Do I need “make up air” in my furnace room.
I have lived here 7 years and only recently was told I need more air or make up air in the room that houses the furnace and hot water heater. It has a regular door and is on the second floor. The man who told me this said his company (heating and air) could do the work or I could get someone else. Said I need 2 pipes, one going almost to the floor and the other through the ceiling or a louvered door. I He said he is surprized it hasn’t affected my hot water heater (not sure why).on’t want a louvered door as I can’t stand the noise when the furnace turns on or does the outside air exchange. I hate the sound of the air blowing but really dislike the whooshing sound when something turns on.This space is close to my bedroom. The room is abaout 44″ wide and 88″ long.
I had to have a pan put in under the hot water heater as the builder didn’t – lots of things he didn’t.
Does this (pipes) make sense.?
Replies
If the "combustion appliance"
If the "combustion appliance" draws room air then there needs to be provision for make-up air. I'm not familiar with the specific rules, and they probably vary from one part of the country to the other.
Just to verify: Your water heater is gas, correct? What about your furnace -- it is a "high efficiency" unit with plastic pipes for air intake and exhaust, or does it draw room air and exhaust through a chimney or flue?
Combustion appliance
Yes, water heater is gas. The installation instructions for the furnace stgates, High Efficiency 90+ Condensiing Gas Furnace. It has two plastic pipes going out the back wall.
The fact that you have 2 plastic pipes (one for exhaust, the other for combustion air) connected to the furnace means that it's only the water heater that needs make-up air--codes call this "combustion air".
Besides the two-pipe method your HVAC guy mentioned, there is another code-approved alternate method that needs only one pipe for supplying combustion air for the water heater . The single pipe must enter from the ceiling or high on the wall so that it terminates within 12" of the ceiling. It is sized at 1 square inch of cross-sectional area of the pipe for every 3000 BTU's that the appliance burns. The pipe can take its air either directly from the outdoors or from a ventilated attic.
For example, if your WH is the common 40,000 BTU size, that means that the combustion air pipe will need to have 13,3 sq. in. of area. No standard pipe is exactly that size, but a 4" pipe gives you about 12.6 sq. in., and most inspectors will pass that in view of the fact that some additional combustion air will be avialable from the room itself.
Make up air
Thank you.
combustion air
Design. I have a question. If you are getting unconditioned air from the outside and also from the room/house are you not introducing unconditioned air into the house as a whole? That seems to negate the efforts to otherwise create a tight house.
Also, if air from the house can inter the mechincal room/closet, it can allow exhaust air from the mechanicl room to inter the house.
Just askin.
As far as air-tightness and controlled ventilation go, of course it would be best for all fuel-burning appliances to have their own supply of combustion air that is introduced into a sealed-combustion appliance like the OP's furnace. Safety would also be improved.
Second best is to have the fuel-buring appliances enclosed in a dedicated closet or furnace room like he has. That way, the cold combustion air is contained within the enclosure where it's actually needed and used, without being distributed thru the rest of the house. This is because furnace rooms generally don't have supply or return registers within ithem.
As for your second question, exhaust from appliances in the mechanical room is not actually exhausted into the mechanical room itself--the exhausts are all piped outside.
Now, as a separate issue, there's also the question of just when air-tightness becomes tight enough to require mechanical ventilation as opposed to relying on infiltration as houses did in the past. Some experts will say that a house can't be too tight; it can only be under-ventilated. Others favor allowing houses to leak a certain amount to achieve needed indoor air quality.
Either of those appoaches raises further problems and uncertainties as to year-round effectiveness and quantifiable results that could be clear enough to be written into the building and energy codes.
This is a problem that generates heated (no pun intended) discussions between homebulders, energy gurus and others, while leaving homeowners frustrated and confused.
makeup air
if you have a conventional furnace ( not high efficiency ) you can make a simple one by installing a dryer vent in the wall to outside about 2 1/2 ft above the floor & pipe it to the area of the burners & down into a 5 gallon bucket stopping about 5 inches above the bottom of the pail
If your in a colder area insulate the pipe
Also take the flapper off the vent & put screening in place of it ( keeps out mice & other critters )
Another method is to use a wall vent as above then get flexible aluminum dryer vent & bend it into the
shape of a pipe like tobacco smokers use
put the loop on the floor with the end extending about a ft above the floor vertically ( there called a Saskatoon loop here )
outside air
they are meant for makeup air & the configerations are to avoid getting a draft
There used to be available a vent through the wall that had a 110 volt heater in it
The damper in it was controlled by a barametric valve , havent seen one lately
When houses are sealed up tightly you need some way to get make up air for any fuel burning appliance
that is not sealed like your water tank ,stove ,wood burning fire place
Gas burning fireplaces in R2000 homes are sealed & do not need makeup air from inside the house
"they are meant for makeup
"they are meant for makeup air & the configerations are to avoid getting a draft"
They will supply makeup air, but they will do nothing whatever to avoid a draft. This is a common misconception that needs to be de-bunked.
Not intending to put words in your mouth, but I suspect that the rationale we've both heard on this is that the loop or bucket acts somewhat like the dip in a p-trap in a drain line where the water in the dip prevents sewer gas from flowing into the house. Supposedly, in the case of the combustion air pipe, a pool of cold air settles in the dip or bucket and this stops cold air from flowing freely into the room.
The trouble with that idea is that cold air won't stop colder air. In other words, the colder, denser, heavier column of air in the inlet pipe will push and displace the pool of cool air in the dip or bucket. It's not analagous to the water in a p-trap because water is many times heavier and denser than air. Just as the water in the p-trap won't stop water from flowing, air in the dip won't stop or even impede the flow of air unless the outlet end of the dip were to be raised higher than the inlet.
Yeah, I've always doubted that the bucket does any good. I think it's mainly for psychological purposes.
It doesn't stop the cold air from flowing, but it does allow some warming of -40 air before it enters the building. I know it doesn't sound like much, but in some cases it helps.
And in theory, you could have airflow without the trap with convection currents.
In terms of controlling convection you could accomplish the same thing with a slight constriction in the pipe -- just something to create turbulence.
One added benefit of the psycho effect is that it might prevent psychos from plugging off their c/a intake pipes.
Now wondering
So, then, having the single pipe you suggested will bring cold air into the house? Thanks again.
Of course it will bring cold air into the house, but it should be confined to the closet or room containing the water heater.
That's where it's needed
Under ideal conditions, cold air would flow in when it's needed--that is, when the WH is firing. Gas water heaters need about 30 cubic feet of air for every 1000 BTU's of gas they burn, and the source of that air (and all air in the house) is from outdoors.
The purpose of a dedicated combustion air pipe is to deliver the needed air right to the appliance that needs it, thereby reducing the cooling effect on the rest of the house while making the combustion air available to the burner.
Your reply
Thanks again.