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Community and Q&A

Can you place an electric on-demand hot water heater in line with a tanked hot water heater?

Gregory Jones | Posted in Energy Efficiency and Durability on

I was given an electric on-demand water heater. I have an extremely long run and it takes quite some time for the hot water to reach the kitchen sink. I was thinking of placing the electric heater inline–that way we would not be running the water down the drain waiting for the arrival of hot water. I am thinking than the electric hot water heater would stop heating once the thermostat sensed the water was hot enough going through the coils. Not certain though…

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  1. GBA Editor
    Martin Holladay | | #1

    Before adding another water heater, you might consider another option: a circulation loop with the pump switch located in the bathroom.

    For more information on this solutions, see Hot water circulation loops.

  2. D Dorsett | | #2

    You can plumb them in series and it will work, but it's an expensive solution, especially if you end up having to upgrade your electric service to handle the massive peak draw of a tankless electric on-demand HW heater.

    If the electric tankless can heat the water sufficiently there's no advantage to putting it in series with an electric hot water heater, which would result in hot water from the tank being abandoned in the plumbing run between the two. It would be more energy efficient for the on-demand heater to draw cold water locally from the cold water pluming near the load.

    An on-demand recirculation pump would be a cheaper solution to install (low power draws), but would abandon more hot water in the plumbing.

  3. JAMES KREYLING | | #3

    And another solution might be to just run a dedicated 3/8" PEX line to the remote hot water tap to reduce the volume of water wasted to get there....(KISS principle)

  4. JAMES KREYLING | | #4

    It can be an expensive solution, if your electrical service is not adequate. But if you can spare the use of an extra 30 or 40 Amps at 240 volts, then the proposed demand heater installation at the point-of-use could be a good solution to wasting a lot of water. Especially if you are in an area that has high water and sewer costs, as I am. There is a scenario in which I disagree with Dana on there being no advantage. If your tank heater is a HEAT PUMP water heater, then there is most definitely an advantage to using the demand water heater downstream of the tank, as the hot water from the tank can cost as little as a third of the cost of the resistance heated water. If the demand heater is a newer generation variable SCR controlled heating element control such as the Steibel-Eltron DHC-E series with electronic control, then the demand heater will only be heating when the local water flow temperature is not up to the setpoint. When the hot water from the heat pump tank arrives, it will vary the heating off so you are not overheating. But, if your demand heater is only flow-activated, as many are, then when the hot water arrives from the tank, you will be overheating the water, and you could create scalding conditions. This is dangerous! Make sure if you attempt this that the demand has temperture control, and is not just flow-activated.
    I am just starting construction on a new home with just this plumbing plan. As the kitchen is somewhat remote from the bathrooms plumbing core, and heat pump water heater (and the Viega PEX manifold,) I am installing a Steibel Eltron12KW DHC-E under the kitchen sink, so I will have virtually no water waste to get hot water in the kitchen. Depending on where you live and the flow you want, a 7.2KW (30A) or 9.6KW (40A) unit might be sufficient, and then your 240 volt electrical feeder sizing would only have to be a 40A or 50A, not the 60A needed for the 12KW. (Yes, this is all easy for me to say since I am an electrician- for the last 40 years...)

  5. Gregory Jones | | #5

    Thanx to all who responded. I have the space in my new 200 amp service panel and that panel is within six feet of the planned placement of the Stiebel Eltron on-demand Tempra 20 hot water heater. Steibel Eltron has confirmed that once the water is at the operating temperature--the unit stops heating water. Since I got the heater for zero cost and only have to pay for copper piping and copper wiring, it is relatively inexpensive to install the heater.
    I recently placed a manual transfer switch into my service panels (yes, have two of them) so I am familiar with wiring and also have done enough plumbing to feel comfortable to plumb in the copper. I did consider using a circulation pump, but one of my future additions to the home will be an on-demand dual boiler system to heat hot water for the main bath and heat liquid for radiant heat in our new addition. I am just not ready to pull the trigger on the boiler just yet.
    Bottomline, I am thinking I will be able reduce my water and sewer costs--not wasting the water (thinking green here). Also, since my current hot water heater is a gas storage unit, I will be using the gas to heat the hot water that "finally" reaches the kitchen sink tap and the dish washer, which is more cost efficient than any electric heater. I have not run the numbers concerning the cost of heating the water for the time I have to run it before the hot water coming from the gas hot water heater. I was primarily thinking that I would save water usage, with only a minimum of electrical use for the short time it is on before the gas heated hot water reaches the sink.

  6. JAMES KREYLING | | #6

    Gregory: Holy Cow! You want to run a kitchen faucet on a 19.2KW heater?!! You could feed just one of the two 50A feeders to it and probably have all the hot water you will ever need for your kitchen faucet, if it will work with one feed. That still gives you 8.6KW, which can make a fair amount of hot water. Check the temperature rise/flow charts before you put in the second feeder.

  7. Gregory Jones | | #7

    Well actually, it will feed the dishwasher, the sink and the washing machine.. But, you have given me food for thought, if I understand what you are telling me--I could set it up initially with only one220 line active...correct. Great idea. Thanx.

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