I'm like Sgt Schultz, I know nothing, NOTHING! I did mess with propane for over 15 years but never had any "formal" training in service, just following Charlie and John around and absorbing info. But maybe I can start you on the logic process to find your own answer. I picked up an older Reznor 75,000 BTU heater for my garage. I had to put on couple parts to convert to LP and new 24 volt transformer. The problem... the heater will cycle every 30 seconds or so. Here is cycle ---Gas valve turns on flame, heats up 1 minute and kicks off. Fan then kicks on and runs for 30 secs and then flame comes back on. Heats up and kicks on and off every 30 secs to 1 minute. Maybe a limit switch??? There is not much electric in the heater--gas valve, and limit switch for fan, and limit switch for gas valve. OK, my first thought is "That pile of junk is 45 years old". Yes, that is a bit blunt. But maybe I'm wrong on the condition, so let's ponder a bit.FIRST thing to do is a thorough inspection. This is where a good serviceman really earns his money. You need to inspect every inch of the heat exchanger, especially on one that old. Since you have it working I assume it is installed? Either get it so there is access or remove it and place it on a bench. Get a good flashlight, a trouble light, and an inspection mirror and start taking things apart. You are looking for a crack or hole in the heat exchanger. Ideally you'd have something like this, but you won't be doing this enough to justify the expense 
Here are a few examples of cracks and other problems. I'm just peeling these off various sites after a quick Google image search. In fact, before you go any further do a Google ssearch for cracked heat exchanger and spend some time reading various sites. In the search I found a web site that has very helpful and interesting info. The home page is http://hphaa.com/index.htm I don't know a thing abut him beyond what he has posted and don't have a clue where he is other than he's serving central Contra Costa ... whereever that is. For heat exchanger info see John's page athttp://hphaa.com/picpages/crackedheatexchanger/crackedheatexchangers.htm    
Let's see, you converted it from methane to propane? So you probably changed burner orifices and the pressure regulator on the gas valve? Did you change the pilot orifice? If not you need to. The pilot fuel mixture will be wrong, it will soot up things, and most important it will produce carbon monoxide. Back to the original topic. As old as it is, I'd venture it never had a flame probe. What you are describing is a classic limit switch malfunction. Remeber the Google search I mentioned? Do it again for furnace limit switch. One site that you might find intersting is this: http://forumm.doityourself.com/gas-oil-home-heating-furnaces/364838-gas-furnace-limit-switch-question.html It seems to describe your problem. If it is a limit switch the solution is simple. Remove it, take it to your local furnace guy and buy a new one. I have to go pick corn, but hope this helps.
Edited by Mike SE IL 9/27/2010 07:46
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Garage heater equipment
