ReplyThat is correct. All it does is run the compressor by up to 1, 2, or 3 deg below setpoint (set by the installer) in cooling season in an attempt to lower humidity. This functionality does not require a variable speed blower.
Regular Member
I've been quoted all manner of heat pumps to replace mine. One of my biggest concerns is humidity control, since I'm used to 40% RH or lower indoors during the cooling season. I understand that my decision to downsize from 3.5 to 3 ton will help keep humidity low by running more, regardless of which type of equipment I buy.
However, I'm trying to decide between a number of dual stage ($$$$) HPs ranging between 16 and 20 SEER, and a number of simpler, less expensive, single stage 15 SEER HPs. Considering the lower ROI for the high SEER units, which all sound great, I may go for the lower SEER, if it can be made to control the humidity to some degree.
My question relates to using the TH8321 with a single stage HP. One dealer said it has a proprietary "overcooling" feature that could reduce the setpoint by up to 3* automatically if needed to reduce humidity.... per user settings. He included it in his quote for a Broan 15 SEER single stage HP.
Is that true about this tstat? I know it's advertised as the model "with humidity control", but I want to be sure that feature will work with any simple HP, with or without a VS air handler.
Professional Member*
That is correct. All it does is run the compressor by up to 1, 2, or 3 deg below setpoint (set by the installer) in cooling season in an attempt to lower humidity. This functionality does not require a variable speed blower.
Regular Member
THANK YOU! That's exactly what I needed to confirm.Originally Posted by RyanHughes
God Speed, Jim. We'll miss you - 2-26-23
It also doesn't dehumidify real well on the milder temp days, as the run time still isn't real long on those days.
An IAQ or HD thermostat with a VS blower work much better, as they can slow the blower down to improve moisture removal, and they also can over cool to dehumidify.
Regular Member
Considering a non-communicating, single stage condenser, with either an adjustable fixed speed or a VS blower. I've compared the product data on the IAQ with the TH8321 and do not see any difference in the two as regards humidity control. Yes, the IAQ will control additional equipment (humidifier, dehumidifier, ventilator), but all I find regarding humidity is the 3* overcooling option, same as for the TH8321. I do not see any reference to controlling a VS blower to lower humidity.
Am I missing something?
Professional Member*
It's all how the installer sets it up, he can tell it(the air handler) to "dehumidify" and it will ramp at different speeds and different lenths of time. This is one advantage of VS.
Professional Member*
When humidity is above the setpoint, the IAQ can send a signal to a variable speed blower to operate at a lower speed setting (typically 80% of cooling CFM) to improve moisture removal. This in addition to overcooling provides good humidity control. There is also a new version of the 8321 that can do the same thing; model is TH8321U1097. It has one set of terminals that can be used for either humidification, dehumidification, or ventilation, whereas the IAQ can do all three.
Last edited by RyanHughes; 10-18-2012 at 01:47 PM.
Professional Member*
DoD is primarily for tiems when your below design conditions for heat load. If the system at or above design conditions is unable ot achieve the desired humidity level, Dod will help, but won't be able ot improve the siutation much in partial load conditions. IT can help shift the percentage of latent capacity, but if runs times are too short, you'll still have high humidity levels. THis is where whole house dehumidifiers work best.
God Speed, Jim. We'll miss you - 2-26-23
It doesn't spell it out. but the IAQ uses its dehum terminals to slow a VS blower.
Regular Member
OK, I've gone through the installation guide for the VisionPro 8000 Series (includes TH8321 with humidity control), the installation guide for the VisionPro IAQ, and now the THX9321/9421 Prestige® IAQ, which has the HD touchscreen.
Neither of the VisionPro products include blower control options for dehumidification. Both the VisionPro TH8321 and IAQ products dehumidify by overcooling. On the other hand, the Prestige® IAQ does have settings and wire terminals to run the blower on "low" speed and will overcool to 3*.
So, to take advantage of the VS blower for dehumidification with a non-communicating condenser, it appears I need to specify the Prestige® IAQ, not the VisionPro IAQ. Is that correct?
Professional Member*
The VisionPro IAQ can do it as well.
Regular Member
No offense to you, but I'd like to see it for myself to be sure. I hate being sold something that's "supposed to" do somthing and then later find out I was misled or misinformed. Could you please give me the page number in the installation guide or a link online that covers blower speed control for the VisionPro IAQ?
Thanks!
Professional Member*
Under 40% in Florida? Ain't goona happen with standard heat pump of any brand or seer.
Regular Member
Never mind. I found it. It's in a wiring diagram for the VisionPro IAQ that refers to "Air Conditioning". I would guess the same wiring would apply for Heat Pumps.
Fig. 16. Typical hookup of variable speed blower for
dehumidification in low speed (contacts normally closed).
NOTE: Connect DHM2 to low-speed fan terminal on air handler
for dehumidification with air conditioner and a
low-speed fan.
Regular Member
So I'll have to set the tstat lower on the new equipment to maintain the same comfort level, I guess. My 13 year old Rheem 10 SEER maintains 38-40% RH during the summer. In the "shoulder" seasons, it gets up to 50% because it runs less.
God Speed, Jim. We'll miss you - 2-26-23
No. I install the Vision Pro IAQ on a lot of VS blowers. It does what I said.
Global Moderator
At our house I use the Prestige thermostat to control a Honeywell TruDRY dehumidifier so we don't have to over-cool the house. You would benefit from a two-stage condensing unit and a variable speed blower when it comes to de-humidifying.
Regular Member
Thanks! A couple of posts back I said "I found it".... in the installation guide for the Vision Pro IAQ.
Regular Member
No thanks on the dehumidifier. I'm considering 2-stage but have to include ROI in my decision matrix.
Professional Member*
I wonder how accurate your humidity reading is as I would have to say that is far drier than typical. In mid summer anything lower than 45% would require near constant runtime- and almost no infiltration.
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