Shop Heater

[Bob Deroeck]

Hi,

I plan on installing a natural gas fired heater in my detached workshop. I'm considering a Hot Dawg from Modine or it's equivalent (Stirling, etc). It would have a power vent, venting to the outside.

I have two questions concerning this type of heater. The first is that there are two options for the heat exchanger metallurgy. The standard unit has an aluminized steel exchanger which has a 10 year warranty. The other option is a stainless steel exchanger. I suspect the primary concern with the steel exchanger is rust due to the products of combustion eventually eating through the metal of the exchanger. Does anyone have long term experience (say more than 8 years) with this type of heater with an aluminized steel exchanger? The SS exchanger adds about $300-400 to the cost of the standard design which is about $500 for a 30,000 BTU heater (24,000 BTU net).

The second question concerns segregated air supply. For about $400 extra you can get a unit which has a separate air supply system to bring in air from the outside via a concentric vent pipe. The thermal efficiency is increased by about 3% because the combustion air is preheated. I don't consider that to be significant. The primary advantage that I can see is that dust in the air is not fed to the combustion chamber of the heater. Whether this is important or not in a shop with decent dust collection is my question. Has anyone with a standard Hot Dawg or equivalent heater that takes its combustion air from inside the shop had any problems caused by sawdust entering the combustion chamber?

All input is appreciated.


Bob DeRoeck

[Lee Schierer]

I don't have any experience with any of these units, but it would seem that the price jump for stainless is excessive. Unless it doubles the life of the unit, it wouldn't pay to go to stainless. Also the outside air option seems high priced, but I think I would go with it to keep sawdust from getting sucked in. Bear in mind that the outside air option probably does not make it vapor safe so take care when doing finishing not to let the solvent vapors get too high.

[Bob Winkler]

An additional consideration is that there is a "system" efficiency gain by using outside air for combustion. This outside combustion air allows for a balanced room air pressure, instead of the alternative "negative" pressure when using inside air for combustion. With a negative pressure, all that makeup air comes in through cracks from the outside, bringing in cold air. In the building science arena, this is often a topic of discussion.

Bob

[Steve Ryan]

Look at some of the high eff units, condensing type? They all require outside air but allow you to vent exhaust with PVC pipe. Consider total system cost of all required parts and installation.

[David Hostetler]

Not directly related, but sort of. The materials and issues are the same between this and automobile exhaust. Stainless steel is WELL worth the extra funds if you are concerned with the long term.

[Greg Portland]

The second question concerns segregated air supply. For about $400 extra you can get a unit which has a separate air supply system to bring in air from the outside via a concentric vent pipe.

Hi Bob,
In my old shop I installed a HotDog with the concentric vent. IMO, the more spraying and/or poorer your dust collection the more the sealed system makes sense. If you have a dedicated spray booth + filter and a really good dust collector then the amount of gunk getting into your heater will be minimized. I was only in the old shop for 2 years so I don't have any long term experience with the hotdog (new shop has radiant heat).

[Jim Neeley]

Look at some of the high eff units, condensing type? They all require outside air but allow you to vent exhaust with PVC pipe. Consider total system cost of all required parts and installation.

A couple of caveats... condensing units require a drain connection, since they get a lot of their energy by condensing the moisture out of the air... second, that condensed water can be very acidic, be careful what you drain it into, or onto.. and third, evaluate the $$$ to get that condensing efficiency.

[Mark Paavola]

Hello Bob, regarding the first question go with the aluminized steel. It is not worth the expense for the stainless in your application. You will not need to have the outside air system either. I am in the HVAC field and have worked on plenty of heaters like this in commercial shops. I have never seen one catch on fire or explode from the dust. They certainly can collect a lot of dust and at some of the shops we would have to clean them out with compressed air. They occasionally would build up enough sawdust to be a problem but these were in shops that had 2" of sawdust on everything in the shop.

[Roy Turbett]

I went with a 20' long natural gas radiant heat tube hung from the ceiling of my 30' x 40' shop. It draws the combustion air in one end of the building and exhausts out the other. Dust quickly settles to the floor because there is not a blower which makes it nice for finishing. And there isn't an open flame. I set the thermostat around 40 and can quickly raise the temperature to a comfortable working temperature of 55 in a short time. I never have any rust on my tools because they are never colder than 40. This is the same heating system used in many aircraft hangers and auto service centers. My woodturning club also installed this system.

[David G Baker]

I have the 75K Hot Dawg. It uses shop air for combustion and if I had it to do over I would pay the extra $ and get the unit that uses outside air for combustion. My shop is 30X40X10 and I keep it at around 50 degrees during the cold months until I need to work in it. The air has a smell that makes me uncomfortable breathing it if the shop has been closed up for a while. I leave the walk through door open for a while to let in some fresh air. The foil that lines the inside wall of the heater is breaking down and blowing all over my shop. I had my installer contact Modine but did not get a satisfactory answer as to why the foil is breaking down. My fear is that the unit may be over heating. If I have to install another heater in the future I will look at another brand of furnace.

[Gary Curtis]

After building a few houses, and investigating heating systems, one little known byproduct of combustion is water condensation on surfaces as they cool. This is the biggest single cause of boilers/heaters rusting out over the years.
If you live in a damp area, the stainless option will add years of life to the equipment.

If you are in Arizona or one of the mountain states, you the aluminized steel won't degrade the heater. In our primary home, we put in a circulating hot water heat system. The boiler came with a 25-year guarantee. The first 16 years seem like they flew by. So having a 25 year security cushion is nice.

In my home near the Oregon border I didn't get fancy in the newly constructed shop. I went to Ace hardware and ordered the 220v portable electric heater. $140. No installation. It puts out 14,000 Btu's of heat and electricity is only 12¢/KwH so it really does the job for peanuts. (14,000 Btu's= 4000 watts).

[Matt Meiser]

Mine is 7 years 8 months old, uses shop air and I'm not sure what the heat exchanger is. It was propane until 12 months ago. HVAC guy said it looked great and was clean inside when he changed it over.

The heat exchanger warranty doesn't mean much if the part isn't readily available and the cost of tearing down the unit to replace it might not be worth it. We ran into that with our house furnace a couple years ago. It was going to be 4 days in the dead of winter to get it, then another day to install and a full day of labor.