Replies

1. 
   charlie_sullivan | Jan 31, 2015 10:21am | #1

   I'd think about it in terms of the balance of humidity sources and ventilation.

   On the humidity sources, how many residents? If you live alone, shower at the gym, and eat out, you'd have very little moisture input, for example.

   On the ventilation side, there are various guidelines for sizing the HRV, some of which are overkill. But there's also the possibility that despite a construction approach that should lead to a very tight envelope, there are some accidental major leaks in the envelope and/or ducts. Have you had a blower door test? Duct leakage test? Are the ducts run outside the envelope?

   I don't like the plan of running the HRV when the heating runs. That means that if you get enough ventilation during moderately cold weather, you'd have too much during cold weather. Or if you have the right amount during very cold weather, you'd have too little in moderate weather. It would be much better to set it to a lower fan speed and run it 24/7, or set it on a timer to run at a low speed some percentage of the time (e.g., on for an hour, off for an hour). The heat recovery works better on a low fan speed. Depending on the type of fan, the electric efficiency of the fan might be better or worse on low speed.

   If the blower door test shows the house isn't leaky, and the HRV is set to provide the minimum acceptable ventilation rate for your occupancy of the house, and you still have too little moisture, you could consider switching to an ERV. Some HRV units have a "swappable core" that you can change out without replacing the whole unit; with other brands you have to swap the whole unit. The ERV core exchanges moisture as well as humidity, so you can recover some of the moisture you are losing.

   A humidifier is usually considered a bad idea. The fact that you are still having trouble maintaining humidity with the humidifier on make me suspicious that you either have a major hidden air leak somewhere, or that the HRV is being run at a very high flow rate compared to what you need. The danger with a humidifier is that, if there is an envelope leak, there is condensation where the air is leaking out, and you can make that problem worse with the humidifier. In your case, at only 20% humidity, I'm not too worried--the problem is more when people succeed in maintaining 40-50% humidity with a humidifier, despite a major envelope leak, where there is moisture condensing in the attic or somewhere similar.

   My questions are:
   How many residents, showers, cooking, etc.
   CFM rating and/or setting of the HRV
   Duct location and sealing
   Blower door and duct leakage testing

2. GBA Editor
   Martin Holladay | Jan 31, 2015 10:27am | #2

   Derek,
   It's your house, and you get to operate the ventilation system any way you want. A large home (3,300 sf) without many residents doesn't need much ventilation. For goodness sake, turn off the HRV for a week and see what happens.

3. 
   davidmeiland | Jan 31, 2015 10:39am | #3

   Get the HRV its own control. If there is an outside air intake to the return duct, seal it. You only have a tight house if a blower door says so--perhaps the HRV isn't to blame.