Furnace Burners Cycling

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  #1  
Old 01-25-03, 12:10 PM
jmsouth
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Furnace burner keeps kicking off

My Comfortmaker updraft gas furnace will run for a while, then will kick of with a contol module code of "open high limit switch". my furnace air temp design is 35-65 air temp rise with limit control calibration main of 195 per label. My actual air temp rise is usually 40-50 deg and a surface thermometer on the induced draft combustion fan housing will rise to about 240 F. Is my limit switch going out or is my furnace getting too hot?
 
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Old 01-25-03, 12:35 PM
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be sure

proper aiflow is present. a dirty filter, coil or blower assembly wil restrict airflow. remove the limit enough to insert temp probe. if temp does not go over 195 but trips, it is bad.
 
  #3  
Old 01-25-03, 01:49 PM
jmsouth
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Performed temp test

I could not put temp probe and limt switch in together so I took limit switch out (left hooked up and taped to where it was not touching anything) and watched it the whole time. The temp probe (used a meat thermometer) never got over 156F and furnace ran for 20 min. without kicking off, and this with higher inlet air temp from house than before. Does sound like limit switch. Also, in additon to limit control calibration main setting of 195F, it also has above that a "max outlet air temp" of 165F. A digital thermometer in house register never got above 115F during 20 min. If limit switch is looking for 195F, then what is looking for the 165 F max. air outlet temp?
 
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Old 01-25-03, 02:38 PM
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high limit

165 is recommended outlet max temp, there is a 30 degree lag in there for some reason, not sure if limit has been replaced with wrong one at some point, or if unit is specked that way due to location of limit. raising fan speed will reduce temp, but not a true fix. how much return ducting do you have? what is the furnace model #?
 
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Old 01-25-03, 02:40 PM
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BTW

if temp does not get above 156, and limit trips rated at 195, limit is opening prematurely.
 
  #6  
Old 01-25-03, 02:57 PM
jmsouth
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Model number, return duct & switch

Model number of furnace is GUI100A016AIN, and is really an "inner-city products" furnace, but told is made by same as Comfortmaker, which is more widely recognized.

Return duct is just one big one for the house that supplies air to the bottom of the furnace. I have changed filters and I don't believe that is the problem, especially since my delta T is about what manufacture suggest (45F) and often even lower (closer to 35F - 40F during recent test). Furnace is rated for about 1,655 CFM, and I plan on doing a flow test based on manual guideline and inlet and outlet temp. This 1,655 CFM is the high speed per manual (1,218 CFM for low).

I believe that switch is set for 195 as this was imbedded in a series of nubmer right below the part number.

I will buy a new limit swith (part number 1320363) Monday. Thanks for your help.
 
  #7  
Old 01-25-03, 03:11 PM
bigjohn
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May I first congratulate you on a very good troubleshooting procedure and heads up thinking. I have worked with more so called techs than I care to admit who would not have been able to diagnose like you did. I'm thinking that the note about max air outlet temp is concerning the supply air oulet but the limit control is monitoring the temp inside the furnace closer to the burners. It sounds like you have a bad limit switch. Is it round/oval shaped with a mounting plate and 2 terminals on one side? Go to www.maplechase.com [you will get an automatic redirect] On the home page click on the UNILINE link. [bottom left] Next page, click on heating. Next page, click on page C73 {fan and limit controls] Does it look like those? They don't have a 195 deg. but they do have a 190. I wouldn't go for the 200 without talking to the manufacturer first. Supco makes some adjustable models. www.supco.com Click on product catalog, on the next page click on thermostats, next page download that catalog section, then scroll down a couple pages and look at the AT Adjustable series. The Supco catalog is has nice color pictures. I don't remember which forum it was, but a couple years ago, we had a gentleman with a broken overhead projector that he used for his Sunday School classes. The temperature sensor that monitored the heat from the light bulb and turned on a little cooling fan had died. The projector was obsolete and they didn't have the money to buy a new one. He was asking if a clothes dryer thermostat would work. I suggested that he try one of the Supco adjustable close on rise types. He bought an AT021 at an appliance store and it worked perfectly.
 
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Old 01-25-03, 04:00 PM
jmsouth
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It is round (maybe 3/4" dia. and maybe 1/2" deep) has a small mounting plate w/ 2 sheet metal screwes. It has two terminals on one side and 1 on the other.

I am downloading adobe right now so I can open the PDF file from the website you sent me to. It will be a while with the dial-up connection I have, but I feel confident we are talking about the same thing.

I think the 190 should be fine, especially since even allowing for error in my meat thermometer, It should be well above where I run at and like you implied, it is much better to be on the conservative side and go down instead of up.

Thanks for the complement and your help.
 
  #9  
Old 01-27-03, 05:43 PM
jmsouth
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Limit switch is good!

Note this entry is duplicate as posted on "Gas Appliance" Forum

The problem with my furnace is not a bad limit switch. I purchased a new limit switch and it still kicked off after about 10 min. run time. My meat thermometer I was checking temp before still did not get above 140 F - 155 F, but it is 6" long and the limt switch sticks just inside the inner insulation of furnace wall. I have a magnetic thermemter that sticks to metal surfaces and is a little shallower than the limit switch. I put this where the limit switch was and it did get to 195F, meaning the limit switch was doing its job.

I have cleaned the filter and registers are open. My burner flames seem to be going in the tube good (no rollout) so it doesn't seem like there would be any venting system problem. Of course the A/C coil above could be a little clogged (big deal to get to on mine), but there are problems that I can't figure. Why is the temp is so much hotter right at the surface where the limit switch goes than the air a few inches in, it if air flow is blocked, why isn't outlet air temp hotter (got only to 114 F @ inside register) and the other is that the temperature where the limit switch reads seems highly erratic. It would go to 180, then down to 170, then up to 190, then down to 180. Sometimes moving about 5 F sometimes 10 or more, but would do in a matter of seconds (highly sensetive thermometer that I believe is accurate). Most of time it did not move more than 3-5 degrees. Could this be a problem with the heat exchanger? I have a forced air blower at bottom and induced draft fan for burner, meaning any leak should pull house air into the combustion air - thus not blowing on limit switch. I stuck my finger in the hole where the limit switch goes and could feel surrounding insulation, but I can't verify its condition. It could be deteriating.

Did a very rough CFM calculation based on furnace BTU/Hr. rating, temperature rise measured with same dital thermometer (after steady state achieved), and formula in owners manual. Although I can't say for sure, it seems that air flow is at or above OEM high flow rating of 1,655 CFM. It just doesn't make sense to me that air flow is defficient when Delta T is often below rated. I usually get 35-45 differential and it should get 45 per manual. The heat exchanger element I see when the limit switch is off looks good, no visible corrosion, rust, etc.

Any feed back on these questions and things that they eliminate would be appreciated. Sorry for the extra long dialog.

Thanks.
 
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Old 01-28-03, 06:15 PM
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i am

thinking evap coil, have seen it too many times.
 
  #11  
Old 01-28-03, 07:52 PM
jmsouth
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Will clean out evap. coil

When weather permits, I will disassemble and clean out coil. I think the foam washable type filter that came with furnace allowes too much dust to get through anyway and may have led to coil getting dirty.

In the meantime, I have turned down the gas to the furnace at the pressure regulator to the point that it does not "cycle" anymore. My 100,000 BTU furnace appears to only be pulling 84,000 BTU's (assuming 1,015 btu/cu ft & correct gas meter). Burner flames appear to still be good and combustion exhaust temp is cooler (about 20 F), so my efficiency is probably up some as well.

Thanks for your help.
 
  #12  
Old 01-29-03, 12:30 PM
bigjohn
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Good questions. I called the Comfortmaker factory and they made me go to the distributor level for a tech rep. So, I called the distributor in Florida and the guy I need to talk to is out of the office until Friday. I suspect that the cabinet insulation may be the issue. I think heat is transfering by conduction from the cabinet to the limit. Maybe the limit's position is in a minimal airflow space in the furnace. I can't see how you would have an airflow problem based on the air temp and Delta T readings you're getting. I'll betcha that the fix is either a higher temp limit or a change to a probe type limit that senses the airflow temperature. I would not and could not recommend/suggest doing either without the blessing of the manufacturer. I want to talk with the tech rep guy to see if there is service bulletin out on this. I'm sure that yours isn't the first furnace where this problem has cropped up. I was thinking that maybe the furnace is overfirng, but the Delta T is a contraindicator. Can you get a guage or manometer and check the gas pressure at the burner manifold? I'm assuming that this is not a new installation?
 
  #13  
Old 01-29-03, 05:03 PM
jmsouth
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Gas Pressure @ manifold

Bigjohn,

I cannot get a gas pressure at the manifold. My owners manual shows a 1/8" pipe tap where you are supposed to get this reading, but my installer did not drill and tap this hole when it was installed 8 years ago (no problems before this) and I do not want to remove this and do that at this time (risk getting metal shavings, etc.). But I may talk with a guy at work who may have experience with this.

I know that the main airflow will not be as high by the limit swith at the edge as in the middle of the furnace due to simple fluid mechanics and that is why my meat thermometer showed so much cooler than my magnetic thermometer positions close to the wall like the limit switch. Also my burner chamber is like 4 big
"M's" in parallel - with the burner at one end, the induced draft fan collector box at the other and the limit swith is right between the two middle "M's" in the part in the middle of the letter, making it right between chambers. Several inches in, as the "M" spreads apart, the distance to a combustion chamber is further away and like I mentioned above, has greater air flow. The metal near the limit swith is hot and I think you may be on to something about the conduction of heat there.

I did another airflow test yesterday based on a formula in my manual as follows:

CFM = (INPUT x .81)/1.08 X TR, where Input is my BUT/Hr that my meter and stopwatch showed to be 84,000 after I turned down the pressure regulator and the TR is temperature rise, which is about 35-40 F, making my CFM around 1,575 to 1,800, which is pretty close to the OEM hig speed blower setting of 1,655 CFM. I still believe my A/C coil is dirty or could us a cleaning even if it is not a contribuitor to this problem. Warmer weather would be nice for this though.

My furnace, with the regulator turned down, does not kick off now, the efficiency is greater (as mentioned earlier - combustion exhaust gas is 20-30F cooler) and my heat exchanger should last longer with the lower firing temp, so I am not disatisfied with where it is at now . Knowing where I am at on the furnace (I never gathered such detailed information on the furnace performance when the furnace was new), if my temperature again rises in the future or my limit swith kicks the furnace off in the future with my input BTU's being the same, it would almost half to be an insulation type issue wouldn't it??

Thanks, you have been a great help.
 
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Old 01-30-03, 03:52 PM
jmsouth
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Gas Outlet Pressure

BigJohn,

As I mentioned my inlet gas pressure tap was never installed, but I did find one on the outlet gas, which I think is what you were asking for anyway. I borrowed a digital pressure gauge from the instrument shop where I work and it only reads about 1.6" W.C. at my new low setting. Turning it back up to where it was when it was cycling, it was about 2.6-2.7" W.C. My Furnace is rated for 3.5" W.C. This really sounds like what I would expect with a now 84,000 BTU/hr and an estimated 92,000 BTU/hr before on a 100,000 BTU/hr furnace. I would think this pretty much proves that my furnace is not "over firing". Even if it is an insulation problem like we both suspect, it is running more efficient than before and heat exchanger should last longer & I have plenty of heat. Burner flames are tuned pretty good too (blue flame). Most likely I may have to revome panel in next few years and replace insulation. I think we have just about eliminated everything but that.

I would be interested to see if Comfortermaker has any previous history of this.

jmsouth
 
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Old 01-30-03, 04:48 PM
bigjohn
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Yes, the pressure downstream of the gas valve is what we we're looking for. An 81% efficient furnace with a 100,000 btuh input should clock at 81,000 btuh output. [ give or take a little] I'm wondering if it's been overfiring all along, running hot and has compromised the insulation somewhat. The only thing that doesn't fit is the temperature rise. With the limit in the space you describe, I'm thinking that a limit with a probe that reaches into the plenum cavity might be more appropriate. I understand what you're saying about the efficiency level, but you actually have to be careful about getting the flue gas too cold. It's a case of more isn't necessarily better. If the flue gas temperature is below it's dew point, water will separate out from the flue gas. The water will contain highly corrosive acid. I'm curious to know what you get if you set the gas pressure to 3.5"w.c. I'm also curious about the size of the main burner orifices.

Go to http://www.andersonforrester.thomasr...ester/home.htm

When you get there, click on orifice capacities. The next page contains a chart that lists orifice capacities for both Natural & LP gas at various manifold pressures. Count the number of burners in your furnace and divide that number into 81,000
to determine what the capacity of each orifice should be. You might get lucky and find they stamped the size on the orifices in the furnace.

Since you're ok for now, you might want to get thru this Winter season and then persue the coil cleaning as well as checking out the orifices and see if there is factory fix/update for the limit. The phone # for Comfortmaker is 1- 931-270-4222.
 
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Old 01-30-03, 06:36 PM
jmsouth
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Burner/Orifice size

My Burners are 25,000 BTU's each (4 for the 100,000). The 80,000 is only the actual BTU's that go into the house, with the 20% going up the exhaust stack. My Manual also says all the Comfortermaker burners listed in my manual (7 different furnaces) are 25,000 BTU each, only the number of burners is adjusted for furnaces from 50,000 to 150,000 BTU's. My orifice size is #41 per manual and will operate in a range between 960-1095 BTU/cu ft and a Specific Gravity of .6 to .66. Still believe I have never been "over firing"

I believe that at about 200-210F combustion exhaust temp, that I am well above the dew point of the combustion gas, but I can check this out by looking at some of my old text books. I don't plan on turning down furnace any more and really only turned down just enough to keep it from cycling.

I can't help but think that the factory did not want a temp probe in the middle of the air stream and that is why the max air temp recommended is 165 and the limit is 195. I would be curious to know what the factory says about this.

I could run a test on it with gas set @ 3.5", but I would have to be quick to get a gas usage on it before it kicked off. Based on what I have seen at two different levels, I think it would be close to rating of 100,000, with minor differences because of variations of gas quality, barametric pressure, etc. I have already put all instruments up, pipe doped the plug, put up all the fittings to connect them, etc.

I do think it is worth pursuing what the factory says about the limit. Thanks for the number.
 
  #17  
Old 01-31-03, 02:51 AM
bigjohn
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With that size orifice, I agree, it's not overfiring: and like I had said earlier, the temperature rise is a contraindicator. I think at the stack temp you have, you'll be ok on condensation. I think the dew point number we're looking for is around 125 deg. F, but I too would have to look it up. The only thing that bothers me about this is how does it run fine for a number of years and then all of a sudden start tripping the limit switch? The insulation and the cleanliness of the cooling coil still merit investigation. Give us an update in the future. I like to see problems thru to their resolution.
 
  #18  
Old 02-01-03, 05:57 AM
jmsouth
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Spring activities

I plan on cleaning the A/C coil in spring, which will certainly help my A/C efficiency and capacity. It has been 3-4 years since I have done that so it is overdue anyway.

I don't know how long my furnace has been cycling. Unless you continuiously monitor the heat temp @ the register, which of course few will do on a routine basis, or you happen to hear the furnace firing and notice that it has already been running, it could have been running this way for sometime. The main noise the furnace makes is the blower and it never shuts off during this limit swith trip and the furnace firing only makes an audible sound for less than a second, making it more difficult to notice. In addition, unless I am heating up my house from very cold, routine cycling by thermostat would shut the furnace off before the limit switch would catch it anyway. Basically I am saying it could have been doing this last year or possbily even the year before that and I just never noticed it since I always had heat and even though I measured the register temp from time to time, it always must have been before it kicked off. I doubt that this problem would go back further than that though.

After I clean the coil in the spring, I will turn the furnace back to where it was (exactly 1 full turn on pressure regulater adjustment screw) and see what it does. I will give you an update at that point. If it doesn't, getting to my heat exchanger to look at insulation looks like I will have to remove pretty much all furnace components (burners, regulators, gas piping, various switchs, combustion blower and piping, etc.) so as long as it works like it does, I probably won't dig in to all that and risk messing up something else that is working now. I will install my magnetic thermometer in place of the limit swith and probably run a test on it every year to see if the temp is stable from year to year or if this is a progressive problem.

Thanks for your help and I will post a reply after I clean A/C coil.
 
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Old 02-01-03, 06:25 AM
busterm
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jmsouth, can't wait for the ending to this interesting saga!

what method will you use to clean your A/C coil?
 
  #20  
Old 02-01-03, 06:37 AM
jmsouth
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Cleaning the coil

I know they make a special acid just for this, but several people that I know say just spraying it with 409 or similar and flushing it with water will do just fine. I will hook up a water hose and put a spray nozzle on it for this, then soak it with 409 or similar mild cleaner, then flush again.

Have any suggestions????
 
  #21  
Old 02-01-03, 08:50 AM
busterm
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No suggestions. My upflow gas furnaces are 13 yrs old and received what I would call light duty. I inspected the evaporated coils last year for the first time and they looked clean to me.

Since I have an almost uncontrollable desire to take everything apart to see how it works, I try hard not to "fix unless broken".

I am also an engineer and retired so I have to work on my diy addiction every day!
 
  #22  
Old 02-02-03, 03:50 AM
busterm
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jmsouth, you have me thinking about evaporator coil cleaning. What about using a pump-up garden sprayer with a solution of auto dishwater powder and hot water? Some advantages would be excellent cleaning/degreasing, good local jetting with minimal water consumption and much better maneuverability than garden hose. Honeywell recommends auto dishwater powder to clean electronic air cleaners so I figure it would likely work well on the evap coil.
 
  #23  
Old 02-02-03, 07:31 AM
jmsouth
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Cleaning A/C Coil

I certainly am not an authority on this, but that would probably work well. I think the main thing is that you have enough pressure/velocity to clean between the fins, but not so much as to damage or fold them over. I would have to let someone else tell you about the compatability between your cleaner and the coil. My guess is Honeywell recommend that because it would work sufficiently, be readily available to most, and would not involve using any harsh chemicals that they would have to worry about idiots hurting themselves with (liability). It certainly would be better than just water.

A local heat and A/C guy told me back in the summer that there are two different kinds of acids you can get for your A/C unit. One for the evaporator and one for the condenser. He said make sure you don't mix them up as one kind would damage one of the coils. Of course I don't know how much he knows about this, but I would think that what you are talking about using would be milder than the acids they use. I am also guessing that after using that solutioin, like your dishwasher, you would rinse with water?

I know there has to be more information out there from professionals who have used different types of chemicals to get different results. Hopefully some of them may read this and give us a reply. The spray rig is a good idea. I think when you use the acid, that is what you normally use anyway (dilute acid with water in tank, then spray). If you browse around the net and find more infor on this, I would be interested.
 
  #24  
Old 02-02-03, 08:20 AM
bigjohn
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I normally clean cooling coils by pumping the system down [IOW- storing the refrigerant in the high side if possible; otherwise I have to recover the refrigerant], removing the coil and taking it out in the yard for cleaning. Outsde I can make a mess that I can't inside. I use a foaming action alkaline based chemical like Renewz or Alki-Foam. An acid based cleaner will damage the metal surfaces as well as me. The alkaline based stuff still requires a face shield and heavy rubber gloves. 409, laundry detergent, dishwasher detergent, won't dislodge the dirt, gunk, pet dander, etc., that is imbedded in the depth of the coil. Think of it this way- first cooling season when the euipment is new, some dirt, mold, etc., gets in the coil. Now, heating season comes along and that stuff gets baked on. Repeat this recipe for several years and it's easy to see why cooling coils get so dirty. The nastiness gets baking into the depth of the coil and may take several applications of a foaming action cleaner coupled with copious flushing to get it out. There are some products on the market that claim you can spray them on and the coil will self clean. Sorry, I've cleaned too many dirty coils to believe those products would deliver as advertised, and so I've never tried them. I've had coils that I couldn't clean. I have had several beauty parlors where the cooling coil was full of dirt, slime, and hair spray. I tried cleaning the first one or two [young and dumb] which I wound up replacing anyway. After that, I sold a new coil straightaway. Virginia KMP makes Alki-Foam, you can see it at www.grainger.com. It's not cheap. My methods may seem like overkill, but bear in mind that I have to warranty my work.
 
  #25  
Old 02-02-03, 08:34 AM
jmsouth
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Alki-Foam

Thanks for the good info. I will check out this cleaner and see what it does. Not having any of the refrigerant equipment you do, I am still going to try to do in place, even if it means doing multiple times. My A/C coil is not in house, but in a closet in a storage room (unfinish) adjacet to my garage so I can get a little more carried away than inside a house. Also no inside pets.
 
  #26  
Old 02-02-03, 12:19 PM
jmsouth
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More Cleaning Question for BigJohn

BigJohn,

I have been at the Virginia KMP website where they detail the cleaners you spoke of sold by Grainger. They are saying that Alki-Foam is for Condensers only and they have a Acti-Klean that is for Evaporators, but can be used on Condensers if heavy foam type cleaning is not required.

Do you think they are just being overly cautious and that since the foam is obviously stronger, it will be better? Could it be that they are just refering to using the stronger cleaner outside only (fumes) as not everyone will do as good as you by removing evaporator coils outside?

Thanks.
 
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