Rheem Gas Furnace problem

Reply

  #1  
Old 12-14-19, 11:26 AM
Member
Thread Starter
Join Date: Dec 2019
Posts: 4
Received 0 Votes on 0 Posts
Rheem Gas Furnace problem

Hi,

I have a Rheem RGDD-06NC-ER gas furnace. It turned on one morning and would not shut off via the thermostat. We had to shut the power off to the unit to get it to finally turn off.

I replaced the thermostat ( a simple 2 wire unit ). I turned the furnace on, and it turned on and heated. I then turned the furnace off using the new thermostat which has an "off" position. The furnace then turned off.

We turned the furnace on again and let it heat the house, but the furnace would again not turn off! Even moving the thermostat to the "off" position would not turn off the furnace, so we again had to turn the electricity off to the furnace to get it to shut down.

This unit uses a Robershaw 780-715U ignition module. Is the relay and all other controls built into this module, and if so I'm guessing replacing this unit will solve my issue of the the furnace not wanting to turn off?

Any help or direction will be appreciated!
 
Sponsored Links
  #2  
Old 12-14-19, 11:47 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 54,245
Received 482 Votes on 452 Posts
Welcome to the forums.

You need to diagnose the problem fully.
You said the furnace won't shut off and that you must turn the power off to reset it.
What is not shutting off....... the burner or the blower ?
Is it still heating until you turn off the power ?

It sounds like the furnace is overheating and then going into a safe mode where the blower only is run to cool the furnace off.
 
  #3  
Old 12-14-19, 02:20 PM
Member
Thread Starter
Join Date: Dec 2019
Posts: 4
Received 0 Votes on 0 Posts
thanks Pjmax,

the unit continues to heat. So it's blowing heated air until we shut off the power to the unit completely.

So i don't think it's overheating since the air is still being heated and blown into the house.

anything else I can provide?

thanks!
andy
 
  #4  
Old 12-14-19, 04:04 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 54,245
Received 482 Votes on 452 Posts
It is extremely rare for that furnace to stay in "burner running" mode unless the thermostat was calling for heat. If it gets to the point where the burner is stuck on again..... you'll need to disconnect the two thermostat wires from the controller and see if that shuts it off.
 
  #5  
Old 12-14-19, 08:06 PM
Member
Thread Starter
Join Date: Dec 2019
Posts: 4
Received 0 Votes on 0 Posts
Ok, so disconnect the two wires when the unit is running? It has a red and a white wire and it's a millivolt thermostat. if i disconnect the wires then the unit should shut down? what if it doesn't?

it's odd because when the furnace is "cold" I was able to shut it off using the thermostat "off" position. then when the furnace had been heating the house for awhile, it would not shut down any longer using the thermostat, even when the thermostat was put back into the "off" position.

this is a new thermostat that I just replaced because of this issue of the furnace overheating the house. a couple of weeks ago the furnace turned on by itself (it could have been colder than the lowest setting of this old thermostat that did not have an "off" position). The furnace stayed on until the temperature in the house was clearly over the setting of the thermostat and it still kept on heating and would not turn off. we had to use the power switch to turn all power off to the furnace. thinking it was a bad thermostat, i replaced it but this problem persists...

thanks Pete!
 
  #6  
Old 12-14-19, 09:50 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 54,245
Received 482 Votes on 452 Posts
That's a pretty old furnace. It has an intermittent spark ignition pilot. Doesn't require a millivolt thermostat.

The 780-715U module gets 24vAC power only during a call for heat. You can connect a meter to Th/24v and Tr/ground to monitor voltage. If there is no voltage present and the burner is running.... that would indicate a possible sticking gas valve.
 
  #7  
Old 12-15-19, 12:04 AM
Member
Thread Starter
Join Date: Dec 2019
Posts: 4
Received 0 Votes on 0 Posts
The original thermostat is a mercury switch 2 wire. Is the new 2 wire thermostat I connected not compatible with this furnace?

Is it possible that the 780-715U needs replacement?
 
  #8  
Old 12-15-19, 07:59 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 54,245
Received 482 Votes on 452 Posts
If the new thermostat says also for 24v systems it should be fine.
You can't guess at that module. Power must be checked with a meter.
You need to determine what exactly is bad.

If the gas valve is sticking..... replacing that ignition module will not help.,
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: