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Old 11-13-2011, 01:52 AM   #1
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Crude outside air feed for older furnace?


I have a 70 year old house with a furnace that im unsure of the age. Its a HEIL NDGG100DF01. The gas furnace and hot water heater sit in the kitchen against an exterior wall. We plan to build a closet around them with louvered doors to box them in so they wont be seen and make a small pantry.

Would it be possible to box in the furnace/water heater with insulated walls and sealed doors and then bring in outside air through the exterior wall with a pipe or one way vent of some sort for an outside air feed? Would this make my home more efficient? Upgrading to one of the new high efficiency furnaces would be nice but i dont have the money to do it right now.

If i were to do this how would it effect the humidity level in the home? As things are, its a battle to keep the humidity up in the winter.Thanks for any replies.


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Old 11-13-2011, 04:44 AM   #2
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Crude outside air feed for older furnace?


If you bring in outside combustion air what you want to do is fine. And it will help with your humidity in the winter, and sumnmer.

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Old 11-14-2011, 09:10 PM   #3
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Crude outside air feed for older furnace?


Anyone else have a comment?
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Old 11-14-2011, 09:31 PM   #4
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Crude outside air feed for older furnace?


Yes, beenthere knows more than the most of us others combined multiplied by one hundred. Listen to him.
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Old 11-14-2011, 10:03 PM   #5
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Crude outside air feed for older furnace?


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Originally Posted by beenthere View Post
If you bring in outside combustion air what you want to do is fine. And it will help with your humidity in the winter, and sumnmer.
I have 30 years in this trade and i cant for the life of me understand how bringing in dry frigged winter air from the out side will help with humidity, other then making it even dryer in the house , and on the same token in the summer we are trying to remove humidity .......?????

Anyway, the amount of combustion air needed is determined by the BTU's of the two appliances that will be in the closed space.Also because its a closed space, smoke, and carbon monoxide alarms that will shut off the appliances if they are tripped should be installed as well. A much easier solution would be to just use louvered doors, and please keep the louvers clean.
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Old 11-15-2011, 04:32 AM   #6
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Crude outside air feed for older furnace?


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I have 30 years in this trade and i cant for the life of me understand how bringing in dry frigged winter air from the out side will help with humidity, other then making it even dryer in the house , and on the same token in the summer we are trying to remove humidity .......?????

Anyway, the amount of combustion air needed is determined by the BTU's of the two appliances that will be in the closed space.Also because its a closed space, smoke, and carbon monoxide alarms that will shut off the appliances if they are tripped should be installed as well. A much easier solution would be to just use louvered doors, and please keep the louvers clean.

If the mechanical room is a sealed room with solid doors. then binging combustion air in from the outside to that room only. Will decrease the rest of the homes infiltration rate. So less dry air is brought into the home in the winter, and less moist air in the summer.

Using theoretical perfect combustion ratio. 14 to 1. If he has a 60,000 BTU input furnace and a 32,000 BTU input water heater, then he needs 1288 CF of combustion air an hour of combined burner on time. Or 20.5 CFM for combustion, plus twice that for the water heaters dilution hood. So in the winter, that comes out to just over 1 pound of moisture/water an hour that is not being removed from the house. In the real world, it comes out to a lot more then that.
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Old 11-15-2011, 12:00 PM   #7
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Crude outside air feed for older furnace?


I disagree.........reguardless of the condition of the mechanical room, (sealed or not) with out the introduction of some type of moisture your home is coing to be dryer in the winter and wetter in the summer. Not to mention in 30 years i have been in thousands of homes all over the mid west and i have never seen a air tight mechanical room in a residential setting.
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Old 11-15-2011, 12:48 PM   #8
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Crude outside air feed for older furnace?


If you can ensure that all of the air used for combustion, excess air, flue gas dilution air, and ventilation air, are satisfied and met coming into the room from outside, then it will allow the humidity that is generated within the home to stay ion the home and not be taken out with the flue gasses as they go up the chimney. This will in fact allow the humidity levels to rise in the main part of the structure.

The amount of fresh air required to meet all of the air types described above would make a considerable amount of fresh air required into the room, and depending on the temp. of the outdoor air could make for a very "cool" furnace room in the winter. I don't know how cold it gets in Illinois, but where I live in Canada, this wouldn't be recommended.

Beenthere, your 14:1 ratio is for combustion air i take it. Theoretical air would include (combustion air 10 cu. ft/1000 btu input, plus 5 cu.ft/1000 btu input of excess air) so 15 cu. ft/1000 btu burner input.

It would also be recommended to add an additional 15 cu. ft./1000btu for flue gas dilution air and an additional 15 cu. ft./1000 btu for ventilation air to replace air being exhausted or exfiltrated from the space (if applicable).

Not saying good/bad or right/wrong, just raising the point that combustion air isn't the only consideration in this situation with a sealed room.
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Old 11-15-2011, 05:14 PM   #9
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Crude outside air feed for older furnace?


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Originally Posted by harleyrider View Post
I disagree.........reguardless of the condition of the mechanical room, (sealed or not) with out the introduction of some type of moisture your home is coing to be dryer in the winter and wetter in the summer. Not to mention in 30 years i have been in thousands of homes all over the mid west and i have never seen a air tight mechanical room in a residential setting.
You did take notice I said help with the humidity. not cure all humidity issues.
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Old 11-15-2011, 05:18 PM   #10
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Crude outside air feed for older furnace?


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Originally Posted by HVACDave View Post
If you can ensure that all of the air used for combustion, excess air, flue gas dilution air, and ventilation air, are satisfied and met coming into the room from outside, then it will allow the humidity that is generated within the home to stay ion the home and not be taken out with the flue gasses as they go up the chimney. This will in fact allow the humidity levels to rise in the main part of the structure.

The amount of fresh air required to meet all of the air types described above would make a considerable amount of fresh air required into the room, and depending on the temp. of the outdoor air could make for a very "cool" furnace room in the winter. I don't know how cold it gets in Illinois, but where I live in Canada, this wouldn't be recommended.

Beenthere, your 14:1 ratio is for combustion air i take it. Theoretical air would include (combustion air 10 cu. ft/1000 btu input, plus 5 cu.ft/1000 btu input of excess air) so 15 cu. ft/1000 btu burner input.

It would also be recommended to add an additional 15 cu. ft./1000btu for flue gas dilution air and an additional 15 cu. ft./1000 btu for ventilation air to replace air being exhausted or exfiltrated from the space (if applicable).

Not saying good/bad or right/wrong, just raising the point that combustion air isn't the only consideration in this situation with a sealed room.
No, the 14 was perfect combustion, so no excess air. I used it as a conservative example only of how much air could be prevented from being drawn into the house.

Lots of people here that go from a nat draft to a direct vent 90%plus notice an improvement in their homes humidity level.


Again, thats an improvement, not a perfect humidity level.
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Old 11-15-2011, 10:42 PM   #11
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Crude outside air feed for older furnace?


Id like to thank you guys for the replies to this. I thought this up taking a shower the other day and wanted some expert opinions on it. I doubt i will actually seal the closet air tight and introduce outside air. Id be better off upgrading my furnace to one of the high efficiency ones that use outside air and since this furnace is older i can see that happening in the next 3-4 years.

The way i see it though, if i WERE to do this, like beenthere stated, i wont be sucking dry outside air into the home to make up for the humid indoor air that gets burnt and goes out the flue therefore the air in the home will retain its humidity level somewhat in the winter time. Ive had humidity levels as low as 16% before the walls and attic were filled with cellulose due to the gas heater running so much. It was a constant battle to keep the air in this place breathable. It is much better now but i still have to run a humidifier in 2 rooms full time during the extreme cold months.

In the summer time when running the central air the humidity is rather nice in here.

Thanks again

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