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Off-grid house needs… gasp!…air!…gasp!

morphocreative | Posted in GBA Pro Help on

I need some advice…preferably before I cut a bunch of holes in the walls or ceiling and find out that this was a pointless venture.

So the details:

900sq foot one level, mostly open plan, highly insulated and sealed home.
Heating climate…like really… a lot. -40 C is not unheard of for stretches of time. More typical is -20 C

The house is powered completely with PV.

Heat is supplied by radiant slab, a bit of passive solar. I am also putting in a small wood stove in the next few days.

Design loss for the house is somewhere in the 17,000 btuHr….yay!…thats one win at least!

I need air exchange in this house! Stale air…a bit of condensation etc. Nothing horrific, but thats probably because I am constantly opening and closing windows.

Now before everyone starts in about the wonders of ERV or HRV…I simply do not have the power in the dead of winter to run these units. Even the “energy efficient” ones.
A few of them are not even recommended given how cold my climate is.
I really can’t put the bath fan on a timer or humidity sensor as it just sucks too much power from the batteries as well.

Like it or not, “I think” I need to put in a few passive fresh air inlet outlet vent openings.

I guess what I need to know is if I put one opening on one side down low on a wall (positive pressure side) and one up high on the other side of the house…will this move the air through the house and exchange the air inside?

Will this leave the interior in a neutral pressure state? Negative? Positive?

Is there a better configuration in regards to locations, heights etc?

Am I out to lunch?

What about condensation in and around the vents?

Any other sage advice?

For the wood stove I would like to put a small air intake right behind it for combustion air. Would it be ok to place it down low right behind the stove near where it pulls the air into the firebox or does high on the wall make more sense.

Thanks a bunch!

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  1. GBA Editor
    Martin Holladay | | #1

    I have lived off-grid for 39 years. You're right -- you don't want to install any ventilation fans. So open your window(s) a crack when the air is stuffy.

    I'm not sure how you intend to heat your radiant slab during the dark days of winter, when you are unlikely to have much electricity. I predict that you will soon abandon the radiant slab, and use only your wood stove. That's what always happens when off-grid homeowners mistakenly install a hydronic heating system.

  2. LucyF | | #2

    I've seen a really interesting product on the 475 building supply website. It is an HRV but it's a one room HRV that maybe you could install in the bathroom or kitchen - the Lunos eGo Room Ventilation HRV. I think it makes more sense than putting random holes in the wall. It is relatively expensive though. If you don't need it when it's really cold, just turn it off and you could cap with the small box cover lined with foam.

    Martin's method is certainly the simplest and the cheapest way to go and saves the most energy. The Lunos is a high tech solution that doesn't require much owner involvement except to flip a switch. Just an idea to consider.

  3. markgimmeshelter | | #3

    I won't disagree with the substance of Martins comment which is that for off grid homes in the North country wood burning and owner initiated ventilation are basic strategies. But my experience in my evolving off grid home of some 35 yrs is that by the time you size your PV system to a scale which avoids back up generators there is a fair amount of time when you have excess electricity which is wasted if it can't be captured and stored. If you are using hydronic based backup heating invest in ECM pump technology like the Grundfos Alpha which can reduce electrical demand for circulation by more then 50%. For electrically efficient powered ventilation with heat recovery consider the Lunos available through 475 building supply or the knockoff called Twinfresh Comfo In any case occupants of off grid homes have to have an informed relationship with their battery and inverter.

  4. SouthGeorgianBay | | #4

    Just throwing a few ideas out there...

    Perhaps a small DC fan in a washroom with its own PV panel that would stand alone from your main system. Although intermittent, it could help, especially in late winter/early spring.

    Is a small wind turbine, to supplement your PV, a possibility?

    Maybe not practical for you (e.g. house is on bedrock, etc.), but something like UltimateAir's water-to-air coil installed at a fresh air intake could mitigate some of the comfort and condensation issues with just dumping -30 air into your house. The pump would draw around ten Watts and bring your supply air above freezing. With the pump on a simple switch low battery voltage issues could be managed.

  5. morphocreative | | #5

    WOW...that was fast!

    The slab is heated with a smallish propane boiler (don't get me started on the topic of efficient boilers! (yes they make physically small/energy efficient boilers...but not for equally small and energy efficient homes) The slab has worked great for 3 years now. The PV system is sized just about right for all my extra in the winter to add in fancy pants HRV's etc., but it works fine.

    I will see what happens with the stove and how it fits into the system...I sized it on the small size so I don't cook myself out of the house.

    Thanks for the link I will check-em out.

    Yes, I plan on replacing my circ pump soon actually. This will free up a bit of amps for sure. The circ pump is the second highest draw on my system after the fridge. (Actually, the top is my lovely wife's hair dryer...which she kindly uses sparingly!)
    Thanks for info about the powered vents as well.

    I have actually used computer cooling fans for doing several things around here and have thought about dropping one into a vent and wiring up a little panel.
    Wind turbine has crossed my mind...left it..entered it etc.
    The cost doesn't fill me with joy...the maintenance doesn't either. The sound is a bit annoying to top it off.
    If I could just figure out a way to harness falling snow!

    So, nobody had any advice about placement of vents...

    The opening the windows thing is getting a bit old around here....the ice build up...can't get em to shut again etc. etc.

  6. kevin_in_denver | | #6 5-10 watts This $60 pump will save enough energy to power this ERV: 5 watts

  7. Expert Member
    Dana Dorsett | | #7

    A 4" PVC earth-tube trenched in for 50-100' and coming up through an unheated portion of the slab can be the ventilation-equivalent of cracking a window. It has to be sloped away from the house and have a drain hole where it 90s up to come up above grade + snow. and protected from precipitation. (You don't want it to collect mold or ice anywhere in the tube- a lot of earth-tubes become mold-breeding grounds at one season or other if they don't have sufficient slope & drain provisions, and retain moisture.)

    A 4" vent near the top of the house with a venturi type cap (not a turbine) can give it both a stack effect and wind-powered boost without the squeak & rattle of a turbine. Eg:

    A 4" cheapo low pressure plastic ball valve runs about $50 USD, and is probably worth it for throttling back the flow when desired.

    One or the other will probably do you.

    If you try out the 4" venturi driven stack first you can go smaller with the earth-tube, or maybe just install a 2" wall-tube with a plastic ball valve for adjusting the flow, and forget about any earth-tempering. But at -40C even a little bit of earth tempering is probably going to be worth it.

  8. morphocreative | | #8

    HAHAHAHA...oh man alive you are taking me back to the days when I contemplated an earthship and earth tubes for air me back about 20 years.....sigh.

    Ya..not going to happen....but thanks for the thought. I love the idea...they are a neat concept for sure.

    Thanks for the link! Interesting.....

    Cool...I haven't seen this one yet....
    I will poke around and see whats what.


  9. fitchplate | | #9

    Did you look at the Vents-us products ... single room HRV's/ERV's, low watts and much better price than Lunos. They have solar powered units, too. Might have to order from Europe.

  10. fitchplate | | #10

    Dana ... I think Keith thinks your teasing him. I have been looking for exactly such a passive pressure-creating vent cap. I also found this one:

  11. morphocreative | | #11

    Yes I had a look at the Vents-us.
    I like the concept for sure...simple enough. It certainly fills the void in the market.
    Not sure if I have the power for it though.

    I know, I know..."it's only 5w"!
    But when you are off grid...5w running for hours and hours and hours at a time adds up. Then you get added complications all of a sudden.
    I need to pull out the generator, gas it up, get her started, listen to the thing rumble away, re gas it up, change the oil, clean and re-oil the filter etc.

    Its a pain in the arse!

    much better and easier to just not add extra demands on the system.

    Nope don't think you are teasing...
    Just that the earth tube thing gave me a flashback.

    Well...that and the idea of digging yet another trench around here sends shivers up my spine. If I never have to dig in this hard packed clay again I will die a happy man.

    I'd sooner throw some money at the problem and add to the PV system than break out my pick-axe again.

  12. fitchplate | | #12

    Keith ... look more closely at the catalogue I sent you (URL). Vents-us has a model with its own solar panel as part of the unit.

  13. morphocreative | | #14

    Got it...thanks.

    Unfortunately/fortunately....depending on my mindset that day.... I am in Canuckistan. Home Despot in Canada does not carry them and Home Despot in the US does not ship to Canada.
    First world problems!

  14. Expert Member
    Dana Dorsett | | #15

    The earth-tube concept came to mind only due to the freeze-up issues you might otherwise run into during coldest weather. The 2" wall-port with a ball valve control probably won't have freeze-up issues sufficient to keep from closing it. Even if it did freeze up a 2-incher is small enough you can probably heat it up enough by other means to be able close it even if it gets pretty iced over and stuck.

    Venturi type stack caps like Empire's Syphon series provide substantially more draw than stack-effect alone, and are a cheap & reliable way to drive the ventilation with out the squeak & rattle (and eventual break down) of a turbine head. With even a 4mph wind a 4 incher pulls 65cfm, 3900 cubic feet per hour. In a 900' home that's a complete air exchange every 4 hours or so in fairly light wind conditions. That 65cfm does not count any additional passive stack effect volume you build in by making your intake port low and putting the stack at 3' or more above the ridge of the house. In higher winds the draw goes up exponentially, which is why you would want a valve to be able to throttle it back when the winds are really roaring.

  15. fitchplate | | #16

    I have (don’t tell Martin) 4 X 4” passive airlets which (as I think Martin might have pointed out once) can get “air locked” and not deliver a flow of any volume without a mechanical exhaust operating (i.e. stove hood externally vented; bath fan). Rather than use the mechanical exhaust-only hybrid approach, in order to save some juice I would like an effective passive draw and these venturi stack caps are promising … particularly when I hear your (Dana’s) numbers on flow.
    This European design looks interesting too: but more complex and expensive.
    Keith … I am back and forth (Buffalo-Toronto) monthly and could bring a couple of Twin Comfo’s across if you order them online for HD store pickup.

  16. kloopster | | #17

    Wood stove burning, small crack in two spaced-apart windows, good socks, a nice sweater and either vodka or a thermos with hot coffee close at hand. I employed this technique to some success in Russia and it worked down to -40 C or so. The earth tube is also a very good idea, and all that digging will be sure to keep you warm and your lungs filled with fresh air.

  17. dfvellone | | #18

    Discovered this thread when searching for a solution to my almost identical situation. Wondering what approach Keith Ahlstrom took and - 4 years later - what his conclusions are.

  18. GBA Editor
    Martin Holladay | | #19

    It's hard to beat the open-your-window-a-crack approach.

    For more discussion on this issue, see "How to Design an Off-Grid House."

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