Hybrid system in reverse

Old 04-18-18, 08:57 AM
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 3
Received 0 Votes on 0 Posts
Question Hybrid system in reverse

Has anyone deployed a smaller tank heater, then used 120v tankless for each shower?
I have a 240v tankless under the kitchen sink because it is far from the water heater, and it works well. The tankless heats the water in the underground pipes until the hot water from the tank arrives.
I thought I could do the same in reverse for the showers, replacing the leaking 70 gallon tank with a 40 gallon tank, then installing 120v tankless in the two bathroom showers in case the water runs cold.
I don't want to purchase a small tank heater if this would not be efficient, but it seems to me that it would be, and cheaper than retrofitting my house to accommodate a tankless gas heater.
Old 04-18-18, 12:58 PM
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 3
Received 0 Votes on 0 Posts
Do any experienced folk have thoughts on this idea?

I found this Stiebel Eltron Megaboost unit at about $350.
I can get a 40 gal tank for about $500.
It seems that keeping less water hot would be worth the cost.
A 70 gal unit is between $600 and $900.

Of course, I would have to run a 240v 40A curcuit from the panel (about 15 ft away).

Last edited by AlreadyinUse; 04-18-18 at 02:15 PM. Reason: More info.
Old 04-18-18, 04:36 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 62,651
Received 1,611 Votes on 1,483 Posts
Welcome to the forums.

I didn't respond sooner because I'm not quite sure what you are doi0ng.
You discussed underground pipes. Is this like a motel ?

You currently have a 70 gallon electric water heater and are considering replacing it with a 40 gallon electric water heater. You have a point of use water heater under a kitchen sink and you want to add two more for showers.

That begs the questions.....
1) where will you put them ?
2) what size electrical service do you have ? Will it support additional heating loads ?
Old 04-19-18, 04:46 AM
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 3
Received 0 Votes on 0 Posts
No, sorry: I have a 70 gal gas water heater (which is 17-yrs old and leaking), and am thinking of replacing it with a 40 gal gas unit, supplemented by a 240v. tankless for the times when the demand is high. I think the tankless will get little use, and the smaller tank will be more efficient. Both would sit in the same location as the existing unit.

I'm just not willing to fork out the money to retrofit a gas tankless for the whole house, so I think this would be an economical and efficient alternative.

I have a new breaker box with plenty of capacity, so I can install a 40A 240v circuit if I can't tap into the one that is already there. It's only about 15 ft from the breaker box to the furnace room.

Thanks for your input,
Old 05-01-18, 05:54 AM
Join Date: Mar 2010
Location: USA
Posts: 4,508
Received 139 Votes on 128 Posts
You need to do a load analysis for your house before adding one let alone two (tankless) point of use electric water heaters. Even though you have plenty of slots in your breaker panel and you believe you have enough amperes coming in, if both tankless units should kick on at the same time you could have an electric overload.

"... (electric) tankless in the two bathroom showers in case the water runs cold. ..."

If the water runs cold then the tankless unit will (must) kick on full blast as if there were no tank water heater upstream. The tankless will be drawing full power.

Electric tankless usually requires an upgrade of your electric service and panel unless the latter were much larger than needed for the house before you started this project. Not counting loss of heat all night due to less than perfect tank insulation, electric tankless costs the same per BTU (or per degree F per gallon) as electric tank heaters which is in most areas of the country a lot more than for gas water heating.

The electric tankless, being downstream of the prmary water heater (gas), will kick on for several seconds each and every time hot water is used. For frequent short usages of water your total hot water use will still have a significant percentage of electrically heated water although this cost disadvantage may be offset by the convenience of getting hot water faster if the tankless unit is close to the faucets or shower.

Last edited by AllanJ; 05-01-18 at 06:21 AM.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: