Hybrid (Heat Pump) Water Heater in a finished basement

Discussion in 'Water Heater Forum, Tanks' started by skylarsutton, Apr 9, 2020.

  1. skylarsutton

    skylarsutton New Member

    Jun 10, 2018
    NE Ohio
    My tank is on it's way out and I need a new electric water heater. I'm between a traditional electric tank or a hybrid (heat pump) tank. Financially there's no question, the hybrid is the better choice and pays for itself in a few years.

    The tank will be in a mechanical room with the furnace, so there will be residual heat it can pull from in the winter (we're in NE Ohio). My concern is if / how badly it will chill the rest of the basement. The mechanical room is unfinished but the other side of the wall is finished and has my media room. It's already pretty chilly down here depending on time of year (I've seen it hit 64 while the rest of the house is 70), so I'm concerned that the energy savings will just be offset by me running my electric fireplace more. I do have a dehumidifier down here and getting rid of that would increase the energy gain as well.

    Does anyone have experience with hybrid heaters in or near a finished space?
  2. Dana

    Dana In the trades

    Jan 14, 2009
    Unless you are using a LOT of hot water (way more than a typical family of 4), during the dry-air months of winter it won't lower the indoor temperature in a 500+ square foot basement by more than about one degree.

    In the humid-air summer a large fraction of the heat going into the tank will be the "heat of vaporization" of the humidity it is taking out of the air, with even less of a temperature drop to the air.

    With or without the heat pump water heater, if any of the walls are NOT insulated to current code minimum (=R15 continuous insulation trapped to the foundation walls with furring or a studwall) it will be well worth it, at least on areas that are unfinished. A 2x6/R19 studwall tight to the foundation wall would also meet code from a thermal performance point of view, but runs a high risk of creating a moisture trap and mold-farm. Insulating and air sealing the foundation walls takes a good chunk off the dehumidification load in summer, and ;lowers the space heating load in winter.

    A middle path that still meets code on on a U-factor basis that would have low moisture risk in a US climate zone 5A climate (=the northern half of OH) would be a continuous layer of 1" foil faced polyiso foam board trapped to the foundation with a 2x4/R13 studwall, with NO interior side vapor barriers (kraft faced or unfaced batts are OK, foil faced batts are not.) Stop the polyiso a half-inch above the slab (or above the high-tide mark if the basement has history of flooding) to keep it from potentially wicking ground moisture or minor leakage into the foam. Install an inch of EPS (not polyiso, not XPS) under the bottom plate of the studwall rather than on the slab to keep the wood above the indoor air's dew point in summer, and as a capillary break between the potentially high moisture content slab and the wood. With the EPS it would not have to be pressure-treated.

    The studwall is not structural, not carrying the weight of the house, and any density EPS is adequate for bearing the weight of an insulated studwall with gypsum board on the interior. A minimum of half-inch gypsum board is required for fire codes in most areas, though some jurisdictions relax that if the foam is foil faced polyiso (foil face EPS would still need it). If the batts are unfaced (or even if kraft faced) the wallboard should to be painted with at least one coat of standard latex primer to provide at least some vapor retardency toward the interior. With an all foam + furring approach the wallboard would not need to be painted in unfinished.

    The band joist & foundation sill also need to be insulated, but using an inch of UN-faced 1.5" EPS foamed in place (cut it a half-inch narrower than the space to allow room for can foam to expand and seal), with R15 rock wool batts trimmed carefully to fit snugly using a batt knife or bread knife. That allows the band joist & sill to dry toward the interior, without excessive wintertime moisture uptake. Any ledge at the top of the foundation needs to be covered with the 1.5" foam (the same thickness as 2x lumber) extending toward the interior over the top of any wall foam & studwall.

    There are multiple vendors of used and factory seconds roofing foam and other foam board operating in NE OH, at well below box-store prices. Used fiber faced roofing polyiso should be derated to R5/inch from a design point of view, even though some products still perform somewhat better than that even after 30 years of service. (I did my basement using 3" reclaimed roofing polyiso & furring through-screwed to the foundation with masonry screws, mounting the wallboard on the furring.) Used XPS needs to be derated to R4.2/inch, since the labeled R is allowed due to the increased early-years performance provided by the HFC blowing agents used. XPS is only warranteed to 90% of initial performance at 20 years, but at full depletion runs R4.2/inch, the same as EPS of similar density. EPS doesn't have that issue. Most roofing EPS is 1.24lbs per cubic foot "Type-VIII" goods, at about R4.17/inch, but Type-II (1.5lbs density) and denser is good for R4.2/inch. So an all-EPS or used XPS solution would take 4" to meet current code minimums.

    Many foam reclaimers advertise here.
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