Make up air
#1
03-16-12, 09:23 PM
Make up air
I am currently building a house and am planning to put in a 1200CFM in-line blower for a wall mount vent canopy over a 36" 6 burner range. My questions is do I need a make up air system and if so where is the logical place? The house will have a bedroom, bathroom and game room above the garage. Total square footage is 2900.
#2
03-17-12, 06:55 AM
Hi rumrunner and welcome to the forum. You have a real Pandora's box for your first question, but we can help.
What is your heat and hot water source? If you have any combustion appliances, they will need to be sealed and well sealed. New construction that wants to be energy efficient will require a tight house. When extra tight they install a H/ERV to provide a source of fresh air and recover some of the energy that would otherwise be lost from exhaust only. Now along comes the question of how to fit in a 1,200 cfm exhaust only fan. If I estimate your house at 28,000 cubic ft and they do a modest job of air sealing during construction, you will experience 1/3 air change per hour. Concerting that to cfm we get a natural air flow of 155 cfm. And if they do a good job of air sealing, your house would have half of that. Needless to say, makeup air becomes a requirement.
Ranch, 2-story, combustion appliances, ac location, basement or slab, and any thoughts as to where you might want a similar 1,200 cfm of air blowing back into the house?
Bud
What is your heat and hot water source? If you have any combustion appliances, they will need to be sealed and well sealed. New construction that wants to be energy efficient will require a tight house. When extra tight they install a H/ERV to provide a source of fresh air and recover some of the energy that would otherwise be lost from exhaust only. Now along comes the question of how to fit in a 1,200 cfm exhaust only fan. If I estimate your house at 28,000 cubic ft and they do a modest job of air sealing during construction, you will experience 1/3 air change per hour. Concerting that to cfm we get a natural air flow of 155 cfm. And if they do a good job of air sealing, your house would have half of that. Needless to say, makeup air becomes a requirement.
Ranch, 2-story, combustion appliances, ac location, basement or slab, and any thoughts as to where you might want a similar 1,200 cfm of air blowing back into the house?
Bud
#3
03-17-12, 02:23 PM
Bud9051,
Tuscan villa style garden home, HVAC's (2) in the attic, foamed insulation, on a slab. Since the roof deck will be foamed with about 5" and should remain a reasonable temperature I was thinking of getting the air from the attic. Although with the foamed insulation the attic will be not be vented. The other alternative is the garage. The unit I'm using for the upstairs is two ton and only 437 ft.2 of living area so I'm planning to drop four or five ducts in a 4 car garage. Maybe use a combination of both?
The vent hood is three speeds and will be used on low most of the time but I want the ability to move larger quanities if necessary.
Tuscan villa style garden home, HVAC's (2) in the attic, foamed insulation, on a slab. Since the roof deck will be foamed with about 5" and should remain a reasonable temperature I was thinking of getting the air from the attic. Although with the foamed insulation the attic will be not be vented. The other alternative is the garage. The unit I'm using for the upstairs is two ton and only 437 ft.2 of living area so I'm planning to drop four or five ducts in a 4 car garage. Maybe use a combination of both?
The vent hood is three speeds and will be used on low most of the time but I want the ability to move larger quanities if necessary.
#4
03-17-12, 03:39 PM
You can't pull the air from a garage, I'm sure code and safety would not allow that. And as you suspect the attic will be just as well sealed as the house.
Your HVAC in the attic, is it heat and ac and does it use gas for the heat?
How will you be heating your hot water?
The point I'm getting at is, any combustion appliances that need to exhaust their combustion products need to be sealed so they are not affected by the depressurization or they need to be protected from it. Protecting a non sealed combustion appliance from that amount of exhaust would mean they cannot be used at the same time. There are some maybe's in there, but it would be difficult. The answer is to make sure they are sealed combustion or everything is electric with no combustion.
Now, even with that, bringing in that much replacement air will still be a challenge. Does the mfg make any suggestions, like ducting it into the existing ducts so it will be distributed through the house. Also, I don't think you can rely on a simple opening to provide that much air. Better would be another fan that would match the speeds and volume of the hood fan.
Check with the mfg to see if they provide any advice or warnings. Also, do you really need a fan that big?
Bud
Your HVAC in the attic, is it heat and ac and does it use gas for the heat?
How will you be heating your hot water?
The point I'm getting at is, any combustion appliances that need to exhaust their combustion products need to be sealed so they are not affected by the depressurization or they need to be protected from it. Protecting a non sealed combustion appliance from that amount of exhaust would mean they cannot be used at the same time. There are some maybe's in there, but it would be difficult. The answer is to make sure they are sealed combustion or everything is electric with no combustion.
Now, even with that, bringing in that much replacement air will still be a challenge. Does the mfg make any suggestions, like ducting it into the existing ducts so it will be distributed through the house. Also, I don't think you can rely on a simple opening to provide that much air. Better would be another fan that would match the speeds and volume of the hood fan.
Check with the mfg to see if they provide any advice or warnings. Also, do you really need a fan that big?
Bud
#5
03-18-12, 09:17 AM
Wow. Pandora's box is getting bigger. I'm asking myself if I need that much air too. I can cut the blower size to an inline 600CFM but I don't want an internal blower in the hood. Too much noise.
The hot water (in the garage) and HVAC (in the attic) will be using natural gas. The builder said we have to change the heating unit from 80% to 90% (not sure what that means) becasue I am using foam insulation.
The only thing the manufacturer recommends is to check with local codes and an HVAC professional. I'm meeting with the HVAC person later this week to discuss this issue.
I really appreciate your advice and warnings. At least I'll have a little bit of an education when discussing. I'll keep you posted.
The hot water (in the garage) and HVAC (in the attic) will be using natural gas. The builder said we have to change the heating unit from 80% to 90% (not sure what that means) becasue I am using foam insulation.
The only thing the manufacturer recommends is to check with local codes and an HVAC professional. I'm meeting with the HVAC person later this week to discuss this issue.
I really appreciate your advice and warnings. At least I'll have a little bit of an education when discussing. I'll keep you posted.
#6
03-18-12, 09:53 AM
The shift from 80% to 90% is probably required to get the sealed combustion I mentioned. Most high efficiency units are sealed, thus they use outside air and would not be affected by internal negative pressures.
I asked around and a pro "Sean" gave me this: "2009 IRC
M1503.4 Makeup air required. Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system."
This is an energy code, but unknown what your area requires. But it is saying you need another fan blowing in to match the speed selection of the fan blowing out and it needs an automated damper to close when not in use.
Others I have talked to have suggested that the majority of the replacement air needs to be directed into the kitchen, rather than throughout the house.
Let's see what the HVAC contractor says.
Bud
I asked around and a pro "Sean" gave me this: "2009 IRC
M1503.4 Makeup air required. Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system."
This is an energy code, but unknown what your area requires. But it is saying you need another fan blowing in to match the speed selection of the fan blowing out and it needs an automated damper to close when not in use.
Others I have talked to have suggested that the majority of the replacement air needs to be directed into the kitchen, rather than throughout the house.
Let's see what the HVAC contractor says.
Bud
#7
03-31-12, 10:20 PM
Hello Bud,
Sorry it has taken me so long to get back. HVAC guy was out of town then I had to go out of town. The builder discussed my concerns with the HVAC installer, I decided to reduce the air flow to 600 CFM max. Like you said "do I really need that much air" for a 36 in stove.
The HVAC installer did not feel there would be any issues at 600 CFM but if there were he would install a make up blower sync'ed with the exhaust. Cost would be around $300.
Thoughts?
Mike
Sorry it has taken me so long to get back. HVAC guy was out of town then I had to go out of town. The builder discussed my concerns with the HVAC installer, I decided to reduce the air flow to 600 CFM max. Like you said "do I really need that much air" for a 36 in stove.
The HVAC installer did not feel there would be any issues at 600 CFM but if there were he would install a make up blower sync'ed with the exhaust. Cost would be around $300.
Thoughts?
Mike
