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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  • Maybe I should Clarify, I wasn't looking to provide combustion air with an ERV.

    The water heater is not in the apartment, the CAZ has not been air-sealed, and the volume of space in the CAZ far exceeds code. The two spaces do share one 1,600 sq ft basement but they are isolated by an air-sealed interior wall. The owner agreed to the addition of make up air to the CAZ so I am not worried about the current plan.

    My own field testing has shown me that depressurization of a living space will throw of the "balance" of and HRV/ERV. I do expect to so see some small relief of depressurization with the addition of the ERV. I will be able to test pressures in all scenarios once everything is installed. I can share those findings if there is interest.

  • As others have mentioned - the ERV is not the way to provide combustion air. At best, the ERV will not change the pressurization of the space (balanced flow). But as the air filters on the ERV load up it is most likely to depressurize the CAZ because the outdoor air filter will load faster than the indoor filter. ERV's have their place but this is not one.

    The existing dryer has coexisted with the existing water heater becasue of air leakage. If you seal up the leakage you need to provide combustion air or expect trouble. Since you are adding additional exhaust (bath fan and second drier) you need to make sure you have enough combustion air for the worst case condition. It is spelled out in the mechanical code and plumbing code.

  • My career started in Laboratories and clean rooms for pharmaceuticals where air flow control was critical. What PH design is doing is exactly the same but trying to do it without the control system.  That's a mistake. Potentially dangerous one.

    At his request, I visited Amory Lovins at his RMI home to solve a problem he encountered after they removed the original wood-burning stove that heated the whole place with a solar water heater. The home was no longer net-zero and he wanted to find out why before adding heater capacity.  The math didn't add up. He had a few guests with metering and controls experience on hand to look at the (oversized by donation) building control system.

    At dinner the second evening, I hopped up and asked if I could check something elsewhere in the home. Turns out, the two fancy ERVs were fighting each other. The fans were cycling between running full speed and off over about a 15 minute period. Very slow and quiet, so hard to detect the problem by ear (someone mentioned measurements!)

    In this case, RMI has so many plants inside the home (Banana farm) that there was more CO2 outside.  The sensors were confused by the other ERV sucking in outside air with more CO2 than the interior air.

    Point is, you need a tiny bit of controls and measurement, designed at the system level, not just per component, and it needs to include failure analysis logic.

    The failure analysis logic is another story. Ask me about my parents $400 electric bill when it's normally just the connection charge of $9.95.

  • Others have already posted to remind - that an ERV is not for make up ventilation.   The Panasonic is balanced air flow only.

    Unless you've measured the CO and combusition gases just above the hot water heater - it is really hard to tell if you've never had a back draft problem.  I'd wager that at times you've had - but short durations and you've just not seen it.   CO levels that trigger the home CO monitors to alarm - only warn you when you are in a SHORT term dangerous condition.   The reason why you NEVER drill holes into gas flues even for screws.... is that it's hard to keep them sealed and those points leak.   Long term low levels of CO (between 8 ppm and 20 ppm) can still cause an injury to the individuals  -- even when this low level will not trigger an alarm.

    You need make up air for the hot water heater,  or you need to add a vent fan to the heater to ensure you're moving the flue gases out.... even if only the pilot is burning.


    What climate zone are you in?

  • Could you maybe just add a draft inducer to the water heater?


    • Y E S!!!   Field Controls DI-1  5-8" pipe lasts 3-7 yrs  puts .15 " neg water on the flue.  2.3 amp 115 volt.  Its just a shaded pole motor and brass bearing go out with heat and dirty use. 

      Tjernlund.com makes models DJ-3  Fits from 3-24"   both work in your case.   I have put a bunch in the past 30 yrs.  Just keep in mind you can still back draft this inducer motor!  but it helps a lot

  • Hi Heidi, long time no see! I hope that  you are doing well. 

    I would not plan on letting that ERV allow for minimizing air depressurization. The dryer is roughly 300 CFM and the bath fan will be about 80 CFM (ideally, but most that I have measured are moving about half of that). The ERV's intake, even with the fan not running, is not large enough to move much air at typical pressure differences. Think about the 4" ducts for the dryer and bath fan.They are moving air with a fan. The ERV, assuming it was not running, would not allow much air to move through it under the small pressure difference that could creat back drafting of the water heater. There are other  factors that could make a big difference on whether or not the water heater might backdraft. The majjor factor is how leaky the basement is to the outside, including leakage paths through the main house. The next major factor is how the flue of the water heater was installed.I have seen many veenting code violations that result in periodic backdrafting, such as not sizing the venting properly. Although not yet a code requirement, it is a good work practice to extend the vent pipe straight up at least a foot, before making any turn (the same goes for bath fans, which is a major contributor to fans not moving the air that they are designed for).

    I might suggest that you take a couple of box fans and put them into basement windows, using cardboard or such to filll in the rest of the window. Then close any doors to the rest of the house in an attempt to make the worst conditions for backdrafting. Turn on any fans that are in the main house and exhaust to the outside. With the fans blowing air out of the basement, run some hot water and see if the water heater backdrafts. You can use incense, a match or your hand (to feel hot air) to check for backdrafting. Close the door to the main house and see if air is moving from the house to the basement. If it is, then the worst case is with the door closed- recheck for backdrafting. 


  • Thanks for the input everyone. The water heater is fairly new so a swap is not an option. I'm now asking for make up air into the CAZ.

    And definitely Bill, I will test as soon as the walls get enclosed and again when all exhausting appliances are installed and running.  Gotta get some good out of that fancy new gauge I bought 2 weeks before the world shut down.

  • On new hot water heaters standing pilot light the Eff is 57-59%   10 yr ago was 51%  20 yr ago was 45%.  50 +yr ago was just 35% with a blow tourch 3" long.   Most B1pipe  flues systems were made for water heater with a flame under tank to keep tank to temp, flue warm   Now the flue is cold and works more of an make up air intake than flue.  So bottom line if the flue now is right - 1/4" slope uphill up to 3'  was 7'   Now we use 2 wall pipe to keep the heat in flue gas.  Its heat that makes the flue gas go up the flue with a standing pilot light.   Just keep away from gas burning flues -  Brick is the real bad flue!!! just takes more heat a lot more heat.   Keep the flue gas dry and above Dew point at end of each run.   Test in Test out each and every time 


  • Take some measurements (before, during, after) what ever you do.


    Bill - MeasurementsRus - Spohn

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