Short version: I need to have a propane unit heater installed for a well-insulated 30'x50' x 14' ceiling garage in northern Minnesota. Should I run metallic cable to 20-amp outlet for a propane unit heater or just run the cable and leave slack for a future installation/installer?

I have a well insulated detached garage and the in-slab radiant boiler system is shot (boiler dead, plumbing bad, looking at >$4k to fix plus keep tabs on winter water lines to maintain it). I mothballed the slab lines with RV antifreeze for potential future use, but I'm planning for a forced-air propane unit heater instead as a replacement. (Won't get a ton of winter use and only needs to be kept about 45 degrees inside to keep water/equipment happy.)

I plan on having a suitable Sterling propane unit installed, and it's going to have to happen while I'm away. I want to get the electrical ready for potential installers. (Still trying to get bids submitted.) I'm told it just takes a 20-amp dedicated circuit, which I've already got thanks to the boiler circuit. I can easily re-route that power up into the corner where the forced air unit will go with some metal-shielded cable.

Do these units plug in with a 3-prong cord or should I leave the metallic cable with slack for the installers to patch it in directly?


Cover your bases. Install a 20A receptacle in a metal box. If the breaker is GFCI then use a standard receptacle. If the breaker is not GFCI then put in a GFCI/receptacle. Since this is a dedicated circuit, ideally you should install a single receptacle rather than the typical duplex.

If the installers want to plug in with a cord, all done. If they want to hardwire, they can do that in the same box. Either way, you've got everything they could need already in place. Worst case, you've spent a few dollars on an extra receptacle that isn't needed, but that's better than the opposite problem of having to pay extra for a second installation trip.

  • How was the old boiler fueled? Natural gas, propane? oil? Or if an electric boiler you will most likely have 240 volts to it which almost certainly would need to be re-wired for 120. Oct 14 '20 at 22:41
  • old boiler was propane, just needed a regular line. Breaker for it is a 20 amp -- I'll just run that to the location for the new heater. Oct 15 '20 at 17:10
  • 2
    GFCI must be in a location where it is easy to reset and test. That would preclude a ceiling or high on wall location. Oct 15 '20 at 19:28

Installer finally got back to me (but not with a bid yet! Yikes these guys are busy.)

Anyway he told me not to use a GFCI receptacle or breaker -- it'll trip too often from the unit motor kicking on. Says there's a carve out for appliances and the usual GFCI rule. Having recently had to uninstall a laundry GFCI outlet (brand new) because a (also brand new) washer kept tripping it during spin, I know what he's talking about.

For future reference for anyone, with the unit to be mounted just below a 14' ceiling, he wants it 2 feet down from the ceiling, 2 feet from the corner where it will be placed.

  • It's less there's a carveout for appliances and more that if it's hardwired, it doesn't need GFCI to begin with Oct 21 '20 at 23:30

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