Intake air setup for garage heater

[matt_i]

I have a typical ducted natural gas burner like you'd find in many basements, in my shop. Previous owner was a HVAC contractor and set it up in a small drywalled enclosure ("furnace room"), sitting on a "box" made of wood. He had a small dryer vent/filter going out the back wall for drawing exterior combustion makeup air, and the room was enclosed by a drywall panel that fits reasonably tight. The room air enters near the floor, there's a filter, and exits thru the ductwork but this is all on the secondary side of the heat exchanger.


Fast forward to me, I built an addition so that the former exterior air intake location is all interior.

I can easily core up into the attic, where there's plenty of cold air and create an intake tube, thinking I would put a filter screen to keep out bugs and any creatures that I don't know about yet.

I'm not sure what type of issues that would create, the only downside I can think of would be a fire that started somehow in the room could probably spread to the attic thru the tube unless I made it & the screen extra heavy duty, but the rarity of NG furnace fires makes me think this isn't a likely scenario.

Looking for your thoughts and advice please on my concept to put the air intake for the furnace room up into the attic above. Thanks!

 

[metlmunchr]

Matt, is this a natural draft type furnace?

 

[matt_i]

No, its a forced draft 100kBTU , fan runs prior to ignition and verifies airflow.

I was thinking a U-shaped tube in the attic and then run the duct all the way down near the floor.

The room is going to get slightly warm when the furnace runs, if I just put a short stub just barely thru the ceiling there's going to be constant temperature inversion due to the hot air escaping and cold air refilling.

If I run the duct (thinking 6" round since I have a few extra pieces) down close to the floor it should be mostly stable temp-wise and just intake what's needed.

 

[metlmunchr]

You're on the right track all the way. Recommendations for combustion air to a relatively small furnace room vary from 1 sq inch per 4000 btu input to 1 sq inch per 5000 btu, depending on the manufacturer. The 6" round satisfies the larger air requirement, so you'd be good there.

Running the duct down near the floor is also good, as is the "trap" at the top. A screen over the end in the attic is fine, but I wouldn't add any sort of filter as the pressure drop across a filter would reduce the effective flow capacity of the duct.

One other point, although it wasn't part of your question. You mentioned the return air from the space enters near the floor. Is the return hard ducted from the return plenum thru the furnace room wall? Or is it just a filter on the return plenum with a return grill cut in thru the wall or door?

The ideal situation is a return duct thru the wall so that 100% of the return air comes from the space. If not ducted, then the furnace room acts as a part of the return plenum and its very important to have enough return grille area to assure that you're not pulling an excessive negative pressure on the furnace room. Negative pressure in the room will cause the unit to pull more air than what's necessary for combustion down your combustion air duct. This will hurt your overall efficiency both by reducing the attic temperature and by causing the furnace to heat some percentage of outside air continuously when the furnace is running, and in your area that outside air could easily be 50-60 degrees colder than your space temperature.