Are HRVs really necessary? - Page 2

Exeric · 12-18-12, 04:35 PM

You are right about the framing. I looked at the framing around doors and with the headers over the doors the transom window clear space would only be about 6" high and door width. The more I think about it the more I realize the door transoms I remember that looked good were in rooms with high ceilings. So that won't work for me after all since I don't have them.

The more I think about the venting duct idea the better I like it. I won't use the through wall approach, but floor ducts instead. They will go through the conditioned air crawl space. (another long term project)

The reason I am liking the venting duct idea more and more is for two reasons. The first is that they will work to get fresh air into the bedrooms, like I said. The second idea just came to me, like a brainstorm.

Usually people don't spend a lot of time in the bedrooms during the day. It's sort of uneconomic to heat them during the day, especially in an older home like mine. However you could use the bathroom door as an easy method of controlling the flow of heat in the house. With the bathroom door open most of the flow through ventilation will go out that door from all parts of the house. If the bedrooms are not heated sufficiently by design they still won't be with that door open.

When you close that bathroom door, and it has a good seal, then all the fan air will come from the bedrooms. If those bedrooms have good flow ventilation from the rest of the house, either through an open door or a closed undercut door, then all the heated air from the communal living space will travel into the bedrooms. If you aren't spending time in the bedroom except for sleeping then in winter what you would do is close the bathroom door an hour before bedtime. If your thermostat sets back at night then at the point point the door is closed all the communally heated air would be sucked into the bedrooms and then out through the bathroom. So the communal airspace heat would act as a reservoir that goes to the bedrooms and bathroom at night.

Of course you could do the same thing during the day. And whenever you wanted the bedrooms to heat up right away just turn the bathroom fan on high for an hour, 80 cfm, and let it's internal timer turn it off after an hour. It seems like it might work pretty well, at least it does in my mind.

Exeric · 12-18-12, 06:37 PM

Actually the transom idea was good because it would take the warmest air near the ceiling from outside the bedroom. One could replicate that anywhere with a through wall register from bedrooms to communal space located near the ceiling in the bedroom. Privacy really wouldn't be affected.

So the coldest air in the bedroom that is near the floor would be sucked out through the crawl space duct to the bathroom. That air would be replaced with warm ceiling air from the communal space. This might work...

Exeric · 12-19-12, 04:58 PM

Quote:

Originally Posted by creeky

lol.
But where are you putting your fresh air vent(s)?

I've been thinking about that question. You're right that it doesn't address the whole ingress/egress question like an HRV does. But from my reading, and I'm learning as I go along just like everyone, there is always going to be some air leakage in any house, even the tight ones. My intuition is that as long as the house isn't super-duper tight a fan pushing air outside will work fine. All the small air infiltration points will balance the fan air going outside. The volume of air that is actually exchanged will be determined by both the fan cfm rating and by the air restriction caused by the tightness of the house. Only in extremely tight houses will the air ingress restriction be enough to cause too low of an air volume exchange. Anyone who has more knowledge about this feel free to correct me. I won't be offended.

If you did it the other way where you used an ingress fan only it wouldn't work as well because cold air would be pulled in at one point. Instead of having cold air slowly coming in at tiny distribution points all over you'd have one very cold drafty place in the house that no one would want to be in.

From everything I've read the only real drawback to having an air egress fan in bathrooms for house ventilation is that they require smaller single floor homes in not extremely severe climates. The other drawback is you have to provide ventilation to the bedrooms when the bedroom doors are closed. But one would also get other advantages by providing for that bedroom ventilation, so it's not really a drawback.

One more thing, and it's a biggie. If using a bathroom fan for ventilation didn't work out
you probably wouldn't be out any time or money. You'd still have a good efficient bathroom fan that you'd want in any case. And you could probably use or modify any air ducting to the bedrooms that you installed. You really can't lose much going this route and could possibly gain a lot.

creeky · 12-19-12, 05:55 PM

thanks for pointing out the framing issue. it reminded me that my grandmother also lobbied for 9 ft ceilings. for a mennonite farm house. circa 1964 (might have been 66).

most of what I've read is that people put their fresh air intakes close to heat sources (ie, behind the wood stove) and away from the bathroom, so that the fresh air is pulled through the house.

the thought behind having a dedicated fresh air vent being that the fresh air should be pulled from a known area (again, clear of combustion gasses or ...) and should enter the house somewhere where the cold air won't affect the comfort in that area.

for me I think the fresh air intake would be perfect behind the wood stove shield. warm the air and provide an extra measure of protection. although, clearly, there is an element of getting carried away here ...

MN Renovator · 12-20-12, 07:17 AM

"I've been thinking about that question. You're right that it doesn't address the whole ingress/egress question like an HRV does. But from my reading, and I'm learning as I go along just like everyone, there is always going to be some air leakage in any house, even the tight ones. My intuition is that as long as the house isn't super-duper tight a fan pushing air outside will work fine. All the small air infiltration points will balance the fan air going outside. The volume of air that is actually exchanged will be determined by both the fan cfm rating and by the air restriction caused by the tightness of the house. Only in extremely tight houses will the air ingress restriction be enough to cause too low of an air volume exchange. Anyone who has more knowledge about this feel free to correct me. I won't be offended."

Just be careful if you have any appliances that need to be exhausted outside, they need combustion air. The most important being your typical natural draft water heater which is in most houses that are equipped with propane or natural gas. Power vent water heaters aren't all that common yet. My house has a natural draft furnace, I need to be careful that if I have the furnace and water heater running for awhile with the clothes dryer and exhaust fans on that the furnace and water heater aren't getting suction to their flue which would essentially suck all of the exhaust gasses into the house and potentially causing serious carbon monoxide issues. Currently my house isn't tight enough for it be an issue but I'm about to close some very large 'air holes', pretty much everything that was identified from my energy audit with thermal imaging while running the blower door is about to be sealed. I may need to retain a fresh air intake to be safe, I'll likely duct it right between the furnace and water heater and have an automatic damper so the cold intake air goes right up the flue while the equipment is running rather than constantly dumping cold air into the house through that opening.

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12-18-12, 04:35 PM	#11
Exeric Apprentice EcoRenovator Join Date: Aug 2012 Location: California Posts: 274 Thanks: 19 Thanked 37 Times in 28 Posts	You are right about the framing. I looked at the framing around doors and with the headers over the doors the transom window clear space would only be about 6" high and door width. The more I think about it the more I realize the door transoms I remember that looked good were in rooms with high ceilings. So that won't work for me after all since I don't have them. The more I think about the venting duct idea the better I like it. I won't use the through wall approach, but floor ducts instead. They will go through the conditioned air crawl space. (another long term project) The reason I am liking the venting duct idea more and more is for two reasons. The first is that they will work to get fresh air into the bedrooms, like I said. The second idea just came to me, like a brainstorm. Usually people don't spend a lot of time in the bedrooms during the day. It's sort of uneconomic to heat them during the day, especially in an older home like mine. However you could use the bathroom door as an easy method of controlling the flow of heat in the house. With the bathroom door open most of the flow through ventilation will go out that door from all parts of the house. If the bedrooms are not heated sufficiently by design they still won't be with that door open. When you close that bathroom door, and it has a good seal, then all the fan air will come from the bedrooms. If those bedrooms have good flow ventilation from the rest of the house, either through an open door or a closed undercut door, then all the heated air from the communal living space will travel into the bedrooms. If you aren't spending time in the bedroom except for sleeping then in winter what you would do is close the bathroom door an hour before bedtime. If your thermostat sets back at night then at the point point the door is closed all the communally heated air would be sucked into the bedrooms and then out through the bathroom. So the communal airspace heat would act as a reservoir that goes to the bedrooms and bathroom at night. Of course you could do the same thing during the day. And whenever you wanted the bedrooms to heat up right away just turn the bathroom fan on high for an hour, 80 cfm, and let it's internal timer turn it off after an hour. It seems like it might work pretty well, at least it does in my mind.

12-18-12, 06:37 PM	#12
Exeric Apprentice EcoRenovator Join Date: Aug 2012 Location: California Posts: 274 Thanks: 19 Thanked 37 Times in 28 Posts	Actually the transom idea was good because it would take the warmest air near the ceiling from outside the bedroom. One could replicate that anywhere with a through wall register from bedrooms to communal space located near the ceiling in the bedroom. Privacy really wouldn't be affected. So the coldest air in the bedroom that is near the floor would be sucked out through the crawl space duct to the bathroom. That air would be replaced with warm ceiling air from the communal space. This might work...

12-19-12, 05:55 PM	#14
creeky Journeyman EcoRenovator Join Date: Jun 2011 Location: a field somewhere Posts: 304 Thanks: 64 Thanked 44 Times in 31 Posts	thanks for pointing out the framing issue. it reminded me that my grandmother also lobbied for 9 ft ceilings. for a mennonite farm house. circa 1964 (might have been 66). most of what I've read is that people put their fresh air intakes close to heat sources (ie, behind the wood stove) and away from the bathroom, so that the fresh air is pulled through the house. the thought behind having a dedicated fresh air vent being that the fresh air should be pulled from a known area (again, clear of combustion gasses or ...) and should enter the house somewhere where the cold air won't affect the comfort in that area. for me I think the fresh air intake would be perfect behind the wood stove shield. warm the air and provide an extra measure of protection. although, clearly, there is an element of getting carried away here ...

12-20-12, 07:17 AM	#15
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	"I've been thinking about that question. You're right that it doesn't address the whole ingress/egress question like an HRV does. But from my reading, and I'm learning as I go along just like everyone, there is always going to be some air leakage in any house, even the tight ones. My intuition is that as long as the house isn't super-duper tight a fan pushing air outside will work fine. All the small air infiltration points will balance the fan air going outside. The volume of air that is actually exchanged will be determined by both the fan cfm rating and by the air restriction caused by the tightness of the house. Only in extremely tight houses will the air ingress restriction be enough to cause too low of an air volume exchange. Anyone who has more knowledge about this feel free to correct me. I won't be offended." Just be careful if you have any appliances that need to be exhausted outside, they need combustion air. The most important being your typical natural draft water heater which is in most houses that are equipped with propane or natural gas. Power vent water heaters aren't all that common yet. My house has a natural draft furnace, I need to be careful that if I have the furnace and water heater running for awhile with the clothes dryer and exhaust fans on that the furnace and water heater aren't getting suction to their flue which would essentially suck all of the exhaust gasses into the house and potentially causing serious carbon monoxide issues. Currently my house isn't tight enough for it be an issue but I'm about to close some very large 'air holes', pretty much everything that was identified from my energy audit with thermal imaging while running the blower door is about to be sealed. I may need to retain a fresh air intake to be safe, I'll likely duct it right between the furnace and water heater and have an automatic damper so the cold intake air goes right up the flue while the equipment is running rather than constantly dumping cold air into the house through that opening.