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ERV and wood stove

Harbor09 | Posted in General Questions on

We are building an approx. 2400 sqft house in south central Alaska. It will be heated by a large wood stove (with backup direct vent heaters that we hope to not rely on except when out of town). We are exploring an ERV and trying to figure out the ductwork placement. For orientation, the downstairs is open kitchen/ living area, a bathroom, mud room, and laundry room, and the upstairs is 3 bedrooms and one small bathroom with shower. Our concern is that we will be relying on convection primarily to heat the rooms upstairs. We plan on putting some vents in the floor of the bedrooms to encourage air circulation with the living area below for more even heating. 

From what we have seen about ERVs, it’s recommended to vent out bathrooms, laundry, and the kitchen, and to vent in to the bedrooms. The questions is, how efficient is the heat retention of the ERV really? We have fears of pumping cold air into the bedrooms upstairs. On the other hand, maybe it will help if all the hot air is rising? It’s one of those things that will be evident with experience, but I’m hoping someone else has lived out this experience and can advise. 

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Replies

  1. Josh_Dillingham | | #1

    Your situation is very similar to mine. I have an open concept first floor with a living room, kitchen, mudroom, laundry room and a second floor with 3 bedrooms a bathroom and another living room, 2000sft overall. Wood stove down stairs in the living room and a Panasonic intelliblance 100 cfm ERV with supply air coming into the 1st floor living room, all 3 bedrooms upstairs and the upstairs living room and exhausting from both bathrooms and the kitchen. When it is 10 degrees outside and 74 degrees down stairs my thermal imaging camera reads that the supply air from the ERV is about 68-70 degrees which is better than I ever expected. The upstairs temperatures in those conditions are typically about 71 degrees in the upstairs living room and 68-70 degrees in the bedrooms so the ERV doesn't have much impact on the temperature positively or negatively.

    Your situation may very depending on your insulation and air sealing. I have 12" thick double stud walls with dense pack cellulose and 24 inches of cellulose in my attic so my house holds heat well, but regardless of the insulation I don't think it would change the efficiency of the ERV.

    1. Malcolm_Taylor | | #4

      Josh,

      It's always good to hear real world data to confirm what we had been expecting based on the specs. Thanks.

  2. matthew25 | | #2

    Lookup the Sensible Recovery Efficiency (SRE) of the ERV you are interested in. Zehnder is 85-93% depending on the model.

  3. Expert Member
    Akos | | #3

    One option is to get a duct heater for your ERV fresh air supply. This would be installed in the supply duct after the ERV so it would normally only need to run a bit to take the edge off the slightly colder fresh air.

    On some of them you can adjust the temperature so you can also get a bit of extra heat if needed to the rooms. Won't be much heat, but might be enough to take the edge off overnight temps.

  4. walta100 | | #5

    Let’s talk about where all the air going up and out the flue pipe is coming from?

    Maybe they make and you are getting a sealed combustion wood stove but sounds rare and expensive.

    If not operating a wood stove in a tight house means opening a window to provide the makeup air or filling the house with smoke.

    An ERV is not intended to provide make up air to replace what went up the flue and my gut says it will not work very well if you try this.

    If the home happens to be leaky enough for the stove to operate with the windows closed by definition you have plenty of ventilation without an ERV.

    If you said your indoor winter humidity was excessive like over 60% then you definitely need more ventilation. My guess is you will be keeping pots of water on the stove in an effort to get the humidity over 20%.

    Walta

    1. Malcolm_Taylor | | #6

      Walta,

      Is there anywhere left that doesn't require dedicated combustion air ducted to wood-stoves? Our code has for decades.

      1. krackadile | | #11

        The IMC doesn't require a dedicated combustion air duct to wood stoves. I'm looking at a design in TN and they adopted the IMC so they don't require it there.

    2. Josh_Dillingham | | #7

      My wood stove has a fresh air intake directly vented to the back of the stove( and it is a small, $600 model from tractor supply). Without the ERV running my CO2 would be in excess of 1500 ppm (maybe much higher, but I've never let it get above that.)

      Although it is certainly true that even "air tight" wood stoves are not air tight to the levels discussed in building science and having a big hole in your building enclosure certainly has a negative impact on energy efficiency. But those things can be mitigated to a great degree and If you have access to free or cheap wood or live in a rural area where power outages can last days or weeks (both of which are the case for me) having a wood stove does seem to make sense.

      And even with the efficiency losses from the holes in my building envelope I am still able to heat my 2000sqft house with a single 12000 BTU minisplit in zone 6 and only use the wood stove on occasion when I feel like walking around in a t-shirt on a 0 degree day.

      1. AdamPNW | | #8

        Josh, you mentioned “having a big hole in your building enclosure certainly has a negative impact on energy efficiency. But those things can be mitigated to a great degree”.
        Mind sharing any details on how you air sealed the chimney? I’m currently installing mine but it’s problematic to say the least.

        1. Josh_Dillingham | | #10

          I air sealed where the class A chimney pipe goes through the wall and all stove pipe joints inside the house with high temp sealant. My stove has one air intake adjustment plate underneath the stove that controls how much air goes into the fire box. On my stove even if you close that all the way it still leaves about a 1 sq inch opening where air can enter. I modified that plate so that I can close it all the way when the stove is not in use. I also remove the fresh air intake from the back of the stove and seal it off when not in use so no unconditioned air is coming into the house from the intake when the stove is not In use. I'm sure you could also install some sort of inline damper inside the fresh air intake duct so you would t have to remove it from the back of the stove but I don't know if one that would seal off the air flow as well as removing It all together and I only use my stove occasionally so it isnt a big deal.

          Even with these measures there are still certain places in my stove (around the edges of the fresh air intake port is welded to the stove and the ash pan) that aren't air sealed so some air exchange is happening despite my best efforts, but on cold nights when I put my hand in front of those areas I don't feel cold air, which I did before all of my other efforts so that is good enough for me.

      2. Malcolm_Taylor | | #9

        Josh,

        I agree. The general advice that wood stoves have no place in very efficient house misses the specific circumstances some of us live in. I have never bought fire wood, and what I use come from logging cut blocks where it would otherwise have been piled and burned with the rest of the slash. We experience prolonged winter power outages, and the density of houses here is too low for our emissions to affect neighbours.

  5. walta100 | | #12

    “Is there anywhere left that doesn't require dedicated combustion air ducted to wood-stoves? Our code has for decades."

    My guess is if you threw 10 darts at a US map 8 or more would land someplace without code enforcement. Yes, 80% of the people likely live in the 20% that does have code enforcement.

    My point is that one needs a very good plan if you want a wood stove in a tight home.

    My guess is if the property is large enough to supply the wood code enforcement is unlikely.

    Walta

    1. Malcolm_Taylor | | #13

      Walta,

      Your comments and this discussion are about wood stoves in high performance houses. The assumption has to be they are going to install a very efficient wood burner, and do it legally - remembering that lack of code enforcement does not in most circumstances mean the codes do not apply.

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