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HRV furnace interlock help.

10875 Views 16 Replies 6 Participants Last post by  Bob Sanders
Installed a Venmar evo5 500 Hrv and tried to interlock to the furnace so the furnace blower comes on when the Hrv starts running when furnace thermostat set to auto. Thanks for the help.
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check the manual.

should involve connecting an output from the hrv to the g terminal. do it wrong and it may backfeed through the stat to y and bring on the a/c if you have it.

running the fan with the hrv is wasteful and uncessary if it draws stale air from a dedicated vent.

This is a terrible way to install an hrv:

..if it's done that way i urge you to change it rather than doing an interlock. the furnace fan may draw something like 300-500w.

with the hrv pulling air directly from the house, not a return it will ventilate just fine without the blower running.
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Just take two wires from furnace R and G terminals to the Normally open and Com on the Venmar.
^watch out for backfeeding.

G and Y are internally connected but isolated from R in t-stats with mechanical fan switches when the fan is set to auto.
Hooked wire from Hrv NO to red on furnace and green to C. Still no change.
Hooked wire from Hrv NO to red on furnace and green to C. Still no change.
That's hooking up two powered wires to the same circuit.
How does the manufacturer dictate the wiring?
The manual doesn't indicate how it has to be hooked up. The terminal block on the HRV appears to have the proper hookups however its not working
Hooked wire from Hrv NO to red on furnace and green to C. Still no change.
Confirm the furnace fan goes on when you cross R and G (it should)
Yes when I jump it the furnace fan starts.
Then the problem is with the hrv. Are you sure you have a normally open contact which closes when the hrv starts?
The terminal block on the Hrv has a NO hookup. What should the voltage be on the normally open contact when the Hrv starts?
If you have no hook up and feel brave, you can always add a relay across the hrv blower motor which would activate the furnace blower. You would however have to open up the hrv and in doing so, void the warranty.

If you don't want to void the warranty probably the best thing to do would be to turn the furnace fan on with a differential pressure switch. You would mount the switch at the hrv and as it starts it causes a pressure difference in the hrv line which in turn activates the furnace fan.

The red and green wires would connect across the NO contact on the pressure switch and the positive/negative pressure lines would connect near to the hrv on the fresh air side.

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Someone previously on here reported that their HRV had relay contacts on a terminal block, but no relay inside the machine. Apparently some models don't come with the relay. You'll have to follow the wires inside the HRV.

PS. Removing the out pipe of the HRV from the return duct, sealing the hole left behind, and letting it draw from inside the house, will allow the hrv to ventilate without the system fan.

I just noticed I have those Differential lines drawn backwards. They should connect to the switch's diaphragm the other way around.
Would you recommend a differential pressure switch to buy? Also (if you know) where is a good place to buy one from? Thanks so much for your help.
My preference would be to use a relay in parallel to the relay for the HRV motor. (if no relay, then in parallel to the motor, adjusting the coil voltage to match) It would be the easiest, as no pressure tubes and taps. If you're not very good with electricity, the pressure switch might then be easier.

For the pressure switch, you'd need to measure the static differential that your unit will create. The switch will have to switch well below that, but higher then any parasitic static differential that will be present when the HRV is off, but the furnace fan is on. (should be zero, but the real world isn't perfect)

I like the relay idea better too, but if you want to go with a differential switch then you really should measure the differential pressure... but you need a meter for that. If you don't have one then this switch will most likely work (but I don't promise!)

The afs 145 or 222 are pretty universal and adjust from .05 to 12 inches... which is probably in your range.
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