HRV ducting
In the middle of building a 4000 SqFt rambler ICF to the roof, triple pane windows, R-50 attic. I am doing my best to pay attention to detail and seal everything up as tight as possible. I will have 3 mini split heads for heating.
My question is in designing the ductwork for the HRV. I recently discovered Martin’s article on exhausting stale air from the bedrooms to pull conditioned air into them and help even out temperatures. I am really liking the idea of this to keep temperatures even throughout the home vs dumping the cold/warm fresh air into the bedrooms.
My current plan is to run a separate 4″ exhaust duct from each of the 3 bathrooms as well as four 2″ ducts (one for each bedroom). I plan on installing inline dampers on each of the ducts that I can control via IFTTT and automate via humidistats etc… As far as the supply side goes, to keep things simple. I was wondering if it would work to allow all of the fresh air to be simply dumped into the utility room where the HRV will be leaving the exhaust ports to distribute the 200 CFM max flow throughout. Would this work? Cause any problems? I would need to undercut the door significantly and possibly install a vent through the door or wall. Thanks in advance, any advice is much appreciated.
Brandon.A.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Brandon,
The 2011 article you are talking about -- "A New Way to Duct HRVs" -- includes a 2013 author's postscript which undermines the article's argument. My postscript notes, "Dr. John Straube, a principal of the Building Science Corporation, has challenged the logic behind the novel method of HRV ducting advocated here by Robb Aldrich. In an article titled 'Choosing HVAC Equipment for an Energy-Efficient Home,' Straube is quoted as saying, 'Ventilation air doesn’t do much to move around heat. ….Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all. Open doors work better than HRV ducting.'”
Further undermining the idea of using a ventilation system to equalize heat between rooms, I wrote a follow-up article, "Using a Bath Fan to Equalize Room Temperatures." I urge you to read that article. It should disabuse you of the wisdom of your idea.
The best way to duct an HRV is to introduce fresh air to the living room and bedrooms, and to pull stale air from bathrooms, the laundry room, and sometimes the kitchen (but never from the range hood).
The idea to dump all the fresh air into the utility room is not going to do a very good job of getting fresh air throughout the house. The tiny amount of exhaust air in each bedroom is not enough to create a pressure differential big enough to pull a significant amount of air into the room. You'll be relying mostly on diffusion. The time delay for fresh air in the utility room to mix and make its way into the bedrooms is going to be pretty substantial.
A few points:
X CFM from the house into a bedroom and X CFM from outdoors into a bedroom are not equivalent in terms of pollution and humidity concentration.
The primary driver of airflow through open doorways is temperature differential. How much there is is highly variable. In general, it provides pretty good pollutant/humidity dilution. Heat movement - it depends.
> Ventilation air doesn’t do much to move around heat.
But it can be pretty good at moving cold from outside. Eg, 30 CFM of 0F air is 2269 BTU/hr. But this delta-T doesn't happen with HRVs/ERVs (ie, it's an exhaust fan issue).
Consider pollution (including CO2 and odors), temperature and humidity. All should be controlled for good comfort/health.
Best is to exceed ASHRAE 62.2 *and* 62.1.
Dampers and automation to avoid ventilation when/where it isn't needed is a good idea. This will help with the high closed door bedroom CFMs (eg, 40 CFM) needed. Multiple spot ERVs can have a similar effect.
"But it can be pretty good at moving cold from outside. Eg, 30 CFM of 0F air is 2269 BTU/hr."
I think the only difference between the scenarios you are comparing is temperature differential. If you had air that was 140F, I imagine that you could supply a substantial amount of heat at 30cfm. I know there's a difference in heat capacity of air at different temperatures, but I think it's slight.
I agree. Another example (probably much more relevant here) is an ERV providing 30 CFM of supply air to a bedroom when it's 0F outside. Say 80% heat exchange, so 450 BTU/hr of cooling. Body heat alone will offset that - but it leaves the other heat losses as a problem.
I'm not sure what you mean by "as a problem". You need the ventilation; it's not like it's optional. You have to account for all the heat losses, of which ventilation is just another one.
The ventilation, in winter, will bring a heat loss. The key is to make sure its effect is not noticeable. Making sure the delivered air temperature is not below 61F from any of the registers is a common strategy, and has worked for us. Some say to make sure the air doesn't blow directly on occupants, but in my experience that is a non-issue. Just put them on the ceiling, and as long as they have a register that directs them out at some angle besides straight down, no one will feel any air coming out.
Thanks for your input everyone, great information and great articles. This helps clear things up. Once again GBA has answered my questions. -Cheers
Brandon,
My ERV is similar setup as you suggest (exhaust from the bedrooms). The fresh air is supplied near the peak of the living space, in the winter time it does a pretty decent job of mixing the stratified air. From my IAQ monitor (VOC, CO2 and PM4), the air in the bedrooms is as fresh as the rest of the house with the doors closed so it does work.
For heat/cool you would be much better off with grouping a bunch of low load zones together (ie bedrooms, bathroom, study) and using a slim ducted unit. Generally most wall mount minisplits are way over sized for a typical bedroom never mind a well insulated one. If you do the design work up front, you can hide the unit and the ducts (AC unit in the ceiling of a larger closet, a couple of 2x6 or 2x8 interior partition walls for the ducts) and can get an almost invisible HVAC system.