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Workshop Furnace

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  • Workshop Furnace

    Hey Guys: More questions about this furnace for my shop.

    I have a woodshop that is 30 ft by 20 ft with 16 ft ceilings (vaulted). The shop is well insulated R30 ceiling and R19 walls. I'm in Colorado. I have an older gas fired furnace I picked up cheap. I run it on natural gas. It is a 100,000BTU unit (way too much). I dialed down the valve (burners) to a very low setting. The furnace keeps the shop at about 65 degrees. The air coming from the furnace seems to be room temperature or a little higher rather than very hot (after I dialed it down). I currently have the unit installed near the ceiling of the shop inside the conditioned space. My thinking was that the intake is taking in warm air and the unit is in a heated space, thus improving efficiency. It is, however, a little noisey and doesn't "look good". This is an 80% whole-house furnace.

    My other option for mounting this unit is to place it on the other side of an insulated wall which separates the wood-shop from a "machine-shop/garage. This area would be in a non-insulated "garage" side of this same building. This area is not conditioned and could be quite cold. I could duct the furnace into the shop and also duct a return from the shop. What do you guys think in terms of efficiency? Inside or outside (but ducted).

    Is it okay to turn down the burners at the gas valve (as I have done)?



  • #2
    Re: Workshop Furnace

    I have some input on this subject:
    1. Turning down the gas pressure is probably a no-no. Gas pressure should only be adjusted +/- 10%. For most furnaces this means that the gas pressure should not be adjusted any lower than 3.15 " w.c. One problem with going too low with the gas pressure is that the temperature rise will not be what it needs to be ("near room temperature air" coming from the vents). When the temperature rise is too low, you'll corrosion and / or failure of the heat exchanger due to flue gases condensing. One other issue I can think of with turning the gas pressure down too low, is delayed ignition. Is this furnace used? If it's "just a shop furnace" you may not care how long the heat exchanger lasts.
    2. Moving the unit into the garage will cause another set of complications. There are safety issues due to the fact that garages typically have gasoline containing items in them (Lawn mowers, cars, gas cans, snow blowers, motorcycles...) Gasoline fumes tend to ignite when you don't wan them to. If the furnace is located in the garage, (I think) it needs to be at least 18" off of the floor. Additionally, the return must be sealed enough so that there's no chance of pulling return air from the garage. (because of the potential for CO poisoning from car exhaust.)
    3. If the furnace is noisy, you may not have adequate return / supply ductwork.


    • #3
      Re: Workshop Furnace

      Originally posted by Reeder View Post
      Is it okay to turn down the burners at the gas valve (as I have done)?
      NO NO NO

      I know what you want to do by dialing down the pressure but this will only lead to a much more dangerous situtation with carbon monoxide. Lowering the gas pressure will impinge the burner flame causing incomplete combustion and producting a byproduct of CO. Not to mention start sooting the heat exchanger as well. Too high of manifold gas pressure will produce the same result.

      What's the proper solution for you?...I don't know but adjusting the gas pressure is not the answer.


      • #4
        Re: Workshop Furnace

        Hey guys:

        Thanks for the responses.

        Right now the furnace is installed near the ceiling of the shop. It is keeping the shop at 65 degrees but will heat it more if I need. I turned the burners down so that there is about a 1 1/2" all-blue flame coming from them. When I got the furnace (used) the flames were about 4" tall with yellow at the ends. This seemed wasteful to me (and way too much heat for this space) so I dialed them down. Actually when the flames were 4" tall, the gases coming from the flue would burn your eyes.

        A couple of questions on this...The burners currently produce a 1 1/2" flame all the way down the burner. Wouldn't this be a more efficient flame? And reference the production of CO; Any incomplete combustion resulting in CO would be vented up the flue right? It seems to me this would not be entering the shops air. Please feel free to correct me if I'm wrong. I may be wrong but it looks to me like the all blue flame is efficient and doesn't use that much gas (gauging by the noise of the gas entering the burners). There's no yellow and the gasses coming from the flue now seem to be fairly "mild" as opposed to the more "pungent eye burning" gas being produced before.

        Reference placing the furnace in the cold air side of the shop, there are no vapors, etc. in this building to cause the flame to be a danger. It's not a "garage". I was simply having difficulty illustrating what the building is. However, your point is well taken about the duct being sealed. Assuming the duct is sealed well (intake and output); what do you think about the efficiency of the unit being placed in this cold environment versus being near the ceiling now where it stays warm and recirculates air in the shop (I do have two ceiling fans as well circulating air--HUGE difference when they are on--then the air is circulated instead of being layered).

        I guess my thinking is this (please tell me if I'm wrong)...It seems to me that if the furnace is warm, there is less warm-up time, plus I don't suffer heat losses through the ductwork that would also be run in the cold side of the building.

        PS> I know some of this may come off as being argumentative. Believe me, that is not my intent (hard to tell tone of voice on a forum though). I'm trying to present as clear a picture of my situation as possible--No argumentative tone here.

        You guys are great and thanks for taking the time to answer my questions.

        Oh yeah...I don't want to get off-topic here but the reason I dialed these down in the first place was that I read a post where someone had a problem with their pilot being "sucked-out" by the burners. This was actually my situation too. Since I dialed them down a bit my pilot light stays on and doesn't get "sucked-out"

        Kneescar: Very perceptive...Yes it's an old furnace and I really don't care if the exchanger life is less. Ignition seems to be fine. And yeah, the furnace has NO ductwork on it yet. It's just sitting there suspended. I intend to run return ductwork but if I end up leaving it where it is, I probably won't run a supply duct (just the return-from the lower part of the shop). Rather than a supply duct,I'd just let it blow into the shop.

        Thanks again guys.


        • #5
          Re: Workshop Furnace

          Turning the gas pressure down at the valve is not the way to go about this. The problem here is the lower pressure effects the saftey aspect of the gas valve. The way to throttle this thing down is to either change out the burner orifices to smaller ones, or you could remove a couple burners and plug off the manifold openings. Be sure not to remove the burner closest to the pilot.


          • #6
            Re: Workshop Furnace

            Gotcha. This makes sense. Thanks.