# Furnace Interlocking for HRV



## HVACDave (Oct 16, 2007)

The alternate method is safest for most installations if you are concerned about feedback through the stat. It appears as though that is done correctly in your case, so all should be good. Are there any reasons as to why you would consider changing it?


----------



## mk5tdi (Mar 9, 2012)

HVACDave said:


> The alternate method is safest for most installations if you are concerned about feedback through the stat. It appears as though that is done correctly in your case, so all should be good. Are there any reasons as to why you would consider changing it?


Thanks!!!!

The reason why I was considering changing it because when I read the description of alternate interlocking in the HRV installation manual, it seemed to me that it was dependent on the type of thermostat because they said "On some older thermostats..." so I was thinking that if I install a new thermostat then do I need to worry about changing this.

Could you elaborate on what do you mean by "feedback through the stat"?

I am just trying to understand how one method is different than the other and what the NC, C, NO terminals do.


----------



## HVACDave (Oct 16, 2007)

The problem with some of the hrv's and stats has been when the hrv doesn't have a set of NO and NC contacts (as yours does) when we energize the G terminal on the furnace control board through a set of dry contacts on the hrv without breaking the G wire that goes to the thermostat then the thermostat doesn't know what to do with a 24 volt input on the G terminal ( as G is designed to be an output terminal from the stat only). The stat would then take the 24 volts coming in on G and activate the A/C, or even the furnace depending on the position of the switching contacts in the stat, and the type of stat used. 

We would commonly install an isolation relay to overcome this problem, but the contacts in your hrv do the same thing, so you don't need to do anything else. Hope that answers your question.


----------



## mk5tdi (Mar 9, 2012)

HVACDave said:


> The problem with some of the hrv's and stats has been when the hrv doesn't have a set of NO and NC contacts (as yours does) when we energize the G terminal on the furnace control board through a set of dry contacts on the hrv without breaking the G wire that goes to the thermostat then the thermostat doesn't know what to do with a 24 volt input on the G terminal ( as G is designed to be an output terminal from the stat only). The stat would then take the 24 volts coming in on G and activate the A/C, or even the furnace depending on the position of the switching contacts in the stat, and the type of stat used.
> 
> We would commonly install an isolation relay to overcome this problem, but the contacts in your hrv do the same thing, so you don't need to do anything else. Hope that answers your question.


Thanks, I am glad I joined this forum, I got two answers to my questions that I have been asking my the company that installed the furnace and HRV for the past week, they never called me back after taking my number.

I just wanted to have an understanding.


----------



## mk5tdi (Mar 9, 2012)

HVACDave said:


> The problem with some of the hrv's and stats has been when the hrv doesn't have a set of NO and NC contacts (as yours does) when we energize the G terminal on the furnace control board through a set of dry contacts on the hrv without breaking the G wire that goes to the thermostat then the thermostat doesn't know what to do with a 24 volt input on the G terminal ( as G is designed to be an output terminal from the stat only). The stat would then take the 24 volts coming in on G and activate the A/C, or even the furnace depending on the position of the switching contacts in the stat, and the type of stat used.
> 
> We would commonly install an isolation relay to overcome this problem, but the contacts in your hrv do the same thing, so you don't need to do anything else. Hope that answers your question.


Just one last question on this topic to close the loop, today I replaced my thermostat that the builder provided with Honeywell Prestige 2.0 IAQ and the Honeywell Equipment Interface Module (EIM), all seem to be working ok so far (call for heat, HRV turns on just the fan on the furnace and no gas is lit, etc)

I left the interlocking in the alternate position, the way you see in the pics, is that fine?


----------



## computergobot (Nov 25, 2013)

*Hijacking*

Well this is sort of hijacking the thread, but here goes:

-I would like to interlock my furnace and HRV in the reverse of what's been shown here.

I would prefer if my furnace simply responded to the thermostat's call for heat, and the HRV would turn on at the same time. I have a very similar, if not the exact same, set up as mk5tdi. Any ideas? I spent a few hours trying today with no luck.

Thanks.


----------

