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.
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.
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!)
A forum community dedicated to Do it yourself-ers and home improvement enthusiasts. Come join the discussion about tools, projects, builds, styles, scales, reviews, accessories, classifieds, and more! Helping You to Do It Yourself!