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Community and Q&A

Phased Construction-Two Electric Tankless Hot Water Heaters?

Inger Peters | Posted in Mechanicals on

Climate Zone 3. Distribution water at tap temperature estimated to be 50-55 (finish water temp as it leaves the water plant). I’m phasing construction: Phase one: 1st floor shop/garage. One shower/one lavatory sink/one shop sink. Phase 2 is the upstairs living quarters that are estimated to be finished out within next 3-5 years. The electric service to the shop/home is 2/200 amp services. We are planning one panel for the 1st floor shop and one for the 2nd floor living quarters. ( I say living turned into a 2000 square foot 3bd / 2bath space-basically a home!).

I’m considering wiring the 1st floor utility room with enough electrical capacity to support two tankless water heaters. The utility room is directly adjacent to the 1st floor bath/sink/sink and was sized space wise for a conventional tank heater AND a water softener. Plenty of room if occupancy loads changed for future owners. (We are only two adults and occasionally host a few house guests)

1st unit would be sized to handle the
primarily 1st floor
1 shower
1 bath sink
1 shop sink
maybe the upstairs dishwasher (2nd floor future-close proximity to 1st floor bath)
Even when house is fully occupied it will be rare/almost impossible that all fixtures would be in use simultaneously. It would have to be done “ON PURPOSE”

2nd unit would be sized to handle
all 2nd floor
2 showers
1 bathtub
2 bath sinks
1 kitchen sink (close proximity to 1st floor bath)
1 clothes washer (we run only run whites/bedding in HOT).

The 1st floor will only be utilized as hobby shop for the next 3-5 years. The 2nd floor space (Phase 2) will be completed on a cash basis however all the hot and cold plumbing lines will be in place (in the 1st floor ceiling/2nd floor floor space) prior to insulation and drywall of Phase 1-1st floor.

Any thoughts on this approach? Gas is not an option I want to pursue. Other alternative would be to go traditional tank style, size for max capacity (or not-maybe just buy a mini), turn it to low when not in use. If the two tankless are viable, would it be wiser to install the 2nd unit upstairs in the Laundry/utility room? There would be space for it in a broom closet sized space. I’m looking at Marey or Rheem 27kW for the 1st unit. I apologize for the length of post…but the devil is in the details!

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Electric tankless water heaters aren't very environmentally friendly -- they put a large load on the electricity grid. Tank-style heaters are much more grid-friendly, since they draw fewer amps.

    Tank-style heaters will allow you to downsize your electrical service, reducing your wiring costs.

    Since you are in Climate Zone 3, you might want to consider installing a heat-pump water heater. A heat-pump water heater will lower the temperature of the room where the heater is located.

  2. Inger Peters | | #2

    Thanks..I'll read up and follow up!

  3. User avatar
    Dana Dorsett | | #3

    Standard tank type heaters sized for the largest anticipated batch draws (usually a bath tub) would be the right default. If placed in a large enough room to manage a heat pump water heater (HPWH), serious consideration should be given there.

    Depending on the local utility, regulators and grid operators, it may be worthwhile to buy a "grid aware" or "smart" "wi-fi" water heater, which has the potential to be put to paid use in supplying grid services such as frequency & voltage controls, etc. While retrofit controls can be installed to turn your water heater into a grid-battery, it's easier/better/more flexible if it comes pre-equipped with the smarts. The largest utility in Vermont Green Mountain Power has a program for retrofit water heater controls ( ), but in the PJM grid region there are also third party "virtual powerplant" companies that will pay homeowners to allow them to use of the water heater for ancillary services, bidding the aggregated water heaters under their control into PJM's ancillary services eg:

    If such a program already exists in your neighborhood, use it! If it doesn't, it might be coming. But there may also be dumber time-of-use rate options to be easily exploited with a smart water heater, which also greens-up the grid.

    Tankless electric water heaters are the opposite of a smart water heater- they impose HUGE intermittent demands, and the power use can't be automatically scheduled or adjusted to even provided peak reduction/demand-response, let alone more valuable grid services such as frequency & voltage control. They also require much more grid infrastructure to support those 10 minute bursts of extreme power load during a shower or tub fill.

  4. User avatar
    Jon R | | #4

    Since building codes are already used to implement energy policies, IMO, they might as well extend them to ban electric tankless and electric tanks without smart controls.

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