Considering ERV pre-heating options (maybe cooling too)
Hello from a long-time lurker but first-time poster!
It’s hard to express how helpful and inspiring this website is to a non-professional like myself. I’m particularly taken by the generosity of spirit shown by all who contribute thoughts, ideas, and experience with such passion. It seems that everyone wants to do their part to help heal our hurting planet–buildings are a huge part. Thank you, thank you.
Okay…end of speech. Some details about our build plans.
We’re designing (with very capable PassiveHouse certified architects) a 7,000 btu/hr-heat-loss small home about one hour west of Ottawa, Ontario. Slab on grade with 12″ EPS above(!) the slab, ICF wall with total 13″ EPS, interior service cavity 2×4 with batt insulation. No concrete floors in this one-storey home (too hard under our feet.) Roof with plenty of cellulose blown in. Triple-pane fiberglass windows (Fibertec). ERV by Zehnder/Paul NOVUS model.
We’re off the grid in a rural area (almost $75K to connect!) so DHW, cooktop and generator is propane-fired. We’ll have solar thermal DHW pre-heat connected to a tank similar to Bradford White EcoStor2 with gas backup, and power vent. Water-to-air coil will boost the post ventilation air temperature (along with a boost in CFM delivery to the rooms) when needed. Beyond this, convection hard-wired wall heaters here and there for the really cold, cloudy days.
Here’s the question:
We’re looking at ERV pre-heat options that don’t involve too much electricity or recirculating defrost modes that compromise efficiency.
Ground loop is nice but $$$ (install and volume of propylene glycol involved). We’d appreciate some feedback about the following concept….
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For winter pre-heat:
A liquid-to-liquid heat exchanger external to the DHW (EcoStor2-by Bradford White) uses the tanks HW to warm a glycol loop in winter to preheat intake ventilation air. This would be a short well-insulated loop between the DHW and ComfoAire ERV.
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For summer cooling:
That same heat exchanger would, for summer cooling, be also be connected to well-temperature water via a valve. The glycol would pick up the cool water temperature and help cool the warm, humid ERV intake air. The condensate would be channeled away by draining it through the washing machine’s drain in the adjacent wall between laundry and mechanical room.
Pros ‘n cons:
Pros: lower upfront costs, all parts of the system are accessible in case of repair. Less glycol in the system means lower costs to refresh the glycol when necessary.
Cons: help me here….extra pumps compared with ground loop? Not sure. Has this method been tried before? Successes? Failures?
Any thoughts would be wonderful—again, a warm THANK YOU to all.
Richard
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Replies
Richard,
I've lived off-grid for 42 years. My mantra is, "Keep it simple."
The only HRV I would recommend for an off-grid house would be one or more pairs of Lunos fans.
For more information, see How to Design an Off-Grid House.
Is it correct that your goal is to create a combined heating/cooling/ventilation duct system with higher than typical ERV flow rates? With open interior doors, you might find that you don't need this complexity.
So honored to hear from you, Mr. Holladay....
It's gonna be an awfully tight house (< .6ACH@50) and the code here in Ontario requires HRV at the minimum. But, I'll check out "How to Design an Off-Grid House.
For the solar PV, we're designing for a daily demand of approx. 5kwh. We're not big electricity users in general, even now, living on the grid in the rental we've occupied for 10 years... Our monthly bills tell us that our daily average use is around 5 or 6 kwh.
Thank you, Jon.
I guess the principal concern is to protect the ERV from freezing. There are several ways to accomplish this, but if we're on solar with batteries and generator, I'd love to come up with something that doesn't add to the daily electric load, at least on the short winter days that are cold and cloudy.
Using the existing ERV ducts -- (the Zehnder "spaghetti" is one possibility, but our consulting engineer is considering a slightly larger elliptical shaped duct to minimize static pressure in the system) -- to distribute heated ventilation air is the concept. If it needs a slightly higher flow rate during those times, we'd probably set the ERV to "party" mode (or so Zehnder calls it).
But definitely, yes. Complexity is not the goal here. And you're quite right: except for my wife's small writing studio where she works behind a closed door, the rest of the house is open door, and pretty much open plan.
Thanks for your thoughts.
Richard
Richard,
A pair of Lunos fans provides heat recovery, so it shouldn't be much of a stretch to convince your local code inspector that these Lunos fans are the same as an HRV.
While Zehnder uses an electric-resistance heater to prevent frost accumulation in the core, other manufacturers (including Renewaire) use simple controls that circulate interior air through the core periodically to eliminate frost buildup. The Renewaire approach uses less electricity than the Zehnder approach. But the Renewaire approach still uses more electricity than a pair or two of Lunos fans.
Martin,
I will definitely look deeply into the Lunos, which I had not known of before yesterday when you mentioned it. Very interesting indeed.
Richard