Had a 5 yo high end home yesterday with a seal attic design. I do not see a lot of those around here, but the ones I have seen ave always either had electric heat pumps or the specialty gas furnaces with the dual PVC vent that provides both inlet and outlet of combustion air.
This home had what I would call typical Carrier 80 AFUE gas furnaces with only the exhaust vented to the outside. The combustion air would have to be made up from the attic and therefore the living area in this design. I also have a concern about the foam touching the metal flue pipe - not a fire concern, but could the heat in that area lead to moisture problems under the foam?
Trying to determine if this is a concern or not. Something just does not seem right about it, but I don’t want to make an issue out of it without facts to back me up.
Here is a picture of the setup. Also, the house was built this way - not modified - no insulation was installed in the floor as is typical for this design.
IMO, this furnace should have an intake routed to the outside of the home. Some manufacturers allow for a wall vent placed in the furnace room for the intake of air, but I always prefer air intake from the outside. As always, refer to manufacturer’s recommendations.
Not vented. It is a sealed foamed in attic design.
Just spoke with an AC contractor I have worked with over the years - he agrees that it is at the very least not meeting the manufacturer installation guidelines and should be looked at.
It will fail if the floor is not sealed, as well. All of the moisture generated from the living space and from the heating/cooling unit will be trapped there and nasty things will thrive and grow. This will happen even if the HVAC unit is removed.
A design that was undoubtedly approved by an official and that probably “meets code” but is a bad idea.
When you allow moisture to enter a “sealed” space, it must have a place to escape.
I think you’d have to do a rough calculation on the amount of air required by the furnace BTUs. If it’s a large attic, the air volume is probably OK. Even though it’s a sealed attic, it isn’t hermetically sealed. Air still gets in from the living space.
I tend to agree, Joe, the house was big, but the attics were not huge due to the design. The flame color looked okay, but then the door to the attic was open when I was looking.
I am going to call it out and have them at least check it out. Performing those types of calculations are beyond my scope and I would rather have a licensed specialist sign off on it than take a chance. Otherwise it was a pretty nice house with few issues. I called the client and explained it to him as well as put it in the report.
I agree it should use outside air .
I disagree With James as the furnace will draw from the home and unless the home has excessive moisture then I do not see how the attic will end up with elevated moisture .
The furnace exhaust goes to the out side so much of the air drawn from the home will now end up out side … Roy
Recommend further evaluation by a qualified person
Roy,
You may be right in a small attic in the winter, but I really do not see it during the summer. You return would have to really leak bad in the attic to pull out enough moisture. Which pulling attic air is not a good idea anyway
Combustion air can only be drawn from the attic when it is adequately ventilated with a source of outside air. That’s common sense. Think about how much air would leave this sealed attic with a negative pressure in the area below it.
We all know how a lack of return in a room can create a positive pressure…and the reverse is also true when a room has a supply duct closed, blocked or improperly sized.
A sealed attic is a sealed attic. This is a sealed attic ceiling, not a sealed attic, with no vent and with uncontrolled communication of air between it and the area below it. Inside of it is a fossil fuel powered device dependent upon combustion air to properly burn with no outside source for that air.
Enough of negative pressure in this attic, considering the short flue with the possibility of very cold ambient air, could even possibly draw flue gases back into the attic and into an of negative pressure area below it.
Sealing the underside of the roof does not seal the attic.
There are air bypasses throughout the house in direct communication with the attic. Mechanical fanscan pull moist air from the crawlspacethrough them, and vice versa…why cannot they, in a room with negative pressure, create a negative pressure in a similarly connected attic space?
Not only that…but when there is leaking ductwork in the attic creating a positive pressure and everything is “sealed” above it…where else can this warm air go but into the cooler bypasses, some running along the colder exterior?
If this were a brand new construction I would anticipate issues and advise, accordingly. If it were an existing structure, I would be looking for and expecting to find problems caused by this set up.
I have seen a water heater in a laundry room have the gases sucked out of the flue and into the living space when the clothes dryer was turned on. I have created negative pressures in bedrooms (with no returns) by simply closing the supply vent and running the air handler. How hard is it to create a negative pressure in an unventilated attic space…and what dangers might be derived from mechanical equipment relying upon a consistent source of fresh air to properly operate?
I did not see that there was a crawl space and if we surmise that there is a crawl space how about if this crawl spaced is not vented .
Where does the air for the furnace come from .
If the dryer is put on now we have increased the lack of air and add the kitchen vent .
I live with my other post ,It needs a supply from the out side and needs to be evaluated by a qualified person .
These type of discusions are important to help teach and help all ,to improve me and other Homies … Roy
I did not see that there was a crawl space and if we surmise that there is a crawl space how about if this crawl spaced is not vented .
Where does the air for the furnace come from .
If the dryer is put on now we have increased the lack of air and add the kitchen vent .
I live with my other post ,It needs a supply from the out side and needs to be evaluated by a qualified person .
These type of discusions are important to help teach and help all ,to improve me and other Homies … Ropy
I think I am saying the same thing, Roy. At least, I’m trying to.
The attic is not “sealed”. It is simply insulated at the roof instead of the floor, but the lack of venting is a potential hazard for moisture as well as sufficient combustion air. Whatever bypasses the attic is using to draw air from can also, under certain conditions, draw air from the attic.
A central heating and cooling system is a closed loop system supply and return from the same box. A central system will not create a positive or a negative pressure within the home. You can not close off a supply duct within a room a create a negative pressure that is talk from out in left field.
The last sentence in his statement I quoted was so far fetched doesn’t need commenting on.
There was not enough info about the attic to make a educated response about combustion air for the furnace. No mention of any attic ventilation soffits or gable. No mention of how many bathroom/kitchen exhaust fans and where they discharge to plays a big factor. Was the foam on the roof sheathing only to what extent was the foam. To many if,ands,and buts here
Its comments like this that gives HI’s a black eye you need gaps at the bottom of the door dumb A** for air circulation supply and return has nothing to do with pressure you can only blow said amount of air into a bottle without removing said amount. Try sealing the door off and tell me if the window or wall blows out or if the room just becomes hot or cold depending on season. You apparently were sleeping during that part of your class. I am trying to keep this simple just for you