DVHP on Southern Maine Coast
We are building a new home: 2,200 s.f., three floors living, southern exposure, closed-cell foam, R-40 and R-60 +-, radiant heat (I know, my wife insists), HRV, little propane stove.
We are seriously considering an AirGenerate ATI66 for hot water.
Here’s my thinking:
Summer, run a supply duct from second floor, exhaust to first floor living area;
Winter, supply and exhaust from outside;
Shoulders, supply and exhaust from outside.
There are only two of us (we aren’t telling the kids we are moving), so recovery is not a big concern. In the winter if the heat element goes on, we are no worse than a conventional electric hot water heater.
We like the cooling and dehumidification in the summer. We are also considering an on-demand. Thoughts are appreciated.
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Replies
Peter,
I'm not sure what a "DVHP" is. But most of your details seem to concern a heat-pump water heater (HPWH), so I'm going to assume that I don't need to learn this new acronym. I'll just ignore it.
Most attempts to duct air to and from a heat pump water heater with an indoor compressor haven't proven very successful. To take just one of your three scenarios: during the winter, you want to introduce outdoor air to the coils of your heat-pump water heater. But these water heaters are designed for indoor installation, where the temperature ideally stays above 50 degrees F all winter. If you introduce outdoor air at -10 degrees F to the heat pump, it won't be happy.
I don't think the outdoor temperature ratings of the ATI-xx units go low enough to be able to use with the intake air ducted from the outdoors in winter in southern Maine, but during the shoulder seasons you could. Running it with intake air below +20F would be outside the specified operating range in the manual. Not only would it operate at a far lower than specified efficiency, it can damage the thing.
http://airgenerate.xbitech.com/Media/Default/Downloads/atimanual_booklet.pdf (See ambient air temp requirements on p.8)
Ducting the output to the upstairs in summer, and into the basement in during the shoulder seasons & winter, and always taking the input air from the basement is the better bet for both efficiency & reliability.
HPWH - got it!
And thank you for your thoughts on ducting. I thought I might be getting a little too clever for my own good.
Sounds like we should shift back to an on-demand system.
Thank you.
What are your thoughts on the penalty to switching to heating element mode when it gets cold? Any different than a conventional electric hot water heater?
Thanks.
Peter,
Assuming that you have a good place to install your heat-pump water heater -- in Maine, that would be a basement; in Florida, an attached garage -- then it makes sense to leave the unit in hybrid mode all winter long. For most brands of heat-pump water heaters, that means that the unit will usually heat water with the air-source heat pump, and will automatically switch to an electric-resistance element when the heat pump can't keep up with demand.
This is the most-energy efficient way to operate the appliance.
In no case will a heat-pump water heater have a lower COP than 1 -- in other words, it can't use more energy than an electric resistance water heater.
In the winter the cost of the space heating fuel needs to be factored in, since half the heat going into the water is from the conditioned space, adding a modest amount to the load. In much of New England the cost of propane is more expensive than electricity, but it varies from year to year.
But there is no simple model that works regarding how much of a space heating load the heat pump hot water heater imparts. If the water heater is in an unheated basement it lowers the temp of the basement a degree or two, which lowers the heat loss out of the house. The tighter & higher R the house, the less of a heat load reduction there is with it in the basement. And if the water heater is in a fully heated space that reduction in space heating load doesn't happen, and half the heat going into the hot water will be whatever it costs to replace that heat with propane burned in your boiler.
Rather than spending money on duct systems & controls, if you're a showering family the money is better spent on a drainwater heat exchanger, which will both extend the apparent capacity of the hot water heater (meaning it uses the resistance elements less). A 3" x 60" or 4" x 48" or longer heat exchanger will return more than 50% of the heat energy that is literally going down the drain. For a 2- person family with cheap electricity the financial case for it isn't very big, but it still has higher "payback" than any ducting scheme you cook up. But if you're a tub-bathing family the drainwater heat exchanger is useless- it requires both the drain and potable flows to happen simultaneously to work.
Separate subject: Is there any way to talk you out of the R40 closed cell foam?
If that foam is thermally bridged by framing it's only delivering a fraction of the performance, and the HFC245fa blowing agent is a very serious greenhouse gas (about 1000x CO2), and at high-R values it's not clear if it ever gets back to dead-even for environmental impact on energy use savings. It's expensive too. From a lifecycle ROI point of view you'd probably do better to spend at least half the foam budget on rooftop PV solar. In a zone-6 ME climate as long as at least 35% of the total R of a wall assembly is outside the first condensing surface (in your case, the interior side of the foam) you don't need anything tighter than Class-III vapor retardency on the interior. So for an R40 wall you could do with as little as R14 on the exterior side of the assembly, R26 in cheap fiber on the interior side, with nothing more vapor-retardent than standard interior latex paint as the vapor retarder. With some manufacturers' closed cell foam that R14 can happen with just 2", which is the maximum thickness that closed cell foam can be safely installed in a single pass too.
With a better description of the wall stackup it may be possible to come up with more specific performance & cost tweaks to optimize it a bit.
For some reason the box to check for "helpful" seems to have gone away. Maybe I was the only one who used it. So it seems I have to write an actual comment rather than just upvoting what Dana and Martin say even though I have little to say other than that you should listen to them because they know what they are talking about they are giving good advice. Particularly about the spray foam, and the drainwater heat exchanger, even though that wasn't your question.
As for the HPWH ducting, the winter is clear: Put it in the basement and don't duct it at all in the winter. It won't be any worse than resistive heating in the winter, even though it's not clear that it will be any better either.
If you want to get creative in the summer, you could, but don't pay too much attention to my advice because I have an unfortunate soft spot in my heart for overly complex engineering solutions. If you position it to have really short duct runs, you could duct both the intake and output to the main living space for the summer. But cooling and particularly dehumidifying the basement in the summer is a good thing, sometimes more important than the main space. So it's only worth it if it's fun for you to experiment with. Two two-foot ducts aren't a major expense.
Thank you all for your insights.
AirGenerate has the ATI66DV which has dual vents. I have contacted them to find out their limitations and report back.
Dana, thank you for your thoughts. RE: closed cell, I had read about flash and bat but heard that there can be a moisture problem. Sounds like you don't think so, so I'll revisit.
Re: PV, this was my preference but our roof line isn't conducive and payback was something like 30-years. The way the house is now designed, we might be able to get 100sqft.
Re: Drainwater heat exchanger. Great idea. Should we go with Ecodrain or do you like another product.
Thank you all so very much!
Peter Whitman
Flash and batt is a moisture problem only if you don't air seal it well on the interior side and don't provide at least the minimum foam-R to total-R ratio for the climate. If you use cellulose it gives you some margin on the R-ratio, but it's better not to push it.
The effectiveness of the horizontal EcoDrain isn't nearly as good as the gravity film type vertical versions, and doesn't have a long track record by which to measure it'd degradation rates. There are multiple vendors of gravity film types most of which are made in Canada. Most vendors will sell direct from their websites to US customers but at a high retail price (so they don't end up competing with their box-store distributors on price. The best "deal" on them I've found in the US is to open an account with EFI (headquartered in MA) with a credit card via phone, and buy them at the distributor's wholesale pricing they list on the web: http://test.efi.org/sites/default/files/power_pipe.pdf
The 3" x 60" and 4" x 48" versions sold through EFI perform at better than 50% energy return at 2.5gpm under standard test conditions set by National Resources Canada. See: http://www.nrcan.gc.ca/energy/efficiency/housing/home-improvements/12356 Longer & fatter are even higher performance (and they make them pretty long up to 10 FEET long: http://www.renewability.com/order_powerpipe_online.html ), but there begins to be a water pressure drop penalty for VERY long ones. Versions from other vendors with a single-slinky outer tube have higher pressure drops than the split-manifold flattened 4-slinky wrap versions EFI carries.
At lower flows the heat recovery efficiency increases and at higher flows it decreases, and 2.5gpm is actually on the high side- most standard showerheads deliver about 2gpm at typical 30-50psi water pressures, and with a 3 x 60 you'd probably be getting something like a 60% return with a low-flow showerhead See: http://terrylove.com/images/power-pipe_recovery.jpg and: http://www.gfxtechnology.com/EFF.pdf
But they must be mounted vertically, and the output needs to feed both the hot water heater and the cold side of the shower to hit those numbers. Since at New England incoming water temps the output of the heat exchanger would still be in the 70sF (even in summer), it's sometimes easier to just feed the entire cold water distribution through the house. The worst that can happen in that configuration is that somebody in the house drawing a glass of cold water while someone is in the shower gets a slug of effectively room-temperature or very tepid warmish water before the cooler water arrives.