Need help to interlock HRV system with my furnace. Currently, I manually have to switch on the HRV whenever the furnace is on since the HRV has no relay switch or proper connection for the interlock. Anyone could give an insight on how to install a relay or other available hacks to interlock with the furnace. Many thanks
The interlock means the furnace fan will start whenever the HRV is turned on. I'm not sure if you're expecting the HRV to turn on whenever the furnace is running but that's usually not how it works.
What is the model number of the HRV?
The easiest way to interlock any HRV is to change your thermostat to an Ecobee Premium and let that control the HRV. That way the Ecobee will start the furnace fan anytime the HRV is running, without the need for interlock wiring or additional relay.
Quote from: Admin on October 23, 2025, 03:44:09 PMThe interlock means the furnace fan will start whenever the HRV is turned on. I'm not sure if you're expecting the HRV to turn on whenever the furnace is running but that's usually not how it works.
What is the model number of the HRV?
The easiest way to interlock any HRV is to change your thermostat to an Ecobee Premium and let that control the HRV. That way the Ecobee will start the furnace fan anytime the HRV is running, without the need for interlock wiring or additional relay.
Thank you! The HRV make and model are Venmar EVO5-700.
My thermostat is Ecobee but it's hard to wire to that location.
Usually the main control for the HRV is located near the thermostat and you can just fish the wires over.
Venmar part #12658 is the relay you need but has been discontinued.
You could buy a White Rodgers #90-294Q relay,
Thank you for the information. I will purchase and install the recommended relay.
Just noticed the plug is a Molex 10-pin marked as follows. Should I still connect #3 to Pin 5 (OL) and #1 to Pin 7 (I)?
Is the relay going to run the fan when the HRV is on or When the fan runs for AC/Heat the HRV is activated?
Thanks in advance!
Pin 1 B
Pin 2 G
Pin 3 R
Pin 4 Y
Pin 5 OL
Pin 6 OC
Pin 7 I
Pin 8 NC
Pin 9 C
Pin 10 NO
Do you have a convector like this? In that case you don't need a relay. Wire C, NC and NO to the furnace to interlock it.
That will energize the furnace fan anytime the HRV is turned on.
Yes, that's the Molex 10 pin connector on the HRV.
I only run two wires from the HRV to the furnace. I connected the NO and C wires to the furnace R and G but it did not energize the fan. I did not connect the NC wire since it's connected to the Thermostat.
Perhaps, I need to install the relay as you initially recommended or maybe the third wire (NC) has to be connected to the thermostat in order for the fan to be energized? Thanks again!
You could call Venmar to ask if your HRV model does in fact have the interlock relay. It would be weird if they included a NO, C and NC terminal if it's not operational. But maybe it's just a generic plug. Do you still have the HRV installation manual? What does it say about interlocking?
It's always best to use the 3 wire interlock method if you have an AC. NC has nothing to do with the thermostat. You disconnect the G wire from the furnace and marrette it with the NC wire from the HRV. Then the HRV C wire is the only wire that connects to the furnace G terminal. The HRV NO wire connects to the furnace R terminal. This will prevent the AC from being energized which sometimes happens to older systems using only the 2 wire interlock method.
But wiring HRV NO to C to furnace R and G should also work. When you turn on the HRV it should start the furnace fan.
If you jumper R and G at the furnace does the furnace fan start? If so, I would say there is something wrong with the HRV or interlock wiring. Where is the main control for the HRV located? Is it close to the thermostat?
I called Venmar and you are right they are just using a generic plug. The Vemar EVO5-700 can't be interlocked to the furnace. The control board is similar to the higher end HRVs by Venmar. The problem is half the board isn't populated with the components to interlock the furnace.
The main control for the HRV is far from the thermostat but the HRV unit is near the furnace. Thanks!