Results 1 to 11 of 11
  1. #1
    Join Date
    Apr 2008
    Location
    Upstate New York
    Posts
    721

    Indoor humidity 60% why ?

    Our house humidity level is 60% and its about 45 degs outside. In the winter last year I was in the 40's when we had the oil furnace heat on.

    Would the new NG furnace create more humidity or not lower it like the oil heat did or is it just because we haven't been running the heat very much yet and the humidity hasn't stablized ?

    I gotta quit thinking of this stuff

    I run a dehumidfier in the basement kept at 50%

  2. #2
    Join Date
    Apr 2004
    Location
    MN
    Posts
    1,779
    Natural gas doesn't burn near as hot as fuel oil, so there is the possibility of it not "cooking" the moisture out of the air. Have any updates to the house been done? eg. new windows, doors, siding, insulation? New furnace? Is it installed correctly? An improperly installed furnace could potentially be dumping exhaust into the house. Natural gas exhaust has a fairly high moisture content. Is your dryer vent plugged causing that air to back up into the house? Or, like you said, it could be the furnace hasn't run long enough yet to dry up some of the moisture.
    A Veteran is a person, who at some point in their life, wrote a blank check payable to the United States of America for payment up to and including their life.
    Gene Castagnetti-Director of the National Memorial Cemetery of the Pacific in Hawaii

  3. #3
    Join Date
    Apr 2008
    Location
    Upstate New York
    Posts
    721
    new furnace this year but its not dumping inot the house it vents outdoors with PVC hi efficiency. Its about 40 degs outside and 80% humidity according to weather.com but the TV said 40%

    Windows and everything else w done 3 yrs ago.. Dryer is electric vented fine.

    Probably once the level drops outside and the furnace runs everyday it will go down. I know we cook and shower etc and the windows have been shut tight

  4. #4
    Join Date
    Apr 2008
    Location
    Upstate New York
    Posts
    721
    is there more moisture in the air from the registers on my NG furnace than on my old oil furnace

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,753
    Quote Originally Posted by doc havoc View Post
    Natural gas doesn't burn near as hot as fuel oil, so there is the possibility of it not "cooking" the moisture out of the air. .
    You can not "cook" the moisture out of the air.
    If 100 grains per pound of air enters a furnace, 100 grains per pound leaves the furnace, weather the air temp is 125, or 210.
    Only it RH% changes, not its weight.
    Once the discharge air cools to the room temp, its the same RH as it was entering the furnace.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Apr 2008
    Location
    Upstate New York
    Posts
    721
    was that Twilli I saw on the debate

  7. #7
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    Have you had your doors and windows opened more than usual prior to having the new furnace installed. Do you have alot of items in your house that retain moisture like alot of wood, wood floors, furntiure, Hows the exhaust in your bathrooms ? Your attic could be adding to the RH in the house a well.

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by beenthere View Post
    You can not "cook" the moisture out of the air.
    If 100 grains per pound of air enters a furnace, 100 grains per pound leaves the furnace, weather the air temp is 125, or 210.
    Only it RH% changes, not its weight.
    Once the discharge air cools to the room temp, its the same RH as it was entering the furnace.
    Beenthere is correct. It is a common misconception that furnaces "cook" moisture from the air.

    Why do houses become so dry in winter with forced air heating, then? And why, at least for the moment, is your house not drying out? Without actually seeing your home I'm going to give you an educated estimation as to what might be causing your high humidity:

    A) You have a fairly tight house, including windows, walls, and doors in the basement.

    B) Your old oil burning furnace used air from the basement for combustion (often referred to as "combustion air". When a furnace, regardless of fuel used, draws air from the surrounding room, this will increase air infiltration from outdoors to indoors. The air the furnace consumes for combustion must be made up from somewhere else. This air in the average house comes through the exterior walls, windows, doors, ceilings, and other unsealed envelope penetrations. From a psychrometric standpoint, if you draw in air from outdoors that is 40 degrees and 80% relative humidity, and then heat that same air up to 70 degrees without adding any additional moisture, the relative humidity at the higher temperature will be around 27%. As you can see, while the outdoor air appeared "muggy" at 80% relative humidity, once warmed to room temperature it now seems almost arid by comparison. That's the wonder of psychrometrics.

    C) Your new furnace, being a 90% model, most likely uses outdoor air for combustion, piped in through one of two PVC pipes. This means no air from the house itself is being used for combustion air. This in turn results in less infiltration into the structure from outdoors, so the indoor air isn't being diluted sufficiently with "drier" air from outdoors, as it was when the old oil furnace operated.

    D) Basements are often a source of moisture, being below grade and not always sufficiently equipped to repel vapor/moisture diffusion through walls and floors. With a new furnace now drawing air in from outdoors for combustion, the moisture building up in the basement may also migrate into the house, particularly if there are supply and return air provisions in the basement, or significant return duct leakage in the basement. This, along with normal household activity such as cooking, showering, laundry, etc, can raise indoor humidity levels during cool to cold weather to higher than desired levels.

    Solutions? You may need some form of controlled ventilation, such as an ERV or a ventilating dehumidifier. You might also see if humidity levels drop as the heating season wears on, being that during the transition weeks from warmer, more humid weather to colder, drier weather, materials in the house will over time diffuse water vapor into the air as the air in the home slowly becomes drier with outdoor air infiltration. But, that's not a sure thing.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
    Join Date
    Apr 2004
    Location
    MN
    Posts
    1,779
    I used the term "cook" as a layman's term. I understand that as the air is heated it expands, thereby "spreading" out the moisture content in a given volume of air. If the house continues to heat up, the Rh will continue to go down. As the air cools, it shrinks and the Rh goes up. I didn't mean to ruffle feathers, just trying to explain the phenomenon in terms that are easily understood. I apologize for the misinterpretation.
    A Veteran is a person, who at some point in their life, wrote a blank check payable to the United States of America for payment up to and including their life.
    Gene Castagnetti-Director of the National Memorial Cemetery of the Pacific in Hawaii

  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,061
    Quote Originally Posted by cvcman View Post
    Our house humidity level is 60% and its about 45 degs outside. In the winter last year I was in the 40's when we had the oil furnace heat on.

    Would the new NG furnace create more humidity or not lower it like the oil heat did or is it just because we haven't been running the heat very much yet and the humidity hasn't stablized ?

    I gotta quit thinking of this stuff

    I run a dehumidfier in the basement kept at 50%
    Two weeks ago we were at higher outdoor dew points throughout the northern US. Your home was at 73-75^F with ??%RH (55?). This week you cooled down inside to 68^F, 60%RH. Every degree of temperature drop increases %RH by 2.5%. 6^F drop = 12%RH increase. The dew point of the outside air is currently 50^F.
    http://www.weather.com/maps/maptype/...nts_large.html
    Your indoor dew point is 53^F. Occupants add moisture to the outside air that slowly infiltrates your home. This explains the rise in dew point. Sounds normal to me. Ok to think about if you understand what's happening. As the outdoor air gets colder, the stack effect pressure will increase. The increased pressure will increase the air change rate of the home. Wind also makes the home change air. My thoughts are that you should make sure you are getting and air change every 4-5 hours when the home is occupied. During spring/summer/fall, adequate air change requires a fan of some sort. A good bath fan operating when the home is occupied would do. The cost of conditioning 75 cfm of fresh air for 12 hours/day for three seasons is <$100. Think about a high efficiency ventilating whole house dehumidifier (thermastor.com) for perfect control of fresh air and humidity throughout your home. For now, operate the fan when the home is occupied until the %RH drops below 50%. I am attaching data from a WI home which is in northern US like you. This home has 60 cfm of fresh air 14 hours per day. Regards TB
    Attached Images Attached Images
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,753
    Quote Originally Posted by doc havoc View Post

    I used the term "cook" as a layman's term.

    I didn't mean to ruffle feathers, just trying to explain the phenomenon in terms that are easily understood.

    I apologize for the misinterpretation.
    You didn't ruffle any feathers.

    Your description though.
    Helps to perpetuate a myth/faslehood. That too many people already believe to be true.

    And didn't address the problem the OP has (of course, that isn't a real requirement to post anyway).

    Although its a lot. If you were to go back through the OP's post. You would see he has always had a higher then normal humidity in his house during the heating season. Even when he had his oil furnace.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event