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wariarr 01-21-2009 07:06 PM

Another cold air question...
I saw the previous cold air return thread. I have maybe a slightly different question and know little about HVAC systems:o. In my house (7 years old) in Minneapolis, I have a forced air furnace in the basement with a cold air vent (flexible 6-8" duct that falls to the floor, shaped like a U pointing upward, and located near the furnace). Last week I measured brisk air flow at 10oF coming from the outside and was wondering whether this is the reason our finished basement runs 5-7oF cooler than my 1st floor where the thermostat is located (usually set to 65F). We have warm air ducts that blow hot air into the basement but maybe no air return back to the furnace.

What can I do to warm up my basement so that it is closer in temperature to my first floor temperature? Is there a simple fix to this problem? Does it make sense to heat the air coming in to the house from the outside? Better recirculate the air?


Hube 01-22-2009 08:13 AM

Wariar; That "U" shaped pipe coming in from the ouside is for combustion(make-up) purpose only.Because of it's "U" shape, it should only admit air from the outside whenever the burner requires it.

The reason it is allowing more air in is because you have no RETURN in the basement. Installing a RETURN or two in this basement area will allow more supply air into the basement,and will eliminate any unnecessary "draw" on this combustion air pipe.
note ;when you install the returns(s) keep them at floor level and far away from the furnace as possible.
Post back if you need more info as the size of the return(s),etc.

wariarr 01-22-2009 11:24 AM

Thanks Hube. I wasn't aware that the air coming in from the outside should only happen intermittently for combustion purposes. In my house I see a constant stream of air flowing in and it is very cold.

Could you help me understand how adding a basement return would help stop the outside air flow? Wouldn't the existing hot air vents in the basement (with no return) tend to slightly pressurize the basement and buck outside air flow? Also our house has a fan in the top floor that seems to remove air out of the house I think.

Also I will probably get professional help to do this. Is it a expensive to add cold air returns from a couple of rooms in a finished basement?


Hube 01-23-2009 09:45 AM

Wariarr; In order to get air OUT (supply) there has to be air IN (return) The reason the outside air is coming in is because since you have no return in the basement the only way for the supply to get any is to get it from the outside.
And you say you think the fan on the top floor is removing air from the house. This is another reason why the outside air is coming in.
Combustion air coming in to an "anti-spill design like you have ( U shape is best ) is only needed as the various appliances like your gas furnace burner,gas hot water tank, any kitchen/bathroom exhaust fans,etc,etc. They all need this "makeup air, but only whenthese appliances operate do they need this makeup air from outside.. This makeup air will eliminate any chance of a negative or positve air effect in the home.(neutral)
Return air is essential in an area where there is supply air. If you have, let's say
2- 5" supplys runs in the basement (a total of approx 60 sq inches) then you should also have AT LEAST the equivalent in return. Return passages are quite easy to install (stud spaces, joist spaces , etc) just make sure you do the math.
NOTE; a stud space of 3- 1/2x 14 will allow approx 45 sq inches of air in. The grille at floor level should be approx 8x14 to allow for the grilles restriction. The return duct path back to the inlet of the furnace should be also be in the 45 sq in. range.
If this is not sinking in then, you would be best to consult an hvac pro.

jdougn 01-24-2009 11:24 AM

Perhaps I'm confused, but if you increase the RETURN air in the basement wouldn't it pull even more air through the outside air supply? Or worse, couldn't it pull air down the chimney?

Hube 01-25-2009 09:21 AM


Originally Posted by jdougn (Post 27162)
Perhaps I'm confused, but if you increase the RETURN air in the basement wouldn't it pull even more air through the outside air supply? Or worse, couldn't it pull air down the chimney?

__________________________________________________ _______________
By installing a return or two in the basement, it will make the basement's supply air stronger, thus 'slightly' pressurizing the area .

jdougn 01-27-2009 11:03 AM


Originally Posted by Hube (Post 27182)
__________________________________________________ _______________
By installing a return or two in the basement, it will make the basement's supply air stronger, thus 'slightly' pressurizing the area .

Respectfully, this is one situation in which it would be wise to consult a professional, as Wariarr suggests, to prevent endangering the occupants of the home. Here is why:

By function, a duct Return is creating a vacuum. It is incorrect to assume that this air will be supplied by the duct system. If the supply air ducts are also inadequate then the increased amount of return demand will Depressurize this entire basement "zone" which includes the furnace.

Here is where it becomes dangerous. Since the furnace has an open combustion chamber air can be pulled back down the chimney. When the furnace "zone" gets depressurized from increased return air then exhaust gases will be pulled down the chimney. Obviously, this puts the occupants of the house in danger of carbon monoxide poisoning!

It is extremely risky to randomly adjust the amount of return air in any house with a furnace without consulting a professional. However, working with the supply air is safe so you might try this. Increase the supply air to the basement then make sure that there is a large opening maintained to the rest of the home. This will introduce more conditioned air into the basement while providing a better return pathway so that this air also has someplace to go. Additionally, supply air will slightly pressurize the basement and make is less likely that exhaust gases can get pulled down the chimney.

Respectfully, Doug

Hube 01-28-2009 07:39 AM

Jdough; Getting a hvac pro in to look at the problem is what I MENTIONED A FEW REPLYS AGO.
And the suggestion you made to increase the supply air into basement is faulty because when you increase the supply, the return is also increased.
Also, a duct return will NEVER cause a vacuum as long as there is supply coming into the area.
And as long as there is a"slight " positive pressure thru -out the home there is no danger of ever pulling in exhaust gases via the chimney.
note; a duct system no matter how well it is designed and sized, will usually only deliver as much supply air as there is return air. In other words it can only deliver out as much as it can bring in.

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