Can I set AC T-stat for 80º and humidistat for 60% humidity?

D107

With our newly closed-cell spray foamed attic, this summer we needed much less AC than usual since the top floor is much cooler. We found 80º quite tolerable, except when it got very humid. Can see set t-stat for 80º and humidistat for about 55-60% humidity? Can the humidistat turn on AC when inside temp is below the t-stat setpoint, and turn off the AC when inside temp is above the t-stat setpoint? Any specific models recommended? Hoping I can wire it myself.

ChrisJ

My Honeywell Prestige does this, as well as the older VisionPro 8000s, but only the ones with the humidistat option.

Mine is set to run the A\C blower slower than normal, I think only 85% of full when it's above 50% RH and it'll also run the A\C for up to 2 degrees (3 deg is default) below my set point.

I'd highly consider a 2 stage outdoor unit and a variable speed blower.
Another, very good option is a whole house dehumidifier which gets piped into the ductwork.

Anything above 50% is nasty IMO and I prefer low to mid 40s.

EBEBRATT-Ed

It can only be done if you have the equipment and controls installed to do this.

We would need to know what you have

D107

T-stat is Honeywell RTH111/RTH221 24V. AC Bryant Air Handler Model FB4ANA036, Bryant Condensing Unit Model #561AJ036-A, 3 ton, 1992. Looking at the Honeywell H46C Humidity Controller.

ChrisJ

I've got a feeling your A\C can't do the slow fan thing.
It could do the over-run deal with the proper thermostat.

But probably your best and cheapest bet, is a whole house dehumidifier. You're talking at least $1K just for equipment, but you'd be able to pull your RH down into the 40s if not 30s. You'll be A LOT more comfortable.

D107

Thanks for the tip on the dehumidifier; not sure our problem warrants a separate unit. Here in NY, humidifity's not a problem in the shoulder seasons, only summer. AC handles it very well; we seem to be ok with 40-45% RH. There are maybe 15+ days every summer when the humidity gets unbearable, even when temps are in mid-70s. So if a humidistat could in those situations turn the AC on at 75 but otherwise keep the AC setpoint at 80 for most other times, that would suffice. Would our current T-stat not work for this?

ChrisJ

You need a thermostat that has a built in humidistat. I say this only because it'll limit how cool it gets. A vision pro with humidity will do it.

I'm amazed you keep it at 80. I run mine at 68 in my bedrooms at night. 72ish during the day.

ChrisJ

I believe this will do it.

http://www.supplyhouse.com/Honeywell-TH8321R1001-VisionPRO-8000-w-RedLINK-IAQ-Contacts-Programmable-3H-2C-Touchscreen-Thermostat

ChrisJ

This should also do it and may be cheaper with Wi-Fi

http://www.supplyhouse.com/Honeywell-TH8321WF1001-Wi-Fi-VisionPRO-8000-Programmable-3H-2C-Touchscreen-Thermostat

D107

Thanks for the Honeywell info. I sort of prefer wired systems; prefer not relying on wireless or wifi. So my original thought for the Honeywell RTH111/RTH221 T-stat along with the Honeywell H46 or H600 Humidity Controllers wouldn't work? That wouldn't limit how cool it gets? Yeah, we used to keep it at 76 but this year--with the attic insulation--we didn't have that hot bedroom ceiling radiating on our bodies. Bottom line is high humidity is by far the biggest factor for discomfort for us. Thanks very much for your informative responses.

ratio

Does you have electric heat? The TH8321U can do true dehumidification by energizing the reheat along with the compressor, that will wring the water out without cratering the temperature. It can also do cooling droop, where the stat allows the temperature to drop up to a certain number of degrees below the cooling setpoint to remove more water from the air. Not all of the 8000 family are wireless, but you do want to make sure it's the 8321U, as that's the only one with the humidity sensor. A wireless sensor can be used with the RedLINK versions, and that will allow humidity control as well.

D107

Thanks, I'll look into that--that cooling droop sounds like a great feataure, but no we have gas-fired hot water boiler.––so maybe that's not compatible. Just want to attach humidistat to thermostat. So far no one has said that the Honeywell RTH111/RTH221 T-stat along with the Honeywell H46 or H600 Humidity Controllers won't work. Wifi generally goes out a few times a year due to a storm or cable issues, so I don't feel it's reliable enough.

ratio

The WiFi is purely for remote access to the thermostat from your phone—search for "Total Comfort Connect" on the app store, it has a built-in demo account. The other style of wireless, RedLINK, is wireless (or mostly wireless) between the air handler, thermostat, and remote sensors if used. It's quite reliable, and the nearly wireless mode just uses two conductors to power the stat, so it can be set up completely without batteries.

In regards to your question, the humidistats you mentioned only provides a switch that changes when the humidity exceeds the setpoint. You still need to find a way to dehumidify. The cooling droop method will work on any type of unit, as it really just allows overcooling to increase the time the compressor is on—that's the only way to remove water from the air. You would want to use a thermostat to limit how far you overcool, since as you've noticed a little dehumidification is worth a lot of cooling. Also, IF the water coil is downstream of the AC coil, and IF your boiler is hot all summer (for, say, domestic hot water production), the reheat method should still work.

D107

Thanks, we don't have an indirect, but a stand-alone HWH, so it sounds like that setup won't work. I'll have to read up on this a bit more so I can understand how this all works. I know humidistats can be wired either in parallel or series, each for a different benefit. So, if we set inside temp for our desired 80º, and humidistat for 45-50%, the prevailing wisdom says it's possible AC may not stay on long enough to bring RH down to 45-50% RH, OR if the humidistat controls system, it will keep AC on to the point where it overcools.

ChrisJ

Both VisionPro's I posted are to be hardwired, the wifi is just for remote access if you wanted it.

My Prestige uses the EIM controller via wireless, and it's extremely reliable. I was hesitant at first, but am glad I went that route. Not because of it being wireless, but because of how nice the system is.


I don't know anything about the models you posted, so I can't comment on them. The VisionPro 8000s, can as long as they have the humidity option which the fourth number indicates that, for example, 8321, the 1 means it does what you want. You would tell it to do dehumidification with AC high fan. If your setpoint is 80, and the humidity goes above your set point, say 50%, it would turn the A\C on and drop it as much as necessary, within your set value, say 3 degrees.

They will do exactly what you want, as well as having easy to read touch screens, programming etc.

ratio

Well, remember that all the humidistat does is tell you if the humidity is above or below the setpoint. If it's wired to bring the cooling on when the RH is above setpoint you could run into issues with overcooling and low system temperatures (causing operational issues). If it's wired to turn off the cooling you could get into a state where the temperatures are uncomfortable.