Furnace diagnosis and repair

[peteb]

Here is a link to the furnace manual. Page 37 is a troubleshooting flow chart. This will help. If you are inclined to work on your furnace, Please educate yourself for safety sake. If you need further help please contact me or call a technician. getting replacement parts and not being a license contractor can be difficult. I can help with that as well. This is what I have done for 40 years.

thanks Leonard, and thanks everyone else!

the manual is with the furnace. The same one as you linked.
the trouble shooting chart inside the cover, that I was looking at is the same chart you referred to on page 37.

I almost certainly will bring in the furnace tech. I just had to do the preliminary inspection in case there was something obvious and easy to fix.

I may try bypassing the pressure switch. The manual isn’t very good at identifying what is what so I only think I may know what and where the pressure switch is.

I tried measuring the capacitance of the run capacitor. I don’t think I was successful. I see two main wires going to each end and two other wires going to each side. Again, I am not 100% sure that I found the run capacitor.


it sounds like the best plan is to bring in the furnace tech as it sounds like standard practice to have the furnace inspected, by a pro.

 

[peteb]

Is this occurring when the furnace panels are removed? On our unit there is an activation switch that will not allow
start up if the panels are not in place.

I got around that by depressing the switch.

it is neat that the upper panel has the little window to view the status lights and the lower panel has the little window to view the burners. At least that is what I assume the lower window is for.

I am impressed by the manual, troubleshooting chart and the fact that the most frequently replaceable parts are accessible.

 

[peteb]

For starters: get the model number and serial number. With that info I can establish the sequence of operation.

this would be useful to help me understand how the furnace starts up and works.

the way I think it works.

fan
pressure switch detects draft
ignition
flame
combustion
heat

The pressure switch must prove that there is a draft before any ignition occurs. The pressure switch is a safety switch, but prone to fail safe. If that switch does not prove (or close) then nothing else will happen. You can jump around the pressure switch to test but do not leave the jumpers on after the test.

if the main fan won’t run, the pressure switch will not detect a draft and will stay open. Is this correct?

if that is correct and the fan is not running then there would be no sense in bypassing the pressure switch.

there is a switch on the furnace that turns on the fan by itself. The old furnace had this switch labeled ‘summer fan’.

the thermostat has two settings for the fan, on and auto.

I just looked it up. This should be set to auto, which will run the fan when the furnace is running. If the setting is ‘on’ then the fan will run continually.

 

[Leonardocoate]

this would be useful to help me understand how the furnace starts up and works.

the way I think it works.

fan
pressure switch detects draft
ignition
flame
combustion
heat



if the main fan won’t run, the pressure switch will not detect a draft and will stay open. Is this correct?

if that is correct and the fan is not running then there would be no sense in bypassing the pressure switch.

there is a switch on the furnace that turns on the fan by itself. The old furnace had this switch labeled ‘summer fan’.

the thermostat has two settings for the fan, on and auto.

I just looked it up. This should be set to auto, which will run the fan when the furnace is running. If the setting is ‘on’ then the fan will run continually.

There are two fans on that furnace. The IDM (induced draft fan) is the fan that comes on first. That fan provides help for the combustion air to draft out the chimney. The pressure switch senses the pressure in the combustion chamber. The fan switch on the thermostat is for the main blower (IBM). If you switch that to "on" your IBM should come on, that will tell if your IBM is working. In the heat cycle the IBM comes on after the piolet is made. Some people like to run their fan continuously for air circulation and filtration. The switch on the side of the furnace is just an on/off switch for the furnace. That should be on. If you look at the flow chart and answer yes to all, you will see the "sequence of operation". Where ever that sequence is interrupted is where the problem is.

 

[lowatter]

The inducer draft fan motor comes on 1st after the thermostat calls for heat. It's the small motor located on the front of the furnace above the exchanger. It has a pressure diaphragm switch that would interrupt the circuit if there's no draft pressure. If this motor is bad they are easy to replace and usually come as one unit. You can test the motor and pressure switch by removing the small rubber air hose going to the switch and lightly suck on the end with your thermostat in heat mode and calling for heat. You should hear it click and if the switch is good and the motor is bad, the furnace will ignite and after a few seconds the blower fan motor will come on and Bob's your uncle.
There are other limit switches in the circuit such as a flame roll-out safety switch, hi temp safety switch etc that interrupt the circuit as well. These are easy to test and their locations are on the wiring schematic on the inside panel.
What error code are you getting and what does the chart say it is?

 

[Old Deaf Roadie]

Calling in a service tech and paying him is a much smaller price to pay than dying of CO poisoning, which is the alternative if you make a mistake.

 

[Leonardocoate]

thanks Leonard, and thanks everyone else!

the manual is with the furnace. The same one as you linked.
the trouble shooting chart inside the cover, that I was looking at is the same chart you referred to on page 37.

I almost certainly will bring in the furnace tech. I just had to do the preliminary inspection in case there was something obvious and easy to fix.

I may try bypassing the pressure switch. The manual isn’t very good at identifying what is what so I only think I may know what and where the pressure switch is.

I tried measuring the capacitance of the run capacitor. I don’t think I was successful. I see two main wires going to each end and two other wires going to each side. Again, I am not 100% sure that I found the run capacitor.


it sounds like the best plan is to bring in the furnace tech as it sounds like standard practice to have the furnace inspected, by a pro.

In my experience the pressure switch is the most likely culprit. It is made of a rubber diaphragm and a bi-metal switch that break easy. Depending on where your location is I might be able to locate an honorable company for you. It would probably be best to go back the installing contractor. Check them out first.

 

[peteb]

In my experience the pressure switch is the most likely culprit. It is made of a rubber diaphragm and a bi-metal switch that break easy. Depending on where your location is I might be able to locate an honorable company for you. It would probably be best to go back the installing contractor. Check them out first.

thanks

the installer and service r is Airductors who have been really good. I plan to contact them soon.

this furnace and the furnace it replaced had a power switch and a fan switch.

i made some progress today.

I lowered the thermostat below the house temp.

I turned the gas on and then the furnace.

the status light was a constant Green . Good.

I tried to turn the fan on from the thermostat and nothing happened. I think it should be in the auto position but I tried it in both positions while trying the next step.

I turned the fan switch on on the furnace. That fan works.
I am confused about the two fans. I believe this is down draft. This is what I consider the main fan. It forces air throughout the house. It is in the upper half while the burners are in the lower half.

then I raised the thermostat above the house temp. The fan shut off, there is the hum and then the 2 blink status light indicating open pressure sensor.

I suppose it may be worth while now to try bypassing the pressure sensor.

I should study the names and functions Leonard describes of the two fans. Then armed with that knowledge follow the troubleshooting chart.

thanks for the link. i need to use a magnifying glass to read the printed version of the trouble shooting chart which makes it really hard to follow.

is it the pressure switch?

or do I have the second fan to be concerned with?

thank you

 

[Leonardocoate]

thanks

the installer and service r is Airductors who have been really good. I plan to contact them soon.

this furnace and the furnace it replaced had a power switch and a fan switch.

i made some progress today.

I lowered the thermostat below the house temp.

I turned the gas on and then the furnace.

the status light was a constant Green . Good.

I tried to turn the fan on from the thermostat and nothing happened. I think it should be in the auto position but I tried it in both positions while trying the next step.

I turned the fan switch on on the furnace. That fan works.
I am confused about the two fans. I believe this is down draft. This is what I consider the main fan. It forces air throughout the house. It is in the upper half while the burners are in the lower half.

then I raised the thermostat above the house temp. The fan shut off, there is the hum and then the 2 blink status light indicating open pressure sensor.

I suppose it may be worth while now to try bypassing the pressure sensor.

I should study the names and functions Leonard describes of the two fans. Then armed with that knowledge follow the troubleshooting chart.

thanks for the link. i need to use a magnifying glass to read the printed version of the trouble shooting chart which makes it really hard to follow.

is it the pressure switch?

or do I have the second fan to be concerned with?

thank you

The IBM (indoor blower) blows air through the entire house. The IDM (Induced draft fan) assist air into the flue for draft. On a call for heat the IBM will shut off (if running continually) until all other sequences are satisfied. It's all about safety. It sounds like you are getting well educated on your furnace. That is a very good thing to be knowledgeable about. Glad to help

 

[D'tar]

Bottom line, if inducer fan doesnt run you will not acheive ignition. If inducer fan runs and your low psi switch(should be two for redundancy) is faulty you will not acheive ignition.

 

[peteb]

The IBM (indoor blower) blows air through the entire house. The IDM (Induced draft fan) assist air into the flue for draft. On a call for heat the IBM will shut off (if running continually) until all other sequences are satisfied. It's all about safety. It sounds like you are getting well educated on your furnace. That is a very good thing to be knowledgeable about. Glad to help

thanks

which fan activates the pressure sensor?

 

[telleutelleme]

When I was a hippy, I lived in the momentarily famous Alice's Restaurant church. Early winter, smoke started pouring out of the forced air vents. I shut down the furnace, called a furnace guy, who said, "That furnace is shot; it's dangerous. Don't use it." So after he left, I went down in the basement and saw that he had removed the nozzle, or whatever you call it., I screwed it back in, fired 'er up, went back upstairs and announced that I had "fixed the furnace." Twenty minutes later, I had to call the Great Barrington fire department. The lesson: don't call a hippy if you want yer furnace fixed.

That is a cool story with monumental provenance. If you have the 8x10 glossy I would love to get a copy.

 

[peteb]

Bottom line, if inducer fan doesnt run you will not acheive ignition. If inducer fan runs and your low psi switch(should be two for redundancy) is faulty you will not acheive ignition.

thanks

So it could be the inducer fan not operating and not activating the pressure switch?

or a bad pressure switch?

 

[Leonardocoate]

thanks

which fan activates the pressure sensor?

IDM (induced Draft fan) Many furnaces have a 10 year warranty on them...parts can get expensive

 

[String Tree]

Ok, so now I am a furnace repairman.


who has diagnosed and or repaired a furnace?


this is new to me. In the past, before I even knew there was a problem, my wife had it handled.

now is not the past.


The furnace is a Rheem’s it has power and the thermostat is operating the flame light won’t light up. It is auto lighting.

i have a green blinking status light with two blinks.

there is a really cool trouble shooting chart on the inside of the removable panel with descriptions of the different blinkings of the status light.

the chart refers to acronyms unfamiliar to me that I need to look up.


I read the motor capacitor is a common failure.


any ideas?


thanks

If it won't light, it could be the Spark Electrode.
The Start Capacitor is for the Fan Motor and nothing else.
If you have a Fan Only Switch and the Fan Runs, it isn't the Capacitor.

 

[peteb]

If it won't light, it could be the Spark Electrode.
The Start Capacitor is for the Fan Motor and nothing else.
If you have a Fan Only Switch and the Fan Runs, it isn't the Capacitor.

It is the induced draft motor that won’t run prior to ignition.

I now know how the sequence goes.

follow the yes branches on the trouble shooting chart from start to finish.

the induced draft fan is the first step.

I will call air inductors tomorrow.


thanks all

 

[Addnine]

That is a cool story with monumental provenance. If you have the 8x10 glossy I would love to get a copy.

Everything from those days is gone, except for my beat up LG-0 and a few memories.

 

[peteb]

I called air ductors this week. Their message said they were very busy now.

I left a couple messages and have not heard back.

the weather warmed up A little. It is not bad.

 

[ale.istotle]

I called air ductors this week. Their message said they were very busy now.

I left a couple messages and have not heard back.

the weather warmed up A little. It is not bad.

I've got an old grainger furnace I think was made by rheem. The draft inducer fan is visible if you take the cover off. Mine stuck once years ago and I was able to turn it manually and the end of the shaft was visible so I lubricated it and never had another problem.

You describe the fan sounds louder than usual and doesn't turn, so maybe you have the same problem. Good luck.

 

[peteb]

You describe the fan sounds louder than usual and doesn't turn, so maybe you have the same problem. Good luck.

the big fan works. And when I was trying to get IT to work I hand spun it.

I see the draft inducer fan. It is completely enclosed. I can’t get a hand or a stick on it. I banged it.