Air return duct to outside - effect on efficiency?

[Daniel J. Stern]

We have gas-fired central heat with A/C in our old 2-story (+ basement) house. 3-year-old Lennox air handler is in the basement. This house has been badly abused in the past, and ductwork has been mutilated. Air return capacity is grossly insufficient: the one and only air return is on the main floor, and it is the ~4" x 10" size of a regular heat register. There are eight supply registers throughout the house, nine if we count the one in the basement, some of which are oversize, so obviously one small register hole just isn't enough. The return duct goes audibly into vacuum when the furnace blower turns on (metal walls of the duct "pop"). To alleviate this condition, I removed the blockoff plate from the return duct in the basement. This plate covered where a humidifier used to be installed. I put some wire mesh over the hole and some spun-fibre filter batting over the mesh. This relieved the vacuum condition on the return duct, but is probably less than optimal (basement is smelly/mouldy/dusty) and may be against code (distribution air intake in the same space as combustion taking place -- don't know if that's problematic or not.)

It occurs to me a flex duct could be run from the return duct (using the blockoff plate hole) to a pass-through in an unused door from the basement to outside, and a rodent/leaf/waterproof exterior air intake could be put on the outside. This would take supply air from outside the house and duct it to the air handler. Now here's the question: What would be the effect on system efficiency by doing so? I can think of two possible answers: Either the very cold outside air would take much more energy to heat up to household temperature, increasing the cost of running the system, or the greater temperature differential between the fire and air sides of the heat exchanger would mean more heat transfer to the supply air, reducing the amount of heat wasted up the flue and not significantly increasing fuel consumption to provide a given household ambient temperature. Obviously the reverse (cool/ warm) of either situation would apply during air conditioning season.

So...which is it?

Please and thank you,

-DS

[Bubba]

You're going about it all wrong. All you need is some cardboard, duct tape, bailing wire and bubble gum. With that you can have a brand new efficienct system that everyone can enjoy. Bubba

[jamesgangnc]

You really need to just fix the return. Using outside air would be insane. Using the basement air is just about equally stupid. The return can just be on the first floor but it needs to be sized properly. It's usually not that big a deal to box off part of a closet and put the return in the outside wall of the closet. Don't you have a coat closet or pantry that is somewhat centrally located on the first floor?

[Zyp]

And Bubba;

Don't forget the thermostat.

[geothermaljones]

If your return duct is "oil canning" from the massive negative pressure & it stopped w/your fix, your drawing the majority of your return through you rigged basement return. Cold outdoor air, in such a large quantity, mixing with the limited indoor return air your getting, will do a couple things.

First, you will start condensing inside the heat exchanger & it'll rot from the inside out, but this won't be a big problem because, Second, the heat exchanger will be cracked from the cold mixed air. Either way CO will be an issue. Typically, in cold climates, the max outdoor air intake to a furnace should be limited to a maximum of 20%. (of heating airflow)

Also, in the summer, depending on you summer design temps of course, you'll likely have very high humidity & a unit that won't shut down. In both summer & winter you'll have no efficiency at all. Don't step over a dollar to pick up that dime. Fix your return duct & add a small flex drier vent for fresh air in.

goodluck geothermaljones st.paul,mn.

[.p.jm]

Save your 'magic words'. The only 'magic words' you have any use for right now are 'Heavenly Father, we commend the spirit of the dearly departed unto your care'.

Congratulations ! You have created a situation that is LIKELY TO KILL SOMEONE, and if it's YOU, you get your very own DARWIN AWARD !!!!!

Bad news - they generally arrive a bit too late for you to enjoy.

[.p.jm]

This was not me!!

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