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cold air duct with new furnace


grnrider10
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I'm replacing my old furnace with a Lennox high efficiency furnace. I live in a very cold location.

When getting quotes I rec'd differing info from 2 qualified dealers re my existing cold air duct.

One said that it should be removed and/or shut off as it would put too much cold air in the house & isn't required with the new furnace.

The other one said to keep it (although it isn't required under code) as it is needed for the hot water heater.

Who is correct?

Thanks in advance for any assistance.

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I think you might be asking about the combustion air intake. Combustion air is the air that's necessary to mix with the gas to make a nice hot flame. If there's a laundry in the mechanical room, you may also need makeup air.

You may nor may not need combustion air vents, depending on lots of stuff.

What furnace, specifically. Is it a Category IV?

What water heater?

Is there a laundry in the same room as the furnace and water heater?

Put up some pics if you can.

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I'm replacing my old furnace with a Lennox high efficiency furnace. I live in a very cold location.

When getting quotes I rec'd differing info from 2 qualified dealers re my existing cold air duct.

One said that it should be removed and/or shut off as it would put too much cold air in the house & isn't required with the new furnace.

The other one said to keep it (although it isn't required under code) as it is needed for the hot water heater.

Who is correct?

Thanks in advance for any assistance.

Why not put a damper on it so that you can control how much fresh air is injected into the system?

I presume from your description that you have a water heater that uses indoor air for combustion. If so, you need to make sure that there's an adequate source of makeup air for the water heater. That can be the fresh air inlet or it can be another source. Ask the first guy where the water heater would get its combustion air if you removed the inlet.

Are there other things in the house that need fresh air? Besides people, these might include a fireplace or woodstove, an exhaust fan over the range, etc.

- Jim Katen, Oregon

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Yeah,

Location is going to be critical. The space where that appliance is installed needs to have a volume of about 50 cubic feet for every 1,000 Btu/Hr that the water heater is rated for. In other words, a 45,000 Btu/Hr water heater requires 2250 cubic feet or a room that's roughly 28ft. by 10ft. with an 8ft. ceiling. If you don't have that kind of volume, you need to bring in fresh air from outside.

As Kurt has already mentioned, a laundry in close proximity can affect that whole dynamic; so, even if you have air coming in from outside, if that air intake isn't large enough, a clothes dryer in close proximity to the water heater can depressurize the air around the water heater enough to cause the exhaust gases to flow backward into the home. Granted, any back draft caused by a dryer will probably get sucked into the dryer and sent outside, but sometimes when that happens the water heater can't reestablish it's draft up the flue and continues to vent into the home. That's very dangerous. Then what if it's a gas dryer? In that case, you need to add the Btu/Hr of that dryer into your calculations for what type and size of fresh air vent needs to be brought into the home.

Want to save yourself all the hassle, as well as some heating dollars? Replace the water heater as well with a direct vent type that has a special combination intake/exhaust vent and isn't affected by dryers or overhead fans. Then, neither of your appliances, assuming that this furnace you are installing is a Category IV furnace getting all of it's combustion air from outside, is using interior air for combustion and there's no need to have open holes through the exterior envelope of your house.

ONE TEAM - ONE FIGHT!!!

Mike

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