Heating, Air Conditioning, Fridge, HVAC/Indoor humdity problems


The past few winters I’ve been having a problem with moisture / condensation. We have lived in the house 15 years and as far as we can remember this is just in the past few years. We’ve noticed condensation on the windows, pants growing mold. Our house is about 60 years, 1300 sq feet and we have a gas furnace.

 Yesterday I purchased a humidity gauge to measure what we had. Yesterdays (outdoor temperature) was close to 70 degrees with low humidity. When I came into the house the inside temp was about 73 degrees and the humidity was about 61%. Since it was a warm day the furnace did not come on all day. I went into the crawl space and found no signs of leaks (no standing water), the vents were all open and clear. I decided to open a couple of windows and watched as the humidity level quick fell to 51-53 %.  I closed the windows after a couple of hours and although the humidity level rose it stayed at 57% until we went to bed. We did not cook , my wife and I both took short showers and used the exhaust fan. Still the humidity level remained at 57%.

 This morning the first thing I did was check the meter, in the room we’ve had the most problems in the humidity was 67%, as I walked thru the house some areas were as much at 74%. This morning the outside temp is 57 degrees with 67% humidity. The only thing that changed over night was the furnace was been used.  

I’ve read a lot lately about using exhaust fans, venting crawl space. Does the furnace make the air more humid?  Any thoughts would be very helpful.


Running Furnaces should help with reducing humidity, you can run your blower fan ,to help reduce the humidity level as stated below.

The combination of indoor moisture sources, air exchange rates, and cold surfaces will determine how much condensation occurs in the home. Activities such as cooking, washing, or bathing will raise the humidity level in your home and often result in some condensation on windows, walls and ceilings for short periods of time during cold weather. Other than causing some deterioration of the finish on wood frames, sills, or casings, such condensation is harmless. Therefore, moderate, intermittent condensation on windows, walls and ceilings is probably no cause for alarm. However, if windows are consistently wet, or water stains appear on ceilings or walls, prompt action should be taken to avoid further problems such as mould growth. Generally, the quickest and most effective response to a condensation problem is to increase the ventilation of the house. This can be done by means such as:

Using exhaust fans that are vented to the outdoors, particularly when cooking, bathing, or washing.
Opening doors and windows to increase the amount of outdoor air being brought into the house.
Installing a heat recovery ventilator (HRV).
Controlling humidity in your home

Outdoor air, when heated to indoor temperatures, will generally be dryer than the air indoors. Thus, increased ventilation will reduce the amount of moisture in the air and, as a result, the amount of condensation, but it doesn’t correct the cause. The cause should be located and corrected. The following table lists possible causes of high humidity in the home and suggests ways to control this moisture.

Causes of High Humidity   Ways to Control High Humidity
Cooking, drying clothes, showering and bathing    
use a range hood fan that is vented to the outdoors when cooking
cover cooking pots to reduce steam and conserve energy
use a clothes dryer that is vented to the outdoors, or an outdoor clothesline rather than hanging wet towels and clothes indoors. Note that your electric dryer may have been vented indoors as a heat-saving measure but this is not recommended any more. Also gas dryers must be vented to the outdoors.
operate vented bathroom exhaust fans during a bath or shower
take shorter showers
deactivate a furnace mounted humidifier
use individual room humidifiers sparingly
Lack of air circulation    
set your forced air furnace switch for summer operation - the fan will run constantly
use a 2-speed fan on your furnace - set it to run continually at low speed, switching to high speed for the heating cycles
pull furniture and stored material away from exterior walls and up off basement floors
open drapes and curtains
leave closet doors ajar
leave bedroom doors open as much as possible
do not block or deflect warm air registers
do not close off unused rooms
Moisture-producing areas such as indoor greenhouses, indoor pools and hot tubs    
keep windows and doors to greenhouses, indoor pools and hot tubs closed
do not draw air from humid areas into the heating system
add a separate exhaust venting system
Exposed earth in basements or crawlspaces    
install ground cover, like heavy polyethylene or roll roofing, overlapped by 10 cm (4 inches) and weighted down or protected by a sand layer
ventilate the space in summer
Outside air supply to heating system is blocked or does not exist    
install a fresh air duct with a variable damper to allow a controlled amount of dry outside air into the home
No exhaust fans or underutilized exhaust fans    
install exhaust fans in moisture-producing areas like bathrooms, laundry areas and the kitchen
run fans for a few minutes after the activity to ensure moisture is removed
use only as needed since heated air is being exhausted
control exhaust fans with a dehumidistat, which only operates when the humidity rises above a preset level
ensure exhaust fans are vented to the outdoors
No Heat Recovery Ventilator (HRV)    
consider installing an HRV
> an HRV provides a balanced ventilation system by constantly replacing the air it exhausts with fresh air
> an HRV can recapture up to 80% of the heat from outgoing air
> this is an expensive item; carefully weigh costs and benefits
Faulty or plugged chimney serving any fuel-fired appliance, such as a furnace or hot water heater    
have a heating contractor test your system
> water vapour forms a large portion of the products of combustion, so unusually high moisture levels can signal a plugged or leaking chimney vent, which is a safety hazard and must be corrected immediately

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Graduated from Private Technical School in 1975. An HVAC/Refrigeration Contractor . Have Unlimited Heating/Piping and Cooling Contractor License.Limited Sheetmetal Contractors License.NATE Certifield. York Certifield Master Heat Pump Technician 1986. Served 33 years combination Active Duty Air Force , Air Force Reserves and Air National Guard in the HVAC/Refrigeration Shop/Mechanical Shop. Troubleshooting/Installing HVAC/Refrigeration Equiptment Worldwide. Retiring in 2003 as a Senior NCO ,and a Mechanical Superintendent.

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