Outbuilding/shop heater questions/opinions

Greetings all, we have an outbuilding (25'x35'x12') with concrete floors that we have all our woodworking equipment in. We're not professionals or anything but we'd love to be able to work in there during the winter months building some furniture and running our CNC machine. The building is insulated with that spray foam stuff on all the walls and ceiling (I'm no expert on insulation but it seems pretty good. I was in there yesterday checking on something and it was -14 degrees outside [SE Michigan] the outbuilding and 22 degrees inside the barn).

The outbuilding has a natural gas line and one of those 30,000BTU gas wall heaters mounted on the entry wall by the large sliding doors. The heater works but doesn't really seem to heat the entire space and, even if it does get a bit warmer, it takes a long time. There is also plenty of available electrical as the previous homeowner was like a "pro" craftsman .

So I've been reading on various forums about the options for heating this space and I love this forum so I thought I'd ask for any opinions people might have on my options. Seems like a lot of people have recommended something like the Modine Hot Dawg system. There are a bunch of those heaters available on craigslist and it looks like they often go on sale, new, at places like costco or Lowes/HD but none of them are the "separated-combustion" type that most people seem to recommend. So I guess that leaves me wanting opinions about:
1). What "type" of system do you recommend for our application (we want to heat the space so that the power tools are warm as well as the people [I don't know if Inrared means it just heats living tissue like a microwave and leaves metal objects freezing to the touch? ])
2). If a natural gas solution like a Hot Dawg is a good way to go, is it a huge drawback to go with the non-separated combustion model (since these seem to be available for hundreds and hundreds less on craigslist or on sale at big box places).
3). What type of BTU range do you think is applicable?


Thanks!

A mobile home counter flow gas fired furnace set on a cottage base would be a good choice as would a gas fired Modine unit heater. Quick, fast to install, fast heat and reasonably priced.

If you want to heat up ~1000' of space with a concrete slab (probably not insulted at the slab-edges) quickly you'll probably want something with 100-150KBTU/hr of output. It would be ridiculously oversized for the 99% heat load, but it won't take all day bringing the place up to temp. On an intermittent-use space where you'd normally set-back the temp 25F or more when you're gone, oversizing is the right thing to do, especially when you have the thermal mass of a slab to overcome.

If you just want to be comfortable while working rather than actually heating up the space, slab & all, radiant heaters directly over the work spaces is fine.