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HVAC Discussions - Heating, ventilation and air conditioning.
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Posted by geothermaljones on January 10, 2008, 8:20 pm
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.
> 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
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| Similar Threads | Posted | | Re: Air return duct to outside - effect on efficiency? | January 8, 2008, 9:14 am |
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> 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