Airflow problem

[jmartin829]

I have a high efficiency propane furnace which I installed myself about 3 years ago. I have a attached garage that is off of my kitchen witha open door way from kitchen to this room. This garage was finished off and is approximately 24 X 18 and is insulated very well. I have been using a vent free wall heater, but recently have removed this heater and was able to get through the concrete plate between the house wall and the concrete garage wall. I ran a 6 inch flex duct off my furnace plenum and installed a 2 X 14 register just below the step of this room. The thermostat is located in the living room and there seems to be about a 4 degree differance between the living room and this finished room which use to be the garage. I do not have a return in this room because it would be very difficult to install one at this time. I am looking to get more heat into thius room, should I increase the size of the flex duct and register to this room? If so what size? The register is located just off the kitchen just below the step about 4 feet off the floor. I shut off most of the dampers going to other registers in the house to try to get the furnace to run longer and heat up this spare room. I have about 8 6 inch runs coming off this furnace right now, furnace works great for the house without a problem. Please help.

[Brandon Whitmore]

Properly sizing a duct goes way beyond the scope of a home inspection. Heck, many heating contractors get it wrong from what I've seen.

With the info. you've provided, I don't think anyone can give you a good answer.

If it were my house, I'd be looking at installing a return air duct/ vent in the area.

[Richard Moore]

If I'm following this correctly, the open doorway to the kitchen is acting as the return and the supply register is on the same wall, close to that door? My guess is the main problem is that the heated air isn't projecting into the "garage" very far before getting sucked back out. Ideally, the register(s) would be at the wall oposite the doorway to the kitchen. I don't know how you would easily accomplish that.

[inspector57]

A 2x14 duct is not very big for that size of room and likely does not throw all the way across the room with enough volume or velocity to service the room. A return duct might help if located across the room but it does not sound like you have much flexibility.

Get a HVAC to do a calculation and design a fix.

[Terence McCann]

In commercial the rule of thumb was 1 CFM per square foot. Your space is 24X18 which equates 432 square feet or, on the safe side, 432 CFM.

I have misplaced my ductulator [:-banghea but found this web site that has HVAC Duct Sizes by CFM. I would love to cross check these but can not at the moment. Assuming they are correct; a 6" round will deliver 300 CFM so you're already short. R/A might help if place on the opposite wall.

The do make supply duct booster fans, like the ones shown, however I can't say for sure they would help. They're cheap enough so it wouldn't hurt to give it a shot.

Good luck!

[Bain]

In commercial the rule of thumb was 1 CFM per square foot. Your space is 24X18 which equates 432 square feet or, on the safe side, 432 CFM.

I have misplaced my ductulator [:-banghea but found this web site that has HVAC Duct Sizes by CFM. I would love to cross check these but can not at the moment. Assuming they are correct; a 6" round will deliver 300 CFM so you're already short. R/A might help if place on the opposite wall.

The do make supply duct booster fans, like the ones shown, however I can't say for sure they would help. They're cheap enough so it wouldn't hurt to give it a shot.

Good luck!

Terry,

I have a Lima Register ductilator and it says a 6" round duct only supplies 120 cfm at .10 friction per 100' of duct.

The Dr. Fix-It site info can't be right, can it? Their numbers say an 8" round duct supplies 500 cfm whereas I've always been told the actual number is at or near 200.

Also, I find 4" ductwork fairly often and flunk it 'cause it can't transport cool, heavy air when systems are operating on A/C mode. The Fix-It site says 4" = 100 cfm, but I've always been told it was closer to 25 cfm.

[Terence McCann]

I have a Lima Register ductilator and it says a 6" round duct only supplies 120 cfm at .10 friction per 100' of duct.

That sounds more like it, thanks.

[Jim Katen]

I have a high efficiency propane furnace which I installed myself about 3 years ago. I have a attached garage that is off of my kitchen witha open door way from kitchen to this room. This garage was finished off and is approximately 24 X 18 and is insulated very well. I have been using a vent free wall heater, but recently have removed this heater and was able to get through the concrete plate between the house wall and the concrete garage wall. I ran a 6 inch flex duct off my furnace plenum and installed a 2 X 14 register just below the step of this room. The thermostat is located in the living room and there seems to be about a 4 degree differance between the living room and this finished room which use to be the garage. I do not have a return in this room because it would be very difficult to install one at this time. I am looking to get more heat into thius room, should I increase the size of the flex duct and register to this room? If so what size? The register is located just off the kitchen just below the step about 4 feet off the floor. I shut off most of the dampers going to other registers in the house to try to get the furnace to run longer and heat up this spare room. I have about 8 6 inch runs coming off this furnace right now, furnace works great for the house without a problem. Please help.

If you have access to the attic above the garage, consider installing a jumper duct between the far end of the attic and the interior of the house, preferably near the area where your main return air grille is. That could show an instant improvement in distribution to that room.

- Jim Katen, Oregon

[jmartin829]

The problem with a jumper duct is my heating system is in the basement and there is no way to get a duct up into the attic. Would just increasing the size of the existing duct improve overall? What good is a 6 inch duct fan going to do for me?

[jmartin829]

I am seeing about a 4 degree differance between the house and this garage space/ living area now. I need more air or even a return. What size supply duct and what size return duct vwould work?

[Tom Raymond]

I'm no expert in forced air systems but here's my take; you are not moving enough conditioned air into that room, and compounding that error by not having adequate return. You need at least an 8" round to satisfy the volume of the room from a single point. Jim's 'jumper duct' is just a jumper, it should connect a point as far as possible from the heat duct in your new room with a spot close to the central return, or the largest return intake possible if your system has multiple returns. It doesn't have to be 'connected' to your HVAC, but it should be well insulated if you run it through your attic. Sizing the return jumper will depend on lots of factors such as the length of the run, the number of turns, and the size and location of the return intake that will be 'driving' it. You could experiment with it, or you may be better served by having an HVAC tech look at it.

Tom