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Community and Q&A

HRV vs. whole-house dehumidifier — Need help choosing

Chris Yarsevich | Posted in Mechanicals on

Hello, I have a 1700 sq ft house located about an hour North of Albany, NY. The house was built in 1998, and I just purchased it last year, it is fairly tight, but nothing exotic. It has 2×6 walls with fiberglass insulation,R-60 in the ceiling, double pane vinyl windows, and vinyl siding. The house has a few year old high efficiency forced air propane furnace and no AC.

Since living in the house we have noticed it seems to be very humid. The mirrors fog up during a shower very quickly, even during the summer months. The basement is damp, in fact soon after moving in we had mold grow on some boxes that we placed in the basement for storage. Since then we have been running a dehumidifier down there most of the year (except winter) and no more mold issues. Finally, during shoulder season and the winter we get condensation on the windows. We tend to keep our house cool (60-65 degrees) during the winter, and it seems like the condensation on the windows is less if the furnace is running more (this tends to make sense as the furnace is drying the air, right?).

Therefore, my question is this. What makes more sense in this application, a HRV or a whole house dehumidifier? I already have full ducting in place due to the furnace and plenty of room in the furnace room in the basement for any required hardware.

I would love to hear the pros/cons associated with both. In the end my goal is to reduce condensation on windows, reduce any mold issues , all while trying to accomplish it efficiently.

Thanks

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Chris,
    It sounds like you have two problems:

    1. You have a damp basement.

    2. Your house lacks a mechanical ventilation system.

    I strongly advise that you not install a whole-house dehumidifier until you have at least attempted to address the two issues I listed.

    To learn more about whole-house ventilation systems, see Designing a Good Ventilation System.

    To learn more about addressing your basement issues, see Fixing a Wet Basement.

  2. Chris Yarsevich | | #2

    Martin, thanks for the reply.

    I just read both articles. I have tried the simple fixes on the basement, cleaned gutters, added extensions to downspouts, etc. After that, we have noticed NO wetness in basement, but dehumidifier runs about 8 months a year set at 55%. For half of those months it's not pulling much water, about a full bucket every few days, the other 4 months it's probably pulling a full bucket every 18-24 hrs (it is a 50 pint unit). Anything else, we are talking huge money to try and fix.

    As far as a mechanical ventilation system, exhaust only does not seem like a good fit, because it is so location specific, only potentially helping one room/area. Therefore, that leaves a central-fan-integrated supply ventilation system or an HRV/ERV. From the article is seems like central-fan-integrated supply ventilation systems can be tricky to install for the fresh air intake and I would worry about that with many nights in the winter being 0 degrees or colder. Can you recommend what I look for in a contractor? Should I being getting quotes on both? Should I steer down the route of an HRV instead?

    Thanks

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Chris,
    If you want to install a mechanical ventilation system, you need to find a local HVAC contractor familiar with ventilation systems. If I were looking for such a contractor, I would call a few up and ask questions like these:

    1. Have you ever installed a whole-house ventilation system?

    2. What type of ventilation system do your recommend?

    3. Have you ever installed an HRV? If so, do you recommend dedicated ventilation ductwork or the use of the ductwork associated with a forced-air heating system?

    The answers to these questions will probably give you an idea of whether the contractor knows what he or she is talking about.

    You can also ask GBA readers for recommendations of good contractors. Note to GBA readers: Chris lives about an hour north of Albany, New York.

  4. Chris Yarsevich | | #4

    Thanks. Any advice on local HVAC contractors would be great, I live in Saratoga Springs, NY.

  5. Chris Yarsevich | | #5

    Martin, can you expand on an HRV vs. a central-fan-integrated supply ventilation system for my house? If using the existing duct work I have in place for the furnace, both types will require my furnace fan to operate, correct? If so, the HRV gains you some reclaimed heat from the return line and better control of outside air mix? A central-fan-integrated supply ventilation system is cheaper, but outside air mix can be tricky and many times too high and no reclaimed heat?

    Do I have the thinking straight here? Anything else I should know before I start talking with contractors and getting quotes?

    Thanks

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Chris,
    The essential information you need to make a decision should be included in the article I linked to (Designing a Good Ventilation System). For further information on some of the topics listed, check out the links in the "Related Articles" box on that page.

    Operating a whole-house ventilation system during the winter will tend to lower your indoor relative humidity. (By the way, it would be a good idea to buy a few hygrometers -- humidity meters -- to install in your house. That way you will know your indoor relative humidity.)

    The best ventilation system is an HRV or an ERV with dedicated ventilation ductwork.

    Hooking up an HRV to heating-system ductwork is not as good as using dedicated ductwork. However, it will be cheaper to install.

    If you want to install a central-fan-integrated supply ventilation system, it's important to verify that your furnace has an ECM blower, or your energy bills will be high.

    In your case, even operating your bathroom exhaust fan for 24 hours a day (without any other changes to your house) might make a dramatic difference in your indoor RH levels.

  7. Chris Yarsevich | | #7

    Thanks Martin. I doubt I would install an HRV with dedicated ducting, I think I would use my current furnace ducts. Therefore, won't the HRV and central-fan system both have to use the furnace blower? I will verify, but I believe my furnace has an ECM blower. It is a newer RUUD unit rated at 92%.

    I will give the bath room exhausts a try. I just installed a new quiet one (110 cfm) to replace the old one that would wake the neighbors (loud!).

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Chris,
    There is a way to hook up an HRV to your forced-air ducts in a way that minimizes the run-time of your furnace blower. Here is a link to an article with more information: Ducting HRVs and ERVs.

  9. Chris Yarsevich | | #9

    Martin, so I went home last night and turned on my upstairs bathroom fan (110 cfm). I let it run from 6pm to 6am. This morning there was NO condensation on the windows except for my daughter's room, which had the door closed all night.

    Is this a true fix or a band aid? Is it wasting energy or heat? I see I can buy programmable switches for the fan. Would it be best to have it run a little every hour (15 minutes or so). Or a few long runs per night (3hrs on/3 hrs off, etc).

    Also, oddly in the middle of the night we had a downstairs smoke alarm go off. This alarm is near the kitchen and often goes off while cooking. Was this a weird coincidence or could it be tied to the fan running all night , creating a draft or something like that?

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Chris,
    Ventilating your home during cold weather is a true fix to high indoor humidity; it is not a bandaid.

    You now have an exhaust-only ventilation system. Many homeowners are perfectly happy with exhaust-only ventilation systems. Your experiment shows that this approach has solved your immediate problem.

    There are many types of ventilation systems. Exhaust-only systems have a somewhat higher operating cost than HRV systems, but they are much cheaper to install (and use less electricity).

    If you like the idea of exhaust-only ventilation, you can upgrade your system if you want.

    I don't know what kind of bathroom exhaust fan you have, but the best model is probably one of the WhisperGreen models sold by Panasonic. They are quiet and extremely efficient.

    If you choose one of these fans, or get a timer, you can experiment with reducing your ventilation rate. (The best timers fit into a switch box, and allow you to program your fan to run for 15 minutes per hour, or 30 minutes per hour, if you want.) My guess is that 110 cfm is on the high side for 24-hour-per-day ventilation.

  11. Charlie Sullivan | | #11

    It is possible that your 110 cfm bathroom fan backdrafted some combustion appliance and it's possible that set off your smoke alarm. 110 cfm would be unlikely to do that unless you had a very tight house, but you said you did. What combustion appliances do you have? (Furnace, boiler, water heater...?)

    I recommend a carbon monoxide alarm to ensure you don't have a CO problem when the fan is on. I also recommend measuring humidity indoors if you are not already doing so.

    If your humidity is lower when the furnace is running, that could be because it's colder outside and so the outside air leaking in has less moisture in it, it could be because more air is leaking because of the stack effect (relatively hot air in your house rising), or it could be because your ducts leak and so the furnace fan drives air exchange with the outdoors.

    If you have a backdrafting problem and want to use exahust-only ventilation, you could install air inlets (really just holes in your wall), but

    With exhaust only ventilation, you will reduce your operating costs if you run it during day only when the exterior temperature is higher. As Martin says, the "green" versions of the panasonic fans have pretty amazingly low electricity consumption.

    I'd list other options but Martin pointed you to an article that has a nice list of the four options.

  12. Chris Yarsevich | | #12

    Charlie, that back draft scenario might make sense. The alarm that went off is on the first floor, right at the top of the basement steps. We normally keep the basement door cracked open, at the bottom of the basement steps is the utility room with a propane hot water heater and a propane furnace. Additionally, the 110 cfm fan that was on was basically one story straight above that, so maybe a large draft was being created up through the two floors.

    What exactly is happening in this back draft scenario? Are combustion gases being pulled into the house, instead of properly being drafted out of the unit's exhaust? There is a carbon monoxide detector right next to the smoke alarm and that did not go off.

  13. Charlie Sullivan | | #13

    Great to know that you have a CO alarm.

    Yes, back drafting in the extreme means that you are pulling air down the chimney with your exhaust fan, and the combustion products all go into the house, not up the chimney. In a milder case, you are not actually reversing the chimney flow, but are just reducing it and some combustion products go into the house.

    The best modern furnaces are "sealed combustion" units that get their air intake from a duct from outside, and the combustion chamber is sealed from the interior air, so that back drafting is almost impossible. If you yours is sealed combustion, it's unlikely that you had backdrafting.

    Other theories could be smoke from a neighbor's smoky wood stove that got sucked into your house, or just dust that was in the cracks through which you sucked the dust.

  14. User avatar GBA Editor
    Martin Holladay | | #14

    Chris,
    Q. "What exactly is happening in this back draft scenario?"

    A. The exhaust fan is depressurizing the house, pulling flue gases that should be exiting your house backwards into the basement. This phenomenon is more common with larger fans (for example, range hood fans), but is possible under the circumstances you describe -- especially if another exhaust appliance (like a clothes dryer) was operating simultaneously.

    The house is a system -- as you are now learning. If the house is "fairly tight," as you describe it, it's best to have sealed-combustion appliances, not atmospherically vented appliances. Unlike atmospherically vented appliances, sealed-combustion appliances almost never backdraft.

  15. Chris Yarsevich | | #15

    Ok, I believe my furnace is a sealed combustion unit. However I don't think my hot water heater is. You can see the pilot/flame if you bend down and look at the bottom of it. It does have a power blower mounted on the top of the water heater than runs whenever the water heater is heating water. The blower exits out a large PVC pipe to the outside.

    Should this powered blower help prevent back draft?

  16. User avatar GBA Editor
    Martin Holladay | | #16

    Chris,
    That type of water heater is called a power-vented model. It is unlikely to be the cause of any backdrafting problems. Based on the information you have provided, my guess is that neither your water heater nor your furnace caused your CO alarm to go off. However, you don't want to mess around with CO warnings. If it happens again, call in an HVAC contractor or gas company rep to find out what's going on.

  17. Chris Yarsevich | | #17

    Just for clarification, the smoke alarm went off, not the CO detector.

  18. User avatar GBA Editor
    Martin Holladay | | #18

    Right. My mistake. Thanks for the clarification.

  19. Chris Yarsevich | | #19

    Do you still believe neither combustion device caused this?

  20. User avatar GBA Editor
    Martin Holladay | | #20

    Chris,
    Yes. But it's only a guess. Do some more sleuthing.

  21. Chris Yarsevich | | #21

    Ok. I think my current plan is to to install a Panasonic fan that can switch between 50, 70, and 110 cfm in the first floor bath. I am hoping the 50 cfm setting running 24/7 will be just right to keep condensation at bay throughout the whole house (according to the tables I have seen this is just about right for 1700 sq ft and 3 bedrooms), but not trigger any late night weirdness/alarms/etc. This will leave the new 110 cfm fan in the upstairs bath to be used only during showers, etc.

    Sound like a good plan?

  22. User avatar GBA Editor
    Martin Holladay | | #22

    Chris,
    Sounds good to me.

  23. Chris Yarsevich | | #23

    Great, I just ordered one!

    I also picked up a hygrometer and placed it in the main living area (open floor plan design). The first day I saw readings ranging from 52-55% RH. This was with 60 degrees inside temp, 55 degree high outside, 30 degree low.

    Does this seem high? I have read the ideal range is 30-50%.

  24. User avatar GBA Editor
    Martin Holladay | | #24

    Chris,
    Yes, the interior of your house is on the damp side -- which is consistent with your descriptions of your problems (a damp basement and condensation on your windows).

    The most likely source of indoor moisture is your basement. If I were you, I would continue to implement basement improvements (along the lines of my suggestions in Fixing a Wet Basement) if you can afford to do so.

  25. Chris Yarsevich | | #25

    I think you are probably correct about the basement being the source. The yard drains really, really well (sandy soil) and there are no obvious leaks or water intrusion. There is a sump and perimeter drain in the foundation that is always dry and the sump basically never runs.

    The basement is finished, but without the recommended foam board insulation standards. It has normal fiberglass insulation with the craft vapor barrier on the inside surface (not against the foundation wall, but against the dry wall), stud walls, and drywall. I think this is probably causing excess moisture when the warmer air hits the foundation wall? With that being said, the basement is never really over 60 degrees and the walls are underground, so probably not a huge temp delta.

    There are a few spots where I can see/touch insulation and the block wall and the insulation never feels moist or wet. There was one problem corner that we ripped out some drywall and insulation when we moved in, but has been dry ever since after installing a down spout extension.

  26. Chris Yarsevich | | #26

    Ok, I have an update and some more questions.

    Two weekends ago, I installed a Panasonic Whisper Green Select fan in my first floor bathroom. It has been running 24/7 for about two weeks, set at 50 cfm. The fan is a super nice unit and very quite, I am impressed.

    We have been having some pretty warm November weather, so the heat has barely been on. The RH inside the house ranges from 58-62% all the time, does not really change with the weather or time of day. With the fan running, condensation on the widows has disappeared.

    I also had a local HVAC company come by to take a look. They thought the fan was a good idea and probably more cost effective than a HRV. He was surprised how high the RH was in the house. He took a very careful look at the basement and saw no water issues, the perimeter drain in the foundation is dry, the sump is dry and basically never runs, no signs of water entering. He was surprised....

    He recommended a whole house dehumifier, but honestly said "this is a band aid" but it's not obvious where the moisture is getting in. However, the quote was 3400 dollars, which seems really high for a 65 pint unit that I can basically do the same thing with a 200 dollar stand alone unit. That $3200 difference pays for a lot of electricity to run the thing!

    Thoughts?

  27. Charlie Sullivan | | #27

    I'd give it a little more time. The humidity must have been higher before. There's a lot of moisture in the building materials, furniture, etc., that needs to come out. As we get into cooler weather it should be easier to maintain a lower humidity, and then once it's dried out you should be able to maintain it more easily.

    You can try the trick of taping a square of plastic to the basement floor and see if it gets damp underneath. If it does, moisture is coming up through the slab.

  28. User avatar GBA Editor
    Martin Holladay | | #28

    Chris,
    Charlie gave you good advice.

  29. Joe Suhrada | | #29

    What is the update here?

  30. Chris Yarsevich | | #30

    I am taking Charlie's and Martin's advice and giving it some time. Then fan is running 24/7, window condensation is gone, but indoor RH is still high about 58-59% the last few days. It does drop a few more points if I have the heat higher (say 68) but normally it is 60-65 inside.

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