How to turn off 1min fan purge after cool??

[Nikolai]

I'm a professional HVAC engineer and was thrilled at the features and price point upon installing a 3M-50 in my home this past winter.

Entering my first summer with the 3M-50, I'm quite frustrated. Nearly everyone in the HVAC industry will agree, most residential HVAC installations can not maintain proper humidity due to chronic over-sizing of cooling equipment. Yet the Radio T-stat line of stats apparently include a "feature" that exacerbates this problem, having the evaporator fan run a full minute after the compressor shuts off, re-evaporating moisture into the air!

Even in my situation, my residential unit is right-sized, and last summer my home's humidity was typically maintained at 45-55%, most often between 45-50%. Enter the 3M-50 t-stat to my system.

Imagine my dismay upon learning this t-stat runs 1-minute fan cycle after cooling terminates, immediately increasing humidity in my home. As a result, my system now has trouble keeping humidity below 55-60%.

In the name of all things good for residential cooling and humidity control, Radio T-stat Company of America, I beg you to disable this "feature" in the next firmware release, or at least provide a means of disabling it. Nearly all residential systems can enable this "feature" anyhow, if so desired by an ignorant homeowner, via dipswitch settings on the controller board or $10 relay wired into the system.

Please please please make this right!!! Otherwise, I am in love with your products, but this problem may be the reason I am forced to return it.

Many Thanks!

[blaine]

Interesting point, that much re-evaporation can occure in 1 minute. Some juristrictions require purging for an increase in cooling efficiency. I suppose another control could be added for both sides.

If you have gas or oil heat (no heat pump) that does not need the thermostat to turn on the fan, then connect the G wire to the Y terminal and the fan will come on and off with the compressor (no purging). Of course then the fan ON switch will not work.

[Nikolai]

Thanks for the note Blaine. I'll pull out my psych chart at work tomorrow and calculate this amount of water re-evaporation. Depending on length of run cycle, I'd estimate it is 5-15% of water otherwised remove during the cycle.

Yes, I have heard of a few recommending this, but always from well-meaning eco-minded "curious enthusiasm", rather than anything reflecting physics or psychrometrics. But seriously though, code-required by AHJ's?

Fan-only operation has been proven to basically turn the equipment into a swamp cooler for whatever duration this involves.

Rather than running the evaporator fan after the end of the run cycle, it would make much more sense to have the compressor and condenser fan run for 5-15 seconds prior to running the evaporator fan at the start of the run cycle... But this can be done between the inside/outside unit and IMHO the t-stat should not ever play a role in this function.

[rustinh]

Nikolai, I'm not an expert on this at all so I have a dumb question. I just purchased the 3M50 today and hooked it up. It's working well but after the AC finishes running I can hear the air turn right back on for a short time. Is this what you are referring to with the fan running for that one minute? Seems like it would be hard on the unit to shut down and then power back on again but again, I'm no expert. Just concerned.

[blaine]

The 1 minute purge is the fan only, the compressor is turned off. At home screen with the backlight on, notice when the A/C is running the word COOL blinks. When the thermostat turns off the compressor, the word COOL quites blinking (home screen and backlight on ) but the fan icon is still on. During that 1 minute fan purging the cold air out of the system, go outside and notice the compressor is not running during that time.

[jkarney]

I know this thread is old but I thought it needed to be resurrected. Being a new owner of a 3M50 ver. 1.94 I quickly discovered that the Tstat (in cooling mode) has a programmed 1 minute fan purge (after compressor shuts off) after the Target set point is reached. The resolution of disconnecting the G wire is not to my liking.

Are there any plans to adding a enable/disable fan purge in the firmware? IMHO this is the best solution be giving the homeowner a true option without disabling standalone fan functionality?

TIA

Jim

[blaine]

Actually there is also a 1 min purge in electric and heat pump (gas/elect in Elect). Gas and oil furnaces have purging built in to their "stack" fan control detection. I understand some of the newer high velocity HVAC have purging built in; heating and cooling.

If you have a gas or oil furnace and the gas/elec switch is in gas, the furnace does not control the fan, therfore you do not need the G control except for A/C and manual. If you wnat to give up manual, then disconnect the G from the thermostats G terminal and connect it to the Y terminal with the existing Y wire. Now the fan will come on and off with the A/C, hence no purge.

[jkarney]

[quote="blaine"]Actually there is also a 1 min purge in electric and heat pump (gas/elect in Elect). Gas and oil furnaces have purging built in to their "stack" fan control detection. I understand some of the newer high velocity HVAC have purging built in; heating and cooling.

If you have a gas or oil furnace and the gas/elec switch is in gas, the furnace does not control the fan, therfore you do not need the G control except for A/C and manual. If you want to give up manual, then disconnect the G from the thermostats G terminal and connect it to the Y terminal with the existing Y wire. Now the fan will come on and off with the A/C, hence no purge.[/quote]

blaine,
Thanks for the quick response and possible solution but a would rather not give up manual control of the fan as a workaround. For clarity I have a 4 Wire Heat/Cool which is gas. I have the gas/elec switch is in gas position. You stated "the furnace does not control the fan therefore you do not need the G control except for A/C and manual". So what controls the fan??? I'm confused now.

Anyway what I find to be puzzling is that my previous tstat which is a 7 day programmable RiteTemp did not exhibit this behavior (nor any tstat that I have used with this unit over the last 10 years). I re-installed the Rite Temp for a test and sure enough when the tstat reached Target Temp (cool Mode) the compressor and fan shut off at the same time. I re-installed the 3M50 and under the same conditions the compressor shut off and the fan continued to run (fan Icon blinking on tstat which indicates a fan run command from tstat) and then shut off after the 1 minute purge.

Based upon my test results: the fact that the fan continues to run after the compressor shuts off with the 3M50 installed has every indication that the 3M50 tstat is programed to operate in this manner.

If you can provide any clarity I would greatly appreciate it. It is my hopes that someone will look at the code and at least confirm or deny that this is expected operation.

Again thanks for the support and awesome Tstat that continues to get better and better through input from all the users out there and people like you that listen to our concerns.

Jim

[blaine]

Sorry, my error, I meant to say your gas furnace does control the fan and G to Y will work; brain fog. Purge has been in the 6000 series of rite temp for years; it was not in the 8000 series. I have seen complaints like yours and also compliments for purge. Perhps the manufacturer could make it optional somehow.

[jkarney]

Thank you for comfirming that the fan purge is expected operation of the Tstat. Yes; hopefully an option will be made available in future firmware updates that would allow the purge to be disabled or enabled as to the users choosing. In the meantime you have comfirmed that this is not a bug but in fact a feature. I am going to leave things alone for now as I want to maintain manual fan control.

Were is the best place to put in this type of feature request for future product development.

Jim