Forced air from my house

[ikessky]

Hey guys! I'm really glad I found this place! Tons of great ideas for when I build my new garage/shop.

For now, I have a standard two car attached garage (22x24 maybe). It has open rafters and our roof has ridge vent only on the garage and maybe six feet onto the actual home. There is a huge screen door into the crawl space above the house. From what my FIL tells me, the rafter construction is a little on the light side, which prevents me from hanging sheetrock or OSB. Keep all this in mind for the next part.

Last year we installed a forced air wood burner in the basement. Most days, it kept the house too warm (like 76-78 degrees). So, I got thinking about my setup for this year and thought, why not put a couple ducts into the garage and dump some of that heat out there? My main concern with doing this is that with the open rafters leading into the crawl space above the house, this will probably heat up my roof and could potentially create ice dams in the winter. So, my idea is this. Put a layer of vapor barrier up on the rafters and then install foam board insulation over the vapor barrier. All the seams and cracks can be taped or have some Great Stuff sprayed in them. This should then create a light ceiling which should keep the heat from causing issues with my roof. What do you guys think? This should allow me to dump some heat from the house and also will slightly warm up my work space (even 40-50 degrees would be a welcome vs. working in 15-25 degrees!)

 

[Costner]

If you use a forced air system to push air into a space that doesn't have cold air returns, you will depressurize the rest of the home and create negative pressure inside. That in turn will **** cold air from the outside which reduces the efficiency of your system and can actually make some rooms colder due to the cold air being sucked in from doors, windows, electrical boxes etc. It also reduces the incoming air temp to the heating plant, which again reduces efficiency.

You also can't install cold air returns in the garage because of vapors and fumes. Not only is this against code, it is also a bad idea because along with the automobile exhaust and vapors from chemicals, you would be sucking in a lot of dirt which will then be distributed throughout the house.

There is a reason most garages are heated by secondary heating systems and this is one of the primary reasons. I think you would be better off investing in a standalone heating source for the garage - either a gas or electric heater designed for such spaces.

 

[ikessky]

Very good point about the negative pressure. Wasn't thinking about that originally. However, my house isn't what I would call air tight though. Any idea how much one 6" duct could affect the pressure? I'm sure there is a formula of some sort.

I had already checked out the building codes in regards to ducting into a garage and certainly wasn't planning on installing a cold air return. The code states that a duct can terminate in a garage as long as an anti-backdraft damper is installed (which I would do for obvious reasons).

 

[Costner]

Well look at it like this - for every cubic inch of air sent into your garage from your heating system, it needs to be replaced. Now if you have a fresh air intake in your mechanical room or better yet one tied directly into the cold air return of your heating system that will be the path of least resistence. However if you have no such fresh air intake, the negative pressure in the home will come from whereever it can - windows, doors, unsealed electrical boxes or in some cases down chimneys or vent pipes (which in the case of gas burning water heaters can actually be a health hazard due to the carbon monoxide being "sucked" into the home).

Either way the cold air coming into the home from outside is obviously the temperature of the outside air - so if this air is coming in through cracks around windows and doors, it can actually make those rooms feel colder due to the resulting draft. The same principle applies with fireplaces - they need combustion air and due to the drafting of heat up the chimney, if there is no external vent providing fresh air, they will **** air from other rooms in the house resulting in a decrease in air temperature in those rooms.

Now since you are burning wood you might not care about the efficiency of your heating system, and thus a decrease of 10 or 15% might not bother you a bit, but at the very least I'd be sure you have some sort of fresh air intake in your mechanical room so the furnace has an ample supply of air rather than it starving itself.

 

[ikessky]

Thanks for the great info! I do in fact have a fresh air vent in the basement. It is a 6"-8" insulated duct that is currently folded up and tucked in by the floor joists. I will untie it and lay it on the floor by the filter box. I don't have the return air duct hooked into the wood furnace, so it is just pulling air off the basement floor (finished basement). We have a NG furnace as back up, but that is the only gas appliance we have. The wood furnace is vented into a 316t stainless liner that is installed inside the clay flue of the chimney.
You're right that I don't mind a little less efficiency from the wood furnace. The wood is free (minus my time and gas for the saw) and I have over 8 cords split and stacked. Last year was our first year burning and we did a little over 4 cords.

 

[Costner]

Do yourself a favor and put your fresh air intake into a 5-gallon bucket. That way the cold air won't just spill onto the floor 24 hours a day. Instead, the furnace will "****" as much air as it needs and pull that air out of the bucket, but when the furnace doesn't need air it won't be dumping into your house.

In most areas the fresh air intake needs to be in a bucket or box by code. Around here some people build boxes out of wood or have them fabbed up in sheetmetal. The cheap route is to just use a bucket, but whatever you use it should be about 15"-24" tall on the sides.

 

[ikessky]

Thanks! I think I'll try something like that. I seem to remember the manual for my wood stove saying something about creating a bend or trap in the fresh air intake, probably for the same reasons that you stated.