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Old 09-25-2008, 08:16 AM   #1
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Hey all,

Currently installing a new gas furnace for the house. New unit replaces an old (50 yrs) oil-fired hot air behemoth.

New furnace is the York PM9 Upblast. 100,000 btuh, 200 CFM.

I have all the sheetmetal connected, ran the vent line last night and have a question on the vent/combustion air piping

1st I have the vent located per the installation manual, using ambient air as the combustion air, therefore I have the single vent pipe running out the wall of the house, above expected snow level, and away from windows. One question I have, and perhaps it shoud go back to York is that they state to have 12" of pipe protrude out of the house, then have a 90 elbow turned down. I like the idea of the 90, but curious as to the 12" distance. I can live with it, but jsut curious.

2nd the manual states that if I am using ambient air, I should not pipe anything to the air intake on the furnace. Both intake and vent connections are on top of the furnace. I was thinking of putting a short piece of pipe and 90 elbow to keep any debris from being dropped into the unit. This was suggested by a contractor friend, but I shrugged it off after the reading the manual. When I was prepping the area for the vent, I was pulling some old wire off the floor joists above and a wire staple actually fell down the vent pipe, and landed in the blower. I had to spend the next 20 minutes taking the vent blower off the unit and inverting to remove the staple. 'Oy!

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Old 09-25-2008, 08:39 AM   #2
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Quote:
Originally Posted by javan View Post
One question I have, and perhaps it shoud go back to York is that they state to have 12" of pipe protrude out of the house, then have a 90 elbow turned down. I like the idea of the 90, but curious as to the 12" distance. I can live with it, but jsut curious.
Not sure but perhaps they're worried if you don't adequately seal around the protrusion you could be sucking indoor air out creating a negative pressure situation.

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2nd the manual states that if I am using ambient air, I should not pipe anything to the air intake on the furnace. Both intake and vent connections are on top of the furnace. I was thinking of putting a short piece of pipe and 90 elbow to keep any debris from being dropped into the unit. This was suggested by a contractor friend, but I shrugged it off after the reading the manual.
Are you describing a trap in the air intake? If so, most manufacturers really want limited restrictions in the intake. I'll bet your manual even says what the max number of 90s they'll tolerate. Bottom line - you want the unit to breath as freely as possible.

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Old 09-25-2008, 10:17 AM   #3
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When I install a high efficient single pipe system, I usually just put a piece of pipe on the intake and route it down to just beside the 6" combustion air intake that is required to sit beside the furnace in this application. This way nothing will fall down into the unit...also install a grill over it to prevent paper, etc from being sucked into the unit.
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Old 09-25-2008, 12:33 PM   #4
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#1, was just questioning 12" from wall, vs. 6"

#2, If the air is from the outside, then you pipe it to the furnace, but if it is ambient, they do not want any pipe connection to furnace. Again, I would prefer to put a 90 degree elbow on it.....

In fact, in the installation manual they tell you that they have already accounted for 3 90 elbows added to the combustion air/vent config (for pipe sizing requirements). They say 1 90 for make-up and 2 for vent.....
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Old 09-25-2008, 12:42 PM   #5
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If I understand correctly, it sounds like you are going to use inside air as your combustion intake air on a high efficiency furnace. Correct?
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Old 09-25-2008, 12:57 PM   #6
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Correct, combustion air will be via the unoccupied basement. Just want to put a 90 on the intake for keeping debris out.
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Old 09-25-2008, 01:27 PM   #7
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While I know York is on the fence about using indoor or outdoor air for combustion, know that in some places you need 5000 CF of air for a setup like that by code.

I just went through this with a friend who bought a new home that was plumbed to use indoor air. A call to Codes office and a Trane dealer confirmed they recommend outdoor air. The concensus was that in older leaky homes this may not be an issue. But if you ever start sealing your home up tight this can be a problem.

I would recommend avoiding all this by plumbing to pull outdoor air. If you truly must pull from indoor air, don't use anything smaller than 1/2" screening and no elbows. Know that even 1/2" screening will restrict air flow by 10%. You can compenstate that by adapting a larger intake pipe.
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Old 09-25-2008, 07:39 PM   #8
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Not sure of how your codes work in New York, but here in Canada, combustion air required is determined by the BTU of the furnace. It does not really take into account the temperature of the air and even if it did, then it would probably make more sense to use a tempered combustion air (drawing from the furnace room with a properly sized combustion air intake mounted by the burners). As far as 5000 CF goes, why would they require that much air if it is tempered and inside your house , but not require that same cf if it has to be drawn through a 2-3 inch intake pipe that is drawing in cold winter air? As long as the manufacturer of the furnace states you can install a one pipe system ( and Trane, Carrier and York say you can) and the local codes allow it, then I am sure he can do it the way he planned.
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Old 09-25-2008, 11:05 PM   #9
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Per the International Codes, for combustion air obtained from within the structure, you must have 50 cubic feet of space for every 1000 btu/h's of input rating on the fuel burning appliances in the room.

So for a 100,000 btu/h furnace...

100 x 50 cf = 5000 cubic feet of space required for the furnace.

Don't forget the water heater as well in the combustion air requirements.

Provided you have the space in the basement to provide the combustion air, you should have no issues with installing a 90* elbow on the intake to keep things from falling in. Of course you should check your installation manual though. On my Lennox, I was required to install a vent limiter disk in the hub of a 3" PVC elbow since I'm pulling combustion air from my basement.
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Old 09-26-2008, 05:09 AM   #10
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Keep in mind that by drawing indoor air vs. outdoor air for combustion, you will lower the efficiency of your system considerably, because you are throwing out air that you have already paid to heat.

Also remember that most mechanical inspectors want the furnace installed per the manufacturers recomendations, the only correction I have had in 10 yrs. is because I didnt have a copy of the IOM near the furnace for the inspector.

Are you lining the chimney for the water heater?

Last edited by 8 Ball; 09-26-2008 at 05:20 AM.
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Old 09-26-2008, 07:14 AM   #11
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In a tightly insulated and sealed home with a gas stove, gas dryer, bathroom and kitchen vent vans and gas hot water tank you are already creating a fair amount of negative pressure in your home. Negative pressure induces additional drafts. Why add more?
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Old 09-26-2008, 06:04 PM   #12
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Javan

Your 1st question of 12" from the exterior wall. The purpose of the 12" is to get the exhaust vent away from the side of the structure to allow the free air movement to dissipate the exhaust. Air currents don't always travel adequately directly near the surface of a wall. As every building and surrounding setting is different, the manufacturers have determined that a 12" minimum will provide the necessary air movement. Especially if snow builds up at the foot of the wall.

2nd...I've installed furnaces by other manufacturers that do say to put a 90 on the intake if using the interior combustion air, but your system may be different. On those systems, I put a 90 deg street elbow on, and install screen meshing if it came in the furnace parts bag. If you want the proper answer for your system, call York technical service.
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Old 09-26-2008, 07:42 PM   #13
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Regarding the Combustion air, We have the gas furnace, but that will be the only thing pulling "house air". The WH will pull from the outside.

Regarding the 12", no problem there, just was curious.
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Old 09-28-2008, 06:36 PM   #14
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Well, lots of progress since the last posting:
-Gas piping run, pressure tested, all is well
-Power run to unit, all checked out.
-Tstat wiring completed

I opened the gas valve, switched on the breaker at the panel, then flipped the shutoff switch at the unit, expecting nothing since I would have to run upstairs and switch the unit to on and increase the set temperature. HOWEVER, the furnace blower started up as soon as I flipped the shutoff switch at the unit, then ran for about 5 minutes then stopped, the control panel flashed a red light 11 times which indicates a few different scenario issues, which I am working out.

My question though is why did the unit start when the switch was flipped to on, and not wait for the tstat. From what I can tell, I was only to wire the R and W contacts on both the Tstat and the furnace control board.

Going to dig more on this in the AM.
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Old 09-28-2008, 07:29 PM   #15
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Quote:
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...My question though is why did the unit start when the switch was flipped to on, and not wait for the tstat.
It's a way of the furnace telling you it has a problem. What does your diag label say a code 11 is?

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