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OT:Furnace Problem

Discussion in 'Uncategorized Threads' started by TC, Oct 20, 2008.

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  1. TC

    TC TS Member

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    Bryant propane furnace. Had furnace shut down for the summer including pilot light. Opened gas shut offs, lit pilot, had wife inch thermostat up one degree above thermometer reading. Sitting next to furnace, heard gas valve open, burner closest to pilot finally lit, several seconds later entire heat chamber lit off, had flame inside far end burner. What is causing this? Tony
     
  2. Catpower

    Catpower Molon Labe TS Supporters

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    The delayed ignition is probably caused from dirt or rust in the cross over tubes on the burners, I don't know how old the unit is but you should have "ribbon" burners, if you tip the far end up they willslide up over the manifold and come out fairly easy, the flat part is the cross over tube, sometimes they can get corroded and cause a delayed ignition. What might be causing the last burner not to light properly some times during an extended off cycle some little friggin bug (spiders) will crawl in the manifold through the burner orfice hole, the orfice will unscrew out of the burner manifold, and blow it out or use a shop vac to suck it out.

    If the burners are rusty, try to clean with a wire brush, thin screwdriver of tip of knife blade, and also clean out burner chamber with a shop vac, or use your wifes, she can only shoot you once,those burners are still available, but are rather pricey, but those units were fairly easy to work on
     
  3. Richshoots

    Richshoots TS Member

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    A spider moved in and the web is blocking the gas flow.
     
  4. TC

    TC TS Member

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    Thanks everyone, pulled the burners, pilot and thermocouple, wire brushed everything, and vacuumed the heat exchanger. Lights off a little better but flames still have about 20% yellow and 80% blue. Doesn't seem to be putting out a whole lot of heat. Getting just under 20* difference in air temperature measured in the ducts about 3' from the furnace. What is a normal range?
     
  5. Catpower

    Catpower Molon Labe TS Supporters

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    Is it natural or lp gas? If it's natural, your gas pressure is low, on the other hand, lp will have some yellow, not alot. It would be a good idea to have a professional come out and check it, because it could be putting out carbon momoxide, with the yellow flames, and the furnace has some age on it, if you have carbon monoxide, and a cracked heat exchanger, it could be very fatal
     
  6. TC

    TC TS Member

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    Propane with a new heat exchanger several years ago. Three strategically located Carbon Monoxide detectors. As stated the blue/yellow ratio is about 80/20. Any idea what the temperature difference should be?
     
  7. Catpower

    Catpower Molon Labe TS Supporters

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    If it's just the tips that are yellow it should be okay, keep an eye out for any sooting though.

    As far as the temp rise across the exchanger, sometimes thats hard to say, as the fan sizes are differant for differant applications in the country, but it should be about 50-60 degrees, but again that's just a guess. I started in the HVAC business 35 yrs ago in NW Iowa, where we had alot of heat, and low cooling loads, so the heat rise would be higher, than it is in Texas, where I have been for the last 28 yrs. It takes more air for A/C so the unit will have a more powerful fan, causing the TD to be lower in heating.

    Let the furnace run for about 10 minutes to get all the duct and equipment warmed up then check the TD it should be 50+ degrees.

    But if the flame is burning good, with just the tips yellow, you should be alright.
     
  8. TC

    TC TS Member

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    Flame is good, some of the tips are yellow, so I think that part is probably OK. Temperature Differential seems to be the problem. How much of a delay between burner lighting and blower start-up should there be?
     
  9. Catpower

    Catpower Molon Labe TS Supporters

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    How old is the furnace, I don't think there are still any still in operation that used a bi-metal fan switch, they were back in the early 70's.

    In the blower compartment there should be a metal box in front of the blower that has the thermostat wires hooked to the bottom of it, inside that box is a printed circuit board, that controls the fan delay, if it is an original, it was a fixed time of about 1-2 minutes, but they made many versions of that board, some had adjustable times for fan on and off, but with out looking at it, I would just be guessing.

    If it still uses a bi-metal fan switch, the burners come on and the fan won't come on until the heat exchanger gets warmed up.

    If the burners are burning properly, which is what you have said, the time can vary, and the output will be the same as when it was new, I mean you really don't need to be concerned, it's doing what it's been doing over the years, but when you sit and wait for it, it seems like alot longer than normal.
     
  10. TC

    TC TS Member

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    1960's vintage. Doesn't have a thermal fan delay. Fan delay is controlled by an adjustable rheostat. Appears to be aprox. 60 seconds, may have bumped it when I moved it to get everything out. Don't know how long it was in the past.
     
  11. Ljutic111

    Ljutic111 TS Member

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    1960 vintage ?? Get rid of it , for the price of a new one and the value of your life and home , it isn`t worth to keep this one going plus the SEER value is probably very low . Old cars are fun -- old furnaces and AC units aren`t .
     
  12. TC

    TC TS Member

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    Replacement not possible untill next year. Still think it is a delay issue.
     
  13. GordonWood421

    GordonWood421 TS Member

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    TC , is there a disc on the burner manifold that allows for adjustment of the air /fuel ratio ? Most times it is nothing more than the type on some BBQ grills . If one is present , and the burner assy is still OK , you should be able to adjust for a bluer flame .

    Charlie
     
  14. Catpower

    Catpower Molon Labe TS Supporters

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    Where is the reostat located?
     
  15. TC

    TC TS Member

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    Used my torch tip cleaners on the burner orifices and followed with pipe cleaners. The flame tip color is more orange, I said yellow which was an error. Most of the flame is blue, the orange only shows up in the tips. The best flame results when the air shutters are wide open so thats where I will leave them.

    Catpower The rheostat is located in the control box. I had to disconect it because the wire from the thermal relay blocked removal of the thermocouple.

    I seem to see some improvement when the unit is cycling during the day rather than when it comes on first thing in the morning. What is an average fan delay time?
     
  16. tom-n8ies

    tom-n8ies Member

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    What fan delay are you talking about? on or off?

    Depends on how it (fan) is controlled. How long is it taking for the fan to come on after the burner lights right now? How long does the fan run after the burner shuts off right now? Does it end its cycle blowing cold air?

    Normally the burner lights and after some amount of time determined by a thermal switch or timer on the circuit board starts the fan, this delay allows the air to be warm when the fan does come on. When the thermostat is satisfied the fan will run a certain amount of time again determined either by a thermal switch or timer built into the circuit board that may be adjustable for length of on and off time. My furnace (different brand) burns for ~45 seconds before the fan comes on and stays on for about 1.5 minutes after the burner shuts off.

    You may need a new fan control, without looking at it it is almost impossible to diagnose your problem from your description.

    Post some pictures of what you are calling the "rheostat and thermal relay"

    The older bryants had a (4 wires) time delay relay and a small wheel you could adjust for off temp if I remember right. The relay had a low voltage heater in it and bi metal switch to turn the fan on. If it is that old it is probably getting flakey and not making good connection every time.

    As far as not puting out much heat you may have a step opening gas valve and only the low flame stage is working.

    Being it is 60's vintage and having multiple issues you should have it checked by a furnace person.

    tom
     
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