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3wr4T8MfuL | Posted in Energy Efficiency and Durability on

The energy auditor has just installed an HRV in our house. The blower door test was approximately 2.5. The HRV is installed in the attic with intakes from two of the bathroom fans. The intake is from the outside and after the HRV connects to the forced air return.

Here are my questions specifically regarding the HRV
1) How fast should the HRV run or how much air should it pull when its run continuously?
2) Since the intake into the HRV is from the bathroom fans, the HRV is connected to a timer switch within the two bathrooms that for either 20/40/60 minutes will make the HRV run at full speed. The cfm in the bathrooms was measured at 30 cfm when the HRV was on max power. This seems way underpowered. One bathroom is 1755 cubic feet, and the other is 792 cubic feet. What’s the best strategy to ventilate these bathrooms?

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  1. davidmeiland | | #1

    I agree that 30CFM is minimal for a bathroom, at least one with a shower that's used daily by a family. I'd prefer to have 80CFM or more so that surges in humidity can be knocked down quickly.

    But, it sounds like you haven't lived with this setup long, so... spend a few weeks with it and see how it works? The goal is to provide adequate fresh air coming in (without over-ventilating) and to remove moisture and pollutants from the house quickly enough, while recovering as much heat from the exhaust air as possible. The whole thing depends on a lot on number of occupants and their habits. How many people in the house?

    To decide if it's working, you probably need to monitor interior humidity, preferably with an accurate instrument. At 2.5ACH50 you are quite likely to need mechanical ventilation running at least some of the time. What climate zone are you in?

  2. 3wr4T8MfuL | | #2

    We are in Zone 5 (Chicago). Prior to the installation of the spray foam in attic and HRV installation, we had a 75cfm fan in the large bathroom that was inadequate -- even after 1 hour of running on a timer, the bathroom was very humid. For the 1755cf/7.5=234 -> That's what I roughly calculated for 8 air exchanges per hour. Within the bathroom - steam shower, toilet, 2 sinks, jet tub. I don't know if the 234cfm calculation however is valid with an HRV in place. We've unfortunately found that 30cfm for the 792 cubic foot bathroom to be inadequate, so I'm guessing its going to be terrible for the bigger bath. I'm just wondering what the options are at this point. It's not like I can make the HRV pull any more air -- 30cfm is at the max setting. Do I unhook the HRV from the intake fan and make a separate intake away from the main fan? Do I simply out another fan that vents to the outdoor somewhere else in the room?

  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "The HRV is installed in the attic with intakes from two of the bathroom fans." Does that mean that your HRV installer didn't remove the obsolete fans? That doesn't make any sense. You need a ceiling grille, not a bathroom exhaust fan, if you have replaced your eshaust fan with an HRV system.

    Most HRVs have controls that allow for a low level of continuous bathroom ventilation as well as a high-speed boost mode controlled by a manual override switch in the bathroom. As you sure that feature isn't available with the model that was installed in your house?

    If your HRV has limited controls, with no high-speed boost, you may have to have two types of exhaust ventilation in your bathroom. If, after living with the system for a few weeks, you conclude that this is the case, you would need to disconnect the HRV duct from the existing exhaust fan and reconnect the existing exhaust fan to the old exhaust duct. Then install a new ceiling grille for the HRV.

    For more information on ventilation rates, see Designing a Good Ventilation System.

  4. 3wr4T8MfuL | | #4

    I'm sorry if I didnt explain the situation my fully.

    Yes the ceiling fans were removed. The grills and locations from the fans were used to place he inlet to the HRV.

    The HRV has a low level continuous mode. The flows when measured were anywhere between 18-22 CFM.

    The HRV does have a manual over-ride switch located in two bathrooms that has a high speed boost. When activated, the flows were measured at between 30-35 CFM. Despite this high flow rate, the humidity in the bathroom one hour after running the shower for 10 minutes still seems excessively high. I don't have any measurements of the actual humidity levels -- just my sense of feel,.

    I guess my question was answered however above. If the HRV ventilation is inadequate, remove the HRV intake, reinstall the fans, and make a new location within with a new ceiling grille.

    Guidelines for flow rates and such seem like a mixed bag. Energy Star/ASHRAE recommendations for a bathroom are 20+ CFM . If flow rates are measured as within the current range but they "feel" inadequate, I would conclude they are.

  5. davidmeiland | | #5

    Your comments about the former bath fan are confusing. If the room is 1775 cubic feet, it's in the range of 200 square feet... large but not palatial. I would expect a 75CFM fan to make a dent in that in an hour of running.

  6. 3wr4T8MfuL | | #6

    You are correct -- the bathroom is over 200 square feet -- Its definitely not palatial. It contains a steam shower, two sinks, and jet tub. Maybe I'm being overly optimistic, however after running just the shower for just 10 min (no steam setting) and turning the 75 cfm fan on for 1 hour via the timer switch, it will take about 4-5 hours before the room feels without any humidity. Is this normal or do I just have just have too high of expectations?

  7. davidmeiland | | #7

    How long does it take for the fog to clear from the glass after the shower is turned off? I know what you mean about humidity being palpable, but what about picking up a cheap/decent hygrometer? Something like this, although I like having a remote sensor too

    I'm guessing the steam shower is going to overwhelm the current ventilation.

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