Connecting a thermostat to a relay to start a fan

Thread Starter

pradhansb

Joined Sep 8, 2017
5
Hi and thank you for being here. I suspect my question is very simple for someone in the know. While I am experienced at wiring in general, I am a newbie to the world of relays. So my project/question is this.

I would like to have my digital thermostat start a fan (simple plug in fan like you'd buy at Home Depot) whenever it the thermostat triggers the air conditioning. I realize the solution is in a relay but exactly how should the relay be connected to the thermostat such that this works? (Just in case you're wondering why, the AC provides plenty of cooling for 2 rooms, so I am simply pushing air from 1 room to the next. I only want/need this fan to run when the AC is on).

I purchased A RIB2410 which has a 24 V coil and simple Open/Close circuit relay and hooking the fan to the relay is not a problem. Specifically, how might I wire the thermostat to the relay to accomplish this on/off task. The digital thermostat simply uses only the R, G, Y, W wiring to the A/C . (It is a combo AC/Heating unit, therefore the W).

So, any help would be greatly appreciated. Thanks for your help.

Pradhan
 

MaxHeadRoom

Joined Jul 18, 2013
23,953
IIRC you would need the the B (black) to power the relay, this is the other side of the AC to R red.
Then the relay coil it would be a connection from the G green to B black. If you have the black tucked behind the thermostat confirm this with a meter first.
Max.
 

Thread Starter

pradhansb

Joined Sep 8, 2017
5
Hi and thanks for the quick reply. There is no black wire at all, just the RGYW wires off the thermostat. The black is typically used for a common. It would come off the controller of the A/C, yes? I could easily run that as the cable carries 5 wires or run it directly from the A/C unit itself. So, black on one side, green on the other. For my understanding then, when the fan (green) kicks on, the circuit is completed and the relay should close, correct?

Thanks again.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
23,953
Going from memory the green supplies the power for the fan relay obtained in the thermostat from the AC red, the green switches the fan relay in the HVAC unit that has the AC black connected to it.
Again, if you locate the black, check with your meter to confirm it.
@#12 may confirm this when he clocks in!.
Max.
 

Reloadron

Joined Jan 15, 2015
6,227
  • Blue or Black – C – Common wire, may be unused by your existing thermostat. ... This is a key to everything as you will need a common wire and it may or may not show up at the thermostat on the wall. Yellow – Y – Air conditioner. This should be the switched power when the system call for the air conditioner.
  • Red – R – 24VAC power from the furnace's transformer. This is the 24 VAC which should always be present. This what gets switched depending on if the system is calling for Air Condition or Heat.
You would want your added relay coil across the Blue or Black and the Yellow which should be the switched 24 VAC for the A/C. You can find where the thermostat wires terminate at the system and add your relay or however you wish to place it.

This is how my home system is wired and not to say yours will be identical.

Ron
 

Thread Starter

pradhansb

Joined Sep 8, 2017
5
Thank you all. Definitely, the green is fan, white is heat, yellow is a/c compressor, red carries the voltage which is pretty standard. I assumed that the thermostat is nothing other than a digital switch and when, say A/C is called for, the red-green and red-yellow switch is thrown to complete those circuits. In times past, my HVAC had me simply connect those wires to test that the fan and compressor were in fact working.

Pradhan
 

#12

Joined Nov 30, 2010
18,223
Post #9 has it right. The old mercury bulb thermostats do not have a ground wire. They are high side switches.
Only the electronic thermostats need a ground wire, and that is mostly to keep the batteries from being the primary source of power for the triacs.

Because the old style thermostats did not need a ground wire, installers used whatever they had as the world changed. Usually blue, brown, or orange for, "common". The first thing to do with any HVAC low voltage problem is to open the thermostat, air handler, and condensor to see which colors were used by the installer. Remember, some of them were hacks and none of them understood electricity.
 
It can get a bit complicated, but generally you have it nailed. Old t-stats didn;t need common. It was just a mercury switch and a bi-metalic thermostat with a heat anticipator. The current in the stat artificially heated the bi-metalic strip making the stat think it was hotter than it was.

You may see Rc and Rh. Connected together, they are R. Rc and Rh are used when there are separate heating and cooling 24 VAC transformers.

C hides - It's the end of the 24 Vac transformer NOT connected to R.

A relay between R and W will get you almost what you want, but it;'s probably close enough.

With AC, the fan follows the state of the the AC thermostat except the digital thermostat implements short-cycle protection. i.e.The system must be off a certain amount of minutes before it can turn on. This protects all sorts of things. Without it, I've had fuse holders disintegrate.

With heating there are options like:
A) The thermostat controls the fan
B) The furnace controls the fan

What does that mean?

1. The furnace has to get warm before the fan turns off and 2. The fan can run longer than when the thermostat ends a call for heat.

The furnace can do it with actual temperature sensors (furnace controls the fan) or the thermostat can do it with start-up and shut-down delays.

There is a device called "add-a-wire" which can effectively add a new wire to an existing install without running a new wire.

Heat pumps and duel fuel stats really get wierd. Dual fuel is when say gas and a heat pump is installed. In Canada, hydropower or electric is much cheaper than fossel fuels. In the US, it's the other way around usually. A heat pump won;t work very well below freezing, so fossel fuels might be cheaper than resistance heating.

You also have heating stages which selects the burner BTU output.
 

Thread Starter

pradhansb

Joined Sep 8, 2017
5
It can get a bit complicated, but generally you have it nailed. Old t-stats didn;t need common. It was just a mercury switch and a bi-metalic thermostat with a heat anticipator. The current in the stat artificially heated the bi-metalic strip making the stat think it was hotter than it was.

You may see Rc and Rh. Connected together, they are R. Rc and Rh are used when there are separate heating and cooling 24 VAC transformers.

C hides - It's the end of the 24 Vac transformer NOT connected to R.

A relay between R and W will get you almost what you want, but it;'s probably close enough.

With AC, the fan follows the state of the the AC thermostat except the digital thermostat implements short-cycle protection. i.e.The system must be off a certain amount of minutes before it can turn on. This protects all sorts of things. Without it, I've had fuse holders disintegrate.

With heating there are options like:
A) The thermostat controls the fan
B) The furnace controls the fan

What does that mean?

1. The furnace has to get warm before the fan turns off and 2. The fan can run longer than when the thermostat ends a call for heat.

The furnace can do it with actual temperature sensors (furnace controls the fan) or the thermostat can do it with start-up and shut-down delays.

There is a device called "add-a-wire" which can effectively add a new wire to an existing install without running a new wire.

Heat pumps and duel fuel stats really get wierd. Dual fuel is when say gas and a heat pump is installed. In Canada, hydropower or electric is much cheaper than fossel fuels. In the US, it's the other way around usually. A heat pump won;t work very well below freezing, so fossel fuels might be cheaper than resistance heating.

You also have heating stages which selects the burner BTU output.
 

Thread Starter

pradhansb

Joined Sep 8, 2017
5
Thanks again for the detail. Yes, I have witnessed that the fan kicks and lingers beyond the heat/ac compressor. But a basic question as regards the functioning of the relay. Does the coil portion of the relay close the circuit? Say, for example, were I to attach the R/W as is suggested (or R/Y or even R/G) to the coil terminals, won't that effectively bypass the thermostat and have the same effect as simply connecting them physically? In other words, won't the heater/compressor/or fan kick on as soon as I connect both these to the terminals of the coil? Or does the coil only "do its thing" when a current is run through?

As I mentioned, its my relay awareness that is lacking here.
 

Reloadron

Joined Jan 15, 2015
6,227
Blue or Black – C – Common wire and the Yellow or Y connection is what the relay coil should go across, assuming the Y is for cooling. The thermostat my not have a Blue or Black Common. You should have a 24 VAC wire always On to the thermostat and that 24 VAC is switched depending on what the thermostat calls for be it Heat, Cool or just Fan. The common generally Blue or Black does not always get routed through the thermostat. Anyway. depending on where you tie into the system you want your added relay across the compressor relay, as Max points out, in parallel.

Ron
 
Ron's got it. As the TS/OP said earlier, the standard T-stat is a bunch of contact closures. That won't get you anywhere.
The C wire is necessary so you can power the digital stat. There's some "power stealing" designs out there as well as ones that use batteries.
and I suggested the add-a-wire device: http://www.venstar.com/thermostats/accessories/add-a-wire/

"Power stealing" uses the open contacts to "steal power" from the furnace when relays are open. The amount of current that can be drawn is limited by the relay coil resistances.

You generally need to add the new relay coil at the furnace or anyplace Common and Y exist.
 

MIke0000

Joined Oct 3, 2019
1
I want to have a device (an exhaust fan not connected to my hvac) turn off when the thermostat kicks on, and turn on when the thermostat turns off. Do you think this would work similar to what has been described on this thread? Thank you
 
Modern furnaces may have almost what you need. There is an EAC (Electronic air cleaner) supply. All you have to do is invert that.

You could monitor blower motor current and base the on/off control to that. Current sensors only need a couple of turns of the wire around the sensor.

The G terminal (Fan) of the thermostat doesn't follow the fan. In heat, the furnace controls the fan because the fan has to run a little longer to remove residual heat. It can do this with time or flue temperature. The fan also gets delayed when there is a "call for heat".
 
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