Duo-matic Olsen Furnace HMS-105-A - HVAC - DIY Chatroom Home Improvement Forum

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Old 01-12-2012, 08:25 PM   #1
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duo-matic olsen furnace HMS-105-A

My furnace was blowing cold air only. I replaced the draft inducer motor because It had overheated and quit working. Furnace still did not work so I also replaced the pressure switch. Also checked the pipe to the roof to make sure nothing was inside obstructing the air flow. The furnace still will not work. The furnace starts and runs a few minutes and then the high temperature switch shuts the furnace off. The draft inducer motor is very hot at this time. The burner flame is about 6-7 inches high. I am not sure if that is normal.


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Old 01-12-2012, 09:11 PM   #2
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Is this a natural gas furnace with an evaporator coil for the a/c connected to the furnace? You might want to have your evaporator coil cleaned. High temperature limit switches usually trip due to lack of or restricted air flow such as from a dirty evaporator coil.


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Old 01-13-2012, 12:21 AM   #3
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If you look on your plenum above the furnace, just beyond the direct sight line of the exchanger you will see a small 1/4" hole (probably covered by duct tape). This is where the installers originally did a temp rise test of your furnace. If you can't find it you can punch your own with a sharp nail & hammer. If you run the furnace and measure the temp inside the plenum and subtract from that measurement the room temp, you will get the temp rise through the furnace. The plate inside the furnace will give you the recommended temp rise for the furnace. This will tell you if the furnace is really overheating and whether the limit switch is faulty or not.
If the temp rise is above the recommended spec's then you need to find out why it's happening now. Dirty or too restrictive air filter/ dirty AC coil / fan motor dying/ faulty motor run capaciter/ too many heating outlets closed off/ dampers on heating vents closed off/ furniture blocking the return air grill.
If the temp rise is within the recommended temp rise then your limit is faulty or you have a cracked exchanger near the limit control.
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Old 01-13-2012, 03:07 PM   #4
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Thanks for your replys. Turns out our furnace has a cracked heat exchanger. We had our DuoMatic Olsen furnace for 24 years and it was a very good furnace. Very economical to run. We are looking at replacing it with an Armstrong. Hope it is as reliable.
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