How can I DIY garage furnace interlock with garage door?

I'm planning to put a forced air ceiling hung natural gas garage furnace in my garage this year. I already have the exhaust done, gas line run, thermostat, etc. (prior homeowner had one installed and took it with him)

My question is this: What's the easiest way to bypass the call for heat signal so the furnace won't run when the garage door is open?

I'd like to generally always keep the garage at 50ish F, so it's easy to warm up to working temps from there for a garage night, and it's always warm enough to thaw cars out, etc. That said, I have two kids... currently 3 and 7, and I know they'll have lots of days they're playing outside, and they'll leave the garage door open. I'm okay with that, they're kids, toys/sleds are in the garage, etc... but I'd like for the furnace to not run constantly trying to heat the earth if they're out there. That said, when they come inside and the door is closed, it would be nice for the furnace to kick back on at that point and catch up automatically.

I imagine there's some kind of limit switch or proximity sensor or something like alarm systems use to sense when doors are open, but i'm not sure how these work from a voltage standpoint, or how i'd use that signal to interrupt the thermostat signal.

Thanks!

You could use a normally open overhead door contact wired in line with the thermostats signal. When the door is closed, contact is closed and furnace runs if thermostat calls for heat. Door open, contact is open and furnace doesn't get the heat signal from the thermostat.

Something like this should work:

https://www.amazon.com/Honeywell-Ademco-Overhead-Door-Contacts/dp/B0006M1I1W

You want to wire the thermostat through a contact that follows the state of the opener. In the power business we would call it an "a" contact; open when the door is open. What garage door opener do you have?

Are we talking about an overhead door or a door for a person?

Garage door it looks like so overhead door was my assumption

You can get a spring loaded mechanical switch that is open when the door is open and holding the switch open.

Or, I am sure that someone sells an integrated relay/proximity switch.

paranoid_android74 wrote: Are we talking about an overhead door or a door for a person?

dculberson's suggestion will work for either/both. If doing both just run the switches in series. That way both doors must be closed for the furnace to run.

Also is easy to do with any magnetic switch for alarms systems.

On most furnaces with an intermittent spark instead of a standing pilot, there are a bunch of orange wires. They tie several proving switches together in series and run on 24 volts. One will be for the door to make sure its closed, one will be a flame proving switch, one will be for a draft inducer fan, and there will be others for overheat conditions. Before the gas valve will open and release the gas into the combustion chamber, all of those proving switches must be in the closed position so that the 24 volts being send out gets back to the circuit board. Adding another "door" switch in series with the rest would be the easiest way to accomplish what you want to do. As soon as the new door switch stops making contact, the gas valve closes and the burner stops firing. Close the switch, and as long as the thermostat is calling for heat, the furnace will go thru its pre purge safety routine and fire again. If you have an old standing pilot unit, then just get a modern door switch and put that in line with the thermostat wire. Just mount the switch down low on a bracket where it can make contact with the door when its closed and pop open when the door opens up.

And when you install your contact switch(es), keep two things in mind: first, this type of switch on an industrial O/H door leads a very hard life (or did when I was in the alarm business), so get a good one and mount it in as protected a way as possible. Second, if your door can stop partly open and be enough to keep the kids happy, make sure the switch is mounted so that "partly open" still registers as "open."

I recognize that these two objectives may conflict. Welcome to real-world alarm system design, where the architect and engineer meet the actual user for the first time.

This board rocks. Thanks for all the replies!

dculberson wrote: You could use a normally open overhead door contact wired in line with the thermostats signal. When the door is closed, contact is closed and furnace runs if thermostat calls for heat. Door open, contact is open and furnace doesn't get the heat signal from the thermostat. Something like this should work: https://www.amazon.com/Honeywell-Ademco-Overhead-Door-Contacts/dp/B0006M1I1W

Perfect. That's pretty much exactly what I was looking for. Reading the reviews, it works within an inch or so of proximity, so some garage door slop won't cause problems, and I can mount it up top rather than on the floor.

Jcamper wrote: You want to wire the thermostat through a contact that follows the state of the opener. In the power business we would call it an "a" contact; open when the door is open. What garage door opener do you have?

Not sure it matters, but it's just a simple chain drive Craftsman. I'd rather not rely on an opener "state", as opposed to the switch physically monitoring the door itself.

paranoid_android74 wrote: Are we talking about an overhead door or a door for a person?

Yes, overhead door. Two actually, but the main door was my primary concern. It looks easy enough to just wire both doors in series though, so I'll probably wire up both.

tr8todd wrote: On most furnaces with an intermittent spark instead of a standing pilot, there are a bunch of orange wires. They tie several proving switches together in series and run on 24 volts. One will be for the door to make sure its closed, one will be a flame proving switch, one will be for a draft inducer fan, and there will be others for overheat conditions. Before the gas valve will open and release the gas into the combustion chamber, all of those proving switches must be in the closed position so that the 24 volts being send out gets back to the circuit board. Adding another "door" switch in series with the rest would be the easiest way to accomplish what you want to do. As soon as the new door switch stops making contact, the gas valve closes and the burner stops firing. Close the switch, and as long as the thermostat is calling for heat, the furnace will go thru its pre purge safety routine and fire again. If you have an old standing pilot unit, then just get a modern door switch and put that in line with the thermostat wire. Just mount the switch down low on a bracket where it can make contact with the door when its closed and pop open when the door opens up.

All good info, but it looks like it'd be just as easy to just interrupt the "heat now" signal from the thermostat, rather than adding onto circuitry at the furnace itself. The furnace will go through whatever checks it would do whenever it receives that signal, and when that signal goes away (door opens) it's no different functionally than the thermostat stopping to call for heat.

Currently looking at picking up the 50K BTU "Mr. Heater" from Northern Tool: (or Menards, or whoever has the best price at the time...). Currently under $400 including shipping to my door with online coupon.

http://www.northerntool.com/shop/tools/product_200577749_200577749