I just recently moved into a new construction, two-story, 1800sq/ft home with a Venmar Constructo 1.0 HRV system. I went down into the crawl space today to check on things and noticed something a little strange with the HRV ducting. The HRV system is integrated with the forced-air heating system. From what I can tell, it's setup such that the stale air intake is connected to the heating system return ducting and the fresh air supply is setup as a supply-return connection (the fresh air is connected to the return air ducting of the forced-air heating system). The odd thing I noticed was looking at the stale air intake ducting (connecting from the forced-air ducting to the HRV stale air intake port), there is a big opening/hole just prior to the HRV. So instead of the stale air only coming from the "returns" in the home, it's also coming from the crawl space (I'd imagine most of the air is coming from the crawl space given how big the hole is and how close it is to the HRV). I know this is only a ~100 CFM system but even when I have it set to high, I can't feel any air being pulled in from the returns (just by placing my hand over them). I'm not sure if I should feel anything but most of the "stale air" is coming from the crawl space and not the actual living areas in the home. Doesn't this defeat the whole purpose of an HRV system in the sense that it's supposed to be "pulling" stale air from the house and "pushing" fresh air into the house? Instead, I feel like it's pulling stale air from the crawl space and pushing fresh air into the house (which is even questionable... can't feel anything coming out of the vents when just the HRV is running, even on high). Looking at the installation instructions of the HRV system, it also mentions this: "Caution: Be sure to duct air outside – Do not intake/exhaust air into spaces within walls or ceiling or into attics, crawl spaces, or garage."

Please clarify: the forced air return ducting is in the crawl space and has a hole in it (uncovered?) just before the takeoff for the HRV?

Not to worry about not feeling air moving. An HRV fan is too small to be detected in multiple-register forced air. In normal operation, the furnace blower will do the heavy lifting.

Two questions:
1. Is the crawlspace vented to the outside or sealed?
2. What climate are you in?

Posted By toddm on 28 Sep 2015 11:19 AM
Please clarify: the forced air return ducting is in the crawl space and has a hole in it (uncovered?) just before the takeoff for the HRV?

Not to worry about not feeling air moving. An HRV fan is too small to be detected in multiple-register forced air. In normal operation, the furnace blower will do the heavy lifting.

The forced air return ducting splits into two prior to reaching the heating system and HRV. Just prior to the HRV, there's a big hole (which is about 10ft down from the split in the forced air return ducting).

Posted By chrs on 29 Sep 2015 01:46 PM
Two questions:
1. Is the crawlspace vented to the outside or sealed?
2. What climate are you in?

The crawl space is sealed. I'm up here in Anchorage, AK so it can get some-what humid (nothing like in the South of the lower 48 though). I was thinking maybe they did it to prevent the crawl space from getting too humid, but it just seems this significantly takes away from the primary purpose of the HRV which is to recirculate the air in the "living" spaces of the home.

I am not liking this setup. Not only does the hrv draw from the crawl, but the furnace could also draw from the crawl. Unless there is a backdraft damper in there. The exhaust from the crawl could help control humidity in the crawl, if it then sucks conditioned air from the house into the crawl, but in some conditions that would make the crawl humidity worse.

One idea would be to seal that hole and monitor the crawl humidity, and then address that only if necessary, and perhaps with a dehumidifier instead. I assume the walls of the crawl are insulated. What about the floor?

But even with that hole plugged, shared ducts like that can be problematic, and could undermine the performance of both systems. I would be inclined to put in a new minimal duct system for the hrv to keep them separate. You might not be easily able to include the second floor in that, but it is not clear that the second floor is getting any air from the hrv now anyway.

But maybe others will be more enthusiastic about making the setup you have work.

I'm with Chrs. You can use the crawl space as a conditioned return plenum but that doesn't explain the ductwork. I'd go back to the builder and ask for a description of how the forced air works in this house and why the hrv exhausts from a location expressly proscribed by the manufacturer. Sad to say it is impossible to underestimate the lack of knowledge in the field about hrvs and ervs.

" In some studies, the majority of HRVs were found to be set up incorrectly. Many were shut off by homeowners who either didn’t understand them or found they were not operating properly."

http://www.ashireporter.org/HomeInspection/Articles/Heat-Recovery-Ventilators/11617

Sharing heating/cooling ducts with a HRV/ERV almost always causes an imbalance and +/- building pressurization. Avoid it if possible. Same for connecting any closed portions of the building to a single heating/cooling duct. Or using not-completely-open parts of the building as a plenum.

As chrs says, seal up the crawlspace and put a dehumidifier in there if necessary.

I took a quick look at the Venmar installation instructions. They show an open end of a duct in the crawl, but that's in the absence of any other return ducts, or as a shorthand notation vs. drawing all the return ducting. It might be that the installer misunderstood that diagram and thought an open end was needed.

But for that style of installation, they also note that the fan on the heating system needs to run for the HRV to work, so they recommend setting the heating system fan to run 24/7. That seems to me like a waste of energy. If you don't need that much ventilation, you could also set up the HRV to only run when the heating system is on anyway, but then you don't get much ventilation during mild weather.