Ventilation strategy
help with ventilation strategy.
My house will be tight, but unfortunately there are 3 leaky wood burning fireplaces. I will do my best to seal these dampers, but they are staying… So essentially I believe these holes will be the leakiest place. I have lived with soot smell in my last house weeks after I had a fire, simply because of negative pressure in the house.
So i guess in this scenario I am a fan of supply ventilation.
Also, manual j shows almost double the heating load vs cooling load. I will most likely be choosing a whole house of ducted and ductless from Mitsubishi. I will probably end up sizing to closer to the heating load, so I can avoid too much supplemental, which in turn would be somewhat oversizing the AC.
Knowing my a/c will likely be oversized, and I have leaky fireplaces, I am leading toward an ultra air whole house venting dehumidifier attached to one of the high static pressure air handlers.
1. I think this strategy will help with the oversizing of the ac
2. I think the supply ventilation will help with keeping positive pressure.
3. I am in zone 3 close to the hot/humid line.
Questions…
Is my crazy obsessive mind thinking the right way?
For those who don’t like this strategy is it only because of the energy penalty of the whole house dehumidifier?
Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Dean,
It's very rare for a house to need a whole-house dehumidifier.
You need to come up with a strategy to seal up your fireplace flues when your fireplaces are not in use. Many people use a Chimney Balloon or Fireplace Plug.
Assuming you can limit air leakage through your home's thermal envelope, your ductless minisplits should do an adequate job of keeping your indoor relative humidity levels in a reasonable range, without incurring the expense of a whole-house dehumidifier.
If you want to install a supply-only ventilation system, you can. Keep it simple, using a 100 cfm fan and two or three 4-inch-diameter ducts that supply outdoor air to the locations where you think you need fresh air. Control the fan with a timer.
Another option to consider for your leaky dampers is a chimney top damper. I replaced an old, bent, leaky damper with a Lyemance. No brand affiliation. There are probably others. I liked that it was a positive seal and also kept out the rain. This was in a mild climate, so a little investigation might be in order in a colder climate with frozen precipitation.
Leaky fireplace dampers aren't necessarily bad.
One reason is that a tight sealing damper may stand a better chance of rusting/sticking closed between seasons, so says the "Leaky Fireplace Damper Association of America" anyhow.
The other reason is that a leaky damper may still provide some draft out of a fireplace. I learned this actual benefit as I researched best operation of our masonry heater. Without that slow residual draw I would have to wait much longer each firing cycle to close the output damper thus trapping maximum heat in the firebrick core. Those last few hidden embers take hours to extinguish themselves even if you poke at them.
Now closing the damper when visible embers are out works for me as I have a firebox door that seals well and an input damper, and because, perhaps most importantly, we have ten smoke/CO alarms for peace of mind. (Sometimes I think our electrician was a bit too literal on code with respect to smoke alarm placement; the Duracell truck will be arriving later this week.)
Conventional air conditioners do poorly at dehumidifying when they are oversized. Minisplits do much better, because they can modulate--run at a lower level when that is what's needed, rather than cycling. Furthermore, if you have several, you can turn on only one and that one might not be oversized.
I think supply only ventilation is safer in a warmish climate than in a very cold climate, where pushing warm humid air into a crack in a cold envelope can create condensation and then mold and rot. On the other hand, pushing dehumidified air into a crack helps.
Thank you for the advice for the chimneys. Martin, so dedicated ducted supply only ventilation... I like that idea, because most likely going ductless in a large portion of the house. Can you help me find more specifics on how to pull that off without the benefit of ductwork and filtration that exists with a central fan strategy. How can I get filtration on that incoming air... and any recommendations on a fan?
Dean,
You want an inline filter box. You can purchase one on the web if you can't find a local source. Here is a link to give you an idea of what a filter box looks like: Inline filter boxes.
You want a supply fan sized to meet your ventilation requirements. There are lots of suppliers; one is Fantech. Here is a link to get you thinking about what is available: Fantech inline fans.
Interesting, the marijuana growing industry sells a lot of inline fans with filters designed to remove odors. If these products interest you, here is a link: http://www.htgsupply.com/Category-Fan-&-Filter-Combos