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Not enough hot water

user-471779 | Posted in General Questions on

Please help my planning… hypothetical… but more that possible…

I am planning a 80 gallon heat pump water heater.. there is a strong chance that this will not meet the demand at times of a family of 5 who all shower in the morning… washing machine etc… and a soaker tub… Aside from the obvious (no soaker tub, spread out the showers, have less teenagers)

2 options..

1. Install a marathon 50 gallon after the hpwh… I can then switch the heat pump water heater to heat pump only mode.. and the marathon would use its well insulated self to maintain the temp…

2. Install a stiebel eltron 36 plus to handle the load after the heat pump water heater starts delivering low temp…It may reduce the flow… but wont experience cold water… I guess part of the question is will this unit not use much electricity most of the time when the water temp from the hpwh is ok.

Which scenario makes the most sense, and uses the least amount of energy? best guess?

Thank you in advance

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  1. charlie_sullivan | | #1

    Neither sounds like a very good option to me. With the Marathon after the heat pump, after a heavy draw you'd have 50 gallons of lukewarm water in the Marathon that you couldn't heat with the heat pump, unless you had a circulating pump to move water between the two tanks. With the electric tankless, you'd need high-current electric service, which could make it expensive to install and perhaps eventually expensive to run, if utilities start charging for peak load. The 36 requires 300 A service to your house, and then three 50 A circuits running to it! In principle that could work, but it seems a little crazy.

    So what I recommend instead is a drainwater heat recover system such as the powerpipe system.

    That works best when you have coincident or nearly coincident showers. It will make your tank go about twice as far, and will help your efficiency rather than hurting it (which the other two solutions would do).

  2. GBA Editor
    Martin Holladay | | #2

    The Stiebel Eltron Accelera 300 has a first hour rating of 76 gallons. One option is to live with the heat-pump water heater for a few months to see if it satisfies the needs of your family.

  3. Reid Baldwin | | #3

    Two suggestions:

    1. Drain water heat recovery as suggested by Charlie - some upfront cost, but less than the options you mention - reduced operating cost. This works great for showers because the water drains at the same time as it is drawn.

    2. Set the temperature higher so that your mixing valve mixes in more cold - no upfront cost but increased operating costs due to higher standby losses and lower COP. You can set the temperature back to around 120 whenever you don't have so many people at home.

  4. user-471779 | | #4

    I am certainly going to look into the drainwater recovery.. I can only use it for the master, the way the drains are set up. I think I am going to install the stiebel accelera as martin suggested and just see if it is enough... Although I am pretty sure that there will be no showers for a while after a bath... I am just hoping that it will fill the bath...

    I am just going to plumb ready for a marathon or a tempra after the accelara.. and plan for the electric as well.

    If I did install the tempra... If It detects hot water coming in... will it not fire up? and then only use energy when the accelara empties out?

    Here is a link to the marathon after a heat pump that Matt Risinger put in in one of his homes.

    Charlie, could you help me explain why I would need a recirc? I imagine that the hpwh would fill the marathon with hot water and it would heat its own water also... essentially i would have 130 gallons of first hour hot water available... Luke warm water would only fill the marathon after the 50 gallons from the marathon and the 80 gallons from the accelera are gone..

  5. charlie_sullivan | | #5

    You would only need to circulate water between the two tanks if you wanted to heat both with a heat pump. If you don't mind finishing heating the 50 gallons with the electric element, that works fine. E.g. set point 140 F, and the temperature in the Marathon droops to 120 F after the big draw. You are then doing 50 gallons of heating the last 20 F with the electric element. If you circulated between the two tanks, you could do that heating with the heat pump.

    I'm not recommending doing the circulation--to complex and expensive. I'm just saying that without it, you aren't doing all the heating with the heat pump.

    What prevents using drainwater recovery on the other drains? This one uses a different geometry, if that helps:

  6. vensonata | | #6

    You could use a geyser heat pump and a 200 gallon tank. Standard electric or solar storage tank, add the geyser and get the heat pump effect.

  7. GBA Editor
    Martin Holladay | | #7

    You can buy 200-gallon stainless-steel tanks -- or larger ones -- but a 120-gallon tank is typically the maximum size that fits through conventional doors, or fits down conventional stairways.

  8. user-2310254 | | #8

    @Dean. I think some behavior change would be more practical than trying to meet your worst-case usage scenario. Can you convince any of your teens to take showers the night before? Can you time washing machine and dishwasher use for when there is no other demand on the tank? (Not that efficient appliances use that much water.) Just asking.

  9. Expert Member
    Dana Dorsett | | #9

    Drainwater heat recovery (even if you have to buy multiples so that all showers could be served) is going to have a lower lifecycle cost than ever more hot water storage capacity.

    Elevated storage temperatures can get quite a bit more out of a tank. It would cut into heat pump water efficiency a bit.

    If you are heating the place with a hydronic boiler, an ~80,000 BTU/hr boiler can run a single shower 24/365 even in cold water country, and you would size the indirect tank for the monster-tub.

    Is the master shower a super-luxe thing with six side sprays?

  10. user-471779 | | #10

    No the shower is large but just just two heads... I really cant justify the deluge.
    I am definitely going to do drainwater recover.. but only two of the showers are on the main level.. and one is a rarely used guest shower.. three showers in the daylight basement with drains in my "no mold, stay dry basement!" . So It realistically only makes sense for one drainwater recovery.

    I see the "power pipe" online is this what we are taking about?

    I wasn't planning on recirculating the water between the hpwh and the marathon.. hpwh on hp only mode... and marathon just does its thing.. but suppied 90 percent of the time with 140 degree water from the heat pump...

    If i went with a geyser and a storage tank.. do they have elements to keep it at temp? If so then what is the difference in my plan..

    I guess what I would really like to know... is would the tempra 36 use little to no energy if 140 temp water was flowing through it? and only kick in when the heat pump cant keep up... For some reason, I just think this makes sense.. some up front costs... but seems like a good choice.

    Although I do have natural gas, and am tempted by a condensing tank vented ... for some reason, I really want to stick with electric, partially because someday I might further explore pv.

    Dana, the plan right now is a tradional air source heat pumps no furnaces, I am in NC... I started to go down the whole house mitsubishi plan.. went very far down that hole.. to the point of having mitsubishi designers out to the house.. The loads are low enough that tradional high seer, zoned system seems to make the most sense. That combined with the fact that no one in my area has enough experience with ducted and ductless mitsubishi. I have one quote still out on horizontal drilled ground source, but don't have that quote back yet... I am already struggling with the insane quotes I am already getting on the traditional air source route.

  11. Expert Member
    Dana Dorsett | | #11

    Fujitsu RLFCD series mini-ducted systems can be more straightforward and cheaper in low-load homes than multi-splits, if you can find a competent duct designer willing to deal with less-powerful-than-traditional air handlers. Among mini-duct cassettes they have some of the best blower specs.

    Renewability's PowerPipe series are probably the biggest volume sellers, but they have competition. The biggest (both in diameter & length) that fits is the best choice, with the fastest payback. The unit costs more, but the installation labor is the same, and the higher return efficiency more than makes up for the marginally higher up front cost. There are others vendors as well. For apples-to-apples comparison purposes Natural Resources Canada developed a test protocol and maintains a list of models with third-party tested efficiency under standardized conditions. (see attached spreadsheet)

  12. user-471779 | | #12

    I just went as far as I could with the low and high static ducted with mitsubishi... It just that I would need so many units, because of the size of the house. I just decided to simplify... I will have 3 variable speed high efficiency heat pumps zoned properly (leaning toward lennox xc25 which is pretty strong. I am putting in 2 one to one 30+ seer ducted fh09 for a couple of applications.

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