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Community and Q&A

Advice on water heaters and air sealing

Brad22949 | Posted in General Questions on

I am building a two-story house, 24 X 40′. The 1st floor is below grade on 2 sides (poured concrete with 3″ of Roxul below grade and 4.5″ covering any above grade concrete)) and exposed to the south and east . All walls above grade are 2 X 6 sheathed with 1/2″ CDX (semi-advanced framing using double top plates) to be filled with cellulose and covered with 3″ of Roxul. The roof is to be made with 2′ heel trusses to allow 2′ of blown cellulose insulation.

The first floor is to be heated via radiant floor system 1/2 pex in a 6″ thick slab sitting on 4″ of Blueboard with heating supplied by domestic water heater (I wish I had read more GBA blogs before this was poured e.g. excessive thermal mass and perhaps not the most efficient system, but the slab is poured and pipe in place).

I live in central Virginia in the foothills of the Blue Ridge Mtns with winter sun hitting the house from 9am to 4 pm. My widows are mostly south-facing with appropriate overhangs for my latitude. I will have a wood stove (with exterior air intake) on the first floor, I hope to have 2 chases with appropriate fans to draw air from the 2nd floor down to the 1st and install grills in the interior doors to allow airflow. I will install a dehumidifier probably a Santa-Fe, on the 1st floor for summer humidity and have an Tamarack attic fan (not to be used at the same time). I plan on using passive ventilation with bathroom fans and kitchen ventilation run by humidity sensors.

Right now I have just finished the exterior framing and am awaiting the trusses I mentioned. I work full time and am building part time and paying as I go.

My questions:

1) Is a hybrid heat-pump/resistance water heater worth the expense as I would only use the heat-pump 6-8 month out of the year as this is one of my heat sources during the winter?

2) What would be a good air-sealing approach as I am building slowly (tape the plywood, Tyvek only, caulking interior wood joints before insulation is applied, air tight drywall, or some hybrid)? I plan on using Tyvek under the Roxul and addressing this along with siding/flashing/ windows, one wall at a time.

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  1. GBA Editor
    Martin Holladay | | #1

    Although it's possible to heat a house with a water heater connected to a hydronic distribution system, you really can't use a heat-pump water heater for such a system. Heat-pump water heaters heat water extremely slowly -- much slower than an electric resistance water heater, which (by the way) heats water more slowly than a gas water heater.

    In other words, a heat-pump water heater won't be able to keep up with the demand if you use it for space heating.

    There's another problem: a heat pump water heater steals heat from the inside of the house to warm the water in the tank. If you tried to use a heat pump water heater for space heat -- or, more likely, 10 heat pump water heaters lined up in a row -- they would be pulling heat from the room they were located in, and trying to send the heat to the building to make up for what they were stealing. It's hard to imagine a less efficient way to use electricity to heat your house.

    For the system to work, you would have to put the 10 heat-pump water heaters in your garage. But it would still be nuts.

    If you really want to heat your house with an air-source heat pump, you should abandon the hydronic piping and use ductless minisplits or a standard heat pump connected to forced air ductwork.

    If you want to keep your hydronic piping, you should get a gas-fired boiler or a large gas-fired water heater like the Vertex or Polaris.

  2. GBA Editor
    Martin Holladay | | #2

    Concerning your air-sealing question: since you are doing the work slowly, and are building your own house, you are in an excellent position to pay attention to air sealing. That's good.

    All of the approaches you listed help reduce air leaks. If you have the patience, and can afford to invest in a few rolls of tape and a case or two of caulk, there is nothing wrong with a redundant approach.

    If you just had to choose one approach, I would probably recommend that you tape the seams of your wall sheathing with a high-quality European tape. That approach will achieve much better results than depending on housewrap (the worst approach on your list).

    If you tape the seams of your wall sheathing, remember that you still need to pay attention to air barrier continuity at the top of your wall (where the wall air barrier meets the ceiling air barrier); and you have to pay attention to penetrations, including electrical penetrations, doors, and windows.

  3. Expert Member
    Dana Dorsett | | #3

    Your R20 XPS under the slab is kind of overkill on both R-value & expense, given your other R-values. If it's not too late, using 4" of EPS (R16) would be more appropriate, (3" if you skip the radiant), and have a lower environmental footprint due to it's much more benign low global warming potential blowing agent.

    A heat pump water heater will be worth it if the alternative is propane, but it's unlikely to serve your heating needs, as Martin has outlined. It wouldn't take 10 of them to meet your likely heat load but it could take three, maybe even four, depending on your window types & size. The money would be better spent on a couple of mini-splits and grid-tied PV. (Or a small pellet-boiler.)

    The whole house dehumidifier is going to be overkill too, unless you insist on extremely high ventilation rates. A Daikin Quaternity as your primary mini-split would let you set the RH and temperature independently, and would likely take less total power use than a Santa Fe + Tamarack attic fan. The Tamarack would provide sensible cooling only, and would leave you with a very large latent load to purge in the morning most summer days, and the Santa Fe would convert that latent load intto a sensible load curing the day. A Quaternity would be running longer lower highly efficient power-sipping cycles, and would be dumping the latent heat of that dehumidification outdoors, not adding to the sensible cooling load.

    Key to getting the sensible cooling load well controlled would be to limit the size of east & west facing windows, and making them a heat-rejecting/low solar gain type. It sounds like you're on the right track with mostly south windows with overhangs to limit summertime gains.

    Running the bathroom fan with a dehumidistat control would be tricky if you were running Tamarack/Santa-Fe scheme since it would run constantly at night during summer, and slow down the recovery ramp for the Santa Fe. Even with a Quaternity you may want to simply run it on a timer.

    Running kitchen ventilation under dehumidistat control is NEVER the right approach- humidity is a LOUSY proxy for the air pollutants from cooking. Use your nose, and some common sense. In a tight house ALWAYS run the range/oven hood fans when cooking, even if its 2-minutes for frying an egg, and leave it on for at least a handful of minutes after the burners are all off (more if you've been cooking for awhile.)

    On a home with very low heat loads it can be tough to find a woodstove that won't turn the place into a sauna. Using a woodstove with some amount of thermal mass like a soapstone stove helps, but pick one with a max firing rate no more than 2x your design condition heat load, or it'll have to be fired at a high-pollution/low efficiency mode much of the time. Cast iron & steel stoves can heat the place up quickly, but don't average the output well when using a intermittent firing strategy to keep efficiency & comfort up- you'll feel the hot flashes during efficient firings for sure with a 2x oversized cast iron beast, not so much with a soapstone stove.

  4. Brad22949 | | #4

    Directed at Martin Holladay: My intention was to use the hybrid water heater in the resistance only mode during our relatively short and mild winter. I am trying to get an idea of the cost benefit ratio of using the heat pump mode during warm weather (being a backup dehumidifier) and resistance as the at least the water heat source during the winter (suggesting I abandon the $2K I naively put into the radiant floor system).
    Directed at Dana Dorsett
    I appreciate the advice on using the Daikin, is the Tamarak still a good idea, normally we have humid nights during the peak of summer, but we see cool and relatively dry nights during some summer and many spring and fall nights. If we pull in 50-70 degree air at 50-70% humidity for a couple hours in the late evening and we allow the house to approach 80 degrees with a 50% humidity during the day before any de-humidification or air-conditioning I wonder if the attic fan is effective and your recommendation of the mini-split over the dehumidifier still applies looking not just at the height of summer but milder climate control. (To put it in perspective my wife and I live in a 1977 mobile home on the property with only a window unit in the bedroom since I often work nights and sleep during the day, we constantly battle mildew and middle of the day 10 degree increases in temp; we can handle heat, all we want is a ceiling fan breeze and decreased humidity).
    I have in mind Pella's Impervia windows, SunDefense Low-E for N,E, and W sides of the house, and Natural Sun Low E for the South. The site for the house was picked and developed to maximize summer shade from the surrounding trees and maximize winter exposure.
    I like the idea for the timer over the humidity setting for both bath and kitchen hood vent, though it was my intention to always run the kitchen vent hood manually during cooking on my electric stove, just have the humidity sensor set to ensure proper ventilation.
    The wood stove issue has been a point of contention with my wife and I. She has always had one and we do live in the woods, so it is a hard feature to give up. We are considering a soapstone wood stove.
    Just of note I am trying to come from a "does this investment outweight the cost of PV's in a low electricity cost state (of course factoring in house longevity and embodied energy). I had the disadvantage of only recently coming to your site and my original "green building" knowledge coming from pre-Regan solar books, a Fine Homebuilding subscription, and various advice from old-timer construction workers and hippies (hence the overkill on foundation, thermal mass, and slab insulation) so I have started with mistakes. Given my current investment, I am trying to find the best path forward.

  5. Expert Member
    Dana Dorsett | | #5

    In a low-cost electricity state it's not clear if using a nighttime ventilation scheme during the less-stick weather would ever pay off if you've already invested in the high-end mini-split for latent cooling, but it might. Though the Tamarack is a much better sealed system than most, any time you have to make yet another hole in the house it becomes an infiltration-risk.

    If you've been living in a mobile you maybe shocked/stunned/amazed at the performance of a 2x6 cellulose + 3" Roxul sheathing and a couple feet of cellulose in the attic. Your heat losses will be quite modest, and your summertime heat gains will be primarily from east and west facing windows. Think "small" on the wood stove- you may only fire it up during the very coldest of weeks, and then only "just 'cuz", not because a mini-split wouldn't keep up. Either the woodstover or the mini-split could be considered the back-up for the other. (Though in a power outage from a winter storm, the wood stove would be very handy indeed!)

    A 1-ton mini-split may even be overkill on the heating end, but probably not on the cooling end, but you may want to opt for a pair of 3/4 ton units. For a two-floor high-R home it's pretty common to use two heads, one per floor to deal with the seasonal stratification issues, were the upstairs unit runs more during the cooling season, the lower more during the heating season due to air-convection. If that's what you do here only one of them (the upstairs unit) would have to be a high-end Quaternity with the precision dehumidification function. The other could be a more conventional & cheaper heating & cooling mini-split.

    Internet pricing on a 3/4 ton Daikin Quaternity is $2500-3K, whereas you can get a pretty good 3/4 ton without the humidity control feature Daikin, if you like, but there are excellent options from Fujitsu & Mitsubishi as wel) for under $1500. eg:

    DIY installation is possible if you're handy, but read up on it first, and you may want to let a qualified certified tech do the final pump-down, refrigerant charging, and final test to keep the warranty valid. (Tech rates will vary, but with two similar units you can probably get the paid-technician cost for final commissioning well under $1K, or even under $500 if you don't screw up the basic installation.) In snow zones it's good to bracket-mount them on a wall above the snow line, protected from roof-avalanche & cornice-fall by roof overhangs (the rake of the roof is better than under eaves), but they're pretty impervious to rain, etc.

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