Gas furnace won't light

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  #1  
Old 12-27-07, 12:08 PM
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Gas furnace won't light

I have a Rheem gas furnace model RGDA-15EE-JR (approx 17 yr old) that won't light anymore. When thermostat calls for heat, glowplug/ignitor energized OK, after approx 30 secs of glow I hear the ignition module click for gas valve, but no gas. After about 2-3 secs the ignitor goes out, then about a second or so later another click and we're done. I've checked for voltage at the gas valve and i get 15VAC when gas valve energized.

My question is do I have a bad gas valve, or a bad ignition module? The reason I question the ignition module is due to the 15VAC - shouldn't this be 24VAC? There is another terminal on the ignition module labeled "TH" which does have 24VAC. Ignition Module is a Robertshaw HS780, 34NL 306A

This all just happened this morning when I noticed the temp in house was low, but thermostat was calling for heat.

Thanks,
Jerry
 
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  #2  
Old 12-27-07, 03:17 PM
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Originally Posted by jrainko View Post
I have a Rheem gas furnace model RGDA-15EE-JR (approx 17 yr old) that won't light anymore. When thermostat calls for heat, glowplug/ignitor energized OK, after approx 30 secs of glow I hear the ignition module click for gas valve, but no gas. After about 2-3 secs the ignitor goes out, then about a second or so later another click and we're done. I've checked for voltage at the gas valve and i get 15VAC when gas valve energized.

My question is do I have a bad gas valve, or a bad ignition module? The reason I question the ignition module is due to the 15VAC - shouldn't this be 24VAC? There is another terminal on the ignition module labeled "TH" which does have 24VAC. Ignition Module is a Robertshaw HS780, 34NL 306A

This all just happened this morning when I noticed the temp in house was low, but thermostat was calling for heat.

Thanks,
Jerry


It certainly sounds like the furnace is trying to switch the main burner gas on, but you are getting no gas to the burner.


That can have several causes ---- no gas to the furnace, a manual shutoff valve at the furnace shut off, the electric gas valve turned off, a control module failure not supplying 24 VAC to the gas valve, a burned out gas valve solenoid.


Does the wiring to the gas valve come directly from the ignition module, or does it go through some other controls? If so, I'd walk the circuit from the ignition module to the gas valve to see if you are getting that 15VAC all the way.


If so, it sounds like you have a bad ignition control module that needs replacement. I'd double check that diagnosis by stealing 24 VAC from another source of voltage and checking to see if that turns the gas on.


I happen to have in my hot little hands a Robertshaw HS780 34 NR-3088 ignition control module. However, it's designed for 220 VAC operation, not 120 VAC like yours. I doubt I will ever find a use for it.
 
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Old 12-27-07, 05:42 PM
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Originally Posted by jrainko View Post
I hear the ignition module click for gas valve, but no gas.
The gas valve should make a click also. At least one you can feel by resting your fingertips on it.
 
  #4  
Old 12-28-07, 03:09 PM
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I also have the same problem(from michigan) same ignition unit hs780, different furnace though"ruud ugda-10ec-er". Only when I reset the bugger a bunch and finally get it going it works fine all day (proper off and back on automatically)until i turn heat down and the next morning comes in which I must give it cpr again. Also every so often my gas regulator buzzes like nobodies business. I read about smacking it with a hammer. I'm scared to do this unless i'm in full body armor.

should i start my own question or would it be better for me to stay in this one? I'm brand new to forums.
 

Last edited by chingchongchang; 12-28-07 at 03:39 PM. Reason: addition
  #5  
Old 12-28-07, 03:36 PM
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Originally Posted by chingchongchang View Post
I also have the same problem(from michigan) same ignition unit hs780, different furnace though"ruud ugda-10ec-er". Only when I reset the bugger a bunch and finally get it going it works fine all day (proper off and back on automatically)until i turn heat down and the next morning comes in which I must give it cpr again. Also every so often my gas regulator buzzes like nobodies business. I read about smacking it with a hammer. I'm scared to do this unless i'm in full body armor.


You need to describe what you observe happening in detail when the thermostat calls for heat.


Gas regulators don't typically buzz --- I'm supposing it's some other part. Is it the electric gas valve that has a control knob on it and several wires going to it?
 
  #6  
Old 12-29-07, 03:43 AM
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Originally Posted by SeattlePioneer View Post
You need to describe what you observe happening in detail when the thermostat calls for heat.


Gas regulators don't typically buzz --- I'm supposing it's some other part. Is it the electric gas valve that has a control knob on it and several wires going to it?
Sorry, yes the buzzing(when it happens) comes from inside a unit with gas lines on each side, also has red on/off switch and two wires going to it. I just didn't know what it was called b4.

As for what happens ... A click, ignitor glows for 20-30secs, another click , ignitor still glowing for 2-4secs more, then a click shutting ignitor off, without gas getting to it. Then it tries this twice more over the next 2 minutes and completely stops. In which I go to the wall thermostat and turn it to off and back to heat and voila! I'm in business and everything is working fine, except for the random buzzing that pops up to say hello every once in a while. So I'm puzzeled as to why once I get it going in the morning it will run fine all day but if I want to drop it 2 degrees it won't kick back on when time to do so.
 
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Old 12-29-07, 04:43 AM
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Originally Posted by chingchongchang View Post
Sorry, yes the buzzing(when it happens) comes from inside a unit with gas lines on each side, also has red on/off switch and two wires going to it. I just didn't know what it was called b4.

As for what happens ... A click, ignitor glows for 20-30secs, another click , ignitor still glowing for 2-4secs more, then a click shutting ignitor off, without gas getting to it. Then it tries this twice more over the next 2 minutes and completely stops. In which I go to the wall thermostat and turn it to off and back to heat and voila! I'm in business and everything is working fine, except for the random buzzing that pops up to say hello every once in a while. So I'm puzzeled as to why once I get it going in the morning it will run fine all day but if I want to drop it 2 degrees it won't kick back on when time to do so.

You have a hot surface ignition system on what? ---a warm air furnace, presumably. But you should have told me that there is a small fan mtor (the inducer motor) that starts and comes up to speed before the HSI glows --- is that right?

Typically, when the thermostat calls for heat, the inducer motor will switch on and when the pressure switch is tunred on by the ventilation caused by the inducer motor, the HSI heats ip for thirty seconds or so, followed by the click which is the gas valve turning the main burner gas on for 4-7 seconds. If the gas ignites, everything stays on until the thermostat is satisfies, then shuts off.


In your case, when the gas doesn't ignite, the main burner gas shuts off and the HSI has another couple of tries at lighting the burner. If that fails, it shuts itself off until you reset the system by turning the thermostat down or shutting off the power.


After bthe HSI glows and you hear the click, is the main burner gas actually turning on? Typically you can hear the gas gushing out. If it IS coming out but not lighting reliably, the HSI may not be in quite the right location, the burners may be dirty or the HSI may be wearing out and not getting hot enough to light the gas in time.


The reason it's operating ewrratically is that it ALMOST works all the time. It's on the edge of failure, or on the edge of working reliably, depending on whether you are a glass half full or glass half empty person.


When was the last time you had routine maintenance work done on this furnace?
 
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Old 12-29-07, 04:46 AM
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Also, how old is the furnace and have has the ignitor ever been replaced to your knowledge? There may be a sticker on the furnace that might record changing a HSI. If so, the date would be interesting to have.
 
  #9  
Old 01-01-08, 12:07 PM
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Originally Posted by ecman51` View Post
The gas valve should make a click also. At least one you can feel by resting your fingertips on it.
I must apologize - the house was getting really cold, supply stores were closing in 30 minutes and I hadn't had any replies yet so I gambled and bought a new ighition module. Once it was installed all was working (and still is!) perfectly. I guess it was indeed the 15VAC being supplied instead of 24VAC, as I checked it with the new unit installed and 24VAC is what I got.

Thanks for your help.
 
  #10  
Old 01-01-08, 02:07 PM
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Cycling Gas Valve.

Some of these problems sound similar to mine.

I have a Coleman furnace Model 2880E766 with a Robertshaw HS780 ignition control module on it.

Thermostat calls for heat.
Draft fan comes on to purge the system.
After the purge, the gas valve comes on and I have a flame.
Gas valve cycles off and on even though there is a flame.
Sometimes the main blower will come on but usually it goes through it's cycle until it locks out.

I am an electrician so I got my meter out and checked a few things. There is 25 VAC at the gas valve except when it cycles off and on. I get a constant 25VAC at connection TH where the safeties connect. There is also a wire connected at terminal RS but the wiring diagram on the furnace doesn't show what this is. I've also been told that my unit probably doesn't have a flame rod so at this point I'm a little lost. It seems to me that there would be a flame rod and it just isn't sensing the flame. I did find a way to force it to run but I don't like doing it.

Any suggestions?
 
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Old 01-02-08, 04:37 PM
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Go to my last response in the thread called "Furnace stops working when..." and answer for us what all applies regarding things I mention in that post.
 
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Old 01-02-08, 06:53 PM
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Originally Posted by ecman51` View Post
Go to my last response in the thread called "Furnace stops working when..." and answer for us what all applies regarding things I mention in that post.


Hello ecaman,


I'm getting a little tired of leading everyone by the hand as well. I suppose that goes with the skill level of the furnace repairman we are advising, though.
 
  #13  
Old 01-03-08, 04:35 AM
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Originally Posted by SeattlePioneer View Post
Hello ecaman,


I'm getting a little tired of leading everyone by the hand as well. I suppose that goes with the skill level of the furnace repairman we are advising, though.
We that come to this site for help are very happy that there are people like you two that take the time to help us by sharing your knowledge. Please keep in mind though that some of us aren't HVAC technicians at all and may need to be led by the hand a little. Personally, I only know enough to log into the DDC system at the military installation I work at and make the system work until the real repairmen come in at 7:00 am. (I'm on call) Thanks to you and my HVAC co-workers, I will probably learn quite a bit about my furnace and may never need to come back to this site for help. I personally want to thank you for the help that you give. I'm sure that there is a family out there who is staying warm tonight because of it.
 
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