Low-tech heated air make up

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I’m looking for an inexpensive way to heat incoming air to an insulated 300 square foot shed I use to spray exclusively waterborne coatings.

There’s a small exhaust fan at one end of the room, and I currently just crack the door at the other end of the building and spray away. Primitive, I know, but so far I have been getting acceptable results. Now, with winter coming I’m looking to heat the incoming air on the cheap.

I am considering using a forced air torpedo style propane heater aimed in the door. With adequate air movement (the exhaust fan), and using strictly water-based products, I can’t see why this would be unduly hazardous, beyond the more garden-variety problems of an open flame inside a building. I’m unsure, however, if the water vapor from the propane would increase the humidity to the point of causing problems with the waterborne coats drying. Also, there would be no temperature control besides ‘on’ and ‘off’.

Another thought I had was to install a natural gas boiler that would have coils running around a large diameter pipe, or something similar, which would lead to the outside. In this way the exhaust fan would pull it’s own air through the pipe to be heated. This is simply conjecture and I’m curious what others have come up with.

My apologies to those with legitimate spray booths, hopefully you haven’t turned away in disgust! I simply don’t have the money to build a proper booth, and we’re talking about five to eight gallons a month anyway.

Anyone have any experience with this type of situation and care to comment? Thanks.

arthur grudko

A pellet stove might do the trick. Low cost fuel is one good feature, and you can get a good used one on Craigslist for $600 to $1200. Don't buy an off brand from Home Depot or Tractor Supply. A used Harman or Lopi is the deal.

rich c.

Water based product have a very narrow window of temp and humidity where they work well. I can't imagine a shed and torpedo heater will keep your work in the factory specs. Moving the product outside and into the shed is even going to cause problems in my mind. But, you don't say if you are in Minnesota or Texas. Maybe you only need to raise the temp 10 degrees.

Gavin

Thanks for the responses. I live in Canada, so I need to raise the air temperature a lot about half the year. I can keep the building at temperature with electric heat no problem, with the exhaust fan off. Parts can stay nice and warm waiting to be sprayed. I just need to replace the exhausted air while I'm spraying. It's worth noting that the exhaust fan is only 12", around 500 cfm I believe.

I actually worked for a guy for a little while who used the torpedo heater method with pre-catalyzed lacquer. I don't know that I would want to emulate this with solvents, but it did work, even in the dead of winter.

I definitely agree that the smaller temperature window of the water based is a problem. I've looked at prices for small air make-up units designed for commercial kitchens and they're still outside my budget.

A pellet or wood stove is not a bad idea, there's a lot of wood heat around here, so plenty of old stoves on the used market. It would also eliminate any concerns with humidity.

rich c.

Opening a door to get makeup air in Canada has to be a horrible way to bring in makeup air. You need to add a LOT of btu to bring up that air temp. Do you do your woodworking somewhere else and bring it to the shed to finish? That trip to the shed will lower the surface temp of the wood. Be sure it gets warmed back up before spraying. Have you ever heard of a earth tube? You bury a LONG 4" PVC tube below the frost line, then bring it above ground some distance away. The air picks up the warmth of the earth on the way into your building. At least the inlet air might be above freezing instead of below 0 deg. F. Makes a better cooler in the summer, but does pick up some heat in the dead of the winter. I still don't like the temp swings you are playing with. I'd look at having a regular shop do the finishing for you during the winter. It won't cost you any money, just figure the cost into the bid.

Jared

I had a setup like yours when I was starting out: a converted garage dedicated to waterborne spraying, a decent exhaust fan, and a cracked door for makeup air. What worked for me through the pacific NW winters (wet, frosty) was to heat the room with two portable radiant oil heaters for a couple hours beforehand with the parts to be painted getting nice and toasty. Keep the paint products warm too, never on the cement floor. Crack the door, turn on the vent fan, shoot the project, and keep the fan on just long enough to vent out the overspray. Then I'd turn the fan off, close the door and get those heaters cranking, sometimes supplementing with a space heater that has a fan. Get the room back up to 60-70 degrees long enough for the finish to flash off and get a good head start on curing. It worked pretty well, all things considered. You'd be into it for a hundred, maybe $150 for all the heaters but it's an inexpensive way to improve what you've got.

Bill in Oregon

Something else you might consider is a negative air machine/scrubber. Someone on the Paint Talk forum posted a home made version using a Harbor Freight blower and hose sucking through a plastic storage tub with a furnace filter taped to the top (having cut out a suitable sized section for the filter)

Gavin

Thanks again, I appreciate the responses. Rich C, you hit the nail on the head with my situation: I have a leased industrial bay not far from my house, which is my ‘main shop’. The price is excellent, but the caveat is that it used to be part of an old fertilizer plant which once had it’s own hydroelectric power supply, and so there is no natural gas. The landlord has been lukewarm on my plans for a spray booth from the start, and so I decided to separate the two. The theory is that I squirrel parts back to my garage as they’re completed, to do the spraying all in one shot near the end of a project. This also has the benefit of not clogging up the woodworking shop with parts, and having a dust free place to spray.

I would have no problem outsourcing the finishing, but to be blunt, there’s no one I would trust to outsource to around here. I do have the option of renting the booth of a guy I’ve worked with in the past, but he won’t budge from $40/hr, which although fair for a fully equipped shop, I feel is too expensive to do on a regular basis. Maybe I should reconsider this?

I’m not married to the torpedo heater idea, I’ve just seen it done with success, and I’m guessing that propane or natural gas is the only energy source with enough b.t.u to work. Hot-rod enthusiasts on various forums I’ve searched seem to use this approach to paint cars in their garages. In sub-zero temps, I would imagine it would take the room too long to recover with just electric heat to be able to spray in a continuous fashion? The power to my house is anemic anyway at 70 amps.

I shouldn’t have said ‘crack the door’. I’m imagining something like a 2’ square hole cut into the wall with a small access door to close when the heater’s not in use.

Anyway, reading my own posts, it kind of sounds like I’ve talked myself into giving the torpedo heater idea a stab. It’s maybe a tad idiotic, but for less than $200, I may as well give it a trial go. I will contemplate the pellet stove idea further also.

Gavin

Bill in Oregon,

That's interesting. Do you mean reusing the air? I did find this unit online the other week while contemplating my problem, and thought it was interesting, although definitely outside my price range:

http://www.cwimachinery.com/product/ecocarbon-woodworking-spray-booths/

I was under the impression that recirculating the air was no-no? Maybe less so with low v.o.c?

Bill in Oregon

The painters use them, including off the shelf scrubbers/negative air machines, to reduce or eliminate overspray in the air. I'd try it without any make up air first and increase the amount of exterior air as needed. I wouldn't try it using solvent based material.

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