Replies

1. GBA Editor
   Martin Holladay | Jul 31, 2015 07:02am | #1

   Walter,
   The short answer is that relative humidity readings are notoriously inconsistent from one hygrometer to another. The usual explanation is that inexpensive hygrometers aren't particularly accurate.

   I would throw out the data from the thermostat, since thermostats aren't designed to provide accurate RH readings. The HRV control is meaningless in this context.

   Sling psychrometers should give good readings, however, assuming that you are using the device properly.

2. 
   wjrobinson | Jul 31, 2015 08:06am | #2

   Humidity where I live is up in the summer...yup. Down in the winter, yup. high in a bathroom while running a shower. Knowing it exactly is has no value to me and may not to you. Relative...humidity... relative.

3. GBA Editor
   Martin Holladay | Jul 31, 2015 09:42am | #3

   Walter,
   Assuming that:

   1. Your air conditioner is operating properly, and

   2. The AC unit isn't oversized, and

   3. There are no unusual sources of moisture in your house (for example, a damp basement, a damp crawl space, or a plumbing leak), and

   4. You are measuring the indoor relative humidity correctly, and

   5. Your indoor relatively humidity is actually elevated, then...

   ... the solution is to install a $250 stand-alone dehumidifier in your house. For more information, see All About Dehumidifiers.

4. 
   WEG | Jul 31, 2015 09:52am | #4

   Thanks. I have a tight house. About 2.7 ACH50. 4" iso on the exterior with 2x4 walls dense packed with cellulose, R84 in attic. That recent article in JLC about the Vermont house with the moisture in the walls has me all worked up. I need to get more dehumidification. I have my 2ton AC running on dehumidify but it isn't doing the job. How can I add more dehumidification? Help

5. Expert Member
   Dana Dorsett | Jul 31, 2015 12:07pm | #5

   It's common in tight high-R/low load houses to need mechanical dehumidification to keep the summertime humidity bounded as outdoor dew points rise. This is a more common phenomenon in the more humid eastern half of the continent, and it's generally worse the further south you go due to the generally higher outdoor dew point averages (= higher latent load from ventilation air.)

6. GBA Editor
   Martin Holladay | Jul 31, 2015 12:17pm | #6

   Walter,
   Dana's point is a good one. In hot, humid weather, it's important not to overventilate your house. Make sure that your HRV is ventilating as an appropriate rate.

7. 
   WEG | Jul 31, 2015 12:49pm | #7

   Thanks,
   It run 20 minutes and hour. I'll be turning it off during working hours and we'll see what happens.
   Walt

8. GBA Editor
   Martin Holladay | Jul 31, 2015 01:42pm | #8

   Walter,
   "20 minutes per hour" doesn't give us enough info.

   1. When it runs, how many cfm is the HRV moving?

   2. How many square feet does your house have?

   3. How many occupants does your house have?

9. 
   AlanB4 | Aug 01, 2015 04:17am | #9

   Has anyone tried the $250 dehumidifier in front of an intake vent, much cheaper then the 3k for a whole house dehumidifier (assuming its effective)

10. GBA Editor
    Martin Holladay | Aug 01, 2015 05:48am | #10

    Alan,
    In Comment #4, I provided a link to this article: All About Dehumidifiers. The article discusses the results of research into the use of a stand-alone dehumidifier placed near the central return grille of a forced-air system. Briefly, the approach works.

11. 
    AlanB4 | Aug 01, 2015 06:02am | #11

    I should have read the article again, i'm sure i glanced at it some time ago and forgot its contents, my bad

12. 
    charlie_sullivan | Aug 01, 2015 11:35am | #12

    Unfortunately, the $250 dehumidifiers are significantly less efficient than the best "whole-house" units. That results in putting heat into the house, as well as consuming more energy, which then means the A/C has to run more as well. But it's hard to justify buying the larger unit unless you have a major humidity problem.

13. 
    wjrobinson | Aug 01, 2015 01:32pm | #13

    Walter, I run a dehumidifier for a month a year. I added a hose drain and put it on auto. Comes September and it's off. My home humidity is at least 50% now and every summer. By February I have to have a moisture maker by the bed as the humidity is too low. But my home is a leaky 80s place.

    I'm with Martin, run the HRV less seasonally and possibly other times depending on air quality number of inhabitants etc.... all discussed here at GBA .