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Discussion Forum

Furnace shuts off – sometimes…

namedpipes | Posted in Energy, Heating & Insulation on December 4, 2002 02:14am

First, we do operate a CO detector.

From time to time our furnace will no longer turn on and the house will cool off until we “kick-start” it by turning the thermostat off then on.

We first noticed the problem last winter right after the overheat sensor failed. It was replaced with a slightly cooler switch. The furnace guy said it would work the same. I don’t know if there is any connection.

The furnace is a Concord 90% Efficient, gas fired, forced hot air unit with a White-Rodgers mercury switch thermostat, 5 years old. When the system is working, the thermostat does keep the house properly conditioned. Moving the temp control makes no difference, ONLY toggling the on/off switch on the thermostat will kick on the furnace. This fix always works.

It can go hours, days or weeks between tantrums.

The Question: (you knew there would be one…) Is the thermostat defective, or is it more likely something in the furnace end?

Any ideas?

 


Edited 12/3/2002 6:29:07 PM ET by namedpipes

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Replies

  1. DaveRicheson | Dec 04, 2002 04:41am | #1

    Bad T- stat. most likley. Check voltage at the time of next tantrum. If you have the proper voltage (black or red wire to white wire), the stsat is bad. If you dont't have the proper voltage, it could be the low voltage transformer opening up when it get hot.

    Dave

    1. KenRubin | Dec 04, 2002 06:03am | #2

      Iagree.  The T-stat is probably bad.  For under $30.00 you can get a good new one and check it out.  They are very easy to replace.  Just remember to cut off the power switch to the air handler before removing / replacing.

      Good Luck,

      Ken

  2. TLRice | Dec 04, 2002 04:44pm | #3

    I disagree with the last two posts because I had a similar problem. The thermostat is, however, a good place to start and easy to check.

    On mine, I got out the voltage meter and checked the wires at the furnace. When the thermostat called for heat (my programmable Honeywell displays the word "HEAT" when it calls for heat), I had 24 VAC at the proper terminal on the furnace control board but it wouldn't fire. I too had to turn the thermostat off and then back on before it would work. This was with a 11 year old Bryant 90 Plus furnace. All integrated circuit board controllers (4). Checked into all of the relays and they worked fine. I talked to some folks that knew a lot more about the details of this particular furnace than I do, and basically got the response: "have to replace the boards", to the tune of $200 to $400 apiece. Had a service guy check it out and he bypassed a timer of some sort, and the problem was somewhat lessened but not solved. The furnace was getting old and was a little undersized so I replaced it.

    Now, I'm not familiar with the Concord furnace, but I would recommend (if you are comfortable doing these things and have the proper tools), check to see if the thermostat is providing the right signal at the furnace. If it is, then you can either troubleshoot the furnace controls of have a technician do it.

    1. booch | Dec 04, 2002 06:12pm | #4

      If the boards are taking a dump consider a surge arrestor on the power side of the circuit. Granted it could be the motor surges that kill the solid state parts on the boards but it would be cheap protection to put a surge arrestor on the load center or at least put in a surge arrestor duplex outlet for the furnace outlet.

      The thing that kills solid state components is voltage spikes. Transient surges from the power line travel to the ss devices and burn a hole in the dope & traces that make transistors work.

      1. TLRice | Dec 05, 2002 05:03pm | #6

        I was told that these Bryant units are notorious for control board failures. Like I said, it was relatively old, and undersized, so I didn't worry too much about saving it. However, with all the electronics in the controls of newer equipment (I now have a Heil H9MPV 2 stage, variable speed furnace) I plan on installing a surge suppressor for "critical" electrcal loads, and that will include the furnace. The furnace, BTW, is direct wired to a dedicated circuit, no receptacle involved. Do you see furnaces with plugs?

        1. User avater
          rjw | Dec 05, 2002 05:34pm | #7

          I plan on installing a surge suppressor for "critical" electrcal loads

          Sounds like a good idea.  How are you planning to do that?

          I suspect the surge protectors we get for computer equipment aren't rated for the loads that a furnace is going to pull.________________________________________________

          "I may have said the same thing before... But my explanation, I am sure, will always be different."  Oscar Wilde

          1. booch | Dec 05, 2002 06:28pm | #8

            Surge suppressors are found in 3 categories. (I'm stretching to remember them.)

            The first is a varistor. these are "lollypops" red 2 legged devices that attach across the lines generally inside of a device (like a plug strip) They are voltage rated at 500 volts or so. when the transient (voltage spike) comes down the wire this shunts the spike to ground. They behave like a rain barrel. they take some quantity of spiking then they 'overflow' generally resulting in a 'pop' at which point the side of the varistor blows out at which point it is no longer effective. This is the cheapest and most common version. These act fast but have limited let thru in terms of about 1 to 5 ka capacity.

            Gas diodes are essentiallyself-contained 'spark plugs' that given enough potential provide a path to ground for the transient. Their threshold is greater in terms of voltage necessary to bridge the gap so they don't provide the voltage protection as well but are higher capacity which can discharge 10ka commonly. This is the modern day version of the carbon packs the telephone company put on incomming phone lines to drain off lightening strikes.

            Supressor diodes have the finest voltage control but the downside is that they can be as fragile as the electronic products themselves. Top end protection at higher voltages could be as little as 10A capacity

            http://www.phoenixcontact.com/index.html

            is a great information source on the types of products available (see Trabtech) These are industrial surge arrestor devices. I only gleaned the types of protective devices from that literature. Other manufacturers have different packaging for the surge arrestor devices. Among the packaging alternatives are surge protective outlets. Hubbell, Leviton, etc all carry the surge protective outlets. Putting one in the load center is also an option if you have a spare pair of breakers. Put it close as possible to the incoming line.

          2. User avater
            BillHartmann | Dec 05, 2002 07:34pm | #10

            "I suspect the surge protectors we get for computer equipment aren't rated for the loads that a furnace is going to pull."

            How do you mean that?

            Are you talking about the voltage clamping levels and amount of energy that it will disapate?

            Are you talking about the current rating? I have not checked, but they use 15 amp sockets and I assume that they are rated for 15 amps.

            But a furnace would typically have a 1/4 -1/3 hp blower and that is most of the load. Maybe 4-6 amps. Slightly more than my monitor and less than the lazer printer.

          3. TLRice | Dec 06, 2002 09:41pm | #18

            There is some commercial power conditioning equipment avaialable and there is "whole house" versions available. I have not done enough research to figure it out. I don't know if it is economically feasible or technically realistic, just a thought I had. Living out at the end of the line, power losses and voltage drops are an occassional fact of life. A generator/transfer sweitch will be installed first or maybe at the same time. The same "critical" loads would be on the transfer switch.

  3. nigelUsa | Dec 05, 2002 04:07am | #5

    Before you start taking things apart. check to see if the air intake and exhaust are both clear. If you have a few leaves in the "breathing tube" the unit will keep switching off.

  4. namedpipes | Dec 05, 2002 07:19pm | #9

    Update, fti

    I replaced the thermostat with a new Honeywell programmable ($40). This was not the fix, although I'll leave the programmable installed in anticipation of saving some fuel costs when the beast is repaired.

    My wife called the gas company when the temp headed south of 50F. They had a heard time believing my description until the furnace shut off for the tech. The overheat sensor was working properly. He wiggled this and measured that and then said the circuit board was bad. They will come by Thursday to install a new one (he thought $150+/- for the part, and about the same altogether for the labor - I'm in the wrong business).

    (Minor rant) The circuit board is a two-layer board with a handful of transformers, transisters and a couple of ICs - not even SMT stuff. There's maybe $10 worth of components there, and this thing lists at $150? I can buy a 2gh P4 motherboard for less than that!!!

    Of course, since he left it has been behaving...

  5. namedpipes | Dec 06, 2002 02:20am | #11

    Well now both the thermostat and the furnace circuit board have been replaced with no improvement.

    The only thing chillier than my house is my wife at the moment...

    Is there any chance the cooler overheat switch could be causing this? Or could the filter be affecting things with too much or too little flow?

    HELP!

    1. User avater
      BillHartmann | Dec 06, 2002 02:26am | #12

      Doesn't the board have any diagnostic lights?

      Without know anything more about the system my guess would be the overtemp. But too little air might cause it to overtemp.

      1. namedpipes | Dec 06, 2002 03:28am | #13

        The tech was guessing when he replaced it - The board looks so 1980's I don't think there is anything like self-diagnostics.

        The main symptom is the furnace simply turns off (including the fans) after a while. When the overheat sensor went, the main blower ran continuously while the burner declined to heat the air. So I'm not convinced it is the overheat...

        The furnace I had before ran for about 100 years without serious problems, until the chimney fell and flattened it. This thing is 5 years old and I'm ready to melt it down.

        What else is there to look at?!

    2. brucepirger | Dec 06, 2002 03:29am | #14

      You should head over to http://www.heatinghelp.com and find "The Wall" forum. Those guys are really great...

      I don't have any help for ya regarding a fix, but I will share your pain in the $150 price tag for a low technology piece of electronics...

      Here's a link:

      http://forums.invision.net/index.cfm?CFID=116382&CFTOKEN=3015e0b-eb9d3686-0d12-4928-9784-c0b691d519c3&CFApp=2&

      1. User avater
        MikeS | Dec 06, 2002 07:23am | #15

        Here's another site to try, http://hvac-talk.com

        Let em know that you did call in a "pro" and you might get a better response.

        Mike

        It's O.k. to think out of the box,           Just don't walk off of the plank!

    3. mike_chrest | Dec 06, 2002 07:42am | #16

      When you toggle the thermostat you are resetting the start-up sequence for the furnace. Since you have a high efficiency furnace it either has a spark ignitor or a hot surface ignitor. There are safetys that check that the furnace has lit after the ignition sequence has started. Its called flame proving. On some furnaces there is a metal rod that sits in the flame of the burner. A small electric current actually passes through the flame through the rod and to ground. Over time soot builds up on the rod and insulates it enough that the circuit is broken. Take some steel wool and shine up the flame sensor. Don't use sand paper. Make sure the furnace has a good ground connection. Check that spade clips (crimp on connections) are tight. Squeeze them with a pair of needle nose pliars if suspect. Try to look up your furnace on the internet and get a service manual if possable. 90+ furnaces are very complicated as you are probably finding out. The lower temp over heat safty shouldn't be a problem unless you have no airflow through the furnace. Change your furnace filter make sure you have air flow. Good luck.

    4. nigelUsa | Dec 07, 2002 02:12am | #21

      The thermostat is just a switch (ok a fancy automatic one). switch on, furnace turns on the forced vent fan to blow out the burnt gas. A pressure switch checks for negative air pressure and then allows for the gas ignighter to glow when this is up to temp for gas lighting the gas valve is opened. While this is going on the "big" fan pushing the heated air kicks on and the wife is happy.

      I have some regular customers and while they were away I went to their house to do some work and the heat was off. The 90+ unit keep trying to switch on. Its under contract, so called in the guy and he replaced the ignighter and the heater worked for the day. Next day samething, called again and they sent the same guy he messed around for 3 hours and said loose wire. Day three, no heat, called again and told them to send different guy he fixed it in 5 mins. blocked inlet vacuumed out the leaves and its been fine ever since.

      Go back to basics and get your money back on the electronics!

  6. mike_chrest | Dec 06, 2002 07:48am | #17

    Was the furnace overheating by itself or did the tech block the airflow to test the sensor?

  7. lancer7540 | Dec 07, 2002 12:00am | #19

    I have seen this problem many times before, it usually is not an easy one to diagnose though.  All modern high efficiency furnaces have an igntion control board on them, and all have a lockout  feature built into them. 

    The short course on how a furnace works: The t-stat send a signal to the control calling for the ignition board to go through it's program sequence to fire up the furnace.  It usually goes in a certain order, turns on the combustion blower, proves that the vent stack is clear of obstructions, initiates the igniton sequence and then proves there is a flame when the gas valve is releasing gas.  If at anytime in this sequence something is not working properly the system shuts down.  The control still getting the call for heat,  attempts another, igniton sequence until either a succesful atempt is made or the maximum number of failure are reached and the board goes into lockout.  (The lockout feature varies from manufacture to manufacturer and runs as little as one failure to as many 10-15.)  To reset the counter, the power to the board must be broken for a short length of time, thus the reason it works when you turn your  t-stat to off and back on to heat.

    I would have the technician come back out and have him observe the furnace go through several of these igniton sequences, I wil bet he/she will find that eventually something will happen, that causes the furnace from properly functioning. ( there are too many to list here)   I know some of you are thinking that this could take days of them sitting there watching, just have them cycle the t-stat on and off till it fails.  While they are there have them set the heat anticipator on the t-stat to the recommended setting by the furnace manufacturer, so that the temperature fluctuations you mentioned are not as great. 

    I said from the beginning that it would not be an easy fix, but hopefully this helps.

    1. User avater
      MikeS | Dec 07, 2002 01:13am | #20

      Try checking for a clogged condensate hose on the combustion blower.

      My 90+ gas furnace was cycling on and off frequently and intermittently.

      During the warmer months it worked fine but the colder it got the more frequently it misbehaved.

      This is what I think was happening. The condensate line was partially blocked so when the furnace ran infrequently in the warmer months the condensate had time between firings to drain out of the combustion blower. As the firings became more frequent the condensate collected faster than it could drain so at some point it collected to where it was interfering with the blower impeller, which caused a reduction in air flow by the blower, since the flue appeared restricted the system would shut down and stay down until the condensate slowly drained and no longer interfered with the combustion blower, then the cycle would start all over.

      I cleared the blocked condensate line and the system works fine.

      Yours might be doing the same only it locks-out after several events as Fred said.

         

      Mike

      It's O.k. to think out of the box,           Just don't walk off of the plank!

      Edited 12/6/2002 5:31:02 PM ET by Mike S

  8. namedpipes | Dec 10, 2002 12:36am | #22

    Another update...

    I believe I finally understand what is happening.

    Symptom: The furnace eventually stops cycling on despite the thermostat calling for heat. I replaced the t-stat. Gas company replaced the main circuit board. Problem remains.

    A year ago, the overheat sensor was replaced. The gas company did not have the proper sensor in stock so they installed a lower threshold sensor (220F). Thay said this would have no effect on operation. We immediately noticed the symptoms above, but it was a mild winter and it didn't happen very often, so we lived with it. (I think the original was 250F?)

    After the circuit board was replaced without improvement, I tried running the furnace with the hatch to the filter bay open. Problem solved. With the hatch open, sufficient air moves through the furnace that the overheat never or rarely trips, so the lockout counter does not shut off the furnace.

    It appears the problem is (A) the overheat sensor trips too soon and (B) the cold return ducts may be undersized. Since the problem never appeared until the wrong sensor was installed, I suspect the bigger part is the sensor. The original installer is long gone anyway, so it better be the sensor!

    Question: Would it be reasonable to expect this service episode ($194 so far) to be refunded, as the problem appears to have been caused by their own work a year ago?

    Second, until the sensor is replaced, the furnace will only work properly if I leave that hatch open. That original installer never installed a flue to the outdoors for makeup-air, so both makeup-air and air-to-heat are coming from the area around the furnace. I DO have a CO detector, and it continues to read 0, but I worry. Am I safe?

    Lastly (or maybe firstly), does this reasoning for the problem "root cause" seem likely, or have I found a covenient garden path?

    Thank you all again!

    1. User avater
      MikeS | Dec 10, 2002 04:50am | #23

      Or ..If the unit was not previously geting sufficent air for combustion then the open filter hatch could be now providing that air via the return duct. 

      Mike

      It's O.k. to think out of the box,           Just don't walk off of the plank!

    2. User avater
      rjw | Dec 10, 2002 06:17am | #24

      Second, until the sensor is replaced, the furnace will only work properly if I leave that hatch open. That original installer never installed a flue to the outdoors for makeup-air, so both makeup-air and air-to-heat are coming from the area around the furnace. I DO have a CO detector, and it continues to read 0, but I worry. Am I safe?

      What kind of furnace?  Natural draft?  Is there a "drafthood" on the front a foot or so above the burners that's open on its bottom?  If so, hold your hand under that opening with the "hatch" open.  Feel any heat when the furnace is running with the blower on?

      If not, ok for a night or two, probably, but you and your heirs can't sue me if you die or get sick because its technically not safe.

      How big is the room that the furnace is in?  Less than 50 cubic feet per 1000 btu of combined furnace and water heater btu?  No?  Leave the door into the rest of the house open.

      And maybe Tim will jump in, but I think you're right, it suggests that the returns aren't properly laid out. 

      Ooou, ooou.  Have you tried running it without the filter, but the lower cover on?  That would be much safer; maybe the filter is slowing down air flow just enough.  Are any of your returns blocked?

      ________________________________________________

      "I may have said the same thing before... But my explanation, I am sure, will always be different."  Oscar Wilde

      1. namedpipes | Dec 10, 2002 03:13pm | #25

        Or ..If the unit was not previously geting sufficent air for combustion then the open filter hatch could be now providing that air via the return duct. 

        It is a 90+ forced hot air.  I *think* the cumbustion air comes in separate from the cold return? There's little to no heat throw into the basement.

        Room is 1300 sq ft, maybe 7' high and the furnace is 112k btu. Water heaters are electric, but there is a massive gravity furnace for the other side of the house (c. 1912) . Not counting that beasty 81 cu ft?

        Filter is actually out at the moment. It was a difficult to find high efficiency filter 6" thick. Very efficient at blocking air flow, that is.  With the filter out and the hatch open the furnace will "starve". Open that hatch and the furnace works properly. I suspect it will work ok with a normal filter and proper overheat switch.

        1. User avater
          rjw | Dec 10, 2002 03:46pm | #26

          It is a 90+ forced hot air.  I *think* the cumbustion air comes in separate from the cold return? There's little to no heat throw into the basement.

          I hope Tim will see this thread, he knows more about this than I do, but, FWIW, a 90+ can have a separate plastic pipe air intake, in addition to the plastic exhaust flue.  If there is, there should be 2 plastic pipes going to the furnace.  Check that the one that is not attached to the small blower motor is actually attached to the combustion chamber box - sometimes that doesn't;' get attached and there is a small;; gap between the two.

          I believe, but don't know for certain, that there are safety pressure switches on the flue which will shut it down if the flue gases aren't getting out, so running with the blower door open (the latch) probably shouldn't affect the proper operation of the flue, especially if they did install the air intake.

          ________________________________________________

          "I may have said the same thing before... But my explanation, I am sure, will always be different."  Oscar Wilde

          1. TLRice | Dec 10, 2002 04:26pm | #27

            Most condensing furnaces (i.e. 90+) have a sealed combustion burner. These can be setup where comustion air is taken directly from outside and where combustion are is taken from the adjacent space.

            Depending on how it is setup, the flow proving switch will prevent the burner control valve from opening when inadequate flow through the system is present. Some induced draft appliances, my water heater for instance, has a high temperature limit switch that will trip the burner control circuit. The unit will not fire if the switch has been tripped. It requires manual reset. The furnace will have a high temperature limit switch to prevent the heat exchanges and other things downstream from over heating. If it is tripping off the unit and it is working properly at 220, the there is a problem with air flow through the unit, and that sounds like the case.

            Unless there is a safety switch (not common) on the door, operating a furnace with the upper door (assuming this is an upflow) will not affect operation of the unit, only the external noise level. The lower door will be where the blower and motor are located, and you can operate the furnce with this door off as well, just that all the return will come through that open door. If gravity vented appliances are in the vacinity, they will most likely backdraft.

            I would recommend that the returns be thoroughly checked out. Also check out all the air flow paths to make sure that none are blocked (look for closed registers, crushed flex, mispositioned dampers, etc.).

            I would also recommend that the service people be asked for a refund, don't let the gas company work on your furnace, and ignore anything posted by Mike S.

            Edited 12/10/2002 8:51:27 AM ET by Tim

          2. User avater
            MikeS | Dec 10, 2002 06:41pm | #29

            Tim

            Specifically which post(s) in this thread do you disagee with and why ?

            Mike

            It's O.k. to think out of the box,           Just don't walk off of the plank!

          3. User avater
            BillHartmann | Dec 10, 2002 06:44pm | #30

            "The furnace will have a high temperature limit switch to prevent the heat exchanges and other things downstream from over heating. If it is tripping off the unit and it is working properly at 220, the there is a problem with air flow through the unit, and that sounds like the case. "

            Did you catch that the unit orginally had a 250 overtemp, but when that failed it was replaced with the 220, because they did not have any 250 in stock.

            I noticed that from the orignal message and figured that was the source of the problem. But I am not that familar with HVAC and not at all with high eff units so I did nt respond.

            Also some one that is good with electronics could instruement the unit to see what it tripping. I don't know if there are any commerical tattle-tail units that are used for some like this or not.

          4. TLRice | Dec 10, 2002 07:35pm | #32

            Bill,

            "Did you catch that the unit orginally had a 250 overtemp, but when that failed it was replaced with the 220, because they did not have any 250 in stock." Yea, I caught that switch change. One of the reasons I would recommend that the gas company stick to reading meters. Even so, should'nt see 200 plus in the discharge of a normally operating furnace. He has, I beleive a 112 MBH output furnace.

            At 1200 cfm and 70 deg return, the discharge temp of the air would be 156 deg, average. It should not be the cause of the problem, but things fail.

            I'd be curious to know some of the details of the operation of this unit, air flow, inlet and outlet temps, but I am no technician.

            A good technician (he hasn't had one look at this yet), that is qualified to service that manufacturer's euipment should be able to figure it out in an hour or less.

          5. namedpipes | Dec 10, 2002 07:52pm | #33

            Wow, lots of responses... If I could get this level of thought from my own gas company this would already be fixed!

            From:  Bob Walker  7:46 am  FWIW, a 90+ can have a separate plastic pipe air intake, in addition to the plastic exhaust flue.It does have an inlet for the makeup air, the original contractor never installed the flue for it, so it draws from the basement.

            From:  Tim  8:26 am  If it is tripping off the unit and it is working properly at 220, the there is a problem with air flow through the unit, and that sounds like the case.With the 220 overheat switch, no filter, filter hatch closed, the overheat will repeatedly trip, eventually causing the circuit board to require reset.With the hatch open, the overheat does not trip (or not often).With the original (250?) overheat sensor, we did not have this symptom, even with the filter and hatch installed. I *think* the correct fix is to install a more reasonable filter and the correct overheat sensor. And also address the cold return somehow.The lower door will be where the blower and motor are located, and you can operate the furnce with this door off as well, just that all the return will come through that open door. If gravity vented appliances are in the vacinity, they will most likely backdraft.There is an ancient gravity furnace in the same basement. (the hatch I mentioned is actually on a filter body that connects between the cold return duct and the lower compartment of the furnace. fwiw, it is a remarkably (air) leaky basement, there is ample opportunity for additional cold air to come straight in through windows, around doors...I would recommend that the returns be thoroughly checked out. Also check out all the air flow paths to make sure that none are blocked (look for closed registers, crushed flex, mispositioned dampers, etc.).All are "hard" ducts, good draw on the ones I've already tested - I'll check the rest, but I think the problem is the size of the returns...

            From:  Mike S  8:38 am  A 6" thick filter ? foam ?, fiberglass ? pleated paper ? HEPA ?I asked for HEPA, got this - thing I can't get new filters for... It is *not* HEPA. Pleated paper.

            We called the gas company because I didn't have a working relationship with a heating contractor and I made an (apparently unwarranted) assumption they would be the most knowledgable. I don't suppose any of you live in the Metrowest area of Massachusetts?

            I originally thought I had a straightforward question - I guess it wasn't so straightford after all. Thanks all for the input and I hope I haven't been a bother...

        2. User avater
          MikeS | Dec 10, 2002 04:38pm | #28

          A 6" thick filter ? foam ?, fiberglass ? pleated paper ? HEPA ?

          Could you run the system with no filter and the filter cover/hatch closed ?

          It might help narrow the problem down between a) an overheat issue due to lack of conditioned airflow and b)a combustion air supply / exhaust gas issue.

          Mike

          It's O.k. to think out of the box,           Just don't walk off of the plank!

          1. TLRice | Dec 10, 2002 07:16pm | #31

            "Or ..If the unit was not previously geting sufficent air or combustion then the open filter hatch could be now providing that air via the return duct." and "Could you run the system with no filter and the filter cover/hatch closed ? It might help narrow the problem down between a) an overheat issue due to lack of conditioned airflow and b)a combustion air supply / exhaust gas issue."

            Mike,

            I edited out that comment as a bad choice and unnecessary, but didn't apply it. That was an unfortunate mistake on my part.

            But since you had to ask, bottom line here, without picking on you too much is, based on the two statements quoted above, I have to assume that you don't really have any idea what you're talking about. Several folks get on these boards and just babble away not really havng a clue as to what is right and what is wrong. (Boss Hog does this alot, but he usually disqualifies his bad advice before spewing forth, by saying something like "I really don't know anything about furnaces, but....) I have been guilty of this once or twice. I have posted messages before checking something out or really thing about it.

            If you beleive that the combustion air for this man's furnace could be supplied through the return system, with the unit operating, you just don't understand how things work.

            To tell someone to run their furnace with no filter to "help narrow the problem down between a) an overheat issue due to lack of conditioned airflow and b)a combustion air supply / exhaust gas issue", is bad advice and indicates that you are not trying to provide any real help based on experience or knowledge.

            "A 6" thick filter ? foam ?, fiberglass ? pleated paper ? HEPA ?"

            High efficiency filters for residential furnaces are common, though usually 4" thick (I think he'll discover that when he finds a replacement) but your question of this again leads me to beleive that you are unfamiliar with HVAC equipment. Disagree with me.

          2. User avater
            MikeS | Dec 10, 2002 08:37pm | #34

            Tim

            I agree that normally the combustion air should in no way be coming from the return duct. My thinking was that if this unit did not have an piped outside air source and was trying to use an inadequate source of surrounding room air ( such as in a non vented closet or an unconditioned basement with no make up air source) that when the fillter cover was removed air was stolen for combustion from the conditioned space via the return duct.

            I was limiting my thinking to either a overly restrictive filter or the above when I suggested removing the filter.

            The 6" is what suprized me

            Mike

            It's O.k. to think out of the box,           Just don't walk off of the plank!

          3. TLRice | Dec 10, 2002 09:09pm | #35

            I believe that if the furnace was taking combustion air from the room, opening the return side to that room (either by opening the filter housing or the furnace cabinet) will create a worse case by reducing the pressure in the room and further staving the furnace.

            I misunderstood that you were talking about additional combusting air bing supplied to the room via the open return. This won't happen, but the idea makes a little more sense than what I thought you meant.

          4. User avater
            MikeS | Dec 10, 2002 09:38pm | #36

            I see your pointMike

            It's O.k. to think out of the box,           Just don't walk off of the plank!

          5. User avater
            rjw | Dec 11, 2002 02:03am | #37

            Tim,

            FWIW, I'vce started some "on the side" testing, comparing draft with the blower compartment cover on and off.  So far, in wide open spaces (older houses, large basement) I haven't seen any effect on furnace drafting (natural draft or 80+) but have seen it affect the water heater, but not to actual backdrafting.

            I have NOT done enough testing to draw any conclusions though, and always recommend that the furnace not be run with the blower compartment cover off.

            ________________________________________________

            "I may have said the same thing before... But my explanation, I am sure, will always be different."  Oscar Wilde

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