Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

How to Wire a Humidistat to a Thermostat

mikeo56 (Structural) (OP)
26 Mar 05 16:08
Any suggestions on connecting a humidistat to a thermostat?
Ranco J10 Humidistat to a White Rodgers Thermostat.
 Thanks, Mike.
cme (Mechanical)
26 Mar 05 16:56
What kind of control sequence do you want to accomplish?
imok2 (Mechanical)
26 Mar 05 18:26
A Humidistat can be wired off of the same transformer as a thermostat but not as one control as they are 2 different functions. The Humidistat control should include safty controls for high humidity and loss of air flow.
DRWeig (Electrical)
28 Mar 05 10:24
You really need to answer cme's question.  If you're trying to control both humidity and temperature with a single unit, it's more complex than just wiring up a thermostat and humidistat.

What sort of equipment are you trying to control?  Is it a dx unit and you're trying to achieve de-humidification?  Or do you have a humidifier too?  Is reheat a possibility?  Do you have a separate de-humidifier?

Best to ya,

Old Dave
mikeo56 (Structural) (OP)
28 Mar 05 22:53
Ranco J10 Humidistat, White-Rodgers 1F86-244 Non-Programmable Electronic Digital Thermostat, Trane 2TFE3025A1D08AA Air Handler, Trane XR-12 Model 2TTR2030A1000AA Condenser, Thermostat low voltage off air handler and humidistat off thermostat. Thanks, Mike.
MintJulep (Mechanical)
29 Mar 05 0:47
As CME as already asked,

what do you want to accomplish?
mikeo56 (Structural) (OP)
29 Mar 05 7:14
Thermostat for temp. Humidistat for humidity. If youv'e done this before offer a suggestion please! Or please explain what more info you may need. Thanks, Mike.
DRWeig (Electrical)
29 Mar 05 12:16

I'm extrapolating from your description that you want to de-humidify using the latent capacity of your DX coil.  This is kinda tricky because you need some sort of arbitration between the two controls.  If your humidistat wants to dehumidify, it will need to run the compressor to do so.  How low will you be willing to let the space temperature get?  You need something in between the two that makes a decision, or else a controller specifically designed for humidity / temperature control using some sort of reheat....

Let me know if that makes sense or not.  I'm in kind of a hurry at the moment, but I might be able to find a controller that you could use later in the day.

Best to ya,

Old Dave
DRWeig (Electrical)
29 Mar 05 12:41

I found a diagram for your humidistat.  It's for a humidifier, not for de-humidification.  Do you have a humidifier in the system?  If so, you can follow what's in the jpg.  Get your 24VAC from the RC terminal on the White-Rodgers t'stat.  For the 24VAC common you'll have to go back to the control transformer unless there's a spare wire pulled down to the thermostat that you can use.

Let us know how you work it out!!

Best to ya,

Old Dave
cme (Mechanical)
29 Mar 05 20:40
I really don't think a programmable thermostat can do a dehumidification sequence. What you want to do is upon high RH, shut the OSA damper, go to full cooling and energize the reheat to maintain space temp.
CountOlaf (Mechanical)
30 Mar 05 11:38
I still think Mike needs to tell us if the air handler is equipped with a humidifier, a reheat coil, both or neither.  Maybe it's embedded in the Trane model number but I don't feel like looking it up to see.
mikeo56 (Structural) (OP)
30 Mar 05 18:37
  I don't belive this trane has a humidifier or a reheat coil. I am not adept at HVAC (wish I was). I have looked into the air handler and there is one set of heating coils and no humidifier. I have seen this combination of thermostat/humidistat controls on many walls in many homes here in S.W. Florida.
  The one time I turned the humidistat to maximum was on another job. I'm sure the humidistat overrode the thermostat and kept the compressor going past the thermostat cut off. Don't know if that is a good thing. Seems like there should be an absolute lowest temperature allowable. That begs the question, is using the AC as a dehumdifier efficient or useful? Ultimately, isn't the whole arrangement designed for cooling?
Thanks, Mike.
SAK9 (Mechanical)
31 Mar 05 3:14
Hello Mike,

Now that you have come out with more information on your system,I would like to offer my two cents worth!I have seen similar installations where the humidistat is meant only to limit space relative humidity to a certain value say 60%.It is not meant to turn a humidifier on as a lower limit on humidity level may not be required in a comfort  air conditioning installation.In such installations the humidistat and thermostat are wired in parrallel so that either one of these can turn the reheaters on.

Adding reheat raises room temperature and brings down room relative humidity.This is certainly not an ideal control system.The reason is even after achieving the set room temperature the heater will be still on to achieve the required humidity level.This obviously means that temperature control will be on a rather 'wide' band.
mikeo56 (Structural) (OP)
31 Mar 05 5:28
Humidity stays so high here in SW Florida, I think the humidistats are used mostly when owners leave their homes, turn the thermostat up to 80 degrees and the humidistat to 20(the range on the humidistat being about 20-80). No way this raises room temp to reduce relative humidity. The humidistat uses the cooloing to pull mosture from the air like a cup of ice draws water from the air creating a puddle around itself - condensate.
What's a reheater?
Thanks, Mike.
DRWeig (Electrical)
31 Mar 05 11:36

Reheat is just a heating coil downstream of the cooling coil.  With a reheat arrangement, you can use a humidity / temperature control algorithm to run the A/C compressor (taking moisture from the air) and control the reheat coil with a thermostat to maintain room temperature.  It's common in commercial / institutional applications where excess humidity is a problem (you'll find reheat systems in a lot of hospital rooms).

Make sense?  In other words, you do allow the cooling coil to be active more than it would just for temperature control but use the reheat coil to keep the space from becoming overcooled.

Old Dave
CountOlaf (Mechanical)
31 Mar 05 12:30
So, we have no humidifier, but do we have a reheat coil or the means to reheat with the primary heating coil?  If the heating coil is not downstream of the cooling coil, then presumably, we don't have reheat capabilities.  If not, then I guess the humidistat is "wired in parallel" like a former respondent said, but all it can do is allow the cooling coil (and compressor and fans, etc.) to remain on  for a longer period of time if humidity conditions are not satisfied.  This will begin to "sub-cool" the space, so I guess the idea is not to set the humidistat too low.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close