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HPWH, HRV, bathroom vent, stove-top vent, make-up air, and balancing house ventilation

Brad Hardie | Posted in General Questions on

So ventilation or rather balanced ventilation seems so distant, especially when using a comprehensive ventilation strategy for all the appliances that need ventilation.

The house will be built in zone 6A, and is all electric, with PV – hoping for Net Zero.

I’m going with a HRV (Zehnder ComfoFond-L), and a stove-top vent hood for an induction stove (low CFM (250ish), it’s hard to find a lower CFM hood). Energy Star or their other newest program “Net Zero Ready” certified homes won’t allow the HRV to operate by themselves without also having source ventilation in the bathrooms….so I’m going to have bathroom vents too I guess, and I’m hoping to use a HPWH instead of a SHW system.

While at Building Energy 15′ in Boston this year someone mentioned a good way to temper make-up air for a stovetop vent hood is to place the incoming make-up air vent at the bottom and behind the fridge, so that when it blows over the coils the cool air will make the fridge more efficient and temper the incoming air also….this only works in the winter though – even in Zone 6A.

So if I did this with my HPWH it would work all year round. Some of the HPWH even have ducted kits available as options. Would the CFM that the HPWH puts out make the kitchen uncomfortable or would the fridge temper the air enough? Would this throw off the balance of the house?

I’ve also considered ducting the HPWH outside. I wouldn’t have the intake ducted from the outside here in zone 6A, but would have the outlet ducted to the outside. Would this throw off the balancing of the HRV or balance of the house?

Or should I just forgo ducting to the fridge or to the outside, and use the HPWH like I planned initially – to duct it too a cool food storage room in the basement. The basement is insulated under the slab (R20) and on the walls (R30). I would probably insulate the ceiling above the cool storage room too, so it didn’t make the floor above cool too. I guess I could put a passive vent in that would discharge behind the fridge right?

What balancing issues am I missing?

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Replies

  1. Stephen Sheehy | | #1

    Your hrv will have exhaust vents in the bathrooms, with boost. Are you sure you also need a separate fan in bathroom to satisfy Energy Star?

    Check with Zehnder, but i suspect they'll say your HPWH vent to the outside will screw up the hrv balance. Plus it adds another hole in your wall.

    I'd put the water heater where it makes the most sense as a water heater, not where it might marginally benefit the fridge, provide summer (and winter) cooling or provide make-up air whether you need it or not.

  2. GBA Editor
    Martin Holladay | | #2

    Brad,
    I'm not a fan of ducting air to and from heat-pump water heaters. These are Rube Goldberg devices that have few benefits. If you don't have a good location to install an unducted HPWH, my advice is: don't install one.

    When it comes to makeup air for range hood fans, here is a link to an article to give you guidance: Makeup Air for Range Hoods. I lean toward the Passivhaus solution: use a recirculating range hood fan -- one with a charcoal filter -- and include a ceiling-mounted exhaust-air grille in your kitchen, as far from the range as possible, and connect the grille to the exhaust port of your HRV.

    Cold air entering your kitchen behind your refrigerator? Not a good idea. Range hood usage and refrigerator compressor run time are probably not simultaneous, and the number of cfm moved by a range hood fan is high. You don't want that much cold air entering your kitchen. Hard to install a good backdraft damper, too.

  3. Brad Hardie | | #3

    Martin,

    Thanks. Yes I've read that article before, while contemplating these issues.
    I've always hated backdraft dampers too!

    I'm hesitant to place a charcoal filter vent in the kitchen - just seems cheap, irresponsible, and an oxymoron from a clean air perspective. Do you know of any good studies in regards to this? I'm not inclined to put one in just because a bunch of Passive Haus builders thinks it's energy efficient and doesn't upset the ventilation balance - we need to think about what is the most healthy too right? I'm just suspect to the ability for charcoal filters to do an adequate job, because they never seem to do as good a job as a vent that leads to the outside.

    I've got plenty of places to consider placing a HPWH in my basement, just trying to pick the best and most beneficial place.....

    Can you explain further why you are not a fan of ducted HPWH?
    Why is cold air running behind your fridge not a good idea?

    What if the duct from the HPWH is 1-2' duct to a cold storage room or up above to a vent behind a fridge?

    If I go with the Charcoal Filter - Is the CFM from the HPWH so powerful there it will have a negative impact on the kitchen or anywhere else?

  4. GBA Editor
    Martin Holladay | | #4

    Brad,
    Q. "Why is cold air running behind your fridge not a good idea?"

    A. I already provided several answers to this question in my last response: (1) Range hood usage and refrigerator compressor run time are probably not simultaneous. (2) Range hood fans move a lot of air, and you don't want that much cold air entering your kitchen. (3) It's hard to install a good backdraft damper on this type of makeup air duct.

    If you install a range hood fan that exhausts 250 cfm, your makeup air duct will be introducing 250 cfm of outdoor air into your kitchen. When it's 10 degrees out, you'll definitely notice that, because 250 cfm is a lot of air.

  5. Charlie Sullivan | | #5

    Brad,
    I like your ideas better than Martin does. I actually think a duct into the kitchen introducing 250 CFM when you run the fan is a good idea, better than 50 CFM each in 5 different rooms. If the kitchen gets uncomfortably cold, that will be good feedback to the range hood user to turn it off immediately when it's not needed. And if the kitchen gets cold, the air going out the exhaust will be cold, and will entail less heat loss.

    I agree with Martin that the timing makes the benefit of cooling the fridge coils minimal, but it's no worse than locating the vent elsewhere, so you might as well. Occasionally the timing will work, and even if it doesn't work exactly, the thermal mass of the compressor is big enough that cooling it significantly could help even when the timing isn't right.

    Maybe in the future when our water heaters and fridges have internet connections someone will write an app that you can buy for $1.29 that will synchronize their thermostats. If you've got a 1.5-ft duct from the water heater to the fridge you are all set to take full advantage of that app, and in the meantime the duct will help a little even if not enough to measure.

  6. Brad Hardie | | #6

    Martin,

    Yes, thanks. I understand you provided several answers in the previous post. The follow up I posted, was not about the range hood, and simultaneously timing the compressors - I was asking different questions. I understand that until the app Charlie mentioned becomes available - this would not be possible.

    These are the questions I was asking:

    1.) Can you provide clarification on why running cool air behind a fridge is a "bad idea" or is it just a cool outside air stove-top make-up air return that you don't think should go there?

    2.) Can you provide feedback on the HPWH cooling the fridge, and/or the cool storage room in the basement via a small duct and how that might unbalance the home ventilation?

    3.) Why aren't you a fan of ducted HPWH?

    4.) I'm hesitant to place a charcoal filter vent in the kitchen - just seems cheap, irresponsible, and an oxymoron from a clean air perspective. Do you know of any good studies in regards to this and how clean the charcoal filters actually clean the air?

    5.) You mentioned in the article you referred me to, that a whole house central vacuum cleaner system can unbalance ventilation, is this the case only if the if the central vac unit is outside the conditioned space?

  7. Brad Hardie | | #7

    Martin,

    Being that I am just over 40, and because he was before my time - I Googled Rube Goldberg. Interesting fellow - and brings some understanding and appreciation to where you are coming from. Heat pumps are complex enough, and adding more to them, only complicates it's function further. Hence why I asked though - because all this ventilation seems complex to synchronize. Thanks for the enlightenment!

  8. Expert Member
    Malcolm Taylor | | #8

    Has anyone done studies on how the location of the make up air for a range hood affects the proportion of air that is taken from the room as opposed to from the ducted supply? Is there an ideal location to minimize the loss of conditioned inside air?

  9. GBA Editor
    Martin Holladay | | #9

    Brad,
    Q. "Can you provide clarification on why running cool air behind a fridge is a bad idea, or is it just a cool outside air stove-top make-up air return that you don't think should go there?"

    A. At the risk of repeating myself: (a) the refrigerator coils are unlikely to heat this incoming air in winter; (b) this duct would either need a motorized damper and electrical interlock feature, or it would leak air, because there are no decent backdraft dampers to stop leakage; (c) this approach would lead to comfort complaints during the winter, because 250 cfm is a lot of air.

    Q. "Can you provide feedback on the HPWH cooling the fridge, and/or the cool storage room in the basement via a small duct and how that might unbalance the home ventilation?"

    A. If you use the GBA search function, you can find many Q&A threads on integrating water heaters and refrigerators. The idea comes up all the time, but it's impractical, for the reasons that you'll read about in the threads. I'll try to come up with some links later today.

    Q. "Why aren't you a fan of ducted HPWH?"

    A. These systems are complicated, and sometimes require additional fans (which use energy). Some schemes increase infiltration and exfiltration through the home's envelope. The amount of energy saved isn't worth the hassle of the mechancial complexity and added maintenance and troubleshooting.

    Q. "I'm hesitant to place a charcoal filter vent in the kitchen - just seems cheap, irresponsible, and an oxymoron from a clean air perspective."

    A. Then don't do it. The standard solution for people like you is a powered makeup air system that delivers conditioned makeup air to the kitchen. Whether or not you will be satisfied with a recirculating range hood depends entirely on what type of cooking you do.

    Q. "You mentioned in the article you referred me to, that a whole house central vacuum cleaner system can unbalance ventilation. Is this the case only if the if the central vac unit is outside the conditioned space?"

    A. The key factor is where the dust bag is located. If it is in an attached garage (outside of the envelope), the system depressurizes the house. If it is indoors, the system doesn't depressurize the house.

  10. Brad Hardie | | #10

    Martin,

    Thanks for the lengthy response.
    In question one could you tell me why you don't like outside cold air.....Just Kidding! But really, "outside" is what I should have said in the beginning, and "inside" is what I should have asked after that in regards to "cool" air going up behind the fridge. I am not a fan of any vent to the exterior that has a damper, because as you have adequately covered, most of them suck (no pun intended).

    No need to come up with links to the fridge/water heater integration......

    Thanks again for responses.

  11. Beenash Khan | | #11

    Excellent question and my apologies to the OP but this post, has me wondering about my set up now. New construction, 6000+ above ground, hood fan at 1500 CFM (Indian spicy/messy cooking LOL, so needed the strongEST fan) and a Geothermal/HRV combo with pretty much whole house spray foam insulation.

    Do I need to worry about cold air being sucked into the house especially with that big of a hood exhaust?

    Thanks

  12. GBA Editor
    Martin Holladay | | #12

    Beenash,
    It is a code violation (as well as a bad idea) to install a 1500-cfm range hood fan without also installing a makeup air system. For more information on this topic, as well as suggestions for equipment used for makeup air systems, see Makeup Air for Range Hoods.

  13. Neil Tarr | | #13

    I agree with Martin's comment #2, charcoal filter and exhaust grill in the kitchen running to the HRV. That's the system we used when building a couple of years ago. It works fine to run the HRV based on humidity and odor. The only thing I would add is the exhaust grill shouldn't be located in the ceiling but high on an interior wall if possible.
    Neil

  14. Graham Fisher | | #14

    Hi Neil,

    Why is it better to place the exhaust grill high on an interior wall, and not in the ceiling? Would a foot from the ceiling be a good placement?

    When building a house, it seems like just about every conversation on GBA is timely.

  15. Beenash Khan | | #15

    In Reply to Martin, would an HRV system not designed to bring in and condition (heat/cool) the incoming fresh air?

    Would one still need a "make-up air system" on top of an HRV or "HRV" is a make-up air system?

    So anything above 400 CFM is recommended to have a make up air system, is that correct?

    Thanks

  16. GBA Editor
    Martin Holladay | | #16

    Beenash,
    An HRV is a balanced ventilation system. It brings in the same volume of fresh air as is being simultaneously exhausted. That's why an HRV can't provide makeup air -- especially such an enormous volume of air (1,500 cfm).

    Every HRV manufacturer states in its installation instructions that HRVs are not intended to supply makeup air.

    Q. "So anything above 400 CFM is recommended to have a make up air system, is that correct?"

    A. That's what the code requires. But even a 300 cfm exhaust fan can cause problems in a tight house.

  17. GBA Editor
    Martin Holladay | | #17

    Beenash,
    Here is a link to an article that explains what an HRV can and cannot do: HRV or ERV?

  18. Neil Tarr | | #18

    Hi Graham
    Placing the exhaust grill in the ceiling is another penetration in the building envelope. Placing on an interior wall is one less penetration to air seal.
    Neil

  19. Beenash Khan | | #19

    I don't mean to be disrespectful to Brad by continuing with my issue but I honestly think this conversation would be helpful to other readers as well.

    Met with my HVAC guy earlier today who's looking into this matter for me but before I meet with him again, I would love to get bit more educated on my options.

    From what I understand from his conversation is, he's looking into 2 dampers, one at the top of the roof where the exhaust will be and the second one, where the ERV (not HRV) unit will be placed?

    He's also discouraging me from using the 1500 CFM as he stated that it would be moving lots of unconditioned air which needs to be addressed.

    So besides the installing the two dampers, do I need to look into upgrading my Geothermal units as well?

    Please be patient as I am not a very technical person.

    Thanks

  20. Andrew Bennett | | #20

    I used to work for Whirlpool Corp. In our training (biased I'm sure) I was taught that to really expell a reasonable amount of humidity a range hood needed to have a minimum of 220 cfm and better 300+/-. That is fir overhead ventilation. Proximity ventilation (i.e. Down draft ala Jenn-Air) actually performed better than overhead at a lower cfm because of it's closer proximity (hence the name..proximity ventilation) to the cooking surfaces. ...how to make that air up? Better listen to Martin and Dana.

  21. Andrew Bennett | | #21

    I was just thinking again...dangerous I know...jenn-Air makes a recirculating downdraft that vents at the cabinet toe kick (for cooktops mounted on a cabinet) and the air passes through a very substantial filter befor exiting. Since nothing is exhauted no make up air is required and it is a much, much better solution than a 1/4" thick carbonized filter.

  22. GBA Editor
    Martin Holladay | | #22

    Neil Tarr,
    You wrote, "Placing the exhaust grille in the ceiling is another penetration in the building envelope. Placing on an interior wall is one less penetration to air seal."

    Your explanation mystifies me. The grille that I describe is not a penetration in the building envelope. The grille is connected to a duct that leads to an HRV or an ERV. The HRV should be inside the building envelope, so all of these components -- the ceiling grille, the duct boot, the exhaust duct, and the HRV -- are inside the building envelope.

    The home's air barrier and insulation should not be penetrated by any of these components. This is easily accomplished in a two-story house with a kitchen on the first floor. In a one-story house, the usual method to handle ducts of this kind is with a service cavity (usually created with 2x4s) at the ceiling. For more information on service cavities, see Service Cavities for Wiring and Plumbing.

  23. GBA Editor
    Martin Holladay | | #23

    Beenash,
    Q. "My HVAC guy ... is looking into 2 dampers, one at the top of the roof where the exhaust will be and the second one, where the ERV (not HRV) unit will be placed. ...So besides the installing the two dampers, do I need to look into upgrading my Geothermal units as well?"

    A. One of the dampers that your HVAC contractor is planning to install -- the one near the roof -- is a backdraft damper to limit the infiltration of outdoor air when the exhaust fan is not in use. This damper has nothing to do with makeup air (the problem we are discussing).

    The other damper -- the one near the ERV -- confuses me. Unless you tell us more, I have no idea what type of damper that is, or why it is being installed.

    Makeup air is the outdoor air that must enter the house when the fan is operating to "make up" for the indoor air that is being removed. The usual way to supply makeup air for a powerful exhaust fan is with a supply fan that introduces outdoor air into your house. If necessary, this makeup air can be conditioned in winter with an electric-resistance heater located in the duct.

    You question about geothermal units confuses me. A geothermal unit (more properly called a ground-source heat pump) provides space heating and sometimes air conditioning. It has nothing to do with ventilation or makeup air.

  24. Brad Hardie | | #24

    Andrew & Beenash,
    Higher CFM volume kitchen exhaust fans are needed more with propane, natural gas fired ranges & stovetops. Much more condensation is produced when using this type of stove. I will be using an induction cooktop, and I will have ZERO fossil fuel appliances in my house (well except for a woodstove(which it's own make-up air source)). Make up air needs are kept at a minimum that way, and I will have only one place CO could be produced (except if my house catches fire). The dryer will also be a ventless heat pump type. As Martin has said a tight house, can more easily develop problems with unbalancing, hence why I posted the original question.

    Beenash:
    1500 CFM is a ridiculously sized exhaust fan...are you commercially cooking for hundreds of people? We have a large CFM fan at the firehouse where I work, and I wish I could record how loud it is when running....I would seriously reconsider your ventilation needs.

  25. Expert Member
    Malcolm Taylor | | #25

    Brad,
    Good point about cooking with gas, but it's not just more condensation:
    https://www.greenbuildingadvisor.com/blogs/dept/green-building-news/hazards-cooking-gas

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