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Old 03-19-2011, 10:48 PM   #1
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furnace does not stay on


Having issues with my Fraser-Johnston forced air furnace (Model PBNU.) Everything seems to work, venter blower kicks on, HSI glows, burners light, heat blows for approx 6-7 mins and then flame goes out (venter blower still runs)? After this sequence it will not restart on a consistent basis. Here is a list of what I have done so far

-I cleaned the flame sensor.
-Changed batteries in programmable T-stat
-changed back to original dial T-stat (changed to prog. back in Nov)
-Turned dial T-stat to lower temp than current temp and furnace still tries to run?
-Disconnected T-stat all together and tied wires together to always call for heat and still shuts off
-No fault lights on HSI control board (White Rodgers 50E47-150) Light does come on during initial reset to verify power

Any help would be greatly appreciated. Local company wants $125 for first half hour on weekend...

Nick

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Old 03-19-2011, 11:00 PM   #2
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furnace does not stay on


Sounds like a high limit switch meaning the air in the unit is trapped (not flowing freely enough) so it's getting too hot inside of the unit and cutting out on high limit/excessive heat. It's a fire safety switch is all. The blower and the vent should stay on to try and disperse the hot air and exhaust gases out of the unit/home after the burners cut out.

If this is the case then there is an air flow problem. Check the evaporator coil to make sure it is clean to allow air to flow and make sure all return air filters are clean so you have enough air coming into the unit.

There is such a thing as a temperature rise that will be listed on the unit somewhere, perhaps on the inside of the door on a data plate with the manufacturer's suggested temperature rise. Use a thermometer at the return to check air temperature going into the unit and at a supply grill (preferably closest to the unit) and check the temperature difference and match it to the temp rise. If the air is going in at 75 at the return and it's blowing out 160 than your temp rise would be 85 degrees and that is usually way out of range. I just used those numbers as an example.

I say if because you have no fault codes so I'm just guessing.


Last edited by Doc Holliday; 03-19-2011 at 11:36 PM.
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Old 03-19-2011, 11:36 PM   #3
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The air temp rise is listed as 45-75 F. When you say check the air coming into the unit, where are you talking about exactly?

So the air coming out of the closest register should be 45-75 deg warmer than the air going in?

Thanks for the quick reply.

Nick
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Old 03-19-2011, 11:46 PM   #4
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The air temp rise is listed as 45-75 F. When you say check the air coming into the unit, where are you talking about exactly?

So the air coming out of the closest register should be 45-75 deg warmer than the air going in?

Thanks for the quick reply.

Nick

I meant the air at the return air grill, the air being sucked back from the living space and into the unit. It is that air that is being heated up and coming out of your supply air vents.

If you have more than one return air grill/vent use the main one, usually the biggest one and located near the thermostat.

Yes, those numbers are the temperature rise and yes you have it right, the air should be coming out within that warmer air range of 45 to 75 degrees warmer from what is going into the unit from your living space at the return grill. Usually we shoot for something close to the middle range (we can adjust the temp by speeding up or slowing down your blower motor-faster blower would mean lower temp rise and the opposite for slower blower motor, higher temp rise) so in your case I'd be shooting for nearest 60 degrees temperature rise, exactly middle of 45 and 75. Anything above 75 degrees and your unit is designed to shut off. Anything within 45 to 75 will and should be fine.

Last edited by Doc Holliday; 03-20-2011 at 12:08 AM.
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Old 03-19-2011, 11:47 PM   #5
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And yes, use the supply vent that is closest the unit as the further away the cooler the air can get so the closer you are to the unit the more accurate the temp coming out of the unit.
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Old 03-19-2011, 11:58 PM   #6
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Btw, on heating mode the blower is usually set up to blow slower as you want it to pick up the heat. Adjusting the speed to a medium hi will do nothing but increase static pressure if there is air flow blockage along with maybe kill your blower motor as it's fighting to keep air moving.

If this is a new problem, the unit cutting off in heating mode, then the speed has nothing to do with it. Air flow does meaning the coil is more than likely dirty (and/or the return air filter is blocked) so not enough air is flowing out and so even then the temperature rise will be off as the right amount of hot air is not reaching the supply vents but rather the air is being trapped/slowed down before reaching the vents by the (blocked) evaporator coil.

Have the evaporator coil checked and cleaned would be my recommendation.

Let us know.

Last edited by Doc Holliday; 03-20-2011 at 12:26 AM.
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Old 03-20-2011, 09:14 AM   #7
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Originally Posted by Doc Holliday View Post
Btw, on heating mode the blower is usually set up to blow slower as you want it to pick up the heat. Adjusting the speed to a medium hi will do nothing but increase static pressure if there is air flow blockage along with maybe kill your blower motor as it's fighting to keep air moving.

If this is a new problem, the unit cutting off in heating mode, then the speed has nothing to do with it. Air flow does meaning the coil is more than likely dirty (and/or the return air filter is blocked) so not enough air is flowing out and so even then the temperature rise will be off as the right amount of hot air is not reaching the supply vents but rather the air is being trapped/slowed down before reaching the vents by the (blocked) evaporator coil.

Have the evaporator coil checked and cleaned would be my recommendation.

Let us know.
This is a new problem and I am willing to accept the fact that the coil is possibly dirty. Is the evaporator coil something I can check/clean myself? If so, I humbly ask where they are normally located?

I intend to call a service tech tomorrow morning, but would love to save myself some money and fix it if possible.
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Old 03-20-2011, 09:36 AM   #8
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I also watched the Fan Limit switch (Dial with 3 prongs.) As the furnace sits idle the dial turns counter clockwise, as the furnace runs it turns clockwise.

I was watching it while the furnace was running and it slowly turned clockwise until the burners shut off. Once the heat cut off the dial slowly starting turning counter clockwise.
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Old 03-20-2011, 09:47 AM   #9
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This is a new problem and I am willing to accept the fact that the coil is possibly dirty. Is the evaporator coil something I can check/clean myself? If so, I humbly ask where they are normally located?

I intend to call a service tech tomorrow morning, but would love to save myself some money and fix it if possible.

Follow the copper linset from the outside unit to the inside unit. The piece of equipment on the inside that the line set is connected to is the evaporator coil located just past the furnace. This is the place where the evaporation and saturation process of refrigeration takes place, the heat from your living space being picked up in the evaporator by the refrigerant.

If you have an access door on top (or anywhere, usually it'll be on top) of the transition from the furnace to the coil then it is easy to open that door and take a look. If not than it can be a bit of trouble as you'd have to open up the transition and closing it back properly would be your next dilema.

To clean an evaporator coil the transition will have to be opened up unless there is an access door.

Can you take pics of the furnace and set up?
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Old 03-20-2011, 10:09 AM   #10
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furnace does not stay on


As you can tell by the picture with the vent motor I have not kept up on the cleaning...
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furnace does not stay on-p3200033.jpg   furnace does not stay on-p3200034.jpg   furnace does not stay on-p3200035.jpg   furnace does not stay on-p3200036.jpg  
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Old 03-20-2011, 10:57 AM   #11
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All of that dirt in the unit only leads me to believe that the coil is absolutely filthy and blocked. You might want to call out an hvac company to clean it and get you going. Have them check out everything in that system like in an anual check up. Although you can clean a coil, this time around I'd leave it to the pros for peace of mind but you can watch and ask questions so you know for next time how to clean your coil.

Invest the money in it my friend, you won't be sorry.
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Old 03-20-2011, 10:59 AM   #12
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I don't see any copper lines leading to the evaporator. I think that it is above the furnace, pretty much sitting on top of the upflow furnace.
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Old 03-20-2011, 11:04 AM   #13
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There should be a 3/4" white pvc drain pipe extruding from the evaporator coil. It will be insulated with black armor flex insulation. Check around and see.
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Old 03-20-2011, 11:41 AM   #14
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Is this little black box what your talking about?
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Old 03-20-2011, 01:21 PM   #15
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You'll have to forgive me ndove, I'm not familiar with systems located in basements. Where I live everything is in attics and are mostly horizontally installed and look like so:



You can clearly see the copper refrigerant lineset looping down into the evaporator coil (one line, the bigger of the two copper lines, is insulated) which is butted directly against the furnace (far side) and the small drain line under the coil in this picture. In this picture the drain line from the evaporator is NOT insulated (but it should be) and is heading off to the left to a plumbing drain line in the wall.

In your pic I have no clue as to what those copper lines are in that black box thing with a ground wire attached.

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