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Old 11-20-2012, 06:51 AM   #1
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Options for reducing humidity w/90% furnace


Hello:

I moved into a new home with 90% Trane Brand (XB 90) gas furnaces. The home has conventional insulation wtih 2x6 walls, batts, and blown attic insulation, etc - no foam. All is good except the humidity level is high- over 60% which is cauisng problems with condensation now (winter). My research indicates the indoor humidity can be reduced by improving "ventilation" via several methods including modifying the existing furnace: 1; add fresh intake pipe from outside to the return supply and 2; move the "inlet" PVC pipe (for combustion) that is terminating outside of the home. I fully recognize this approach will reduce the efficiency of the furnace but to be quite frank, I'd much rather pay a higher bill than deal with serious consequences of humidity, particulary the moisture/vapor that may be condensing within the insulation, etc. Im also aware that I can add HRV/ERV/dehumidifier, but there is a cost with this as well. I would much rather modify the Trane XB furnace to bring in fresh air and use the internal air for combusiton to burn up the moisture.

Please let me know your thought with the above approach. Are there any consequences with moving the inlet PVC pipe to the inside and what size duct would I need for the fresh intake duct? The two furnaces are located in the basement along with gas fired water heater.

Thanks for all that respond.

Tom

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Old 11-20-2012, 07:44 AM   #2
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Options for reducing humidity w/90% furnace


The thingts you mention will have no reasonable results.You must first discover where the humidity is coming from.If for instance you have moisture coming from the basement,nothing you do to the furnace will do anything.
It may mean that you will need a dehumidifier like what Reasearch Products make.It may mean that you need major plumbing done with weep trenches .
You can't put a bandaid on a headache.

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Old 11-20-2012, 08:07 AM   #3
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Options for reducing humidity w/90% furnace


If you are getting humidity levels that high, not enough exchange of fresh air from the outside. Has nothing to do with the furnace, it is that your home is too tight.
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Old 11-20-2012, 08:29 AM   #4
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Options for reducing humidity w/90% furnace


Your home is too tight and you should have a HRV installed. This will recover about 80% of the heat from the air being exhausted. If you take the air from the house for combustion that could be dangerous if your house is too toght and won't allow enough air back in around the doors and windows. Then you have a negative pressure in the house and it can suck air in through the attic and exhaust fans and range hood and create other problems. You then would need to install a 6" insulated combustion air pipe to the furnace room AND a 5" fresh air pipe into the furnace return duct to meet codes and be safe. Then you are bringing -10 to -40 deg air depending on where you live in and having to pay to reheat it. See where the HRV makes sense/ Google Lifebreath heat recovery ventilator for more info. Where do you live? I have a very tight home and a HRV and know your situation. Lifebreath makes them for Lennox and calls them Healthy Climate so you can check theirs or buy a Lennox if that is easier.
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Last edited by yuri; 11-20-2012 at 08:33 AM.
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Old 11-20-2012, 09:05 AM   #5
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Options for reducing humidity w/90% furnace


Thansk to all for responses. My humidity level hovers around 50% upstairs and in the basement and about 44% on the main level. There are three furnaces for - one for each level, with attic being 80%. The home is in Northern Virginia.

I will explore an HRV, or some other solution, but is it worth for me to first add a fresh air intake duct to see if that improves things because this will be highly cost effective, simple, and most practical? Isn't the functionality similar to what an HRV would provide? If I have to go the HRV route, do I need to add one to each furnace or can I use one to connect all three (two in next to each other in the basement, and one is in attic)

I've discussed this with the builder but unfortunately, he has limited knowledge on HVAC and the sub who installed the equipment is not that much better. I'm doing independent research and will then sit with both of them to discuss this..

Thanks again for responses...
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Old 11-20-2012, 12:11 PM   #6
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Options for reducing humidity w/90% furnace


Why three hvac systems? How large is the home? As for the attic being 80% you have problems, such as bath vent venting into it. The attic should be around what the outside rh is, or can be lower as I have seen with mine during Winter, around 13-21%.
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Old 11-20-2012, 04:15 PM   #7
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Options for reducing humidity w/90% furnace


Before just adding a fresh air intake or changing your furnace intakes to use inside air. I would see what happens if you run your bathroom fans 24/7 for 2 or 3 days.

While more expensive to do. An HRV can repay for itself in energy savings, over just bringing in cold fresh air in the middle of the winter.
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Old 11-20-2012, 05:14 PM   #8
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Options for reducing humidity w/90% furnace


Try run the exhaust fans to see if there is any change but that may not necesarily work if the house is real tight and may suck air back in thru the range hood or dryer and depressurize the house. You need to find an EXPERIENCED contractor who understands HRVs and installs lots of them. Not too many do. You can get one larger one and run it like a dedicated system but that is where you need to find the right contractor. We cannot see your ducts etc from here so you need to get a local guy to do that 4U.
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Old 11-21-2012, 07:08 AM   #9
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Options for reducing humidity w/90% furnace


thanks Yuri and others for responses...much appreciated. Since the home is new, less than six months, it may be best for me to just wait and see if it dries out and monitor the humidity levels. I can then explore an ERV, HRV, dehumd, etc to mitigate the moisture problem.

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Old 11-21-2012, 06:26 PM   #10
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Options for reducing humidity w/90% furnace


other sources of moisture. skim coated dry wall, framing was wet when enclosed. i have heard of housed taking months to dry out when new. run the bath fans 24/7 for awhile and see what happens.

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Old 11-21-2012, 07:50 PM   #11
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Options for reducing humidity w/90% furnace


So true, the drywall compound, primer and paint, materials, fresh concrete that almost every time got wet during constrution all drying at the same time.
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Old 11-21-2012, 10:01 PM   #12
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Options for reducing humidity w/90% furnace


The home is new and has no means of fresh air intake? I thought most states mandated that in their building codes. If you don't have a HRV or ERV at least a barometric fresh air damper into the return air ductwork should have been installed.

If you want to reduce humidity in the house for the least amount of cost, open a window or two slightly. Humidity always trys to find a balance and it will move from inside the house to the outside, but you need to give it a way to get out.
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Old 11-21-2012, 10:05 PM   #13
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Options for reducing humidity w/90% furnace


how about check if CFM is configured properly?

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