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wiz561 03-03-2011 08:15 PM

Huge Vent in intake trunk run

We moved into our new house a few months ago, and whenever the heat turns off, there's a big bang coming from the (unfinished) basement. After a few months of off and on investigation, I discovered that if the basement door is open a few inches, it will slam shut when the heat turns on. If the basement door is open, there is also no bang in the vent when the heat turns off.

This led me to believe that maybe there's a hole in the intake trunk that's sucking the air in downstairs and creating a large suction. I started to look around and discovered a rather large vent near the end of the intake run. The vent is maybe 16" by 16". I also don't have any other gas applianced down there (have an electric water heater), so I don't have to worry about it pulling air in from that exhaust.

Is this normal? Wouldn't you want the heater to suck the air in from the living area, and not the basement? I am also concerned that it's pulling air in from the sump pit, which is something I would think you wouldn't want (radon?).

So, should I leave it how it is or tape over it?

Thanks in advance...

gregzoll 03-03-2011 08:40 PM

There should be a cold air return in the living spaces (ie Living Room, Bedrooms). As for the basement, it should not if no duct work heating or cooling the space.

wiz561 03-03-2011 08:49 PM

Thanks for the response. I've been googling around tonight and found this thread here...

I have two vents downstairs, which I closed off because we're never down there. But what it sounds like from the above thread, in order to meet code, you have to have a vent on the intake in the basement.

Now I have a feeling that because of such a huge draw where it'll slam the door if it's open even a foot, the vent might be too large. An option would be to tape over it or get magnetic vent covers to block them off.

I'm still curious to know if this is a good idea or not. Part of me thinks that the builder knows more about it than I do, and the vents are there for a reason. The other part of me is concerned the vents are too large and drawing possibly bad air from the sump pit.

Thanks for the response.

HVACDave 03-04-2011 02:47 PM

If it is causing the door to slam when the furnace comes on then obviously the furnace needs the return air from that location and probably more. If you decide to block off the vent and further reduce the amount of return air your furnace is drawing in, then you may create additional problems by overheating the heat exchanger in the furnace. If you want to eliminate the vent, then you need to create additional return air from other locations in the house to tie into the return air duct.

You should check local codes as to where these need to be located, and perhaps have someone come out and look at your system for a few bucks and give you some options. Are your basement ceilings finished or accessable?

Jackofall1 03-04-2011 02:58 PM

Here's what I believe you need to look at, a hole that large in the return air duct in the basement is too large and should be partially blocked off. You should open the supply air's into your basement to provide a balanced condition between supply and return.

THe test of balance would be what happens to the basement door when your furnace turns on, when is stops moving that is when you know you are close to a balanced condition.

Some will argue that the heat in a basement that is not used is wasted heat, I subscribe to having warmer floors, heat in the basement rises, therefore it serves to keep the floors warmer and is not necessarily lost.

However when it comes to AC, I close the vents in the summer, both supply and return.


wiz561 03-04-2011 02:59 PM

Thanks for the info. The basement is unfinished and everything is accessible.

There's lots of intake vents all over the house in the other rooms. I kinda figured that if it's not pulling the air in from the basement, it will just pull more air in from the other vents located in the house.

Plus, I would think that you would want more air to be pulled from inside the living area rather than the basement. We also have a whole house filter attached to the furnace and I'd rather filter the air from the living area than the air in the basement.

I never thought about the exchanger overheating, but I guess that's a possibility. I would think that it wouldn't overheat though because there are lots of other vents, but it would be good to hear if anybody else can comment about this.


Jackofall1 03-04-2011 03:03 PM

Wiz561, when was the last time you looked at your furnace filter. Keep the filter clean and the heat exchanger should not over heat, unless you are using a filter that is not compatible with your system. IE too high a MERV number/rating.


HVACDave 03-04-2011 03:15 PM

If you reduce the CFM through the furnace and burn the same amount of fuel you will deffinately have a situation where the heat exchanger can overheat. You need to conduct a temperature rise accross the heat exchanger to ensure you are putting the correct amount of air past the heat exchanger. Easy to do, put a thermometer in the supply duct, and another in the return duct and measure the difference in the two temps. take the front cover off of the furnace and check the rating plate for recommended heat rise and see if you are in the temp range listed. If not speed up or slow down fan until you are in the range. Maybe you should just cut a return grille into the basement door.

Jackofall1 03-04-2011 03:35 PM

HVACDave, that really is jumping to conclusions, an opeining that size in the basement 16 x 16, will short circuit any RA from the upstairs, and cause all the return air through the path of least resistance ( the basement door) this is why the door is closing, by cutting in a vent in the door, you are not allowing the RA system to pull uniformly from areas that have RA's cut in.

That said, balance the basement air flow, (which will close off the RA substantially in the basement) and then for interest sake differential temperature reading can be taken.

Also ensure that the filter doesn't have a MERV rating higher than I believe 8, as this could have a greater detriment to airflow, than the RA opportunities.


wiz561 03-05-2011 11:34 AM

Thanks for all the great info. I ended up covering 3/4 of the large vent in the basement, and closing the two vents down there half way. The door doesn't slam shut anymore but rather gently closes.

It seems like overall, it is venting the house better. Before, the master BR was the farthest from the system, and there was very little air movement. Now, you can somewhat feel air being moved around the room when the system is on.

I'm also going to be changing the filter out soon. It's a Lennox "Clean Air" system and the filter for it is rather large (16x25x5). I believe it's a MERV 10 filter. Since this is a new construction, the filter is rather clogged with debris. I'm going to be replacing it with a 3m filtrete 16x25x4 until I can order the original online.

Thanks all for the great info!

Jackofall1 03-05-2011 12:09 PM

Your very welcome, I figured you if you got that RA closed up it would dramatically change the house comfort level. I would go until the door doesn't move at all.

Use a silk scarf to measure how much flow you will be surprised!


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