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How to tell if I need a HRV?

BuildingNewb | Posted in Energy Efficiency and Durability on

So at the conclusion of my home energy audit, it was recommended that a heat recovery ventilator be installed because my house (built in 2002) is already too tight.

I always though excessive wintertime moisture was a good indication that a HRV was needed. However, the humidity level in my home drops to about 17% in the dead of winter. Isn’t this an indication that I already have a ton of air infiltration?

I’m very confused and wonder if the auditor performed the blower door test incorrectly. Could the forced air system be the culprit for the dry air?

Thanks in advance!

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  1. user-2310254 | | #1

    We would need to know the results of your blower door test.

  2. Expert Member
    Dana Dorsett | | #2

    All houses (even leaky ones) need ventilation. With tighter houses balanced ventilation where you KNOW the intake path for the ventilation air beats exhaust-only schemes since some of that path is usually through soil or other less-than-ideal path.

    Unbalanced HVAC or HVAC that is designed to take in ventilation air whenever the air handler is running can indeed be the cause of excessive winter dryness. The duty cycle of the air handler is highest when the weather is coldest, which is when the outdoor air is driest. Ideally ventllation systems would be controlled independently of heating & cooling loads.

  3. GBA Editor
    Martin Holladay | | #3

    Building Newb,
    There are several ways that a forced-air heating system can increase infiltration rates, especially if ducts are located outside the home's thermal envelope, or if leaky ducts are located in joist bays adjacent to leaky rim joists.

    In an older house, the decision to install an HRV is a judgment call. You are correct that very low indoor humidity is associated with air leakage, and I agree that is evidence that you may not need an HRV at this time. (Once you finish sealing the air leaks in your home's thermal envelope, however, and tracking down your forced air system's duct leaks, you may need to reconsider.)

    Other signs that your air isn't very fresh are lingering odors.

    Note that if you home's high infiltration rate is due to operation of your forced air system, your home's occupants may not be getting enough fresh air during the swing seasons -- spring and fall -- when your forced air system is idle.

    In other words, it's complicated.

  4. BuildingNewb | | #4

    You're correct, the ducts are located in the attic and the blower door test revealed that some air was in fact coming through the ducts. Dust also accumulates on registers from time to time. I intend on getting all new ductwork soon.

    I just find it hard to believe that the house is as tight as stated (80% of bsi standard) when there is such low humidity levels in the winter time. Especially when I can see daylight from basement under bay windows and feel significant drafts where the cantilever is located over the second floor. It doesn't seem to add up. Could a house that is 80% of bsi standard have such low humidity rates? We don't cook too often but take plenty of showers (with exhaust fan running) and have no indoor plants. We do have a baby so the bottle sanitizer is continuously putting steam into the air. I will say that last winter the thermostat was set to 60-65 most of the time depending if we were home or not. Now with the baby the thermostat is constantly on 68-70. Not sure how much of an impact this will have on increased humidity levels.

    The temperature here has been approx 20-49 degrees the last 2 weeks and humidity levels are hovering around the 30% mark.

  5. Expert Member
    Dana Dorsett | | #5

    Leaky ducts in the attic have notoriously high parasitic air handler driven infiltration, even if the house measures less than an IRC code-maximum (for cold climates) 3 air exchanges per hour @ 50 pascals.

    If there is any way to bring the replacement ducts completely inside the insulation and pressure boundaries of the house (sealing up and insulating the old duct & register penetrations of the ceiling plane) the consequences of duct leaks drop pretty dramatically, though you'll still want the new ducts to be reasonably balanced for room to room temperature and air-handler induced pressures.

  6. BuildingNewb | | #6

    Unfortunately bringing them inside the thermal void) boundary isn't an option. The new ductwork will have a considerably longer main trunk with small lengths of flex coming out of it. Everything will be sealed with mastic tape or silicone prior to insulating.

    Right now there is a very small main trunk with long lengths of flex, sometimes exceeding 25ft.

  7. Jon_R | | #7

    Indoor humidity is also highly effected by indoor moisture sources and so is not a good indicator of air exchange rate.

    In mild weather with no wind, even fairly leaky homes will be under ventilated. A 10+ ACH@50 home only *averages* to adequate ventilation.

    For around $100 you could buy a CO2 monitor and look at the unoccupied decay rate under various conditions (HVAC use, outdoor temp, wind, etc). A big change with HVAC use indicates a duct leak/pressure-balance problem.

  8. user-2310254 | | #8


    Perhaps you could use a split mini system? If you eliminate (or minimize) ducts, it will be easier to seal the home and bring the HVAC inside the conditioned space. You could leave the attic ventilated and just concentrate on improving how it performs.

  9. BuildingNewb | | #9

    That's definitely the practical solution but I just think those things looks ugly to tell you the truth.

  10. Expert Member
    Dana Dorsett | | #10

    "...a considerably longer main trunk with small lengths of flex coming out of it..." isn't exactly a prescription for operating efficiency, though short sections of flex off a hard-piped trunk can be a significant improvement over a "flex Medusa" or even a (significantly better) "flex octopus" approach.

    If replacing the ducts, spend the time/money for a real Manual-D duct design, even if you have to hire a third party engineer to specify every duct ell & tee.

    A mini-split option is probably beyond your budget but doesn't have to be a wall-coil blobs taking up valuable wall space. There are ceiling cassettes that look quite a bit like ceiling registers, as well as mini-duct cassette that can be tucked in the top of a closet serving 2-4 nearby rooms with fairly innocuous standard register grilles, etc.

    It's a lot easier to air seal around refrigerant lines & penetrations than ducts, and to build air tight insulated enclosures over ceiling cassettes. But any mini-split solution is more expensive to install than a full duct system redesign/replacement, and the operating cost isn't always going to be cheaper than condensing natural gas (but it can be less half the cost of propane, depending on the local markets.)

  11. GBA Editor
    Martin Holladay | | #11

    Building Newb,
    What does "80% of bsi standard" mean?

  12. BuildingNewb | | #12

    I apologize, I meant BPI standard. It's apparently a metric for determining the ideal amount of ventilation.

  13. GBA Editor
    Martin Holladay | | #13

    Building Newb,
    Do you know the actual blower door results in air changes per hour at 50 pascals?

    If the BPI standard is a target, and you say that your house air leakage is at 80% of the target, does that mean that your house is even tighter (with even less air leakage) than the BPI target?

  14. BuildingNewb | | #14

    3,797 cubic feet per minute of air exchange is the standard and 3,045 cfm was measured. So yes, tighter than the standard.

    I called shenanigans because the winter air is so dry in the home. Someone else is coming back to perform another test.

  15. GBA Editor
    Martin Holladay | | #15

    Building Newb,
    It sounds as if your home's thermal envelope has a low level of air leakage, which is good. That would tend to support the conclusion that air leakage is driven by your forced-air system.

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