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Coleman Model CGU gas furnace
When troubleshooting, understand you are working with potentially lethal voltages and a highly flammable gas. If you do not have the ability to do these operations safely, do not attempt them.
The Coleman CGU model gas furnace has had many forms. It started out as a standing pilot furnace used primarily in mobile homes. It uses a single burner that burns in a barrel type heat exchanger. This particular model has added a secondary heat exchanger for use as a high efficiency furnace. The video below starts out the series on how this furnace works and problems related to it.
The Coleman CGU sequence of operation is shown below.
When the burner fails to either fire or the ignition control fails to sense flame, the inducer will continue to run with no flame. This is the indication that there is a problem. The video below covers how to troubleshoot this problem.
This video shows what happens when there is a flame rollout because of a blocked vent and a failed pressure switch.
The video below covers the replacement of the the Robertshaw 780 control with the S86 Honeywell control with a flame rod added.