Author Topic: Can a furnace blower be wired to turn on a HRV, without using NO/NC terminals?  (Read 909 times)

Offline needair

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Hi Folks,

The situation relates to a Venmar EVO5 500 HRV. The HRV has NO/NC/C terminals, but according to the manufacturer the NO/NC terminals are blanks. Otherwise it has I/OC/OL terminals (for optional bathroom switch, which is not installed), and Y/R/G/B terminals for it's own dedicated control switch.

The goal is to have the HRV start when the furnace blower starts - whether due to the furnace doing a heating cycle, or due to the fan being enabled through the furnace's thermostat. Furnace and thermostat are connected using C/G/R/W.

There are lots of discussion on interlocking so that a HRV turns on the furnace blower, but not nearly as much the other way around. I'm wondering if lack of NO/NC support on the HRV is a deal-breaker for this scenario?

Thanks in advance

Offline Admin

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The Venmar EVO5 500 does not have the ability to interlock with the furnace fan.  They don't sell any optional relay kit for that model of HRV.

You're right, the HRV's are designed to start the furnace fan if they are interlocked, but there is no way to turn on the HRV whenever the furnace fan starts.  The reason for this is that the HRV doesn't use 24V.  It's just a communicating signal.

You may be able to find a 3rd party thermostat, with dry-contacts, that you could use to have better control over the HRV.

I use the Honeywell Prestige 2.0.  I wired my HRV to the U1 and U2 dry contacts.  The thermostat allows me to schedule my own ventilation run times or will use the ASHRAE algorithm.  These days most furnaces are equipped with ECM fan motors, so leaving the fan running continuously is normal.

Offline needair

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Very interesting, thank you for the insight. I'm glad I did not unpack anything from boxes yet...

I was hoping that I might be able to connect OL on the HRV to the G on the furnace. But if OC/OL is not 24v, then I guess that's not doable.

Were you able to get the Honeywell Prestige 2.0 in Canada?

Edit: If I connect a normally open relay to the G 24V wire, and tie in the OC/OL wires, might that work? Presumably then when the G wire is energized, the circuit will be made across the OC/OL terminals, which then would turn on the HRV?

Offline TurboLed

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I have a similar issue with the OC/OL terminals but it's reverse. I'm trying to connect a bathroom fan timer that is intended to use those terminals to another system using 24V... If I could do anything, step-down voltage, etc.. that would work with that timer I would be happy to know.

Offline TurboLed

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Please can anybody measure the voltage across terminals OC and OL? I cannot find this information anywhere...

Offline Admin

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Quote
Very interesting, thank you for the insight. I'm glad I did not unpack anything from boxes yet...

I was hoping that I might be able to connect OL on the HRV to the G on the furnace. But if OC/OL is not 24v, then I guess that's not doable.

Were you able to get the Honeywell Prestige 2.0 in Canada?

Edit: If I connect a normally open relay to the G 24V wire, and tie in the OC/OL wires, might that work? Presumably then when the G wire is energized, the circuit will be made across the OC/OL terminals, which then would turn on the HRV?

I bought the Honeywell Prestige 2.0 from WWG in Ottawa, Canada.

I believe you want the HRV to start whenever the furnace fan starts, either in heating or from turning the thermostat fan switch on.  When the thermostat fan switch is turned on it will energize G with 24V.  But when the thermostat fan switch is in the auto position, and the heat activates the furnace fan, the furnace board relay will energize the furnace fan based on timing, not necessarily a 24V signal on G.  Your relay idea could work but may work only when the fan switch is turned on and not on a demand for heating.

Assuming touching the OL and OC wires together, while they are connected to the HRV, will start the HRV you might be able to use a current sensing relay.  This would clamp around the neutral wire of the furnace fan motor then close a relay anytime the furnace fan is running.  You just have to figure out if the furnace uses different fan speeds for heating, cooling and continuous fan operation.  You may not want the HRV to run when the furnace fan is in cooling, but with this setup the HRV would run anytime the furnace fan is running.

You could try using a SPDT relay with 120V coil.  The furnace board may have an ACC terminal which energizes with 120V anytime the furnace fan starts.  Install a wire on the ACC terminal and another wire to the furnace neutral wire, then connect those 2 wires to the 120V coil on the relay.  Then try connecting the HRV OL to the NO terminal on the relay and the OC to the common terminal on the relay.  Again, this would energize the HRV anytime the furnace fan starts including in cooling.  Using the B and G terminals, on the HRV, should accomplish the same thing.

I will check tomorrow and let you know if touching the OL and OC wires together will start the HRV.

I have a Lifebreath HRV RNC155 hooked up to my Honeywell Prestige and the HRV supplies 12V.  The problem with my setup is I lose the ability to control the high and low speed HRV fan speeds.  The Honeywell dry contacts allow me to control 1 speed only, so I wired my HRV to run on high speed, when the dry contacts close in a call for ventilation.

Offline Admin

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I did some reading and it sounds like the Vanee / Venmar HRV's use resistance values in their controls, to control the low speed, high speed and off function of the HRV.  If you jumper B and G, or OL and OC, the HRV should run in high speed.  But like me, you will lose the ability to control both the low and high speed HRV fans.  You will have to chose 1 speed to use.

Vanee / Venmar rebrand and sell a lot of their HRV's to other manufacturers.  Apparently the resistance values vary, depending on brand name and model.

Another option would be to use a SPDT relay with 24V coil and a Aprilaire #51 24V Current Sensing Relay.  This way you're using low voltage wiring instead of line voltage wiring and a 120V relay.

Start the furnace fan and measure the amperage on the neutral wire.  I believe you need at least 4 Amps to close the relay or, if not, you can wrap the neutral wire around the CSR a few times to increase the signal.  Connect one side of the CSR with 24V from the R terminal in the furnace.  Connect the other side of the CSR to the 24V coil of the relay.  Connect the common side of the relay to the C terminal in the furnace.  Now connect either OL or G from the HRV to the relay NO terminal.  Connect either OC or B from the HRV to the relay C terminal.

That should start the HRV in high speed anytime the furnace fan starts.  You can just unplug the HRV in the summer if you don't want it to run.

Offline Andre P

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If your furnace control board has terminals for EAC, use that to power the HRV. The eac terminals are energized coincident with the blower operating.