than the electric w/h being downstream doesn’t it?

Is it possible that the electric heaters only went to the sinks?

No, the builder’s rep tried to explain the set-up but I couldn’t quite catch his explanation at the time. It all had to do with providing instant hot water at fixtures with no cool water for any period of time. His explanation was that even a tankless, on-demand gas fired heater will provide a few seconds of cold water until the lines are purged and the tankless has had time to bring the water up to temperature. This configuration, along with recirculation pumps, greatly minimized that situation. After this discussion and thinking about it some I think I understand it better now.

Ahhh, small tanks for hotwater now, and flash heaters for greater demand. Never seen it, interesting.

It would seem like the lines between the heater(s) and the point of use would be cold in any event - unless they are recirculating the water through the electric heater.

There were recirc pumps apparently coming off of the electric w/h’s and now that I know (or think I know) a little more about the installation I’m tempted to go back to analyze the design some more. If the electric w/h is upstream of the tankless then I don’t see how a recirc system would work. I inspected the components of the system for general installation, leaks, temperature, etc but not the design so I’m still somewhat unsure of how it all worked as a system.

It is hard to tell from the picture, but I would expect the electric heater and recirc pumps to be downstream of the tankless heater. That way the stored hot water is ‘instantly’ available and the gas fired heater will replenish that supply for as long as needed. Actually, that sounds like a pretty good system - if I had the recirc equipment and lines in place I might add that feature to my tankless systems.:slight_smile:

Yes, but that would seem to violate my city’s ordnance that there can be no new electric resistance water heating as the primary source of hot water. This was a brand new home in the city limits with all the permits & a CO. I’m going back tomorrow for other tasks and I will try to take more photos and I will ask the builder if I can talk to his plumbing installer about this design.

OK, after a return visit for other purposes I did find out that the small, 6 gallon w/h is downstream of the tankless and has the recirc system and the builder agrees that does not meet the current city energy conservation ordnance requirements but that this particular home was started prior to that ordnance and therefore was not subject to it. The city agrees and has issued a CO.