Miller 90% AFUE Furnace in my recently purchased home


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Old 02-16-15, 06:32 PM
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Miller 90% AFUE Furnace in my recently purchased home

Hi guys,

Been a while, but I've been busy and moved to a new home, one I own, that has a Miller G5RC series (90% AFUE) updraft LP furnace. I did my homework prior to buying and this home has used only 620-635 gallons of LP last 5 years prior to my buying it and those are the numbers from the LP supplier. Anyways, Here's been my problem.

This Furnace was installed approximately 20 years ago into this home, located in the basement and is currently a 1 pipe non direct venting installation.

In the past 10 years it has went under many improvements. Newer Anderson windows, doors, and reroofed, insulation of about 11" in the attic, new siding and soffits, etc.

We we've lived in the home since Thanksgiving, and in the past few weeks I have had trouble with the furnace and the differential air switch and the exhaust outlet and condensation liquid backup in the system. I have replaced the inline drain and all the tubing. The nipple broke off the old unit and it was in poor condition, I've cleaned out nastiness in the P-Trap and refilled it with water, and all that. Now, here we are in the craziest of cold weather and the furnace goes out Sunday morning! wake up to 50 degrees, so I go down and check items out, and get the differential air reset, make sure my drains working good and it is.

Now that I restart the furnace, switch laundry and fire up the drier. Two things happen almost at the exact same moment. I feel freezing air coming down from the area of my plumbing for the bath tub several feet away and have my wife told me that our shower/tub has no water as its frozen. Yea, I have down draft going on in the tub pipe access, so Negative pressure issues. YAY ME

Our basement door is internal to the house and leaving it open has everything happy for the moment, but I don't want to leave it this way and its not a real fix IMHO.

So, my question is to fix my air pressure issues, can I do one or both of these two things.

modify my furnace to a 2 pipe, direct vent system, added a external supply combustion air line per the directions I have from Nordyne/Miller to draw air from the outside. my exhaust line is 2", so according to the documents, I can add a 2 inch inlet to the unit, so this should improve the furnace operation, reduce the air pressure switch problems, and cut my negative air by some degree. A HVAC guy I work with, a fellow tradesmen, suggested adding a 2" PVC air inlet for the driers air needs, said just plumb it in by the unit and have it open end to about 12" off the floor, and this 2nd air vent solution could reduce the remaining air requirements so as not to have negative pressure? I only have the LP furnace and a electric drier in the basement for air consumers.

Sorry this is so long, but I am trying to be detailed and specific.

Thanks Grady and crew in advance!
 
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Old 02-16-15, 07:35 PM
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I would suggest a concentric intake/exhaust vent system instead of two separate pipes.

I honestly don't know what size pipe the dryer would need for make up air. When an air intake is installed, I would suggest using something with some kind of damper. No need in leaving an open hole in the wall.
 
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Old 02-16-15, 08:31 PM
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So, did you get the frozen pipes going??
The 2" pipe for pressure relief for the dryer will do very little, needs to be larger, and when it runs all the way to the floor it becomes a siphon. A trick if you go this route is to bring the pipe in through the rim above the concrete, to a "T". Leave the top open and run the bottom down as planned to just above the floor. The open end of the T breaks the siphon while the pipe running down to the floor will still direct the incoming air down where it will mix and be less objectionable. The breaking of the siphon also reduces the effective stack effect pressure so it will bring in less air when you do not need it. When the dryer runs it will create plenty of negative pressure to make the pipe/duct work.

What do you have for a water heater?

Definitely add a combustion air source for the furnace and check to see if your return ducts in the basement are well sealed. If there is a return in the basement it needs to be matched to a supply so it is not also depressurizing the basement. Also, supply leaks to the outside will cause the furnace to pull additional air from the outside to compensate.

Bud
 
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Old 02-17-15, 03:58 AM
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electric hot water heater, and also there's a sealed off chimney for a wood stove down to the basement, say 18" from the floor is where its sealed off, and it seems sealed well

60watt bulb thawed the pipes, thank god!
 
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Old 02-17-15, 07:07 AM
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If you can seal any openings in the attic, like around the plumbing stack, it will prevent some air leakage. But, the pipes freezing is something you will need to figure out. Normally warm air is flowing up around pipes when there is a path from basement to attic and that could tend to keep that path warmer. Seal it and it might get colder.

Bud
 
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Old 02-17-15, 09:42 AM
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I'm hoping convection keeps warm basement air going up the pipes and keeping them and my wife happy. I'm hoping that the T you describe if installed allows for some outdoor air inlet.

If I understand you correctly, i need a larger then 2" T inside my basement at the rim with the long leg going outdoors, the two short legs pointing at the basement ceiling and floor and the leg towards the floor to go down to about 12" off the floor. Approximately how large do you suggest?
 
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Old 02-17-15, 09:46 AM
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concentric piping

I will look at the concentric setup but that is going to have to likely wait until the weather atleast breaks, 7 degrees F isnt very friendly to shutting down the furnace, and all that. Wife seems content to let the basement door sit open until spring when we can fix it on our terms
 
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Old 02-17-15, 03:28 PM
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If you can deal with it until the weather breaks, I don't blame you a bit. I'd do the same thing.
 
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Old 02-17-15, 05:28 PM
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Sizing is difficult as you want the air to come in with a minimal pressure. Just a guess, but a 6" duct would be a start. However, we are guessing in the blind and some pressure testing would tell us if there is a problem and how big it is. Energy auditors conduct what is called a CAZ (Combustion Appliance Zone) test where they determine the worst case configuration for doors and all exhaust fans, the maximum potential for backdrafting. With a naturally drafted furnace I believe the target number is -5 pascals worst case. But once you provide a direct combustion air supply for that unit, if it is truly sealed, then there is no safety target.

Would be a shame to create a passive make-up air supply (the big hole) and then no longer need it.

Bud
 
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Old 02-17-15, 06:29 PM
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so here's my plan of action-

get thru the current cold season. Install a concentric vent kit and supply furnace with fresh, outdoor air.

Check air pressure or see if its drawing from the pipe access area of my tub once this is installed with the basement door closed. Proceed to next step if this remedies air issues to this point.

See if the dryers exhaust air warrants additional fresh air supply or causes air to get pulled from pipe access. If not great, if it does then I'll probably enlist the help of the home inspector/energy auditor I had to see if a passive make up air supply is warranted.
 
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Old 02-17-15, 07:43 PM
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Sounds like a plan to me.
 
 

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