Hey Pros- I'm seeking a check on my logic. I'm finishing out part of a basement into an ADU that will have a bath fan and a stackable washer/dryer. The remaining part of the basement contains a natural draft water heater and another laundry set up. The water heater and existing dryer have coexisted for years with no problems. My concern is that the addition of that dryer and bath fan could depresurize the apartment and cause the water heater on the other side of the wall to back draft. I have (sorta) airsealed the partition wall between the ADU and the CAZ but want to do more to prevent the apartment from succking on the rest of the basement. I'm thinking of installing a Panasonice spot ERV to run at a low continuous flow with hope that its intake will provide make up air if the bath fan and dryer are running at the same time. In other words, they would suck through the ERV before sucking on the rest of the basement. I can do a worst case pedressurization test once everything is in but we're not there yet.



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  • Its all about the flue.  Change the Hot water heate with power vent or tankless or some thing with out a flue.   I tend to keep power type

  • The dryers can draw 300+CFM each and will definitely depressurize the CAZ, you need dedicated make up air for the dryers AND the CAZ.


  • Hi Heidi
    I agree with the others, that an ERV is not a make-up solution. They are more "mitigators of losses" and tend to be "pressure-neutral". (Any imbalance between supply and return, meaning less return for recovery, means, well, less recovery and that delta load is still a loss.)

    The best way to use an ERV in this condition is to capture exhaust/waste heat already being thrown away and leverage that recovered energy into OA for your purposes. That is VERY high-level. I do not know your building situation, crossings of any rated partitions, party-walls, etc. so am treading very lightly with that suggestion.

    Definitely work on the CAZ sealing/definition.

    Another suggestion, also mirrored in earlier responses and as part of your work on the CAZ:  Add a combustion air fan for the WH. I undertand that replacing the heater may not be in the cards right now, but including the relative age of that in your assessment (e.g. 8 year old heater on a five-year warranty with poor water quality, might change your approach to replacement with a heat pump type.)  As Sean noted, put money there, if it makes sense.

    If the dryers are gas-fired, you may need combustion air for them anyway. If electric, that requirement goes away, at least by model gas codes, so a bona-fide make-up air system may be in the cards to offset pressure differences.

  • ERV's are not make up air solutions. Use ERVs instead of bathroom fans. If you have forced air, add the makeup air from outside in 6-8" duct to a filter box to the return duct where it will again get filtered and fired or cooled... Cheap and easy here. Tightening things up makes this more important.

    Certainly changing types of appliances etc. is for future maintenance and upgrades and comes with its own drawbacks like turning a room into a cooler to heat the water ;0)  Good time to soundboard design challenges. Cheers to you all--Cody

  • Ok but if you seal up where the water heater is - where is the air needed for combustion coming from? - do you have a high low pipe in the area

    Before spending a ton of money doing this & that for an old water heater - replace it with either a highly insulated elkectric one or direct vented one which has the supply & exhaust air both being piped in & out

    Will an ERV help - yes & no as it will allow more air in then it expels when depressuized but, air & water seek the path the least resistance so...

    • Along this line, I'd recommend installing a heat pump hot water heater, that will operate using 40% of the energy of a standard resistance electric hot water tank, and also cost less to operate than your fossil fueled unit.  

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