Custom Fan Speed Switch w/ Thermostat for PTAC Unit?


(Dan) #1

I have a Packaged Terminal Air Conditioner (PTAC) system in my home that is nothing short of terrible.
I’ve attempted to install smart Thermostats to it in the past, but scrapped everything because of one main issue. The wiring for the PTAC has both high and low fan speeds. After extensive research, I was not able to find a compatible smart thermostat that allowed for two fan-speed wires. Instructions from the PTAC manufacturer said to use either the high speed or low speed fan wire for the G slot in the thermostat.

I don’t really want that. I want to be able to control the fan speed, which is really useful for quick temperature changes and when we have a lot of people over.

My idea is to use a zwave relay/swich of sorts that has two inputs for both fan speed wires and one output which will be connected to the thermostat. Is this possible? Has anyone ever done this before? I don’t know where to start, what relay/switch to get or even how to properly search for this.

I apologize if I’m not using the correct terms and any help would be greatly appreciated!

(Eric) #2

an easy way is to insert manual SPDT switch to select high or low speed.

Or install MIMOlite which has SPDT relay, which can be driven by automatic routine for high load events. Low is typical, good, quiet, slow.

No multispeed tstat required.

(Dan) #3

Thank you! I definitely want something I can connect to smartthings and control from outside the house.
MIMOlite seems like a bit of overkill for just two wires though, especially at the price (I’ll have to buy 3 of them for my 3 units). Do you know if there are smaller alternatives to this?

Every search I make leads me to this micro controller:

Would that fit my needs?

(Paul) #4

No. You need a dry-contact relay that is Single Pole, Dual Throw (SPDT). This means if the relay is “On”, one set of contacts is connected, and if the relay is “Off”, the other set of contacts is connected. At no point is any voltage supplied to the relay contacts by the relay itself.

The MIMOLite has a contact that is “Normally Open” (NO) and a different contact that is “Normally Closed” (NC). You’d connect the NO contact to the High fan input, and the NC input to the low fan input.

There are other Zwave dry-contact relays, but I believe they are all Single Pole Single Throw (SPST). This would not work for your situation.

The Aeon Labs Micro Controllers all supply 120V to their contacts when they are “On”… so they aren’t dry contact relays.

This wiring can be tricky, so please post questions if you have them.

(Dan) #5

Now I understand. You guys are amazing help. Thank you!