blower fan cycles on and off


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Old 03-04-07, 10:25 AM
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Question blower fan cycles on and off

Hi,
I have a York diamond 90 furnace, about 8 years old. The blower fan goes off about a minute after heat cycle starts (the flame is still on) and even when the set temperature is not reached, then after about 20 sec it goes on again. This is repeated until the set temp is reached. If I remove the filter that is in front of the fan, this does not happen.
Any suggestions on what might be wrong?
Thanks very much,
Rao.
 
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Old 03-04-07, 10:59 AM
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Furnace Fan

What you describe sounds like either the fan motor overheating or the capaitor for the motor breaking down. The motor on this furnace is probably controled by a timer on a control board. If you have a volt meter & know how to use it, we can help you find out if the fan is getting power or if the board is the problem.
 
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Old 03-04-07, 11:36 AM
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Furnace fan

Thanks Grady.
Yes, I have a multimeter and know how to use it. Please help.

Rao.
 
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Old 03-04-07, 11:55 AM
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If I remove the filter that is in front of the fan, this does not happen.
Is the filter new??? What MERV # is it. We have run into some where a very high MERV filter is to much for some blower motors
 
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Old 03-04-07, 11:56 AM
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Multimeter

The easiest thing to test, if your meter has the capability, is the capacitor.
Turn the power off to the furnace & remove the capacitor from the fan (usually mounted on the blower housing). Short the terminals of the cap with a WELL insulated screwdriver. Set your meter to its capaitor furnction & read across the terminals of the cap. Compare your reading to that marked on the capaictor. If more than 10% different (low) replace the capacitor.

If the capacitor checks out, find the control board. There should be a terminal marked "heat" or "heating speed" or something similar. With the furnace operating, monitor the voltage (120 volts AC) between this terminal & neutral. If the fan stops & you still have 120, the board is not the problem.
 
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Old 03-04-07, 12:03 PM
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You will have to determine if power is still feeding your blower when the blower stops. If you can't trace the "run" blower wire ( a wire color that is hooked up to the blower that is NOT green, white or brown...and indeed is connected) to it's connection somewhere back in the furnace where some relay or other control is,... then if you are handy, you could just (with the furnace turned off) cut the "run" wire (not any of the other colored wires I mentioned) and twist them back together for the moment. Then restart the furnace and make sure that when the blower is running, that when you volt probe your meter at the 250 volt setting, for the 110 volt test between the now-twisted run wires and a ground (even the metal of the furnace itself), that if you have 110 you know that at least your volt meter is making good voltage contact. Then wait for the burner to quit with the flame stil going, and see if you still have juice or not, trying to feed the blower.

If you do, obviously the motor is history.

If the voltage quits after the blower motor shuts off with the burner still on, then you have some other issue.

Possibly even a thermostat issue or some weak/pitted? relay switch that is held 'closed', energizing the blower motor.
 
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Old 03-04-07, 12:04 PM
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Fan

Originally Posted by Ed Imeduc
If I remove the filter that is in front of the fan, this does not happen.
Is the filter new??? What MERV # is it. We have run into some where a very high MERV filter is to much for some blower motors
Ed makes a very good point & something I obviously overlooked. Sometimes it is just that simple.
 
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Old 03-04-07, 12:14 PM
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filter

Originally Posted by Ed Imeduc
Is the filter new??? What MERV # is it. We have run into some where a very high MERV filter is to much for some blower motors
Yes, this is a new filter. I don't see any merv#, but this is one of those blue, very thin, $.50 filters from Homedepot.
 
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Old 03-04-07, 12:46 PM
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Filter

The filter is not likely to be the source of the problem but it was worth a shot.
It does seem odd that the problem disapears with the filter removed. Everything else, furnace door in place, etc., remains the same, just the filter removed?
 
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Old 03-04-07, 01:14 PM
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Do you have an A/C A-coil unit above this furnace?
 
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Old 03-04-07, 06:39 PM
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blower fan cycles on and off

Thanks to you all.
Yes, the problem is with just the filter off - actually two filters. For simplicity sake I said 'filter' earlier. As shown below, the xxxx denote filters. For some reason, there are two frames, one vertically and one horizontally which fit one filter each. The blower motor is at position 'O'. When the problem started there was a felt like filter vertically and a 3M high efficiency filter horizontally. Thinking that these might be restricting air flow, I replaced both by the thin ones. Still the problem persisted and just by taking those off (everything else in place), the problem disappears.

...................|..........|
...................|.....^...|
...................|.....|....|
____________|..........|
...................x..........|
...................x....O....|
.air--->.........x..........|
...................xxxxxxx..|
___________________|
Forgive me, but I'm not sure what the A/C A-coil unit is. My guess would be yes since this system is for both A/C and heating. What does it look like or how should I be able to tell? How might that be contributing to the problem? I will check the capacitor and control board tomorrow and report back.
Another development is I called a professional and he said that the fan/motor is defective and needs to be replaced. I, being clueless, wanted to have an expert opinion before I shell money on replacements that may be unnecessary.
Thanks again.

Rao.
 
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Old 03-05-07, 12:51 PM
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What do you get if you go to just fan on??????
 
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Old 03-05-07, 03:12 PM
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If you have an air conditioning coil above the furnace, the reason your furnace might stay going with the filters out, is even though the air conditioning coils might be plugged pretty good...with the filters out, you are able to ram more air through the coils than with the filters in place. The filters might be the last straw due to pretty well partially cloggged air conditioning coils in the plenum right ontop the furnace. If you see copper lines going into your furnace plenum...that is where your coils are.

(Now I have to go back and read your OP to see if there is any issue with high limiting)

Edit: Just got back from re-reading your OP. It is unclear from your post as to how long your blower runs for after the flame is going, before it shuts off. Be as specific as you can about the time it is on and the time it is off, and what the burner flame is doing at those times.
 
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Old 03-05-07, 06:28 PM
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blower fan cycles on and off

Progress:
The answer to Ed Imeduc is - even when the fan is set to continuous ON, blower just cycles on and off (with no burners turning on, of course).
DaVeBoy - Yes there are copper tubes going in above the furnace, so it looks like I have the a/c coils. Could these be plugged badly? Are they accessible to clean?
Regarding the cycle timings, it goes like this. T-stat calls for heat, the vent motor goes on, the ignition is lit and flame comes on, and blower motor starts. After about 2 min, blower stops. Flame is still on at this time. After about 1 min, the blower goes on again for 1-2 minutes and stops while the falme is still on. After another minute or so, the blower starts then in a couple of minutes, the set temperature is reached, flame goes off and the blower stops - till the next call for heat, and then the whole thing repeats. All this with air filters in place. When the blower was stopped, it didn't feel really hot, but just warm to touch.
I think my multimeter doesn't have the capacitance capability, but I saw the tech testing the cap and the reading was 13, the cap rated at 15 mfd. I didn't test the control board.
If the motor is giving up, does the cap need to be changed too at the same time?
Thanks,
Rao.
 
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Old 03-05-07, 07:33 PM
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I've been following this thread and I noticed you didn't check blower voltage at the board when the motor shuts off. It's hard to help if we don't get all the info. As someone mentioned earlier if you have voltage at the motor terminals when the blower shuts down. the motor is bad, if the you don't have voltage the board is bad. To answer your question about the capacitor, yes you should replace the cap. if you replace the motor.
 
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Old 03-05-07, 09:37 PM
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The answer to Ed Imeduc is - even when the fan is set to continuous ON, blower just cycles on and off (with no burners turning on, of course).
This is back to the motor then or the fan relay board
 
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Old 03-06-07, 03:14 PM
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blower voltage

On the board there are 4 wires grouped under 'circulation blower' and labeled cool, heat, park, park. The park wires (red and blue) read 190 and 150 V while the motor is running and both read 120 when the motor stopped.
Not sure if this confirms the motor is bad. If it is, isn't it affected when the filters are not there. I am really naive, can a motor work partially (which is good enough when no filters are there)?
My other question is if the motor runs normally while there is no filters (and so more airflow), there may be some clog at the a/c coils (as DaVeBoy said), which still is a problem. I'm wondering if it's possible rule that out and/or to look at or clean the coils.
Thanks to all for reading this and making suggestions. I'm more informed now than when I started out.

Rao.
 
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Old 03-06-07, 04:33 PM
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Voltages

The voltages on the "parked" wires are not abnormal. With the blower running, you need to check voltage between the "heat" and neutral terminals. With luck, the blower will quit while you are monitoring that voltage. If said voltage drops to or near zero, the board is bad.
Since capacitors are cheap & easy to replace, maybe the best thing to do is to replace the capacitor & try it. If the blower now works ok, it was an inexpensive & easy fix. If not, you are out less than $20.
 
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Old 03-06-07, 07:28 PM
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Voltage at the "heat" terminal is 120 while the motor is running or stopped.
As suggested, I will try out with a new capacitor first.

Rao.
 
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Old 03-07-07, 04:13 PM
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Thank you poster to your very concise description as to what is happeneing. It is not with your a/c being clogged. If it were, the FLAME would be the thing that shuts down. Since it is just the blower that ashuts down, we are all on the same page... back to motor, cap or board,as most likely, not figuring of course some partially bad connection somewhere.

If I got called to such a scenario as yours... one test I woud do for sure would be to quickly meter test when the blower stops with my test meter set to ohms. I would quiclkly SHUT OFF THE FURNACE, then disconnect all wires that connect up between the board and the blower motor (you can leave a green ground wire still connected). Then I would check ohms between the wire that runs the blower and the casing of the blower motor itself to see if there is short. This test must be done within that one minute down-time time frame . This test must be done with wires off, because if you leave them on, this invalidates the test and you will wind up with full continuity, making you THINK there is a short, when there might not be. But, if the wires are disconnected and you have full continuity...even ANY showing of a reading in ohms for this test..the motor is bad and has some short in it to ground.
 
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Old 03-07-07, 04:20 PM
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After that whole spiel, this is the bottom line to keep in mind: With AC motors, sitting disconnected from a power source...you should have no continuity ever between a hot or even a neutral wire of the motor, and the CASE of the motor. With AC motors, unlike in DC, the power has to flow through the windings, insulated from the metal case, and go to neutral, without ever passing through the case... which then, if it unwantingly did so, would obviously cause current to flow through the metal of your whole furnace. (Something you don't want, obviously)
 
 

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