Search the forums

Loading

Fresh air line to Furnace

workdog2's picture

I presently have a fresh air line running outside through the rim joist in a basement to draw outside air into the cold air return duck. This is a foil type line like used for a clothes drier, it's insulated with a fiberglass jacket. The jacket gets wet in the winter months from condensation where the warm air meets the cold air. I'm insulating the rim joist in the basement with foam board & spray foam, can't seem to come up with a good way to seal around the jacketed foil area. Any ideas ? Thinking about removing the fresh air line. Is it Energy efficient?          THANKS

(post #115064, reply #1 of 17)

The duct is needed (and required by code) to assure that the house is under "positive pressure" and hence not apt to draw in fumes from your furnace. Don't remove it.

How much of the duct gets wet? If only the first few inches then likely the work you're doing will eliminate the problem. Getting a little airflow in the area will help too -- if there's a heat register in the area that you've had shut off, crack it open a hair.

If more than a few inches of the duct are getting wet then either there's too much air flowing through there or, more likely, the humidity level's too high in your basement. The case of too much air would be caused by an air leak to the outside somewhere else, possibly in one of your hot air ducts.

I presume you have a high efficiency furnace? What kind of water heater do you have?


What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115064, reply #2 of 17)

I've seen this outside air duct mentioned before, I think here, but I haven't had one in either of the houses I've owned that had forced air, and the whole concept doesn't make sense to me....especially now with "sealed" combustion furnaces.
What's the specific code language-and the theory?
(I completely understand the need for make-up air in a non sealed combustion furnace setup,
just don't understand how an outside draw in the return duct could be useful...)

(post #115064, reply #3 of 17)

I added one to my house many years ago because I was getting a lot of condensation on my windows even with storm windows.  It allowed the inside and outside air pressure to equalize better so less air would be drawn through the windows.  It really worked.


Last year I put in a high-efficiency direct-vent furnace and I closed off the intake.  Of course, my house still runs at a lower pressure than outside because I still have a chimney (for the water heater), but apparently the difference isn't as great as it used to be with all of that furnace heated combustion air going out.

(post #115064, reply #14 of 17)

A couple of possibe reasons for an outside air/ventilation air duct:


1)  To prevent problems in combustion appliances in an extra tight house (i.e. backdrafting because they get starved for air).


2)  To provide make-up air for exhaust fans in the house ... again if the house is tight.


Some energy codes require the ventilation air intake ... and require that it be controlled with a motorized damper interlocked with the furnace fan.


If you have the potential for high humidity in the house, it is a VERY good idea. Most tightly built houses in most of the US have this potential.


I'm surprised though that an insulated fresh air duct gets moisture on the outside of the insulation. You either have VERY high levels of humidity in your basement or the insulation is really not doing anything for you. Even a mildly insulated duct should show no signs of condensation ... assuming consistency of insulation. If the moisture is on the duct itself, the insulation and it's vapor barrier are not installed properly.


Consider installing a motorized damper before closing it up to prevent cold air from settling into the furnace/return air plenum. A good quality damper will seal itself with a weatherstripped lip inside of it.

There ain't NO free lunch. Not no how, not no where!

(post #115064, reply #15 of 17)

Also,


Remember ... controlling air leakage through fans will generally be more energy efficient than allowing the wind to ventilate naturally through all the cracks in the house ... which is why energy codes require tight sealing combined with fan assisted ventilation.

There ain't NO free lunch. Not no how, not no where!

(post #115064, reply #16 of 17)

All your points are pretty much true. I guess I've just seen way too many poorly/cheaply done HVAC systems, and anyway technology has moved on, with more sophisticated systems for air exchange available...

(post #115064, reply #17 of 17)

Theory and availability of products is one thing ... getting someone to provide a well done job is often a problem. Especially when you are talking about technology and techniques that are relatively 'new' or less traditional than many of the ways people have done things over the decades.


New things can be great, but if not done properly ... just like in the old days ... it just won't work right.

There ain't NO free lunch. Not no how, not no where!

(post #115064, reply #4 of 17)

Don't know if it's code here in MI. About six years ago when the high efficiency furnace was installed the company wasn't even going to put the duct in,but I had them install it. Not sure but I think because I had read about it back than in FineHomebuilding,seems it stated adding a fresh air duct will replace stale air being circulated over & over. I have a gas hot water heater vented through the roof by the way of a metal chimney.


Doesn't seem right to just cut a hole in the blue board for the foil duct than spray foam around it, they probably should of install 4" sheet metal duct , better to insulate around.I don't like the tin foil stuff. 


The condensation is just around the fiber jacket by the rim joist where the foil duct penetrates it, a couple of inches.

(post #115064, reply #5 of 17)

Yeah, likely that condensation is about 50% due to a poor connection and air leakage at the joint. Make sure the connection is tightly sealed and then cobble up additional insulation for those few inches. Wrap the added insulation tightly with plastic to prevent condensation inside the insulation.


What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115064, reply #6 of 17)

 Were thinking a long the same lines...Thanks for your opinion Dan, if people would only do it right the first time around !!!. Going to dig deeper to see if its worth while having the duct,maybe check into EnergyStar.com for some information & look into the area code requirements. Thanks again for your time.


 

(post #115064, reply #7 of 17)

It may be that your house is leaky enough that you don't need it, but then it won't do any harm either. I've been trying to figure out how to add a duct to our house (cold air supply from attic was removed when we had the HE furnace installed), but it's hard to figure something out with a finished basement (and when you want to be on the other side of the house from the furnace flue).


What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115064, reply #8 of 17)

I installed a fresh air intake but used conventional duct work.  However, the secret is to wire up one of these dampers to the burner circuit so that the vent only opens when required.  I installed the damper as close as possible to the intake.


307107


Even so, on the coldest days I will get a little frost around the first 2 inches of duct near the intake.  You can find the part at Smarthome.com.  It was about $65.

(post #115064, reply #9 of 17)

Well, I hate to disagree with everyone, but this type of ductwork is about the dumbest thing you could install in a house.  If you have a high efficiency sealed combustion furnace, you don't need anything like this in the first place.


This is the next worst thing to leaving your windows open in the winter to vent your house.  If you want to exchange air in the house, do it with a proper air exchanger that saves and recycles most of the heat that is lost in the exchange process.


These ducts are supposed to be equipped with balanced dampers that will only allow air into the house when the drop in house pressure causes the damper to open, but they still leak cold air like crazy.


I've seen homes that had so much cold air coming in these types of ducts that the furnace couldn't warm the house on a very cold day.


Edited 5/13/2008 7:55 pm ET by BoJangles

(post #115064, reply #11 of 17)

If his furnace is in fact a SEALED combustion high efficiency model, then I agree with you.  OTOH, if he has an unsealed combustion high efficiency (so-called) model, then the fresh air intake is necessary to prevent negative pressure and drafts.


Remember that cold air can only enter if hot air can get out.  I know that in my case, every time the furnace kicked on, the drafts started up because of a lack of combustion air.  I recently resided (and resealed) my house which made the problem worse because the drafts were then concentrated at doors.


With the fresh air duct, the drafts stopped and the house has been much more comfortable.  The damper I installed is only open during burner operation, so there's no chimney effect possible.  A day's leakage when the damper is shut is roughly equivalent to someone leaving the door open a few seconds too long when coming in or out.  I was worried about the temperature in the basement dropping when I put in the duct and damper, and actually put a thermometer down there to monitor it, but no such drop happened.


As far as fresh air for human consumption  is concerned, that's a whole other matter entirely.

(post #115064, reply #12 of 17)

IIRC, he has a conventional gas water heater that needs makeup air.


What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115064, reply #13 of 17)

If his furnace is in fact a SEALED combustion high efficiency model, then I agree with you.  OTOH, if he has an unsealed combustion high efficiency (so-called) model, then the fresh air intake is necessary to prevent negative pressure and drafts.


That's a fact.  It's better to have controlled air flow than to encourage leaking through all the poorly sealed windows or doors.


Hard to imagine a 6 year old furnace that's not sealed tho'.


And to clarify.....I was responding to the OP, who had this duct running in to his cold air return.  If you want to provide makeup air for a furnace, this is not the place to put it.  It should be installed directly in to the furnace combustion air intake area. This will not freeze you out of your house like his setup will.  His situation will still create all kinds of drafts because your furnace can suck air from all over the house in an attempt fuel itself with combustion air.


 


 


Edited 5/14/2008 8:25 pm ET by BoJangles

(post #115064, reply #10 of 17)

I'm with Bo, your killing your efficiency. With a newer house or a older house that is sealed well, you do have a problem with stale air but there are things made to deal with that and keep your heated air. Check out these links, they exchange the old air for new air but capture the heat you've already added to the air. Its more money then what you did but not a ton. I want to say most are $500 or so.

http://www.renewaire.com/index.php

http://www.airmechanicalinc.com/renewaire_erv.html

http://www.energyfederation.org/consumer/default.php/cPath/30_2247

http://yourhome.honeywell.com/Consumer/Cultures/en-US/Products/Ventilation/