@Kristin, moisture on window
@Kristin, moisture on window glass at sub-zero temperatures isn’t all that surprising (depends on window vintage). Water dripping from basement walls is something else altogether. If you can identify the source of the moisture, the solution will flow from that.

In what part of the country are you located? How old is the house?

If basement is dry in the summer, as you indicate, this suggests an internal moisture source since exterior moisture (from groundwater and/or improper site/roof drainage) would likely manifest throughout the year. Inadequate spot ventilation (from showers, laundry, and/or cooktop) is the most common interior moisture source, but this would also manifest year round. OR perhaps you have elevated indoor RH throughout the year but don’t realize it since there’s no condensation?

Do you have any unusual moisture sources such as a steam shower? Is it possible a humidifier is installed on the furnace that you’re not aware of?

Is the furnace (and presumably, a gas water heater) atmospheric or closed combustion? (The former will have a metal flue, while the later will have a PVC flue.) Gas combustion appliances generate a LOT of water vapor, but that moisture gets vented to the outside with other combustion gases. Your CO detector should alert you if there’s a problem… the fact that you’re alive suggests that’s not the case 🙂

Do you happen to have an unvented gas fireplace insert?

A humidity & temperature data logger (e.g., Onset Hobo) may reveal clues, especially if a pattern emerges.

Does anybody know how long
Does anybody know how long defrost cycle of hrv continues when once it started ?

Although it’s true that super tight homes can have the opposite problem (high wintertime RH), that’s generally only true for smaller homes in terms of the occupancy-volume ratio. And even those homes can have low RH during colder weather (the colder the air, the drier it will be). Keep in mind that even a tight home will exchange its entire volume with the outside at least once or twice a day, plus contributions from central ventilation, clothes dryer, range hood & bath fans. That’s a lot of dry air in cold weather!

As for your question — a steam humidifier will not cause any issues with your ERV. The only air that passes through its core is outside air and exhaust air from the house.

As an aside, in my considered opinion, an ERV operating at 20 to 40 CFM is largely irrelevant in a 6,200 ft2 house given the natural air changes through your building shell. I generally don’t specify or recommend ventilation systems on homes that size unless required by code.

@NS, here’s another
@NS, here’s another interesting factoid: The pressure differential you described is probably on the order of a few tens of Pascals at most. A blower door is typically operated at a 50 Pa differential, which is enough to slam a door shut or cause ears to pop. As a point of reference, an equivalent reduction in atmospheric pressure (and oxygen density) occurs when you walk up a 10 or 15 foot hill!

Ok, I’m familiar with that
Ok, I’m familiar with that ERV and how it operates. I’d be surprised if it caused the pressure differential you describe. The fan is only rated at 40 to 50 CFM, which is less than the average bath fan.

Nevertheless, regardless of what’s causing the differential, your home is not being robbed of O2. For every cubic foot of air being sucked out of the house (by fans, winds, stack effect, whatever), there’s a cubic foot of outside air entering, somewhere. Otherwise the differential would continue to grow indefinitely!

When something causes the house to go negative (or positive), the imbalance between infiltration and exfiltration is only transient until the two reach equilibrium. If the shell tight, a fan will activate leakage paths that otherwise may have been static. If the shell is tight enough, the fan’s flow volume might decline. But one way or another, the “in” and “out” will be always be equal over time. The transient imbalance is probably on the order of seconds, although admittedly that’s not something I’ve tested.

Bottom line, if you actually have sustained elevated CO2 levels, it’s likely because the fan CFM + natural leakage isn’t enough for the volume and occupancy of the home, not because the fan is operating in exhaust only mode.

Yes, according to the manual,
Yes, according to the manual, that is how it works below 30F and even more so below 20F. I am using a Panasonic FV-04VE1 WhisperComfort. I agree with your logic, it should be passively pulling through the vent plates. When I opened the door this morning, there was a woosh of air into the room. I know there can be slight pressure differences between rooms, but this was different. It was almost as if there was less air in the room and fresh air rushed in. I definitely feel like a train hit me and my lungs are sore today. When I immediately went outside breathing fresh air was a boon.

I am sure the HVR is working as it had been fine until last night, and last night was the first night it had to work below 20F which is when it goes into its defrost mode. Regardless, I am going to order a CO2 meter/alarm and an O2 levels meter/alarm to be sure what actually happens.

Are you sure it’s operating
Are you sure it’s operating in exhaust only? By definition, for every CFM of house air exhausted, a CFM of outside air is brought in through induced infiltration. If an exhaust fan pulls the house negative, any difference between exhaust and induced infiltration is transient. Otherwise, the negative would continue to grow over time.