Humidity in an A/C'd room


Old 05-27-18, 07:10 AM
Thread Starter
Join Date: Mar 2013
Location: USA
Posts: 73
Received 0 Votes on 0 Posts
Humidity in an A/C'd room

First off, Thanks for all the input. I love this site. Situation - I have a 1 1/2 ton Trane supplying a 600 sq' master bedroom. The room faces south east with a triple double hung window ( average size, New ). The house is a 1 1/2 story, so part of the room " ceiling" is drywalled roof gables so there is more heat gain. The air handler is located about 40' from the compressor and is located in the unconditioned attic. I live in southern NY, LI.
Problem - The master bedroom is noticeably more humid than all other parts of the house, meaning if the house is at 35% humidity, the master will be about 43%. I've tried the slow fan speed, high fan speed. I've had the thermal expansion valve replaced. No change. Any thing strike you as obvious that I should check out ??? I and all A/C professionals are baffled. I've had 3 different companies.
Sponsored Links
Old 05-27-18, 09:31 AM
Join Date: Jan 2011
Location: USA
Posts: 4,660
Received 55 Votes on 50 Posts
Is there a return air vent in the room?
Is there a gap under the door into the room?
Old 05-27-18, 10:17 AM
Thread Starter
Join Date: Mar 2013
Location: USA
Posts: 73
Received 0 Votes on 0 Posts
Yes..., Return air is just above the thermostat. The gap under the door is not larger than what is typical in a house. The zone outside the bedroom is also A/C'd. No issues. The master bath door is kept closed after showers and has an adequate fan vent for steam, no mold problems etc.
Old 05-27-18, 10:41 AM
Marq1's Avatar
Join Date: Sep 2016
Location: USA MI
Posts: 5,637
Received 238 Votes on 225 Posts
So how is the rest of the house cooled, separate unit?

If you had two units supplying different rooms generally they should be the same considering temp setting, outside temp and outside humidity.

I'm not sure the average person can really even "feel" humidity that is only 8% difference.

So what is the problem?
Old 05-27-18, 07:11 PM
GregH's Avatar
Super Moderator
Join Date: Jan 2001
Location: Manitoba
Posts: 10,111
Received 26 Votes on 25 Posts
If you are comparing humidity levels within a house you have to make sure the temperature in the areas you are comparing is exactly the same.
The reading you are using is % RH which means the amount of % of moisture it holds compared to what it can hold at the current temperature.
Turn the temperature lower in an area and even though there is the same amount of moisture the % reading will go up because the air at the lower temp can hold less moisture.
Old 05-29-18, 04:45 AM
Thread Starter
Join Date: Mar 2013
Location: USA
Posts: 73
Received 0 Votes on 0 Posts
GregH..., Yes. I have 3 zones in the house, all separate units. If every zone is set at say 72, the humidity is noticeably greater as you walk into the master bedroom. It's noticed immediately even though the T-Stat reaches it's set temp of 72 degrees.
Old 05-29-18, 05:42 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 24,836
Received 435 Votes on 398 Posts
I would assume there are a number of factors. A 8% difference is not much, and as pointed out... some of that difference could be due to the average temperature in the room, the volume of air, height of the ceiling (more warm air up there... warm air holds more humidity), cold air return could be insufficient for such a huge room, no vapor barrier in bathroom walls means some of that moisture diffuses through walls... open doors as you go in and out... not running the fan long enough, (I don't know of anyone who runs a bath fan long enough to reduce the humidity to 35% so don't imagine that all that moisture stays in the bathroom) There can also be greater air leakage in a room with large windows... and all windows leak "some" air. (outside air usually = more humid air) and there is more solar heat gain in a room with lots of windows during the day, more heat loss at night if its cold outside. If ANY ONE of those is true, or if all of them are true to even a limited amount (like 1% each) it could easily add up to your 8% difference.

You say the temp in that room is exactly the same as the others, but there can be a margin of error in your readings too. The temperature at the thermostat may be the same, but that doesn't mean the average temperature in the entire room is the same. Volume of air and average temperature of that air has to be considered. In such a large room I would expect there would be a large quantity of warm air near the ceiling, and that's where cold air returns would need to pull from, from an air conditioning perspective.

Try setting the master bedroom thermostat a little higher or lower or set the fan to circulate and see what happens.
Old 05-29-18, 08:44 AM
airman.1994's Avatar
Join Date: Dec 2005
Location: VA
Posts: 5,795
Received 8 Votes on 8 Posts
If you are trying to get to 35% you are never going to get there. An AC is not going to get you to 35% unless its designed for that. 50% yes. Id say maybe you are a bit oversized but since they don't make a one ton system its as small as they could go. Id add a Ductable dehumidifier to the system and be done with it.
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: