To the best of my knowledge something has to make glycol, water or actual steam to be effective. Maybe improvising with a wall mount Paloma-Style could accomplish similar to a boiler??? Many brands besides Paloma but similar compact mounting on a wall etc.!
I found something interesting on Amazon for tiny areas. Note that after scrolling down to product description when you get to the below link ... as to how many BTUs are transferred per water temperature. So it is not like it has to be even hotter than 120° F to do some good.
Based on the water temp - BTU chart ... I am thinking that some of us could even improvise with a compact wall mount unit like 2Biz & some others are using. Of the below example would not be big enough fo a tunnel. On the other hand it possibly could be a good fit for equipment room heating &/or for heating a dog wash. IMHO it makes a lot more sense than an out of sight electric bill.
Does anyone remember what Zoro did last year for cyber Monday? I need about $1600 stuff (well, I want, not need) I have a 20% off that expires Sunday. Trying to decide if I want to gamble and wait for Monday. A 30% flash sale would be nice. Are you listening Zoro Gods?
I got in on some 5x more durable entrance mats for the laundromat facility & a spare 3/4HP motor for our huge Taco deicer pump. They had the best prices on those items before the 25% discount we got before Dec 8th. Thanks Greg for starting this thread. The entrance mats we got less than a year ago from Sam's Club turned out to be less quality & not durable enough for our foot traffic.