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Old 10-28-2006, 09:38 AM   #1
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Small boom from lennox furnace


My Lennox furnace stats up normally then while the furnace is operating normally the burner will go out while the thermostat is still calling for heat. The blower and ventor exhaust fan is still running, then within a few seconds the igniter will start clicking the burner will reignite and a percussion sound (small boom?). The burner relights and will run normally. The service guy agrees this is not normal and wants to start replacing the most expensive part to rule out the problem. Last year I replace the flame sensor b/c the thermostat was calling for heat light and then go out while the thermostat still is calling for heat. I replace the flame sensor b/c the repair guy wants to charge me to clean it each time. Any relationship with the prob now and the past probs? Any help out there would be appreciated.
Mike

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Old 10-28-2006, 11:34 AM   #2
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Small boom from lennox furnace


I don't have an answer but I do have a similar problem.

My Utica boiler will get the call for heat and begin to go through the ignition cycle. The igniter will start clicking and then comes the gas, the system ignites. It runs for about 1 to 2 seconds and then stops. There is a pause and then the igniter goes again, the gas comes and it lights.

On the second relight it continues to run and has no problems.

We have had a boom a couple of times on the second relight that I attribute to this. The short cycling is wasting energy but my service guy could not determine any problem and as with you could only recommend that we start replacing expensive parts until it stops.

So, no answer just letting you know you are not alone.

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Old 10-30-2006, 09:34 AM   #3
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Small boom from lennox furnace


Hi mhenry48

Before I get too involved into your question I will lay out some possibilitys. There are several things that cause hard ignition (boom), the most common one is dirty burners. The second most cause is low gas pressure. The dropping in and out could be caused by one or both of them. The other possible cause could be a sooted heat exchanger, however this does not happen very often. The flame sensor that most people are so quick to change or blame for the problem is the most misunderstood part of the newer furnaces. I will explain it to you, the flame sensor is nothing more than a stainless steel ground rod. When you superheat air it becomes ionized, the ions creat a ground path between the burner and the flame rod. When the ignition module sees enough resistance it will allow the gas valve to stay open. But if it does not read the proper resistance it shuts down and tries again. It will do this on most ignition systems a total of three time before it goes into lockout. You could try pulling and cleaning the flame sensor, MAKE SURE TO TURN POWER OFF TO FURNACE, the flame rod has voltage on it at all times. Use a wire brush to clean DO NOT USE SANDCLOTH OF ANY TYPE, this leaves a silica residue when heated turns to glass. But from what your have in your letter you may have further problems to adress. If you can give us a little more detail we can pinpont the problem.

Good luck
Rusty
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Old 11-04-2006, 11:12 AM   #4
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Small boom from lennox furnace


Thanks for replying Rusty,
I think you are correct the flame sensor is fine now. I will try to detail the problem that is going on now.
Once the furnace starts everything lights and ignites appropriately. Then anytime after 2-5 minutes more or less, the burner will go out thermostat still calling for heat, ventor fan and air handler blower continues to run. In a few seconds the flame sensor will start clicking, as it does at normal start up to light the burner. In some cases this is not necessary the burners already have fire. Sometimes the burners have gone out and need to light, and then it may take 1-2 attempts to relight. There may be a boom and sometimes not, the furnace will run as normal, unless it goes through the same sequence of events later. It could do this 2-3 times during the cycle. Or, the furnace may shut down as normal when the thermostat is no longer calling for heat.
You mentioned possible dirty burners or low gas pressure.
1st the dirty burners there are 5 venturi or burners with small orifices. How do you recommend they be cleaned?
2nd How does a person determine low gas pressure?

Sorry I did not get back sooner out of town this week. I look forward to hearing from you again on this issue. Thanks again for the help.
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Old 11-04-2006, 01:22 PM   #5
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Small boom from lennox furnace


Hi agaim mhenry48

Inside your furnace will be a white sticker, It has a model # starting with G. It sounds like your furnace is a G11, but I can't tell intirely. The problem it sounds like you are having is the ignition control is malfunctioning. The one your unit should have on it if it is a G11 is a Robertshaw. This will be a gray box that is located next to the inducer assy. If this is so replace it with a Honeywell S8610U1003 and your problem is solved. Keep in mind however that the two controls do not look the same and wire slightly different. There is not enough difference however that you cannot do it. And its ok you didn't get right back, we are here to try and help you. If you can let me know your unit, I can pinpoint the problem for you.

Good luck
Rusty
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Old 11-04-2006, 03:50 PM   #6
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Small boom from lennox furnace


Thanks again Rusty,
I could not find a G11. The model # on the furnace is 80MGF3-75a-1. If I understand corectly the ignition control may be intergrated or is the PC board....? Looking forward to your response.
I hope I do not sound cheap or stupid. I just enjoy working on these things myself or with a little help fixing them myself.
Mike
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Old 11-04-2006, 10:36 PM   #7
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Small boom from lennox furnace


Hi mhenry48

Now were getting somewhere, Ok it said in your earlier response flame sensor is ok. Did you pull it out and clean it with a wire brush? If you answered yes to this question, pull yourself up a chair and make that faitful phone call to your local Lennox dealer, the reason I said pull up a chair is I don't want you damaging your head before attempting these repairs. The particular furnace you have if it is old enough has a direct spark ignition system, but if you are lucky it is new enough to have a sure light system. It is imperative that you make sure the flame sensor is removed and cleaned. These boards and kits are very expensive, usually starting around $200.00 and going as high as $350.00, now you know the need for the chair. Let me know if you need any further help. And like I told you earlier this is why I joined is to help those who have the knowledge, thats why its called DIY. So no need to appologise, there is no cheap in my industry, just thrifty.

Good luck
Rusty
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Old 11-05-2006, 08:48 AM   #8
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Small boom from lennox furnace


Thanks again for the info. Fortunatuly, I was in my chair when I read this, but was floored when the lennox service KID cleaned the sensor 3 years ago and said if this did not work it would require changing parts to find the problem. He could not be sure which part was causing the problem. it could cost me $200-400 per trip and part. He suggested it would be eaiser that we may want to replace the furnace for new. I replaced the sensor for a new one last year hoping that would solve the problem. It did correct part of the problem that did not cause the furnace to go into watch dog mode. I do not have a problem with calling the service man back. But I do not want him to start changing parts to solve this problem. As I metioned previsouly, I enjoy doing this.
Is there anyway we can continue? What you mentioned before it may be the ignition control. This seems to be the most logical is this control done from the PC board? I chhecked around the internet they are around $200
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Old 11-05-2006, 04:44 PM   #9
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Small boom from lennox furnace


Hi mhenry48

I first wanted to let you know, unless there is a reason to changeout furnace due to heat exchanger failure or because you just want the upgrade. Leave ther furnace where its at. This was a pretty descent little furnace and I believe has at least a 25 year heat exchanger warranty. Now back to the problem, I have went a little more indepth of your previous responses. I need to know if you have a voltmeter that can read A/C volts down to 25 volts. If you do, this is going to make helping pinpoint the problem a ton easier. If you can let me know I will stick out this problem with you until its repaired. If I do not respond right away, just hold on I will as soon as I can. Once again I am here to help till you are happy.

Good luck
Rusty
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Old 11-08-2006, 09:38 PM   #10
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Small boom from lennox furnace


Hi Rusty,
Sorry I was not able to get back right away. Thanks for sticking with me on this. I do have a volt meter that will go down to 25 volts. I as well will be in and out for the next week or so. I will check back as much as possible. I look forward to hearing back from you.
Mike
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Old 11-09-2006, 12:08 PM   #11
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Small boom from lennox furnace


Hi mhenry48

I will need you to locate your pressure switch, once you have done this. Turn your voltmeter on to the voltage I previously specified. Start the furnace up, put one lead on each of the terminals of the pressure switch, do not remove the wires to do this just slip them back enough you can make good contact with your meter. Once the furnace fires up keep an eye on your meter, if at any point during the flame on cycle the switch shows voltage, this means you could have a restricted flue or a inducer assembly not running up to speed. Remove the flue pipe from the top of the furnace if this happens. Use a wood match, light the match and blow it out after it has started burning the wood. Hold the match about 1/4" above the opening in the pipe and see if it draws the smoke into the pipe. If it does chances are your flue is ok, if it does not you will need to find the cause of the restriction. If this runs into a brick chimney it could be a Bird, Squirrel, or even sand from the deteriorated mortar. If you find a bunch of sand you will want to line the flue. Let me know where this gets us and I will go to the next step.

Good luck and hang in there
Rusty
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Old 11-13-2006, 08:01 PM   #12
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Small boom from lennox furnace


Hello again Rusty,
Back after a long weekend. I have tried the voltmeter the one difference is I could only set it on 10 or 50. Once I did this it would peg the meter on 10 or 50. It also started cycling on and off rapidly. I checked the flue it has a very good draft. It is funny you should mention the inducer assembly. If I am correct this is the fan/blower that is attached to the flue. I asked the service guy previously why it is so loud when it starts at the beginning of the start up cycle and all during the heating cycle. Any correlation? I look forward to hearing from you for the next step.
Mike
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Old 11-14-2006, 09:49 AM   #13
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Small boom from lennox furnace


Hi mhenry48

It sounds like we are getting to the bottom of your problem, If any inducer on any make or model makes a high pitched squeal or rattles badly. It needs replaced, no two ways about it. As I have said in previous replies to others, the inducer has to pull a given amount of air through the burners and the heat exchanger. I feel that if the tech you talked to would have been a true professional tech. He would have done something for you about it then! I looked up the part #s for you, but first we need to identify your furnace. In the center of your furnace is going to be what is called the vestible limit. This is going to have two wires on it, it will be aprroximatly 2"x3 1/2" rectangle. If yours sets flat on the front of the furnace, your part # is 10k51 or 10k53 or 43k51 inducer assemblies will all work. If the area around the vestible limit is recessed however you will need to use part # 11k98 pressure switch kit along with the inducer assy. Let me know if I can help further but I betting when you change that inducer you will have fixed your problem.

Good luck
Rusty
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Old 11-15-2006, 08:55 PM   #14
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Great I look forward to giving it a try. Any suggestions on the best place to purchace it online?
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Old 11-15-2006, 09:49 PM   #15
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Small boom from lennox furnace


Hi mhenry48

I went to a website that is pretty easy to navigate, I am pasting the URL so all you have to do is click on it.
http://www.cozyparts.com/lennox%2Dparts/.
let me know if I can help further.

Good luck
Rusty

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