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Community and Q&A

Heat-pump water heater: Air volume, etc.

Tobeywan | Posted in Mechanicals on


Lots of good information here about Heat Pump Hot Water Heaters, thanks! My situation is unique, so I thought I’d get some feedback before I pull the trigger and purchase 2 Stiebel Eltron 80-gallon units to install in place of 4 40-gal resistance hot water heaters servicing 4 2-BR units in a multifamily building.

I am thinking of installing these units in a small mechanical room (5’x8.5’x8′ tall) that also houses two ~80 gallon pressure tanks for the well. It will soon also have a water treatment system with salt canisters and green sand to deal with arsenic. I would replace the existing door with a locking security screen door to connect the air space with the staircase (8’x4’7.5′ tall), landing (6.5’x8’x8′ tall), and hallway (4’x12’x8′ tall) immediately outside the room. Then there is a fire door that I would install a specially designed louvre system into to connect with the front entry area (5.5’x5.5’x7.5′ tall). By my calculations, this would give me almost exactly 1600 cuft of air volume. I thought I would also add another louvre system to the fire door at the top of the steps to connect with an upstairs hallway, landing, an additional staircase and an additional hallway on the third floor. I didn’t measure this space, but it is at least 1000 cuft more, perhaps 2000cuft including the third floor.

I will be installing mini-split heat pumps in each of the units (total 6, including 2 1BR units on the third floor; LED lighting; and eventually solar panels once I have the load reduced considerably. Paying for all of this by including utilities in the rent going forward.

The mechanical room shares a wall on both sides with a bedroom in each of the ground floor (1/2 basement) units. There are resistance base board heaters in all the hallways.

1. Would you install these units in this space to be shared by two separate units? Anything you would do differently?

2. Do I need to be concerned about noise in the bedrooms?

3. I would plan to set the temperature very high (149 degrees is what rep said on the phone), put in mixing valves, and install low flow shower heads. Anything else I can do to make sure tenants have enough hot water at peak times?

Any other thoughts or considerations are welcome. Thanks in advance!


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Would you install these units in this space to be shared by two separate units? Anything you would do differently?"

    A. Your mechanical room measures 340 cubic feet. The installation instructions forbid the installation of one Stiebel Eltron Accelera 300 water heater in a room smaller than 800 cubic feet. Your mechanical room is less than half that size -- so it is much too small for one water heater, must less two water heaters.

    As the instructions note, failure to follow installation instructions will void the factory warranty. You assumption -- that installing a screen door changes the definition of a room -- is not based on fact.

    Q. "Do I need to be concerned about noise in the bedrooms?"

    A. Possibly. The answer depends on the construction method of the partitions surrounding the mechanical room and the sensitivity of your tenants.

    Q. "I would plan to set the temperature very high (149 degrees is what rep said on the phone), put in mixing valves, and install low flow shower heads. Anything else I can do to make sure tenants have enough hot water at peak times?"

    A. The higher the water temperature, the lower the COP (efficiency) of the heat-pump water heaters. There is no way that two 80-gallon heat-pump water heaters will ever match the hot water output of four 40-gallon electric-resistance water heaters.

  2. STEPHEN SHEEHY | | #2

    My biggest concern is noise. We opted not to install one in our mechanical room after listening to one, and it would have been far from the bedroom. Sleeping next to two HPWH is likely to be unpleasant. And that means unhappy tenants. I'd spend the $$ on pv and just replace the existing water heaters when necessary.

  3. user-626934 | | #3

    The newest models from Rheem are SIGNIFICANTLY quieter than any other model we've seen/heard installed...and we've heard ALL of them.

    160 gallons of stored hot water at 120F-125F for 4x2-bed apartments should be more than enough...especially after adding low-flow fixtures.

  4. NormanWB | | #4

    The Rheem units can also be ducted.

  5. Tobeywan | | #5

    Thanks for your reply Martin. When I spoke to the rep at Stiebel Eltron he suggested that a screen door would connect the air spaces. He didn’t say anything about voiding the warranty. I realize its not an ideal scenario. But, what is the worst that could happen?

    As far as hot water output, would installing two of these megaboost units resolve that issue at peak times, if it indeed presents?

    Totally open to this not working, but want to make sure I explore all the options before I abandon the idea. In the end I need to make space in this mechanical room for the water treatment system and would prefer to do something that lowers the electrical load (even slightly) rather than consolidate the water pressure tanks, since they are bladderless and not leaking — they ought to continue working fine for years. The water heaters are older models and will need to be replaced at some point soon anyway.

    The only other idea I’ve had would be to add an insulated sunroom/entry way at the other end of the hall (faces due south), to house the units. This would give the added benefit of additional roof space for PV...but the quote for the slab alone is $3800.

  6. GBA Editor
    Martin Holladay | | #6

    OK, maybe I need to modify my answer.

    I don't know whether there will be enough movement of heat and air through the screen door (without a fan) to prevent the temperature in the mechanical room from dropping below 40 degrees F. It's interesting that the Stiebel Eltron dealer is endorsing your approach. If I were planning to proceed, I would ask the Stiebel Eltron dealer to put a statement in writing to the effect that this installation does not violate the Stiebel Eltron installation requirements or warranty terms.

    I know that the first-hour rate for these water heaters tends to be lower than for gas heaters or even electric resistance heaters, but I trust John Semmelheck. If John Semmelheck says that the capacity of these heaters is adequate for your apartments, I stand corrected.

  7. Jon_R | | #7

    The Rheem HPWH above allows the use of a duct, which will improve air flow (causing less temperature differential and more efficiency).

  8. BillDietze | | #8

    I don’t know about screened doors, but the discontinued GE HPWH allowed installation in tiny rooms if there was a louvered door with sufficiently large louvered area. I believe they allowed installation in otherwise too small a room if the louvered area was greater than 240 sq. in. Putting two units in such a room seems a stretch.

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