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Community and Q&A

Heat-Pump Water Heater Plus Tankless Water Heater

ermackey | Posted in General Questions on

I have a Rheem Marathon and do not want to replace it. However, we occasionally run out of hot water when we have company. This is rare, but inconvenient. Here is my quandry:

AELP has a heat-pump rate of 5.9 cents/kwh. HOWEVER, that rate also comes with a peak charge of basically $10/peak kwh on top of the total Kwh. The peak is computed at (Highest kwh usage in any 15 minute interval during billing period) * $10 + (5.9 cents * total kwh per billing cycle). Therefore, if your highest peak usage in any 15 minute interval was 12 kwh, you would have a $120 peak charge on top of the billed total kwh. Why? Because we are hydro and they want consistent usage and could care less about time of day.

Given this situation, I am considering installing a Rheem RTEX-08 AFTER the tank. My understanding is that this tank does not activate unless the water drops below the set temperature (is this correct)? It also has a max GPM of 4.8 – enough for our two showers when we are not using the tankless. In an emergency, we will still incur a +8 kwh peak charge. But this should be very rare and it is there, if needed.

1. Is it correct that the tankless will not activate unless the water temp is below the set temp?
2. Is this the best use for my situation?

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  1. Expert Member
    Dana Dorsett | | #1

    Unless your rates are very unusual, demand charges are normally on the peak draw which would be measured in average kilowatts over the 15 minute period, not kilowatt-hours. But maybe it's per kilowatt-hour. A 4kw load for 15 minutes would be 1 kilowatt hour, but the average load during the period is 4kw. So, would they be charging you $40, or would it only be $10? (If I read your description correctly, it would be $40.)

    Either way, a 10 minute shower with even a fairly wimpy 8kw tankless like the Rheem could be pretty pricey, eh?

    Do the math:

    An electric tankless regulates the output temperature by feeding in more power. If the incoming water is warmer than the setpoint it's power use is zero. But, if you're running out of hot water when somebody starts a 10 minute shower the water entering the tankless is likely to drop below a very tepid 90F, maybe even below 80F. At 80F in and 105F at the shower head, at 2gpm (about 1000 lbs per hour) you're talking 25,000 BTU/hr or ~7kw.

    If it averages 6 kw for a 7.5 minute shower interval after which it's off, the 15 minute average would be 3kw, and that shower costs you $30 plus the energy cost.

    Basically, any time you have demand charges being assessed, the LAST thing you want in your house is an intermittent but heavy load such as a tankless water heater!

    A better solution (if it fits):

    If you have at least a 5 foot or taller vertical section of drain downstream of the main shower you'll get much better "payback" out of a drainwater heat recovery heat exchanger feeding both the water heater and the cold side of the shower mixer (or the whole cold water distribution of the house, if it's easier to plumb.) At 2gpm a 4" x 48" or taller gravity film type heat exchanger recovers 50% or more of the heat going down the drain and puts it into the incoming water stream, which roughly doubles the showering time you get out of the Marathon making it less likely to run out, and it will pay for itself in power use in fairly short years at AELP rates.

  2. ranson | | #2

    I also second what Dana said about hot water heat recovery.


  3. Jon_R | | #3

    Hopefully company is a predictable event. So you can switch on option A or B above as needed.

  4. ranson | | #4

    Following up on Jon's comment: Option B, installing the mixing valve, has safety benefits even if you don't raise your tank temperature. I've been scalded more than once when contractors have accidentally cranked up the water heater to full blast. (I still don't understand how one accidentally does this.) The valves still let a little overheated water through, but they help.


  5. ranson | | #5

    A tankless heater in series with your tank should work, but I would consider two alternatives:

    A: A well insulated, extra storage tank in series with your main tank. Like the tankless option, it would only draw significant power when you draw down the main tank. However, you might be able to get away with a much smaller heating element in the new tank, say 1.5kW. (The downside of a smaller heating element in the second tank is that if you do fully draw down all of your hot water, the new tank will take longer to recover.)

    B: Increasing the temperature of your tank and adding a thermometric mixing valve to mix cold water back into the water. This effectively increases the amount of hot water available and is very cheap to implement. If you bump the temperature above 140F, it has the added benefit of preventing legionella growth.

    Both of these will increase the available hot water. Both will increase your standby losses. However, it wouldn't take many $80 charges from the power company to make standby costs look insignificant.


  6. ermackey | | #6

    Thanks for the helpful comments. I bought the mixing valve and will install it this weekend. I did some research and my best option for a drain heat recovery seems to be the Ecodrain A1000. This is because I do not have the clearance between any of my drains and sewer in my crawlspace and the long vertical sections of my drain pipe is behind structural elements. Therefore, I need a horizontal greywater system like Ecodrain.

    The problem is I cannot find anyone who sells a horizontal system. Ecodrain has been unresponsive to my emails. Anyone know an online source for a horizontal drain heat recovery system?

  7. Expert Member
    Dana Dorsett | | #7

    If you have a section of 11.5-12' of horizontal drain to work with you might be better off with the B-1000 than the A1000:

    But if they don't respond to emails or calls it's pretty hard to get more info on them or order them.

    Without the gravity film to spread it over the whole surface as with vertical heat exchangers it takes a much longer version (like the B1000) or a lot more turbulence (as with the A1000) to get much heat transfer out of it.

    Call them- some companies are terrible about responding via email. They might be able to advise you on whether you get better return efficiency of the B1000 vs. A1000 too.

  8. AppliedBuildScience | | #8

    i have this system using a Rheem 80 gallon Heat pump water heater with a Stiebel Eltron Tankless in series. The heat pump water heater works great all year but on those rare occasions with many guests using the showers the tankless kicks in to supplement and provide unlimited hot water....its a creature comfort that has really changed the game for me... additionally i turn off the HPWH middle of deep cold winters to avoid that cold air being pumped into the house....just not worth running when it's -10 degrees outside.

    1. etiennef | | #9

      @Emil, sorry for high-jacking your question, I have a very similar situation. I dont have any concrete answers for you, but yeah your reasoning sounds good to me.

      @AppliedBuildScience, You have the exact setup I've been thinking about for a while...
      Over here in Quebec Canada, the electric utility offers a "dual-fuel" rate.
      The regular electrical rate is around 0.06$ per kWh (Canadian$, I'm rounding up numbers and keeping it simple)
      With the "dual-fuel" rate you get your electricity at 0.045$ kWh all year round.
      When the temperature falls under 12C (55F) the kWh goes up to 0.26$ Which doesn't happen all that often, or for that long.
      The idea is to use electricity all the time, and when the temperature drops, use the alternate fuel. This is fairly easy to do with the HVAC (Gas and heat-pump) but I'm looking for a way to do it with the Hot Water too.

      My thought is the have the hybrid heat pump do it's thing, then hook up the tankless after the hybrid tank. When the temperature drops under 12C (55F) I would "disable" the heat-pump and the tankless would automatically take over if the inlet temperature falls under X. (optionally, I'd test to see if disabling the electric element altogether on the hybrid would bring any cost saving).

      My question is, does the tankless stay off if the inlet temperature is high enough ?
      Does it trigger any faults on the unit ?
      I'm guessing your tankless only turns on when the hybrid tank cant keep up, do you like your solution so far ?


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