Do I need to run a dehumidifier in a new house?

[novis1]

Hello,

I have just moved to my new house in Kitchener, Ontario. Some of my friends who moved 2 months before me in the same neighbourhood are running dehumidifiers in their basements. I have my furnace fan running with a settings 20/40(20 min. running and 40 min. off) since it's installed in the basement. So, my question would be - do I also need to run a dehumdifier in my house?

Please note I do not see any dampness in my basement. However, I noticed the RH 50% -65% range in furnace control device.

I would really appreciate any suggestions in this regard.

Thanks in advance.

[amd]

Definitely not at this time of year. (or ever in a new southern ontario house, provided that it has a/c)

If you find condensation on the windows when it gets cold, increase ventilation*; showering, cooking, and building materials drying out increase the relative humidity.

*You may just have exhaust fans if the builder cheaped out (HRVs is preferable)

[teddy bear]

Definitely not at this time of year. (or ever in a new southern ontario house, provided that it has a/c)

If you find condensation on the windows when it gets cold, increase ventilation*; showering, cooking, and building materials drying out increase the relative humidity.

*You may just have exhaust fans if the builder cheaped out (HRVs is preferable)

How do you keep a the inside of a home in Southern Ontario <50%Rh during wet cool weather? When the outside air dewpoint is +60^F or +80 grains/lb. of air, outside air infiltrates the home. In fact a home with good IAQ needs a fresh air change every 5-6 hours at a minimum when occupied. The basement walls/floor are 55^F-60^F which is below the outside dew point. Insulation on the walls and carpeting on the floor but the moisture in the air can condense on the cool walls/floor surface. After a several years of high spring/summer/fall spells of high dew points, the space will smell like a basement.
Ask an a/c contractor: Do I need a dehumidifier? The sincere a/c contractor responds: Not unless your a/c is oversized or your home leaks excessively.
Think about your answer. When the outside dew point is near/above any of the inside surface temps, mold growth will eventually follow.
The next question: Can your a/c maintain <50%RH during typical weather in a green grass climate? Yes????
My calculations show that the amount of moisture that must be removed from a home to maintain <50%RH dependson:
The amount of fresh air infiltration/ventilation,
times the required reduction in moisture content,
plus the moisture from the occupants resperation/activities.

On the most hot, 70^F dew point days, 70-100 pints per day of moisture is removed by a properly setup a/c cooling 8-10 hours/day. This will maintain <50%RH, <55^F dew point in minimally ventilated (.2 ach)home with typical occupancy.
During damp cool weather, 65^F dew point outside air, 40-60 lbs. of moisture must be removed to maintain 50%RH.
During the ideal spring/fall weather with a 50^F dew point, no dehumidification is required.
In fact with outside dew points of <50^F outside infiltration/ventilation is .2 ach will remove the moisture from the occupants and keep the home dry.

Do I need a dehumidifier? A better answer is depends on the outside dew point, the amount of fresh air, the number of occupants, and how dry do you want the house tobe. Agree?
Properly ventilated, green grass climate homes with normal occupancy activity need a properly set up a/c and adequately sized dehumidifier for ideal comfort and IAQ. Slightly oversized or undersized a/c has very little impact on the problem. I am open to rebutal.

Regards TB

[amd]

Well, a whole house dehumidifier salesman will always be biased on this matter.

Being in sourthern Ontario, I can tell you that basements in newer houses don't have moisture problems in the summer.

Usually when it's humid, it's also warm and mechanical cooling tends to be used. (if not, use of a dehumidifier during humid weather might be a good idea, but not in september/october)

During damp cool weather, 65^F dew point outside air, 40-60 lbs. of moisture must be removed to maintain 50%RH.

If the indoor dewpoint creaps up to that of the outdoor dewpoint with the windows closed during mild/humid weather, there's a huge infiltration problem.

If ventilation is provided, moisture might be a problem only 5-10 days per year here.

I'm no fan of dehumidifiers because 500-1000 watts full time adds up quickly.

[teddy bear]

Well, a whole house dehumidifier salesman will always be biased on this matter.

Being in sourthern Ontario, I can tell you that basements in newer houses don't have moisture problems in the summer.

Usually when it's humid, it's also warm and mechanical cooling tends to be used. (if not, use of a dehumidifier during humid weather might be a good idea, but not in september/october)

WI and SO Ontario have similar weather. Don't you agree that the amount of moisture in the outside air that is infiltrating/ventilating into our homes plus the moisture from the occupants will determine the moisture content of the inside air. So on days when the outside dew point is <50^F, the inside moisture will be the result of the amount of outside air infiltrating plus the amount of moisture added by the occupants??
Thats what keeps our home dry during winter weather.
Spring and fall has many weeks of 60^F-65^F dew point weather. EXpect that the dew point in the home will be a little higher because of internally generated moisture. A 70^F inside temperature with a +65^F dew point is 80% RH.
Anway thanks for the concession on having an occasional need for dehumidification. Unfortunately or fortunately, most homes do not have adequate fresh air change during the warmer months of the year. I have been documenting a several WI and FL homes. During this time of the year, the dehumidification load is about 10-25 lbs. per day. When the outside dew point drops below 50^F with .2 ach, the dehumidification load drops to near nothing. Also when the outside dew points drop to 10^F, .2 ach reduces the indoor %RH to <35%.
O S dew points of 65^F with .2 ach plus 2 adults/2 kids is 25-30 lbs. of dehumidification per day to maintain <50%RH.
All of this is solid high scholl physics. Green grass climates that want good indoor air quality which includes fresh air ventilation need a good dehumidifier as part of the program.
Thanks for participating in the discussion.
Regards TB

[amd]

Almost no mechanical ventilation in the summer is the norm - do those whole house units bring in and dehumidify fresh air?

Keep in mind that using an air exchange unit with an ERV core in the spring/summer and HRV core (I don't know if units with interchangeable cores are available) in the winter is the best solution, since refrigerated dehumidifiers consume a fair amount of energy.

[tedkidd]

Simple answer - set the dehumidifier to 50-55 and let IT decide if it needs to run.

[teddy bear]

Well, a whole house dehumidifier salesman will always be biased on this matter.

Being in sourthern Ontario, I can tell you that basements in newer houses don't have moisture problems in the summer.

Usually when it's humid, it's also warm and mechanical cooling tends to be used. (if not, use of a dehumidifier during humid weather might be a good idea, but not in september/october)
kly.

Well, a (whole house dehumidifier--omit) Subsitute(a/c) salesman will always be biased on this matter. A little humor!
We have a home owner that is telling us that his neighbors and he have damps basements running dehumidifiers. Some have fresh air ventilation and some do not. All of my monitoring shows that normal occupancy with varing degrees of fresh air infiltration/ventilation and no major cooling loads result is indoor dew points slightly above outdoor dew points depending on the rate of air change and amount of moisture from the occupants. Clearly cautious homeowners do not want a basement that smells like a basment. You are a HVAC contractor which includs V fr ventilation and should include another H for humidity.

Most IAQ experts including ASHRAE suggest an fresh air change every 5-6 hours when a home is occupied. The fresh air purges the indoor pollutants and renews oxygen. We should do this at a minimum when occupied. This means about 75-100 cfm of fresh air when occupied. A 15 mph summer wind will make make most homes breathe at this rate. On a cold winter with a wind of 7mph, most homes will get the same amount of fresh air. So we are left with warmer, calm weather that the homes are more stagnate. You need to operate mechanical ventilation during the calm weather and the home is occupied. HVR/ERV/Ventilating Dehumidifier or something that will provide an air change in 5-6 hours. You also should maintain <50%RH throughout the space including the sensitive basement areas.
Considering that no additional mechanical fresh air may be needed during cold or windy weather or when unoccupied, we are only talking about <25% of total time supplemental mechanical fresh air ventilation. Consider that some of that time, the outside air is dry or wet. This decreases the actual dehumidification required. Your a/c will maintain <50%RH during high cooling loads.
In the final analysis, with a high efficiency dehumidifier, <$150 per year of supplental dehumidification is required to keep a adequately ventilated home <50%RH. Home with fresh air ventilation and controlled humidity are healthy and comfortable.

If the indoor dewpoint creaps up to that of the outdoor dewpoint with the windows closed during mild/humid weather, there's a huge infiltration problem.
If ventilation is provided, moisture might be a problem only 5-10 days per year here.

I'm no fan of dehumidifiers because 500-1000 watts full time adds up quickly.

Almost no mechanical ventilation in the summer is the norm - do those whole house units bring in and dehumidify fresh air?
Yes, any of the Santa Fe/Ultra-Aire/Honeywell dehu will operate free standing and are ductable to bring in fresh air, mix it with house air, filter the blend, and circulate the air throughout the home via heating/cooling ducts. Keep in mind that ERVs remove only a small portion of the moisture in the fresh air. ERV homes need supplemental slightly less dehumidification. ERVs exhaust wet air from baths etc, will transfer moisture to the dry air comming into the home, not a big deal but.

Not to beat a dead horse, but. An air change in 5-6 hours is not a "huge infiltration problem" but it is ideal when occupied.
You are right, watts add up but $100-$150 per year is a small price comfort and excellent indoor air quality. If you think not tighten up as much as possible and save the money.
Thanks again for your sincere effort to fend of a dehumidifier salesman.
There about 2 million cheap dehumidifiers sold per year in North America.
There cheap dehus are expensive to operate, not very durable, and many do a poor job of maintaining low humidity compare to the premiun units
Regards TB