A “Pretty Good” Ventilation Plan
RichardMay
| Posted in Mechanicals on
Hi all… I’m trying to piece together a ventilation strategy for a ~2500sq ft two story 2017-built code-minimum home in CZ 2A. I’m hoping someone can point me in the right direction.
* Looking over some of Dr. Joe’s articles on BSC, it seems having a small fresh air intake on the return side of the central HVAC is a valid (though suboptimal) strategy for make-up air. This solution requires two dampers — one manually adjusted to throttle airflow and the other to auto-open/close with AHU fan activation — plus a high-MERV filter. Anyone here using this strategy and can comment on its effectiveness? Are there any recommended in-line filtration and/or damper assemblies on the market that would be ideal for this application?
* In lieu of the above, I could use that small Panasonic ERV but I’m not sure I’d see enough benefit to justify the expense. Are these things useful in CZ 2A? It seems like people use them mainly up North to keep heat and humidity in — I need to keep heat and humidity out.
* There will be cat litter boxes in the laundry room upstairs. This seems like a good exhaust location as I can ventilate the stink along with the rest of the house from this room. The lowest cfm bathroom fan I can find is the Panasonic WhisperSelect set to its minimum (30 cfm) which might still be more than I need. Perhaps I could run it at a 50% duty cycle with a fancy wall switch? Also looking at variable radon fans though it’s hard to discern cfm specs at their lowest setting.
* Here in CZ 2A the indoor dew point is substantially lower than the outdoor dew point pretty much 24/7/365. Am I correct in thinking a slight positive pressure within the building envelope, which would be deadly for houses in the Frozen North, might actually be a good thing here? I envision a protective effect on leaky wall assemblies, pushing ~55-60F dew point air into the walls, displacing whatever swamp air might be sitting in there, and assisting with drying any bulk water that may have leaked-in. Or should ZERO pascals across the building envelope be my goal? I haven’t seen anything on BSC about this. But then again I still have ~45 papers queued up to read. 🙂
* I plan to convert the attic to unvented via ccSPF. Given my plans to install mechanical ventilation, should I cut the required holes and position the vents before the foam goes in? I would think the job would be a lot more difficult once foamed…
* Regarding make-up air for the actual (high-cfm) bathroom fans plus the range hood, I see people crafting wired solutions to ramp up intake cfm when these types of exhausts are activated. Are there any feedback-driven solutions on the market that use barometric transducers to ramp intake cfm up/down as needed? Seems like a good way to compensate for wind pressure, stack effect, duct leakages, half-clogged filters, clumps of dust on fan blades, etc. Has this ever been tried in a commercial product? (yes, I’d need a real intake solution here, not the hacky HVAC solution above)
Sorry for so many questions but I want to get these details hammered out before I finish moving in. It’ll be more difficult if I have to circle back later.
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Replies
I'm also in CZ 2A, and through this site I've found quite a lot of info on why an exhaust-only ventilation system is a bad idea in a hot-humid climate. I've got one of the Panasonic Green bath vents that I do run for hours on end when the weather is dry outside, but that only means maybe 50 days per year. It seems like the podcast discussed this topic on their dehumidification episode.
Conventional wisdom is that HRV's are for cooler climates, and ERV's are more appropriate for 2A. I don't see why an ERV would not work for you. It will bring in some warmer and more humid air most of the year, tempered somewhat by the heat and humidity exchangers, but that's the unavoidable nature of fresh air in 2A.
I'd say you should do anything you plan to do before putting in closed cell foam, provided you won't need access to the area later. Closed cell is hard to cut.
Richard,
Before you do anything, consider hiring an energy rater to conduct a blower door test. The test will give you data points to guide your efforts and also help to identify problem areas.
Yes, with AC running, a slight positive pressure is beneficial for wall moisture and not pulling in mold. Because of stack effect and wind pressures, zero pressure everywhere isn't possible.
Hi Richard
We built a house in CZ 4, trying for exhaust-only ventilation. Here is what we learned:
1) The Panasonic Whisperquiet fans we used are INCREDIBLY sensitive to back-pressure. We installed them "normally" and they sounded like lawnmowers and delivered maybe 1/3 of the rated airflow. (After we re-ducted one of them with a short, straight run of 6" duct, it performed as advertised.)
2) My house turned out tighter than expected (0.54 ACH@50 Pa), which is great, but it meant that I needed to add a supply air fan to the system. Fortunately, I had a spare duct for the purpose.
3) My plan for dehumidification was to run a portable dehumidifier when needed, as is often done up north. This was a big bust: in Summer, the dehumidifier ran 24/7 just to maintain RH of 58%. The thing added about $80 / mo. to my power bill, and the noise was intolerable. We are now trying to figure out a better form of dehumidification, and it will be an expensive retrofit. I urge anyone building in the South to incorporate dehumidification into their ventilation / air quality plan.
4) I bought an AirVisual air quality instrument, and it has been very helpful in tuning the ventilation rates to achieve our goal CO2 concentration. If we had simply set the system to ASHRAE ventilation norms, we would be overventilating by 100% to 200%.
5) This is slightly off-topic, but let me add: I wish I had hired someone to do an energy model during the design phase. I wound up doing the energy model in the end anyway, and I learned a lot from it.