Furnace Question

[sroxberg]

I have a ceiling mounted Modine Furnace that is acting up.

It receives a signal from the Thermostat to turn on and it does. The burner ignites and begins to heat up.

The fan doesn't start so the burner shuts off after about a minute. So I'm guessing that the Thermacouple is good, and that either a relay to turn on fan is bad, or the fan motor is shot.

Any tips on how to determine which part to replace?

[Mr_Mike]

Multi-meter. It is simple analog stuff.

Place multi-meter on volts and start up the unit probing the motor leads. If you get voltage, its likely the fan, if not dig deeper.

[DieselDennis]

Also see if the unit possibly has an air movement sensor.

If it's not sending voltage to the motor, check for a sensor issue.

[Robert Adams]

Clean the thermocouple with some 400 grit or finer sandpaper. Most likely culprit from those symptoms. 

    What happens is the thermocouple isn't providing enough power to keep the valve open. The thermocouple may look fine but it can have an extremely thin coating on it that's just enough to insulate it. It takes very very little to insulate it.

[blackhat]

It's either the motor / capacitor or whatever tells the blower to start. Which Modine  and does it have a terminal strip where the thermostat connects to the unit?  Can you put your hand on the motor and see if it's hot when it should be running?  This shouldn't be hard to diagnose.

[Duane N]

Clean the thermocouple with some 400 grit or finer sandpaper. Most likely culprit from those symptoms. 

    What happens is the thermocouple isn't providing enough power to keep the valve open. The thermocouple may look fine but it can have an extremely thin coating on it that's just enough to insulate it. It takes very very little to insulate it.

This would be the first thing I would check and if your thermostat has a way to turn on the fan without turning on the heat just flip the switch to fan on and see if the blower runs to eliminate the blower motor being the culprit.

Possibilities:

Thermocouple, fan relay, fan motor or fan motor capacitor or as DeiselDennis suggested maybe a sail switch that tells the heater the fan is running but most heaters need to warm up a bit before the motor starts up.

[Cecil]

Is this a high efficiency furnace.  They have a little tube drain.  There is a trap or something on that tube, basically a canister with an in and out.  Pull the tube, and canister and clean it out.  Mine was doing that and the furnace repair man showed me how to take it off and clean it. Basically it is plugged and fills with water.  When it cannot drain, the burner shuts off.  It will slowly seep out and the furnace can fire again.

FWIIW, that company will get all my plumbing and furnace business going forward.  He did not need to show me how to do something they charge for, but he did!

[Cecil]

I did some followup research.  It is the "P" trap on the condensate drain you are concerned about.  There are a handful of YouTube videos.  They are easy to find once you have the correct terminology.  Searching the web for doohickeys does not work so well

[Robert Adams]

I did some followup research.  It is the "P" trap on the condensate drain you are concerned about.  There are a handful of YouTube videos.  They are easy to find once you have the correct terminology.  Searching the web for doohickeys does not work so well

If it's an upflow it's usually inside and if it's a cross flow it's usually screwed to the bottom onpu the unit. But they are all different. 
  
    Also that trap has to deal with the condensate and the rain water that falls in the pipe. I haven't seen a rain cap for the 2" or 3" pvc flue pipe that wasn't $300 so it'll stay open to the rain. 

    Also beware of the ECM blowers. They are very failure prone and very expensive. You spend way more in repairs than the tiny amount of electricity that it saves you. Stick with a regular motor if you can.

[daddo]

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