A modest proposal
Regarding the eternal "should the stat be set to FAN=AUTO, or FAN=ON", a 2-way choice.
In a humid climate advice is to always use AUTO, because in a 5-minute period after a cooling call ends, the coil is wet and fan action would evaporate that water back into indoor air. Right? Yet some times people wish to use "FAN ON" for a variety of reasons. The 2-way choice forces a compromise.
If there were a 3rd choice, where the stat guaranteed the fan off for that 5 minutes, and only THEN would obey the "ON" command or setting... that would appear to deliver the benefits of "FAN ON" combined with the humidity control of "FAN AUTO".
Should be easy to design if there is a microprocessor controlling the functions. I submit that the 2-way choice is a throwback to mechanical controls, and we might adjust our thinking toward the future.
Thank you for reading -- Pstu
Somebody offers this as an add on board, I saw it at a AHR convention, can't remember who.
Anything I say here is only my opinion. Even if you understood what I said.... What I said may not even be what I meant.
Many digital thermostats already have a feature similar to what you propose. It's often called "circ", which is a timed circulation interval between run cycles of the furnace or a/c. Often used in conjunction with high end filtration to provide filtration between run times of furnace or compressor.
Even so, an evap coil can stay wet for some time after the blower shuts off. Some condensate will run off and collect in the pan, but the fins can remain moist, since the environment surrounding the coil is nearly saturated itself. Best way for a homeowner with a "circ" feature on his thermostat is to experiment with run times and monitor humidity levels, to see if he can achieve a happy medium. With fresh air intake there's the added reality of loading the home with moisture inbetween run cycles in humid climates.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.
The Infinity control has that feature, but I don't think it shuts down for 5 minutes rather it shuts down for 2 or 3 minutes but there could be some type of time built in to adjust.
Not in Houston but NW AR is kind of humid. Our new Honeywell VisionPro IAQ Tstat has Circulating (Circ) where it comes on randomly on average about 1/3 of the time the AC isn't running. The new York VS furnace has Fan On/ Circulating settings at 100%, 70% and 40% of blower capacity which in my case is huge at 2100 CFM. I chose 40% or around 840 CFM to cut the noise, uncomfortable hurricane coming out of the vents and power usage.
Originally Posted by shophound
I use Circ to even out the temp and humidity across a fairly large house and would like to use it at night due to closed bedroom doors (cats!) if Mom would only let me.
Carrier does drop out the fan for a few minutes after a cool cycle. Believe it is on more than the Evolution/Infinity too. Wise move.
evaporate that water back into indoor air
How much really does get re-evaporated into the air? In Tucson when the coils are wet enough to have any water I imagine that it would be a miniscule amount.But im sure somebody has had the time to figure this out some where.
Originally Posted by pstu
Aut or on, is a continuation of the old stats.
The thermostat manufacturers have not as of yet. Invested in the logic function and monitoring of the A/C run cycle to implement a timed off function of the blower.
Probably because a lot of furnace/air handlers have a built in blower delay off on the control board. So the stat wouldn't be able to shut the blower off as soon as the A/C cal ended anyway.
Carrier is an exception. But, they are owned by UTC, and use their controls.