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Furnace Room Venting?!

9.5K views 7 replies 3 participants last post by  3onthetree  
#1 ·
My small utility room consisting of furnace/laundry/bathroom is just off the kitchen and leads into the garage where our water heater is. Now that I'm fixing up the room, I'm wondering why I have floor and ceiling vents in this room. As it stands, there is a 9" square hole in the floor with metal grille but it has always been blocked with a magnet cover to stop the cold air coming up from the crawlspace. I did notice last time I had duct cleaning done, they used this access point to get into the ductwork. The ceiling has a small rectangular hole about 4"x10" with a flat, flimsy grille that leads into the attic, but I just found out there is no venting at all and just 9" insulation up there resting right on top of the hole. It appears to me while these vents exists, it's not suiting its purpose of airflow (if that is the intent). My question, do I really need them or can I close them off for good since they weren't being used anyway?!

The room itself 8' x 11', detectors in this as well as other rooms, and the door to the kitchen is fully louvered. The door itself is rarely closed at all, just for privacy when using the bathroom. Any advice on the matter is appreciated. Thanks!
 
#2 ·
If you have fuel gas appliances, you need combustion, ventilation, and dilution air for the flue gases. A clothes dryer can require makeup air as well. There is a specific equation to determine the size of these openings based on your BTU ratings of those appliances. A louvered door unto itself may not meet those requirements. By connecting directly to outdoor air through the crawlspace or attic you can also reduce the sizes of the vents required.

If the furnace was upgraded to an efficient direct-vent, then there would be a reduction, or even elimination, of a requirement for those vents.

637479
 
#3 ·
If you have fuel gas appliances, you need combustion, ventilation, and dilution air for the flue gases. A clothes dryer can require makeup air as well. There is a specific equation to determine the size of these openings based on your BTU ratings of those appliances. A louvered door unto itself may not meet those requirements. By connecting directly to outdoor air through the crawlspace or attic you can also reduce the sizes of the vents required.

If the furnace was upgraded to an efficient direct-vent, then there would be a reduction, or even elimination, of a requirement for those vents.
Thanks for the reply. I just recently had a new furnace and air put in about a month ago and it is a high efficiency unit but they just have a 2" pvc 45 elbow on top of the furnace drawing air from the room, not ran through the wall and outside. Not sure, but any research I did seemed like either method is fine, but typically newer installs go out so you are pulling fresh air. Obviously, I know nothing about all this myself but since I was fixing up the room after this install, I started to look into it as I wondered why I have these extra vents in the room.

Like the diagram shows, I also have my inlet air coming up from the floor. Is it okay that it is drawing air straight from the crawlspace? I've seen mixed comments about whether it should come from outside rather than under the home.

As for the outlet air, that is just going up to the attic via a small vent in the ceiling and the insulation was resting right on top of the hole. If I clear the pathway of insulation, should I vent it through the attic and out the home or can it just stay as is? I noticed the picture above seems to just show it going up to the attic. Again, research has been varied for me but I did see some comments that nothing she vent to the attic, but instead outside. Perhaps this is a different situation because it's just room air instead of some sort of exhaust/gas?

Thanks again for the help!
 
#4 ·
Not an hvac pro but if you have a new high efficiency furnace it should have both exhaust and intake running to the outside. If it draws combustion air from that room then cold air must replace it either from the vents or the louvered door. Both are unnecessary and a waste of heat. Replacement air for dryer still needs a path but that door may suffice.

Make sure the exhaust is not dumping into that room. When you say " outlet air" what are you referring to, exhaust from furnace?

Bud
 
#5 ·
My furnace exhaust is venting outside through the roof. I used the term outlet air to refer to the hole in my ceiling because it matched the term used in that diagram above in this thread. I guess in an effort to get replacement air into the furnace room (which also has dryer and bathroom), the original work with the old furnace included cutting a hole in the floor and ceiling, then putting grilles over them without any actual ductwork.
 
#6 · (Edited)
Is the exhaust going to the roof a pvc pipe or metal?

As for the high and low vents my guess has always been an attempt to br sure all incoming air does not get blocked. Here in Maine snow can get high or junk can block the low but less probable both would be blocked. It would also provide some natural air exchange to the outside to ensure that utility room does not accumulate unwanted fumes.

Maybe one of the pros can give a real answer :).

Bud

Since my coffee is starting to work I did some searching for related numbers, FYI.
 
#7 ·
This is a real answer :whistle:

Your room's combustion air requirements were designed to follow the drawing in Post#2, which is two permanent openings passing air through from outside for a confined space (704 cubic feet). You have now installed a direct vent furnace, however according to your description they have installed it as a non-direct vent installation, using intake air from the room, like this:

637570


This means that you need to keep those two permanent vents for outside air open and unencumbered. Note that it is usually recommended that a direct-vent furnace installed in a confined space (or laundry room) pull it's air directly piped from the outside, not from inside the room. So if they had instead run the 2" PVC intake air through to the roof (which the manufacturer may actually specify in their instructions), then you probably could close off those vents.

Your clothes dryer, if high capacity, would need makeup air. Also, a toilet exhaust fan (which is required) also requires makeup air. We will assume the louvered door accomplishes this, but that means you cannot replace it with a solid door because the exhaust fan can not interfere with the combustion air from the vents.

9" square hole in the floor
The ceiling has a small rectangular hole about 4"x10"
If you want to do a calculation for fun, each vent opening needs a free area of 1 square inch per 4,000 Btu/h for both the furnace and gas clothes dryer combined. This is after you account for a metal grille having a 75% free area. So the air provided by the vents are 60.75sqin at the floor and 30sqin at the attic, so you are allowed 120K Btu/h.

I did notice last time I had duct cleaning done, they used this access point to get into the ductwork.
There should not be any connection from the floor combustion air vent to your conditioned supply/return ducting for the house. This vent could have a separate duct that runs through the crawl space to an outside wall, but then they couldn't have accessed the rest of the HVAC ductwork through this. If there are ducted combustion vents, that could change the calculations of the size of the vents required.

The door itself is rarely closed at all
It makes no difference on any of the requirements - the fact there is a door that can be closed is all that matters.