Retrofitting an Air Handler / Water Heater
Hello. Our home, off-grid southwest of Ottawa, Ontario (-13F design temp), has a heat load of 9318 Btu/h. The air leakage is .57ACH50 for the 1381 conditioned square feet, mostly open plan on one floor. Layout image is attached below.
Here’s the story: In the five years since we built it, the house has required heating between late November and the first week in March–only early morning and late afternoon on sunny days, and maybe mid-day when it’s cold and cloudy. We hard wired some radiant panels (1.950 kW) to pass inspection, and find that a total of around 3kW of resistance heat satisfies our heating needs on very cold days.
Our lithium batteries, from a 2013 Chevy Volt, enjoy the rapid discharge, but are recharged by a propane generator that uses almost 10x the Btus in the propane than the heaters produce. Kind of a 10% AFUE.
It is glaringly clear to me now that we’re burning through obscene amounts of propane and getting virtually nothing for it. Back in 2017, the only design we got from our energy consultant was for an in-floor radiant heating setup with two tanks in the small mechanical room, based on the Polaris, but modulated down to 91F in the second tank. The design looked like vintage Rube Goldberg, and our plumber couldn’t even do an estimate for it–he found it impossible to understand.
A possible solution (or two)?
I read Martin Holladay’s piece about the Dettson Chinook low-Btu furnace (https://www.greenbuildingadvisor.com/article/finally-a-right-sized-furnace) which struck me as a possible retrofit. Since our mechanical room is centrally located, I figure that a supply duct to the adjacent living room wall would be in effect a single point source.
But, there’s our wonderful Polaris water heater with two connectors for a “heating system” just waiting to be used….
Could a hydronic air handler connected to the Polaris be worth considering? Given our heat load, finding one small enough is a challenge. I imagine the installation would be less cumbersome than a Dettson Chinook Compact, since there’d be no additional venting required of the air handler.
Oh, and we’re not all that concerned about air conditioning since, thanks largely to our 38″ roof overhangs, no sunlight gets in the house from early June to late September.
Thank you for reading this far. And, thank you in advance for any insights/wisdom/advice you choose to share.
Richard, in gratitude.
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Replies
Congratulations on your high-efficiency house.
I don't know what a Polaris is. But if you're burning propane for your energy source, burning it directly is going to be the most efficient way to get heat.
Hi there,
The Polaris is basically a domestic water heater that can also be connected to a water-based heating system. Typically, and particularly in our case, it was suggested to us as part of the (very complicated) floor radiant proposal. Here's a link: https://www.hotwatercanada.ca/polaris/
OK, yeah that would work well with hydronic convectors. The 94% efficiency is good, you won't lose any efficiency using it for heat.
The question is whether you want to run it through a heat exchanger, or directly. If direct you'd have to use potable water plumbing throughout. You'd also have to take precautions to prevent bacteria from growing outside of the heating season.
Thank you for your comments and ideas--they're very helpful.
I'm considering a fan coil installed on the mechanical room's ceiling, facing a through duct to the living space. The design would use a heat exchanger (preferred in my jurisdiction) to separate the potable side from the heating side. (The Polaris heater, my only combustion device uses outside intake air.)
Your mention of bacteria growth: I understand that a timer operating the circulating pump of the fan coil side of things every so often keeps growth down. Alternatively, UV light? (Sounds fancy and complicated....)
Can you help me with the following?
I'm trying to understand air distribution from what is a single-point source. If the FCU is running in long stretches at say a starting speed of 200CFM, its output should fill our 11560 CF space in roughly an hour. Given that the envelope is so tight, and the space is mostly open, wouldn't that result in consistent temperatures, given enough run time? At CFM50, our blower-door test indicated leakage of 111 cfm/minute, well below a fan coil running at 200CFM.
A fan coil with a variable speed ECM motor could modulate down to where the temperature is maintained over time, right?
What am I missing here? Am I basically on the right track?
Again, I so appreciate your willingness to consider this for me, a rank amateur who wants to cut down on our propane use. Thanks!
What you're missing is that the house is losing heat continuously, so you need to be providing heat faster than it's lost.
The heat transfer from air flow is 1.08*(temperature difference in F)*(Air flow in CFM).
So let's say your fan coil produces air at 100F, your rooms are at 70F and your fan coil produces 200 CFM. That gives 6000 BTU/hr or about 1.75kW. So that's less than you need.
You also have to worry about heat distribution. There's a reason that houses use ductwork.
An air handler is very doable. Check out the equipment from First Co. I think they call what you are considering, Hydro air. What you are considering is common in the US warm climates.
If you have existing forced-air ducting throughout the house and want to swap in a hydronic air handler, your best bets are either a First Co VMB (supports cold water cooling and hot water heating) or an Enerzone CAH (hot water heating only). Given the efficiency of your house, you can size down on total CFM.
If you are running new ducts for this project, consider the Hi-Velocity HE-Z which works with 3-4" diameter ductwork. It's small enough to knock through a 2x4 with a hole saw so you can retrofit more easily in a house that doesn't have easy access points for standard low air speed duct work.
Seems to me you have chosen to live off the grid but are trying your best to live like you are on the grid. I think forced air blowers and heat pumps simply require more electric than you can produce and store reliably.
If propane is your generator fuel, I would think a vented propane heat that requires no electric would be what you want.
Can you really live off grid without a wood stove?
https://www.amazon.com/Ashley-Hearth-Products-Lightning-Regulator/dp/B07XTBV8SB/ref=asc_df_B07XTBV8SB/?tag=hyprod-20&linkCode=df0&hvadid=385309235643&hvpos=&hvnetw=g&hvrand=8806491502381836008&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9022854&hvtargid=pla-848846895294&psc=1&tag=&ref=&adgrpid=75980285662&hvpone=&hvptwo=&hvadid=385309235643&hvpos=&hvnetw=g&hvrand=8806491502381836008&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9022854&hvtargid=pla-848846895294
Walta
Hi, and thanks for your reply all those (5) months ago. (About 15 minutes ago, I received an email from Taunton saying "your question has a new reply!" or some such. Talk about delays.)
Life off the grid would certainly point to using wood for heat. The house we rented nearby (2.5 hours north of Syracuse, NY) for 14 years had a wood stove even though it was grid connected. We loved it. My wife and I are approaching 70 years of age and decided to leave behind the "schlepping" when thinking about our new build. (This was in 2017).
Going with Passive House standards in our design meant we'd not have to heat as much or as often. It has certainly turned out that way.
We do have propane delivered for a generator + stove + high efficiency condensing tank water heater (designed for space heating in addition to DHW). We can connect a small fan coil to the water heater which puts out the needed range of BTUs (9318@design temp + 140%) at the needed range of CFMs for our space. The house is small, slab on grade on a single level and is mostly common area. The fan coil will be located roughly in the middle of the space.
The fan coil's blower operates at various speeds using as little as 4 watts, (up to about 48W) and we're looking for a low wattage pump like a Taco 006e. (The coil's gallon per minute ranges from .39 to 1.65.)
Also, the fan coil is one of those "low temperature" kinds which means we get the required range of BTUs from 115-130F supply water, and return water cool enough to allow the water heater to condense.
It appears this setup will hardly register with our 48-volt batteries/inverter system, and will be a massive improvement over what we've worked with since the house was built (resistance heat with generator running at 1/4 load).
The trick now, I think, is to locate a contractor who would be comfortable installing a not-your-everyday system. I believe it's mostly a plumber's job with a side skill in electrical hookup. The carpenters who built the house could easily install a couple of transfer ducts for the return air, suggested by both a consultant we Zoomed with (Corbett Lunsford) and one contractor we've contacted.
Again, I appreciate your time and thoughts! (People have been so generous and helpful. Among others, Dr. John Straube offered a series of detailed answers to a cold email...)
Cheers,
Richard