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Community and Q&A

Water-to-air heat exchanger for space heating?

Cjb80 | Posted in Energy Efficiency and Durability on

My rough calculation for a heat load shows that I will need about 18,000 BTUs of heat (climate 4C, 2100 square feet, R40 walls, R60 ceilings, triple pane windows, air tight). I have been trying to figure out the best method of heating the home and up until recently I was settled on using a wood furnace as the home will be off-grid.

I was concerned with how to duct and distribute the heat in the home. I intend to have an HRV so there will be one set of duct work for that system already. I was trying to figure out how to share the duct work and I came across the Ultimate Air water-to-air heat exchanger system. They’re claiming that the unit can output up to ~8000 BTUs with their system. Then I started to wonder if I could get a bigger heat exchanger to achieve a higher BTUs… Upon some cursory searches, it appears that larger heat exchangers are indeed available.

We’re planning on using propane for water heating, and this could be an inexpensive method of satisfying the need for space heating (in terms of equipment) in the near term that also doesn’t use much additional electricity. Another alternative would be to use a wood furnace to provide hot water for space heating (via a water-to-air exchanger) and as a hot water source during winter months.

Is there any information about how efficient these types of systems are? How about any thoughts about the reliability and viability of this heating solution?

(on a related note, all of the duct work will be within the building envelope)



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  1. GBA Editor
    Martin Holladay | | #1

    Lately, I've been answering a lot of questions about the best way to heat an off-grid house. I've lived off the grid for 38 years, and many of my friends also live off the grid.

    I strongly advise anyone who is building an off-grid house to choose a heating system that doesn't require any electricity, unless you live near a fast-flowing stream and plan to install a hydroelectric system.

    During the winter, when your needs for space heating are at their highest, you won't be generating much electricity with your solar panels. During the dark months of winter, almost all of your electricity will come from your backup generator, which is a greedy beast that needs lots of gasoline or propane. It's crazy to run your generator just to keep power-hungry fans and pumps operating -- the fans and pumps of your heating and ventilation system.

    There are three ways to heat an off-grid house:

    1. A wood stove. (No blowers allowed.)

    2. One or more propane-fired space heaters with through-the-wall venting. (Choose a model that doesn't require any electricity.)

    3. An old-fashioned oil-burning parlor stove fed by gravity. (This method requires an elevated oil tank.)

    I gave this advice to one family, and they ignored it. After two winters of struggle, during which they fed huge quantities of gasoline to their generator, they disabled their old heating system and installed a wood stove.

    By the way, the best way to ventilate an off-grid house is by learning how and when to open your windows a crack.

  2. Cjb80 | | #2

    I'm with you on this. :)

    My question is primarily concerning using a water-to-air heat exchanger and ducting as a viable means of heating (off-grid or on-grid). Secondly I am curious about using a wood stove for heating the water in the heat exchanger (and potentially heating water as well) . This would take about 300 watts for circulate the water and run a blower, so it seems to be within reason even for winter time solar collection.


  3. GBA Editor
    Martin Holladay | | #3

    First of all, 300 watts is a huge load for an off-grid house. Remember, at this time of year most days are cloudy. On the rare sunny day, you might get 4 hours of good sun (10 a.m. to 2 p.m.).

    I wrote about the heat-exchange coil offered by Ultimate Air in "Heating Options for a Small Home," an article published in the February 2011 issue of Fine Homebuilding.

    In that article, I wrote, "One ERV manufacturer, Ultimate Air, offers a hot-water coil unit (including a heat-exchange coil, an insulated metal cabinet, a Grundfos circulator, and a control unit) for $936. If the Ultimate Air coil is supplied with 160°F water and 200 cfm of airflow, it can provide 8700 Btu/hour of heat—not much, but enough for a small Passive House. Full heat output from the Ultimate Air coil requires 200 cfm of airflow. Because the typical ERV supplies only about 50 cfm of fresh outdoor air, you’ll need a second fan that can provide about 150 cfm of recirculated indoor air to the coil. The indoor air needs to be mixed with the ventilation air from the ERV to bring the total airflow across the coil up to 200 cfm."

    Your original question mentions using a "wood furnace" to make hot water. Presumably you meant a boiler, not a furnace. I hope you aren't considering an outdoor wood-fired boiler -- a type of heater that is inefficient and a high user of electricity.

    It's possible to install a stainless-steel coil in the firebox of your wood stove, and to connect that coil to a hot water tank located one floor above the wood stove. The water will circulate by a thermosyphon, requiring no electricity. Note that this type of setup will void the warranty of most wood stoves and many not be approved by your local building inspector.

  4. wjrobinson | | #4

    More numbers to crunch. I install heat systems though only for my builds and my ongoing customers. I am doing a small system right now for someone where using the water tank for heat as well as domestic hot water was brought up.... Anyway, several considerations later that idea didn't make sense. For example, the lifetime of water heaters is short unless one spends much more than a basic priced unit. And then you have to think about heat exchangers or not and circulators and more. By the time all is said and done more conventioinal methods of heating a home and heating domestic water start make more sense than trying to combine... or pull off a woodstove or solar panel or all together...

    You can have fun experimenting with all that though...

    Add up all your parts including all the plumbing, everything, wire, switching and more.... see what you are getting into.

    Last rules... less is more sometimes.... keep it simple...

  5. Expert Member
    Dana Dorsett | | #5

    What HE said!

    The lifecycle cost of off-grid electricity is extremely expensive, north-of 50 cents/kwh for any application that requires a battery & power-inverters to run 120VAC pumps & blowers. More than half of that lifecycle cost is in the battery- some manufacturers are claiming new-school lithium ion will run half the lifecycle cost of deep-cycle lead acid, but it's a bit early to tell if it'll actually work out that well. If you're lucky you might get your lifecycle per-kwh cost under 40 cents, but don't count on it. (The all in delivered price for residential retail grid power in much of zone 4C USA is about 10 cents, not sure about B.C., but I can't imagine it being much more. You spend more than 5x that for off-grid AC power.)

    Either way, propane + off-grid power is going to be enormously more expensive to operate than a higher efficiency EPA-rated wood stove, with far less hardware and fewer maintenance issues to be concerned with.

    An outdoor wood boiler has pretty low system-efficiency compared to a wood stove too, even without factoring in the off-grid power needed to run the thing. In that climate a house with those R-values won't have much of a heat distribution problem. For reference, I have about twice the heat load on a comparable sized house and an outside design temp of -15C ( that's probably something like 10C lower than your 99% outside design temp), and I can heat all but the over-windowed family room comfortably with just the wood stove in the living room. If I swapped out the ~100-125 square feet of U-0.5 window in the family room for U0.20 triple panes it would heat that room too.

    With a design load of less than 20,000BTU/hr you'd be relying on the thermal mass of the house &/or the wood stove to keep it from turning into a sauna. There are some smaller soapstone stoves in the ~35-40,000BTU/hr range with enough thermal mass to be able to heat reasonably with intermittent firing at a burn rate high enough to be in it's high efficiency/low emissions range. (I'm thinking maybe the Hearthstone Tribute, a little gem of a traditional soapstone half-cube that takes 16" wood that has a ducted combustion air kit option. And no, I don't have any financial relationship with that company.)

  6. Cjb80 | | #6

    Alright, well, I am now realizing that there is a constraint that I haven't mentioned, which is that we would prefer to not have a wood stove inside of the home; but rather in an insulated garage that is outside of the home envelope and air barrier. Therefore, I suppose the two options to distribute the heat would be using a water-to-air heat exchanger or ducting air through a wood furnace.

    I do like the idea of using the thermosyphon with a boiler. It seems that this would be a power saving measure, but perhaps it would be quite a bit of complication when a ducted furnace would be more conventional and simple.

    Anyways, thanks for the feedback!


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