Blower Runs Continously

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Old 01-15-19, 05:29 AM
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Blower Runs Continously

Inter City products Corp Model DGHH075A012IN gas furnace about 45 years old. Strong like bull.

Background:

Heat would not engage, buzzing noise from timer control board on turn-on (had been buzzing for a year, but kept working). Replaced pressure switch. Now system would engage, with timer board buzzing, but without flame. When I applied pressure to the buzzing relay, the system ran perfectly, but I couldn't stay in the attic all the time--my wife occasionally needs the trash taken out and the oil changed in her car. It's impossible to change the oil in an old Volvo while in the attic. I've tried. The car won't fit up the attic stairs.

So, instead, I replaced the timer board. Of course the replacement has a different physical configuration and connections. I tried to match the connections properly, but evidently failed. The system would go through the entire turn-on but no main circulation blower. After a cold couple of days, I got an HVAC trained, but inexperienced friend to help. He connected a wire to the "S3" connector and the system cycled through and the blower came on. Glorious heat!

The house was very cold so the system ran for a couple hours. I went to bed. At 3AM when I got up for my 50-something mid-sleep wake up, I noticed the blower was still on, even though the house was warm.

I checked the system this morning. I pulled the wire from S3 and connected it to "heat" in the blower section. Same behavior. Note: on the old board, there was a jumper connecting "heat and cool." It worked that way forever (unless I just got it wrong which is always a possibility).

Another note: The unit does not have a fan limiter switch, as far as I can tell.

Ideas? I can provide pics of the old board, new board with how it's connected now and the schematic from the new board.

Thanks in advance.

az in sc
 
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Old 01-16-19, 05:36 AM
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Something that old usually had a combination fan and limit controller with the sensor placed in the furnace's air exit plenum. Once the thermostat was satisfied, the fan contacts on the fan/limit controller would keep the blower energized until the sensed temperature reduced to the point where the fan contacts opened. You were getting heat at no fuel cost. If your furnace doesn't have a fan/limit controller, then the blower should de-energize when the thermostat opens.
 
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Old 01-16-19, 06:21 PM
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On edit: I thought this was a no heat issue. Post pictures so we can see what is going on.
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The fan timer board has nothing to do with ignition.

At 45 years old that furnace should be professionally cleaned and checked out. The older furnaces did last longer but heat exchangers don't last forever.

I show that as an intermittent ignition system. A spark lit pilot light. More than likely.... that's where your problem is going to be based on the age. That should use a Honeywell S-8610 type ignition board. Pressure switches very rarely need to be replaced. To date..... I have replaced one. A technician will use a manometer to check actual available vacuum. People have said that changing a pressure switch has fixed their problem. It's usually the pressure switch is set slightly different and will work ok on the available reduced vacuum. Guessing at parts gets expensive.

You should have a furnace wiring diagram on the door there. You need to trace the 24vAC that supplies the ignition module. It will pass from the thermostat thru limit switches and the pressure switch. If the voltage is on 24v and TH terminals.... the ignition system is at fault. If the voltage is not there.... it's a problem limit or pressure switch issue.

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Last edited by PJmax; 01-16-19 at 07:23 PM.
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Old 01-17-19, 04:26 AM
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I read the post as a problem controlling the main circulation blower. I didn't consider it a heating problem.
 
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