Proper size/amount of cold air returns.

Old 08-15-13, 06:00 AM
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Proper size/amount of cold air returns.

I'm confident that I don't have a large enough air return for my furnace system. I was wondering if people had recommendations for how to solve this problem the cheapest way possible, and with as little damage as possible to the walls of the house.

1. Old Janitrol furnace in a tri-level home about 1,450 sq/ft.

2. It is installed in the attic of the home, and the vents are on the ceilings of the 1st and 2nd floor. (There is ductwork, a slab, and gas supply for a basement in the furnace to heat the rooms 2 rooms in the basement and 2 heat registers on the first floor, but the unit was removed after a flood).

3. There are 6 vents for the working system, all in the ceilings. 2 in the living room and 1 in the kitchen of the 1st floor. 1 in each bedroom and the bathroom on the 2nd floor.

4. The vents are the 1' circular type.

5. There is only one return, located toward the ceiling of the living room, in what is probably the middle of the house on the 1st floor. The hole is roughly 28" wide by 7" tall.

I keep the heat pretty low in the winter. The Honeywell digital thermostat is kept at no higher than 64 degrees and is located at the top of the stairwell in the middle of the house on the 2nd floor. I haven't had to put much strain on the unit at those temps so far, but haven't lived here in a "bad" winter yet.

The problem is, unless i were to stand on a chair and feel right in front of the vent, I can't even tell it's working. I used to just dismiss this as a quirk of having a system that tries pushing the air down, but I think it's the return now. I'm used to houses that have heat vents you can feel from at least half a foot away if not more.

The unit is very old and likely horribly inefficient to begin with, and I plan on fixing that as soon as I can afford it. My heat bills seem to be quite high for how low I keep the thermostat. I'm hoping there is a somewhat cheap way to fix this without ruining any walls in rooms I've already finished and doing minimal damage to rooms I haven't worked on yet.


And let me know if you need any more information.
Old 08-17-13, 01:57 PM
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I can't visualize the setup.

What size is the furnace? Post the model number if you have it.

Post the duct sizes. (supply, return, duct branches, etc.)

If you post your gas consumption for the coldest month of the year, I could probably tell you if it's in line for that type of house.

High gas consumption causes:

1. Furnace not set up right - burning too much or too little gas (underfired/overfired), not moving enough air

2. Furnace is < 80% efficient - does it have a continuously burning pilot and draft hood?

3. Ducts are leaky or poorly insulated; attic systems are inherently inefficient, but you might be able to make improvements

4. House has serious deficiencies - missing or inadequate insulation, high leakage

I would start with the furnace; if there's a problem, you'll have to call a pro, but there are a couple of safe DIY tests you can do to determine if the furnace isn't burning the right amount of fuel and/or there's an airflow issue.
Old 08-18-13, 09:27 AM
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I'm trying to visualize it too. You had a furnace in the basement. Normally just an air conditioner would be installed in the attic with the ducts in the ceiling. A common location for the return was in the hallway directly below the air handler. Cold air falls so that works good.

You have a furnace/AC unit in your attic which means now your heat and cold both come from the ceiling registers. Now your furnace has to work harder since you've lost the heating from the basement and hot air rises.
Old 09-03-13, 07:27 PM
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Hi Muggle,

I'm not sure what "size" the furnace is, or what exactly you mean by that. If you meant how many BTU's, I believe I have that below (please let me know so I can understand the question).

Here's the specs inside the unit:

MODEL: GU075-3 REV: C.S INPUT: 75000

February was the highest usage at "139 Units", as opposed to the "7 Units" I used in August.

(Units are measured: 100 cubic feet (1 ccf) = 1 therm (approximate). I figured you knew this but would post it for any of the readers of the forum.)

The house is a tri-level that is roughly 1,600 sq/ft. The furnace, as I said before is in the attic (attic not heated and/or counted in square footage). The thermostat is at the top of the stairwell to the 2nd floor. Heat goes to the 1st and 2nd floor from the ceiling (no output to the basement). Basement is "wide-open" to the rest of the house with no way of closing it off without major construction (wouldn't want to do that anyways).

Now, when you ask for "duct sizes", did you mean sq/in on the face at the grates, or how long the ductwork is back to the furnace for each vent/return? I'll have to get back to you on that one.

About the "High Gas Consumption Causes"....

1. I actually wouldn't be surprised if it was burning too much or too little gas. I did a LOT of tinkering with it when I was trying to get it to work after it was sabotaged by the previous owners after they were foreclosed on. Thanks to this forum and a friend of mine I got it working without having to hire a professional, but maybe paying that up-front cost would have saved me money in the long run.

2. I'm quite sure the furnace is less than 80% efficient. I was told from somebody I know they're referred to as "Junkitrols" today and that it's a pretty old unit. It DOES have a continuously burning pilot, but I don't know what you mean by "draft hood". I looked up the term, but I'm still not sure what to look for or how I would know.

3. I'm fairly confident that the ducts are insulated well in the walls as the house seems to be insulated very well. In the attic, there is no insulation on the duct-work. As far as "leaky".. I don't know how to test for that, but even if I did, I don't think I'd be able to do anything about it now without looking at some serious rehab costs.

4. I am fairly confident the house doesn't have these deficiencies. In the summer, I don't have A/C and if I don't open the windows at night when it dips to 60-65 degrees it will still be 82 degrees in the morning when it was 87 degrees the day before.

Hi PJmax,

I never had a furnace in the basement. I bought the house rehab/foreclosure. There is a slab, gas line and existing duct work to where one should be, but there isn't anything there. Surprisingly, I believe that this house was designed to have two units. The attic unit only gives heat to the 1st and 2nd floors through 6 circular ceiling vents. The basement unit (if there was one) only gives heat through two wall returns in the basement, and 2 floor returns on the 1st floor.

There was an A/C unit installed outside the house at one point, but that wasn't here when I moved in either.

The return is located pretty much directly below the unit on the top of the wall on the center of the house on the 1st floor, which is just to the right of the stairwell in the massive living room. It's not the prettiest spot for it, but if I wanted to put it in a hallway it would have to be on the 2nd floor which wouldn't be as efficient.

I only have heat in the attic now, not A/C, although it looks as if the heat was pushed from the attic unit when it was available since the copper pipe was cut (they not only took the A/C unit when they left, but they cut the copper pipes all the way up to the attic furnace).

So with hot air rising, I understand that even with the most modern and efficient unit/system I would be at a disadvantage.

I'm just concerned that without proper "return" to the system, there isn't enough air "coming in" so there isn't a lot of air left to "go out". As I mentioned in the initial post, unless I stand on a chair and put my hand to the ceiling vents, I can barely even tell that hot air is coming out. I don't believe a bigger blower could be installed since it takes up most of the lower cavity of the unit. I just have to believe there would be a way to get more FORCE out of those vents.

Thanks guys.
Old 09-05-13, 07:07 PM
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Many years ago, I was taught there should be at least 20% more return duct capacity than supply. More recently, I've heard of 30-35% more. The additional, I'm told, is to help compensate for the air flow restriction caused by people using "high efficiency" air filters.
Ideally, you should have both high & low returns. Use the high returns in the summer to pull the hot air off the ceiling & the low returns in the winter to pull the cold air off the floor thus helping to "pull the warm air down".

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