Many gas heaters have a minimum recommended operating ambient temp set point of 50 degrees to avoid condensation induced heat exchanger corrosion.
I am in SE PA and have been looking to heat my ~1000sq ft well insulated space. I went through all the options for NG unit heater, NG house furnace, and mini split. I ended up going with a Fujitsu mini split install. The install is in progress now, so I can't tell you how well it's working just yet, but per the research it should be fine.
In researching, I found most home gas furnaces seem to have a 60 deg min set temperature on the documents. Combine lower set temp or over sized furnace with shorter run times and you will most likely rot out the heat exchanger. They will at least try to deny warranty if they can claim it was run too low. Reznor has a stainless heat exchanger option on their unit heaters, so that is an option. But for home style furnaces it's a potential restriction.
My space is split into two sections with only a doorway size opening between. For the minisplit setup I ended up pricing out two options - one with a larger compressor unit (30k btu) to run two heads inside and then another option with two completely separate units (15k btu, each). Fujitsu has some rebates going that made the two unit option cost effective. Plus, with two units I will have redundancy and significantly higher efficiency of the smaller units. Downside is two outside units hanging on the wall verses a larger single unit outside and a bit more install work. Per the performance documents, the two setup should output more heat at all temps, even though the nominal rating is the same.
Why did I go this route?
The division in the space cut back the ease of a unit heater install. It would take two to do right and one would be hard to exhaust because of location. The house furnace option would work and potentially give the option of adding A/C, but it would require ducting to the second space. This which would take up valuable room inside the work space or end up in the attic, taking up space and needing to be fully insulated. I also wanted low set temp and kept running into the low temp set disclaimers and warnings of the same from installers.
I finally ended up on Fujitsu, partially because of their low temp setting for 50 degrees as quick set on the remotes. Went with the XLTH option just for a bit of piece of mind as the record low temps here would occasionally push the recommended low temp limits of the normal units. Specs are for rated heat output down to -15f, which we've hit once in the last 40 years, so that shouldn't be a concern. Efficiency should be good with about 30 SEER and 14 HSPF.
Plus, this year has been crazy with rain. The space has been extra humid with little chance to dry down and now I have mold. With the mini split I have AC and can regulate summer humidity, a major bonus.
The cost was a high, but only a bit higher than a high efficiency gas furnace with some minimal duct work, without A/C. Probably about the same cost as gas furnace/duct work with A/C or even a bit cheaper. However, it would be harder to get the a/c cost option past my wife as a separate item. With the minisplit, it's just part of the package.
I will update when it's 0 degrees out and let you know how it's going.
