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Community and Q&A

Best way to supply fresh air to finished basement?

Anne3A | Posted in General Questions on

My apologies in advance for the length, but since I may not be able to get back here for a few days I tried to anticipate questions and provide sufficient detail.

Our home has approx. 2500SF of finished basement that includes a kitchen, media room, game room, bedroom, and bathroom. The kitchen has a vented range hood and the bathroom has an exhaust fan just outside the shower. There are no operable windows. Two french doors in the bedroom are bolted 99% of the time, a hall door that leads to the lower driveway is not used, and the other two doors lead to garages, only one of which has been used periodically for parking. The house is about six years old. It has multiple HVAC systems, including one for the basement.

Several years ago, we had an AprilAire ERV installed for the main floor because we were concerned about the potential for backdrafting due to a large range hood over a 48″ Wolf dual fuel range. We also wanted to be able to filter and condition fresh air because of severe allergies, high pollen count, and high humidity. Previously we added AccuClean media filters to the systems on which the builder had not installed AprilAire media filters, including the basement system.

We hoped to install an ERV in the basement when the budget permitted. What we’ve learned is that the ERV causes our indoor humidity to rise significantly about 4 – 5 months of the year. Due to a mild winter and very wet spring, it was even more of a problem this year. We currently run four 70 pint portable dehus in the master suite and kitchen/hearth room area, as well as in the basement and upstairs. They’re a (literal) pain to empty, sometimes twice a day, as well as noisy and unsightly.

My HVAC contractor is a good person, and I trust him, but in our area everyone sticks to what they’ve always done. We were his first ERV installation. Here, in our warm humid climate, even multimillion dollar homes are built with “builder basic” systems that do no more than meet code.
So, if I want to try anything else I have to research it and hope to make the best choice for our circumstances. Initially, he encouraged me to simply set our A/C temp lower to let it dehumidify the basement. That’s fine for the summer months, but during the shoulder seasons when it’s very humid it doesn’t help. We’d have to lower the A/C to the mid 60s, and when we tried that we just ended up feeling chilly and damp. It also doesn’t address the need for fresh air in the basement.

I’ve looked at the Ultra-Aire ventilating dehumidifier, but am not sure it’s the answer. I’ve also read suggestions about putting a portable dehu into a closet with a louvered door and running a hose to a drain. I’m wondering if a simple fresh air port with a disposable filter plus the closeted dehu would be a more cost effective solution. We have a closet in the basement within six feet of the HVAC return and should be able to replace the door on it easily enough. One wall is adjacent to the larger garage, so we could run the drain hose out that way.

I appreciate you reading this far and look forward to your suggestions. Thanks.

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  1. Expert Member
    Dana Dorsett | | #1

    Where are you located?

    Was the foundation air-sealed and insulated?

    Is the basement slab insulated?

  2. dickrussell | | #2

    Wow, you've identified several problems.
    1. You want fresh air supplied to the basement.
    2. You have a high interior humidity problem in the shoulder seasons.
    3. You have a potential backdrafting problem due to a large range hood.

    As to fresh air to the basement space, depending on where the bathroom down there is located relative to the stairway, you might be able to provide a louver in the stairway door and change the vent fan in the bathroom, perhaps to one with multiple speeds and with a speed boost switch with built-in timer. The fan could run at low setting continuously, drawing air from upstairs, presumably freshened by the ERV.

    I was going to ask where you are located, as a lot of potential solutions are very climate-dependent. Then I noticed you said "warm and humid climate." Search on this site for something I saw not too long ago about the selection of ERVs not being the right choice vs HRV in some mixed climates. Human occupancy produces interior humidity, and if your fresh outside air is humid, the inside air is just going to get more humid unless you actively dehumidify, regardless of whether you have an ERV or HRV. Dehumidifying In the shoulder seasons with the A/C system, when there is little cooling demand, gives you the usual "oversized A/C" result: cool and clammy interior. I'm not sure if I understand your proposed location for a dehumidifier, inside a closet with a louvered door. You need to have unimpeded flow of air to the dehumidifier, and surrounding the unit with walls, despite a louvered door, strikes me as isolating the unit from the air to be dehumidified. Furthermore, if you are running four dehumidifiers already, I have to wonder if you have a basement moisture problem, contributing to that coming in with outside air and being added by human activity.

    Finally, on adding that ERV to address a "potential for backdrafting due to a large range hood over a 48" Wolf dual fuel range," Do another search for the piece on makeup air for large range hoods. While an exhaust device such as range hood or clothes dryer may just throw an ERV or HRV into imbalance so as to provide some additional makeup air, the range hood still can depressurize the house sufficiently to cause backdrafting of combustion units that aren't closed systems (with dedicated outside air). The proper solution for a large range hood is to have a separate makeup air duct bring outside air directly to the vicinity of the range, with a motorized damper connected to the range hood.

  3. GBA Editor
  4. Anne3A | | #4

    Thanks for the quick replies. I'll check back next week in case there are more questions.

    Dana, we did not build this house (or have it built) so we don't know the details of the construction. Based on observations of other houses under construction in this area over the last decade, my guess is that the basement slab was not insulated and nothing was done over and above code requirements (which are not always enforced.) For example, originally the master bath vent fan just dumped moist air into the attic - we had to get it vented outside. Also, the range hood for the basement kitchen was not ducted; we caught that before we used it. The house was inspected by both the local building inspectors and by an independent, ASHI inspector before we bought it. During our first year we discovered and corrected many problems.

    Dick, your suggestion about the exhaust fan in the bathroom would be easy to implement and the door from the main floor to the basement could simply be left open.

    The dehu in a closet with a louvered door idea is something I read late last night. Perhaps I need to reread it.

    I was told when I first looked into the ERV that I was silly to be concerned about makeup air for the range hood, given the size of the house (>8000 SF.) The ERV has a control with settings from 0 - 100% in 10% increments, so when we use the range hood we ramp it up. During rainy weather we turn it off and I bake or use the microwave instead of the gas cooktop. I may use the sideburner on the outdoor grill more often during the shoulder seasons.

    Martin, perhaps we need to reduce the use of the ERV according to the humidity readings at various times of day. Based on what I read last night, it seems that conventional wisdom on ERVs/HRVs has changed over the past few years.

    I hesitated to post the house size initially, but those four dehus cover 8000 SF above grade + the basement, so maybe the moisture issue isn't quite as bad as it may have seemed at first. Sorry for leaving that out, but you might be surprised at the nasty comments that information sparks.

    We don't have actual water intrusion problems, but the house could certainly be sealed better. We've put a lot of time and money into rain water mangement and dealing with wind driven rain to try to prevent problems. I also may be unrealistic about what indoor RH can be achieved. Because of various health issues, I try to keep the whole house at no more than 45% and prefer closer to 40%.

    Thanks all.

  5. dickrussell | | #5

    I was curious as to whether the numbers on humidity removal fit together, so I did some ballpark calculations. Suppose those four dehumidifiers are emptied on average once per 24 hours, but are full each time (or half full twice a day). That comes to 35 gallons, or 292 lb water/day, or 12.2 lb/hr

    Suppose the ventilation or air leakage is displacing 72 F, 45% RH air with humid outside air with a 75 F dewpoint. In terms of absolute humidity, the drier and humid air masses have 0.00752 and 0.01882 lb water/lb air. Thus the air makeup would have to be 292/90.01882-0.0052) = 25,841 lb/day. With 80 F air density at 0.0736 lb/cuft, the air makeup comes to 351,100 cuft/day, or 14,629 cuft/hr, or 244 cfm. Looking at it another way, and using an average ceiling height of 9 ft, the house volume is 8000 x 9 = 72,000 cuft, and 14,629/72,000 = 0.20 ACH.

    For a house that size, with normal family occupancy, 0.20 ACH is a lot. Using ASHRAE standard 62.2, and assuming five bedrooms, the ventilation rate ought to be 8000/100 + 5*7.5 = 118 cfm.

    If the above numbers are reasonable, they suggest that the ventilation rate under those conditions is double what it ought to be. On the other hand, they do indicate that the bulk of the indoor humidity removal problem might well be just from ventilation with humid outside air, and not necessarily from a basement with a moisture problem. I have made a lot of assumptions in these calcs, but actual numbers collected over a few days on condensate removal, air flow, and outside humidity could be used to refine the calcs.

  6. Anne3A | | #6

    Just a quick note - the Frigidaire 70 pint dehus have buckets that hold just over 16 pints. I would guess that on average we dump about 30 pints per dehu per day or 15 gal. total. We set them at the lowest setting (other than continuous run) of 35% in an effort to obtain 40% - 45% throughout the house.

    The average daily humidity in June here is 75%, but recently it's been >90%. The avg. temp (high & low) has been 78 for the last week, which is about 2 degrees above avg. for early June. We generally see a significant warm up in late June that continues through early September, but this year the heat wave is starting now.

    The builder who's doing some repairs to our home (not the same guy who built the house) mentioned that he also runs a 70 pint dehu in his finished basement and empties it twice a day. Based on some conversations over the weekend, that seems to be pretty typical here.

    BTW, the dehu in a closet idea came from the Building Science website and was mentioned in several others.

  7. Expert Member
    Dana Dorsett | | #7

    The outdoor relative humidity % is an irrelevant number without the temperature to which it's relative. Instead, track the outdoor DEW POINT, which moves more slowly than %RH, and a measure of the absolute humidity. When you ventilate with 50F/90% RH outdoor air into a 70F house, it's RH at 70F is a healthy 44% RH. When you ventilate a 65F basement with 80F/45% RH air, that same air at 65F is a mold-friendly 66% RH. Play around with this pychrometric calculator (or ponder a psychrometric chart) to get a sense of what this is about:

    If your goal is to keep the upstairs 50% RH (or less) @ 70F during the shoulder and cooling seasons, whenever the outdoor dew point is over 50F, reduce the ventilation rate on the ERV to the absolute minimum. (Find a nearby or weather station for dew point tracking.)

    In the basement, setting the dehumidifiers to 55% or 60% is usually fine, and setting it to 35% is close to insane. The only reason you'd need to ever set it that low would be if you have excessive ventilation at too high an outdoor dew point, or were recovering from a flood or something.

    I re-skimmed your posts- where in the world is this little 8000' weekend getaway shack? Even at 78F average temps it makes a difference if it's in San Diego vs. Philadelphia. data is pretty easy to view for a quick grasp of climate and air humidity, but my sense is that you're over ventilating with overly-moist outdoor air, and the graphing the dew point history would be telling.

  8. ronjohnsonusa | | #8

    More details are required for the suggestion, or you can just try googling!!!

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