Momentary Power Loss or Drop (Long post)

[MikeS]

Long time lurker, first post, thanks in advance.

Got a perplexing service call regarding a new gas furnace in a crawl space that gets caught in a cycle-- the blower will sometimes slow down and then power up, sometimes just once, sometimes it'll power down & ramp up 10-20 times. Not normal behavior for this unit. The phenomenon can happen at any point in the cycle, either heating up or in the cool down phase.

HVAC company suggested "neutral backfeed". Not sure how they arrived at this conclusion without test equipment. Any rate they say the problem is electrical in nature so I was called & it's my baby now.

With a clamp-on ammeter I observed the amperage drop from 5.6A to 2 and bounce back and forth 5-6 times.

Peripheral info:
The unit acts up more when it's cold, and usually at night.
House built in 1991.
Square D QO Panel 200A.
The current furnace is new ~6 mos. The previous unit circa '91 did the same thing. There is an identical unit in the attic that operates flawlessly.
I've checked & tightened all connections in the panel & meter base.
I replaced the unit's breaker, wire, thermostat wire and disconnect switch.
All flue & duct work was replaced with the unit, as well as the gas meter and the thermostat.
HO reports they have lost a fair amount of electronics over the last few years, Microwave, Washer, TV, VCR, X10 switches. Contacts blew out of one A/C Compressor and Disconnect Switch Fried on the other (these happened about 2 yrs ago).
Ordinarily I'd say defective unit, but the fact that the previous one did the same thing indicates otherwise.
What am I missing?

 

[SG-1]

Can you take some voltage measurements while the equipment is malfunctioning ? What is the blower motor voltage ?

Is it posssible, if the motors are 120V, that the one in the attic is on the other line ? Is anything else always running when the motor slows down ?
During the winter my line voltage is lower at night.

Measure the voltage at the unit while the motor is running slow. Start right at the motor terminals if possible. If the measurement is bad move back toward the service one step & check again. You could start at the service and work toward the motor.

By one step I mean any device or connection in the motor circuit.

Work Safe &
Good Hunting !

 

[MikeS]

Yes, motor voltage is 120. The attic unit is on a separate line & different phase in the panel.
As far as anything else running during the slowdown, not consistently, there was a moment when we thought the kitchen stove set the thing off, but now it'll drop out regardless of the stove.

 

[readydave8]

If possible I'd run a circuit directly from the panel to the furnace and see if this makes the problem go away. It sounds like you have already checked connections, so what if there's a bad splice concealed in a wall?

Then again I had a similiar problem with motors and finally found the neutral bar in the panel to be broken. Since it was still in place it made connection most of the time, when I started tightening lugs it twisted and so thats how I found it.

Only the circuits below the break were affected (obviously once I knew what was wrong), but before I saw the break I couldn't see what the affected circuits had in common.

 

[charlietuna]

You really need to know what the voltage and amperage is doing--does this unit have any variable speed controls in it -- does the motor have multi speed taps inside the unit? If so how are they connected--maybe wrong slpice there ? Consider investing in a data logger --- easy and professional way method to troubleshoot most problems. Set the unit up and leave --have the customer call you the next time it occurs--pick up the logger--download the info -- charge the customer for your labor and a fee for data logger rental..

 

[Volta]

Check the neutral serving the building.

 

[jwjrw]

Check the neutral serving the building.

My thought exactly and even more when I read about the electronics burning up.

 

[electricman2]

Check the neutral serving the building.

The OP states "There is an identical unit in the attic that operates flawlessly"
If that were the case, looks like both units would be affected.

 

[jwjrw]

The OP states "There is an identical unit in the attic that operates flawlessly"
If that were the case, looks like both units would be affected.

I have seen so many weird things caused when a poco neutral was bad or going bad I would still want it checked on poco side.

 

[Volta]

The OP states "There is an identical unit in the attic that operates flawlessly"
If that were the case, looks like both units would be affected.

Maybe . . .

 

[hunt4679]

I have a 2 year old furnace that has done that since day one its not the blower motor on mine its the power vent. No power issues and was told its normal?

 

[nakulak]

Long time lurker, first post, thanks in advance.

Got a perplexing service call regarding a new gas furnace in a crawl space that gets caught in a cycle-- the blower will sometimes slow down and then power up, sometimes just once, sometimes it'll power down & ramp up 10-20 times. Not normal behavior for this unit. The phenomenon can happen at any point in the cycle, either heating up or in the cool down phase.

HVAC company suggested "neutral backfeed". Not sure how they arrived at this conclusion without test equipment. Any rate they say the problem is electrical in nature so I was called & it's my baby now.

With a clamp-on ammeter I observed the amperage drop from 5.6A to 2 and bounce back and forth 5-6 times.

Peripheral info:
The unit acts up more when it's cold, and usually at night.
House built in 1991.
Square D QO Panel 200A.
The current furnace is new ~6 mos. The previous unit circa '91 did the same thing. There is an identical unit in the attic that operates flawlessly.
I've checked & tightened all connections in the panel & meter base.
I replaced the unit's breaker, wire, thermostat wire and disconnect switch.
All flue & duct work was replaced with the unit, as well as the gas meter and the thermostat.
HO reports they have lost a fair amount of electronics over the last few years, Microwave, Washer, TV, VCR, X10 switches. Contacts blew out of one A/C Compressor and Disconnect Switch Fried on the other (these happened about 2 yrs ago).
Ordinarily I'd say defective unit, but the fact that the previous one did the same thing indicates otherwise.
What am I missing?

did you check to see if it's a multispeed motor, and if the main board is calling for different speeds ?

 

[MikeS]

I have seen so many weird things caused when a poco neutral was bad or going bad I would still want it checked on poco side.

OK so, how do I convince poco to check their neutral?
They put a data logger @ the meter for about a week. Everything looks OK there. Phase to phase everything about equal--only minor deviations, absolutely nothing that corresponded with the unit acting up.

Re: Checking the voltage @ various points along the run; I got a consistent 124.3V - 123.9V. Even when the unit was acting up.

The motor is not a variable speed.

The unit is a forced draft.

I misunderstood the HO about the attic unit. It is not identical. Smaller & natural draft. However, the previous crawlspace unit which exhibited the same issue was a natural draft unit, So I'm not inclined to think draft or gas supply issues.

 

[SG-1]

The low ampere readings appear when the motor is running slow or normal ?

 

[MikeS]

The low ampere readings appear when the motor is running slow or normal ?

Low amps appear when the motor slows. If it matters, it never got down near zero, bottomed out at around 2 and then ramped back up. The unit at rest (between heat cycles) hovers around 1.2A

 

[MikeS]

New Data

New Data

HO was anxious to help diagnose. So I had them shut all 240V Breakers off for a bit (Range & 2 A/C Compressors). Unit cycled like crazy and last night ligfhts flickered with the cycling. The phenomena is now repeatable. I haven't had an opportunity to go back yet, but this is seeming like a loose neutral. Correct?

 

[ptonsparky]

Normally with a loose Pico neutral you will notice some lights going bright as others dim. Any mwbc will exhibit the same behavior. What you describe now could be a poor connection of any kind.

I have a variable speed blower on one of my furnaces. It speed is determined somewhat by the back pressure in the duct work and also varies along with the stage of heating. This is a zoned two stage unit. Is your unit switching backz forth between the two stages? Did they put in new thermostats?

 

[Volta]

OK so, how do I convince poco to check their neutral?
They put a data logger @ the meter for about a week. Everything looks OK there. Phase to phase everything about equal--only minor deviations, absolutely nothing that corresponded with the unit acting up.

The phase to phase voltage may be steady even when the neutral voltage varies. They need line to neutral measurements.

Re: Checking the voltage @ various points along the run; I got a consistent 124.3V - 123.9V. Even when the unit was acting up.

Might be good, though if the other hot to neutral at the same time was 116 or so at the same time it may still imply resistance in series with the neutral.

If that neighborhood has metal water mains connecting several houses, you may read an abnormally high current on the GEC to water to street to another house to the transformer/utility if the neutral is compromised.

 

[gar]

100207-2327 EST

So far the reported symptoms seem to be inconsistent.

One confusing item is the report of flickering lights. This is not consistent with constant voltage at the furnace if there is some relationship between light flashing and speed variations.

If at the problem furnace the 120 V supply voltage at the furnace input terminals is quite stable while the motor speed is varying, then the problem is internal to the furnace. Ideally as mentioned you need a recording volt, amp, and power meter. At the very least a recording voltmeter.

If the furnace input voltage changes with the changing speed, then the problem or most of the problem is most likely external to the furnace.

One assumption is that the furnace is only supplied with 120 and no 240 goes to the furnace.

.

 

[SG-1]

Low amps appear when the motor slows. If it matters, it never got down near zero, bottomed out at around 2 and then ramped back up. The unit at rest (between heat cycles) hovers around 1.2A

I was considering an obstruction in the air flow. That should have caused the opposite to happen. Higher current when slow/lower current when fast.

Are you measuring a steady voltage at the motor terminals during speed changes ?

Does the motor have a capacitor ?

What type of lights are flickering ?