HVAC questions.

[skinny_tom (aka boney)]

It seems that there are some experts here that haven't minded anwering some questions in the past. If I may bend your keyboard for a few minutes I would really appreciate the help.

 

My furnace has been doing a little thing lately. It's a side mount in the attic. I can't find a MFG name but luckily there was a booklet nailed to the rafter nearby. The model number is GB3AAV.

 

Here's what it's doing:

 

For a while it runs fine. Then, for a while, it acts like it's going to start but doesn't... I hear a fan (inducer fan?) the ignition of the gas, but when it's time for the blower motor to start, the gas goes off and the blower motor runs for a bit then shuts down. Immediately, it starts it's process again, and goes through it's normal heating cycle fine.

 

Now, for the second time in as many months, it "false starts" several times and shuts off, leaving us with no heat. The little LED light indicates that the ignition lockout has tripped, presumably after we've had the Ignition Proving Failure 3 times. (I'm getting this from the sticker on the front of the unit and the copy of it in my handy-nailed-to-the-rafter manual.) If I unplug the unit, wait a minute and plug it back in, the codes are lost and it goes back to functioning normally. For a while.

 

I figure I can replace parts, as I'm failry mechanically inclined, but troubleshooting the possible electronic components in not my forte. Here's the questions:

 

Is this most likely a flame sensor problem? Should I replace it or "clean it with sandpaper?" Is it possible that it's loosing it's ground and that's what's causing it? I'm not sure I can adequately measure the electrical current to it (.5 microamps- hey, isn't that a 5 milliamps?) All the other "troubleshooting" tips for these error numbers appear to be handled. Am I headed in the right direction? Anything I'm missing? Anything else I should look out for?

 

Thanks in advance for any help you can provide.

[Fugu]

I aren't expert but my money is on thermocouple.

 

That's the "flame sensor". It operates by the expansion of dissimilar metals at varying rates when heated generating a current*.

 

Also, by that description you might be able to see why they are prone to failure.

 

Look for a little metal probe in there, right where the burners make flame. Remove it and take it to the local furnace shop and get a replacement. In my experience there aren't too many different designs for these- both times I've needed one (one furnace, one gas fireplace) it's been a matter of calling, getting told maybe we'll have one, taking it to the shop and them saying "oh, that's just like this one" and handing me one for a modest cost.

 

In short, the failing thermocouple isn't telling the control board that the flame's lit and it's shutting down as a safety to keep from gassing you. The one in my fireplace was intermittent for a while before quitting altogether.

 

 

 

 

 

 

*gas fireplaces often have a thermopile, too, which is similar but thicker, and sits in the pilot flame- this actually provides the voltage for the control board to operate in my fireplace- there's no power supply to it but the miniscule current from the thermopile powering the circuit reading the status of the thermopile in the big flame. Cool, eh?

[Aluminum_Butt]

Hope you get your problem solved here, but have you checked out this place: Linky

[T__]

I agree with Fugu on it probably being the flame sensor.. They usually don’t outright go bad but just get soot covered or oxidized then won’t hold the gas valve on long enough..

 

If you can find the flame sensor (might look like thick nail) & it should be in the flame path somewhere) just take a piece of Scotch pad & clean it.. If you can’t find the flame sensor it might be part of the ignitor (the thing that gets red hot just before the flame lights).. Those are difficult to diagnose if they aren’t completely burnt out as sometimes they will get hot enough for ignition but have incorrect resistance for correct gas valve feed back (you will need to find the correct hot resistance then measure that with an ohmmeter) ..

 

In any case start with the flame sensor as those are usually to blame for the symptoms you are seeing..

 

Twisty

[snod]

Sounds like the thermocouple. Most Home Depot/Lowes type stores will have them in stock for about $15. Normally not hard to swap. Removing and replacing the thermocouple does not effect any parts that could leak gas, so it is a low risk for a DIY repair. I keep a spare thermocouple laying on top of the hot water heater. Most furnaces and hot water heaters use the same part. My experience has been that cleaning them is to no avail. Some times adjusting the "nub" end more into the pilot light flame is all it takes.

[skinny_tom (aka boney)]

Hope you get your problem solved here, but have you checked out this place: Linky

 

Haha! There's a forum for everything!

 

Thanks for the feedback. I'll get up there this afternoon and see if I can get it swapped out.

[T__]

Hope you get your problem solved here, but have you checked out this place: Linky

 

Haha! There's a forum for everything!

 

Thanks for the feedback. I'll get up there this afternoon and see if I can get it swapped out.

 

I believe Carrier calls their flame sensor a “Flame Proving Electrode”.. See if you can find that is the booklet you found.. If so, start with that by cleaning it (should be close to the hot surface ignitor) .. The flame sensors on the electronic ignition furnaces are not a standard thermocouople like on a pilot lighted system..

 

I’m not sure on your furnace but if it is like on the similar EI furnaces I have worked on you only get so many ignition tries before the thing shuts down & has to be manually reset.. Look in your booklet on how to re-set the darn thing__ some have a button,, & some require the power to be switched off then back on,, & some might require the thermostat to be lowered below the turn on threshold.. (should be in your book)..

 

Twisty

[tallman]

Mine is doing the same thing.

Supposedly, thermocouple is OK and a pressure switch was bad, according to fault code.

Replaced worked fine, then same problem.

Supposedly the exhaust vent diameter was OK, but it was enlarged to max as the problem seemed to coincide w/high humidity/rain, worked OK, then same symptoms, again.

Turn it off, then on, will start and fire up, mostly.

Other times, not so much.

Leave it on and same symptoms until eventually, it fires up.

Or not.

Still look at the thermocoupler?

[Fugu]

Still look at the thermocoupler?

 

I think thermocouple is high on the list of things to check with any furnace problem, and I think it's there for a good reason.

 

Worst case, you put in a new one for $20, and still have the problem. Now you can consider that "aggressive maintenance" and keep the old one for a spare.

 

I've never had luck cleaning a thermocouple- but an interesting issue has been brought up- new furnaces with flame sensors that are not thermocouples and these sound like a whole different thing. I don't know anything about such new fangled hi techness. As I said initially, I are not expert. I've just replaced thermocouples in about every gas fired heating widget I've ever owned, and that's all that has ever gone wrong with any of them.

 

If you run a google image search for thermocouple you can compare what you've got to that and see if you have a thermocouple or a new whiz bang flame sensor.

[snod]

but an interesting issue has been brought up- new furnaces with flame sensors that are not thermocouples and these sound like a whole different thing..... I've just replaced thermocouples in about every gas fired heating widget I've ever owned, and that's all that has ever gone wrong with any of them./quote]

 

+1

 

I don't know anything about the self lighting set ups. If the furnace is new enough to have a spark plug of some type, I don't know if will still have what I think of as a thermocouple?

[RFW]

I aren't expert but my money is on thermocouple.

Yes, except there is no thermocouple. Furances mostly use flame ionization sensors (I know about these things because I designed one for a German furnace company I worked with).

 

- If the ionization sensor probe (just a rod of metal) has carbon buildup, then this can cause problems.

 

- The connector at the end of the sensor rod can cause problems because the current that flows there is less than 100 microamps. Oxydation can interfer with this current. Unplug and polish the connector with scotchbrite.

 

- Initially when the thermostat calls for heat, a pilot light ignites and its flame is supposed to envelop the last 3/4" to 1" of the sensor rod. Once the sensor detects ionization current, the main gas is turned on and is ignited by the pilot light. If the pilot flame is too weak or directed away from the sensor rod, the flame ionization current will be insufficient to trigger the sensor circuit and the main flame will not start.

 

In the case of the above, increase the pilot flame and be sure that it is enveloping the end 3/4" or so of the flame sensor rod. Redirect it if necessary.

[T__]

Still look at the thermocoupler?

 

I think thermocouple is high on the list of things to check with any furnace problem, and I think it's there for a good reason.

 

Worst case, you put in a new one for $20, and still have the problem. Now you can consider that "aggressive maintenance" and keep the old one for a spare.

 

I've never had luck cleaning a thermocouple- but an interesting issue has been brought up- new furnaces with flame sensors that are not thermocouples and these sound like a whole different thing. I don't know anything about such new fangled hi techness. As I said initially, I are not expert. I've just replaced thermocouples in about every gas fired heating widget I've ever owned, and that's all that has ever gone wrong with any of them.

 

If you run a google image search for thermocouple you can compare what you've got to that and see if you have a thermocouple or a new whiz bang flame sensor.

 

 

Fugu, the Carrier GB3AAV uses one of those new whiz bang flame sensors.. It also uses a hot surface ignitor to light it’s flame (no pilot light).. No gas/fluid filled thermocouple either like on the old manual gas valve furnaces..

 

Twisty

[Fugu]

Fugu, the Carrier GB3AAV uses one of those new whiz bang flame sensors..

 

About which it should be stressed I know nothing.

 

I do know that some relatively new furnaces still use thermocouples, though, and I had a furnace with an ignitor (no pilot) and a thermocouple.

 

No gas/fluid filled thermocouple either like on the old manual gas valve furnaces..

 

I've never encountered a gas or fluid filled thermocouple- just dissimilar metals using the Seebeck effect.

 

I did search up some information on mercury flame sensors that are filled with mercury and use its vaporization to signal the furnace "brain" that the gas has been ignited.

 

Here's some interesting reading that includes mention of gas sensors in valve assemblies (ATTN TALLMAN). By the way I know nothing of the source, just googled it.

[WURTY]

replace the spark module.

The gray colored box that has all the wires connected to it.

 

 

If by chance you don't have that spark box and yours is a circuit board type then get the checkbook out and call the a/c man

[skinny_tom (aka boney)]

If by chance you don't have that spark box and yours is a circuit board type then get the checkbook out and call the a/c man

 

Funny you should say that. Today I experienced the super secret society of HVAC servicemen. It's an inpenatrable wall of coverall clad people behind which all HVAC knowledge and parts are kept. They all looked at the flame sensor I had removed from my furnace and carried into their shop and said, "we'd be happy to come out and look at your system." Meanwhile I'm waving the Flame Sensor around pointing to it, suggesting that it would be far easier for everyone involved to just sell me one. "A service call is $89 and includes a full system inspection."

 

I finally suggested that if they don't sell me one, I'll be forced to come home, get on the interweb and buy one from someone else- making the point of saying "giving the money to someone else." They were not interested.

 

For the record, Home Depot, Freidman Brothers, and all the other hardware stores visited today have Thermocouplers, but they do not have Flame Sensors. Before today, I didn't know there was such a thing or what the difference was. It's been educational to say the least. I've also discovered many other things about my furnace.

 

Then I found Shorty. I've aready sent an email. In the mean time, I polished up the old flame sensor nice and shiny, and reinstalled it. We'll keep our fingers crossed, but so far...

[Deadboy]

My experience repairing my system was similar....no local shop wanted to sell me parts...so I found these guys : http://waterheater.stores.yahoo.net/index.html and was very satisfied....answered my questions, sold me parts and over all treated me like an adult, not some interloper into a secret world...looks like they have a good variety of flame sensors, most for around $10.00.

 

Good luck!!

[ESokoloff]

I polished up the old flame sensor nice and shiny, and reinstalled it. We'll keep our fingers crossed, but so far...

 

Not sure but you may have a flame rectification type sensor.

It uses the flame of the pilot or main burner to complete an ac (alternating current) to (or from) ground & knock off half the sine-wave thus converting it to DC that the electronics senses & thus proves pilot or ignition of main burner.

A good ground is also important here so make sure that the other half of the circuit is clean as well as the flame rod.

[ESokoloff]

Something else to look for would be the induced draft motor/fan & insure the vent pipe/cap are functioning properly.

Though I'm not sure if that would cause ignition lockout.

[daveco]

there are, of course, many senarios. you may have a problem with the circuit board that controls both the ignition and or fan sequencing. if the fan comes on after the flame goes out the sequencing of the fan relay may be out of kilter. this would cause the high temp limit to trip and this would signal the board to turn off the flames and turn on the fan to cool it down and the process would happen all over again after the limit resets. or the board could, because of a failure, turn off the flame when the blower kicks in or if the flame sensor is indeed coated with a film of carbon the burners could shut down just as the blower kicks in. both could cause the unit reset and start over and over. also if the pressure switch is just on the verge of cutting in or out it could cause a continual reset of the process. i rarely see a flame sensor bad. my money is on the board, but it can be tricky to diagnose if it's intermitent, even to a tech. hard lock outs require unplugging and plugging the unit back in, soft lock outs require only to turn the heat/cool switch to off and back to heat, to reset. which type of lockout depends on the frequency and or severity of the said problem. i'd have to say the main reasons people are reluctant to sell parts to those who are not in the business are, of course huge liability issues (didn't i see a post about lawyers?) and the dangers of working on a gas appliance. hope you get it fixed. Dave

[Knifemaker]

Take a look at the gas valve and iginition modual and get the part numbers off these parts. The manufacture of these part are usually Robertshaw, Honeywell, or White Rogers who will OEM these parts to Carrier. With the part numbers off these controls I probably can get you a specification/troubleshooting sheet emailed to you. You more that likely have a thermocouple or hot surface ignitor problem. Another problem if you have a standing pilot (one thats on all the time) you may be experiencing a draft problem causing the flame of the pilot to wonder off the tip of the thermocouple causing the main gas vave to lockout because the gas valve is loosing the 30 milivolt signal from the thermocouple. Becareful no one has addressed the fact you may have a thermopile system which is 250-750 milivolt system if you have this the cost of the themopile generator is between 25-30 bucks. I do not know if you are using a natural or LP gas system but on LP low gas pressure can cause the regulator to "freeze" up and not allow gas to the furnas.

 

It is VERY doubtful you have a retification system in a domestic heating control system these are usually a more dedicated system like a boiler.

[ESokoloff]

 

It is VERY doubtful you have a retification system in a domestic heating control system these are usually a more dedicated system like a boiler.

 

Sorry this is not true.

In California (location of said unit) standing pilots & thermocouples went out 25+ years ago. Long before control modules with LED indicators.

 

Tom, you may find some trouble shooting help here.

 

If I was not clear in my prior post let me restate that you make sure that the burner has a good/clean ground as well as the control module. Not a lot of electrons are used for this task so cleanliness is next to Godliness on the connections

 

 

Good luck with this.

If you stop posting,

 

 

 

well...... can I buy your KTM for cheep

[Fugu]

boney did you see the discussion on testing your flame sensor in the link I posted?

[Jerry Johnston]

Happenned to us once when the pvc pipe that runs outside froze up. The two pipes were two close to one another. When I extented one of them away from the other, no more problem.

[skinny_tom (aka boney)]

boney did you see the discussion on testing your flame sensor in the link I posted?

 

I've gone back and looked at all your posts carefully. Am I missing it?

 

(Nice and warm in the house this morning.)

[Fugu]

boney did you see the discussion on testing your flame sensor in the link I posted?

 

I've gone back and looked at all your posts carefully. Am I missing it?

 

I just looked, too - not sure what happened to it. I found it again, here's the text- the bit on testing those flame sensors (mercury type) is towards the end. Tallman, there's also mention in there of gas line pressure sensors buried in the valve assemblies that might be of some small help to you.

 

I am curious that you may have one of the lesser used White Rodgers brand "3000" or "900" series "Mercury Flame Sensors." They are used in conjunction with a number of their gas valves such as the "36C series and other valves. I have never seen them used with any other brand gas valve than their own, White Rodgers, so if you have another brand gas valve what is written below probably does not apply. They come in a direct plug-in type which plugs directly onto the gas valve and a spade connector type which has the push on flat blade "spade" connectors. The spade connector type are really scary because you can inadvertently plug the wires onto the wrong terminals (bad idea= potential BOOM!)

 

Often folk confuse these with "Thermocouples" because the pilot flame end looks exactly like a thermocouple, and there is the apparent "capillary tube" similar to a thermocouple which comes from the sensor end which senses the pilot flame. Also, because these are not as common as thermocouples, less informed supply house men who work the Parts Counters sometimes even think these are "Thermocouples" and call them "Thermocouples." I know Scott has worked on many of these and is intimately familiar with them. He just didn't pick-up on the possibility that you may have one of the lesser used Mercury Flame Sensor setups for whatever reason. Also, with the advent of Hot Surface Ignition systems with their accompanying solid state control modules, I am not aware of any manufacturer still using MFS systems in current production furnaces and rooftop equipment. They are, however readily available as are the compatible gas valves which they mate to and which I will describe below.

 

Here's the BIG difference between a "Thermocouple" and a "Mercury Flame Sensor:" The part of the "Thermocouple" which actually senses the flame is really technically called a "thermocouple junction". It is two dissimilar metal wires which are fused (or welded) together at the end where they are (enclosed and protected by an outer metal case) immersed in the hottest part of the pilot flame. When heated, the thermocouple junction actually produces D.C. voltage from the energy of the flame similar to the way a solar panel cell produces electricity from the energy of light. The output of a thermocouple is a very minute voltage (30 millivolts) but it is sufficient to operate a very small electro-magnet which is the "holding" part of the "Push to Light" mechanism of most standing pilot gas valves found on older furnaces and water heaters. What appears to be a capillary tube which goes from the thermocouple end (which is immersed in the pilot flame) to the gas valve is actually a coaxial wire with the copper exterior being one conductor and the inner insulated wire being the other. Most Thermocouples connect to the gas valve with a hex nut and if you look closely you will see that the "button" type end has a piece of insulator disk to separate it from the flared copper exterior. When the nut is tightened the electrical connection is made firm to the internal circuitry of the gas valve.

 

The "Mercury Flame Sensor" on the other hand is an entirely different animal all together. The only thing that is the same is the appearance of the sensor end which senses the flame and what appears to be a capillary tube which goes from there to the gas valve. I emphasize "appearance." Unlike the tube of the thermocouple, what appears to be a capillary tube on the MFS is actually just that, a capillary tube which carries the pressure from the sensor end to a set of "Bellows" at the gas valve. The bellows or diaphragm actuates a single pole double throw switch mechanism which first energizes the ignitor, be it spark, hot surface, or glow coil, via the normally closed contact, and then, upon proving the pilot to have successfully lit, energizes the gas valve to send gas to the main burners via the normally open contact (and other related circuitry). There are some MFS's which have spade connectors with field or factory connected wiring, but with most, the bellows/switch assembly simply plugs onto the gas valve. If you took what you thought was a "thermocouple" off of the gas valve by using a 3/8" wrench to loosen a hex nut you do in fact probably have a thermocouple. If, however on the other hand you "unplugged" it from the valve and if it has three either round or flat prongs on it and a 3XXX- series or 9XX- series number on it, you have a MFS.

 

Here's how a MFS works. The sensor end, capillary tube and spdt switch assembly at the gas valve end are filled with mercury. When the flame sufficiently heats the sensor end, the mercury in the sensor end tube vaporizes and forces the remaining liquid mercury through the capillary tube to the bellows/switch assembly at the gas valve end of the capillary tube. The bellows move against a non-adjustable calculated spring tension and with sufficient pressure exerted "snap" a single pole double throw set of switch contacts from one position to the other thereby opening the normally closed contact and closing the normally open contact. This then de-activates the ignitor, be it spark, hot surface ignitor or glow coil, and energizes circuitry which allows the gas valve to open for the main burners. There are other critically important things which happen to allow the main burner gas valve to open, but for general explanation purposes, it's a simple switch-over from pilot lighting and proving to be on, to the main burner gas valve then opening for the main burners.

 

If this is what you have, the MFS's themselves are very reliable, and I would dare say that most of those which are replaced are probably not defective. They are even more reliable than the operating thermostat in your kitchen refrigerator. Have you had to ever replace one? Probably not. I've seen guys replace both the gas valve and the MFS simply because they did not know how to diagnose which was bad. If you have the MFS unplugged from the gas valve and have a propane torch or even a heavy duty cigarrett lighter handy and a continuity light or ohm meter and are familiar with how a spdt switch works, just find out which two of the three plug pins are "made" with the sensor "cold" and then put the end of the sensor in the flame as if it was actually in the pilot flame itself. CAUTION!! be sure to keep it vertical as it would be in the pilot flame- what you want to do is have the vaporized mercury vapor push down on the liquid, not have the vapor go up the capillary tube!!! When the end is heated up to the temperature the pilot flame would heat it you will lose continuity on the two pins you had it on when the end was cold, but should then "have" continuity between the third pin and one of the original two pins. Whichever of the two original pins gives continuity to the third one will be what is known as the "common" pin because it is "common" to both the normally closed and the normally open pin.

Having said all of that about the MFS, on this type of system the gas valves are usually the culprit. Many have an integral gas pressure switch which you cannot get to and which fail frequently. I've seen them where the epoxy or plastic which hold the tiny gas pressure switch in place gets loose and slightly moving the wires will allow it to temporarily "make" contact and function. They also have a "pic" coil which energizes for the pilot gas and electrically latches in and must stay latched in when the MFS unit switches over from "no pilot sensed" to the "pilot proven on position." The reason for the integral gas pressure switch is obvious, but it together with the pic coil protect the system from unexpected momentary power drops or gas pressure losses which could have the gas valve be "open" upon a momentary loss and resumption of either power or fuel. Were it not for the pilot gas pressure feature and the Pic coil feature raw un-ignited gas could roll into the combustion chamber and wherever else building up for the eventual "retry of the pilot when the MFS cooled down and switched back to the "no pilot sensed position." The result could be disastrous! When working on this kind of stuff it is really scary to think what could happen, so I must caution you to be sure you know fully what you are doing and if in doubt, just compare the cost of a service call to the cost of a house or worse lost lives. In our immediate area at around 3:00 AM a house blew up and killed two people apparently while they slept. When I say "blew up" I am not exaggerating. The house was literally flattened and the surrounding trees were littered with clothes, pieces of furniture and building materials hanging where they snagged in the tree limbs and branches. Scott and I rode over that way this evening and although we could not get really close because the area was ribboned off, it was a very ugly and sobering scene with even the two houses on either side substantially damaged from the force. The cause has not been determined, but it will most likely either be a natural gas build-up or something like a propane gas grill stored inside with the tank valve accidentally left on together with a ruptured or leaking rubber hose. Because we work in the industry and are acutely aware of the need for extreme care and caution when working on gas units, we will be following the incident to find out what was the cause of this tragic incident. Be careful, and do not construe anything written herein as advice to do any of this yourself. I am writing this only as courtesy advice for your qualified technician to only remotely consider in his diagnosis of your problem, and because I have not seen or even heard first hand of your problem, can in no way have any part in what or how it gets fixed.

[ESokoloff]

.....the MFS's themselves are very reliable, and I would dare say that most of those which are replaced are probably not defective.

 

Mercury switches for flame sensing are kinda old school & I don't think they have been used in heaters for sometime now (decades).

 

As long as the capillary tube remained intact, the only failures of this type of switch that I ever came across was dirty switch contacts.

[T__]

(Nice and warm in the house this morning.)

 

So what all did you do? Just clean & re-install the original flame sensor?

 

Twisty

[Fugu]

Mercury switches for flame sensing are kinda old school & I don't think they have been used in heaters for sometime now (decades).

 

I think the difference is your location in CA. The last two furnaces I've owned have both been less than 10 years old, and both have ignitors and thermocouples.

 

Maybe Tallman can use the information since he's in FL.

[ESokoloff]

What type of fuel?

Natural gas, propane or oil?

 

In Tim's case, if he has a natural gas unit, perhaps this pressure switch is sensing forced or induced draft?

 

Need more info but it kinda "smells" that way.

[tallman]

What type of fuel?

Natural gas, propane or oil?

 

In Tim's case, if he has a natural gas unit, perhaps this pressure switch is sensing forced or induced draft?

 

Need more info but it kinda "smells" that way.

 

That's exactly what the fault code read.

Replaced pressure switch.

Then, not so OK.

Still talking it over, unit is "new" only 1/1/2 years old.

May need a new brain, well, maybe me too, but the unit might.

Apparently they aren't cheap. Hooray for warranty.

We'll see.

[ESokoloff]

So it sounds like this pressure switch perhaps was not bad but rather for some as yet understood reason the combustion chamber is not adequately breathing?

 

The bad news is that the service tech(s) do not have a handle on what's going on but the good news is your not out of pocket?