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Furnace (on dedicated circuit) starts and lights (on other circuits) dim?

Furnace (on dedicated circuit) starts and lights (on other circuits) dim?


Old 02-15-05, 12:02 AM
sparks n arcs
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Furnace (on dedicated circuit) starts and lights (on other circuits) dim?

I recently ran new 12/2 to my old furnace. Furnace is gas but is quite old (actual age unknown). The house was built in '58 and I guess it could be that old. But the furnace is on its own 20a breaker and when it starts the lights throughout the house dim just for the start up period. Should I be looking at the furnace as the culprit or should I be looking elsewhere? Currently have 100a service with a Square D QO breaker box. The range, clothes dryer, and the water heater are all gas. It is now 2am and all that is on is the light in the computer room, the computer, and the furnace. When the furnace kicked on just a minute ago the light in the computer room dimmed for about 1 second.

Thanks for the help again.
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Old 02-15-05, 07:36 AM
Join Date: Sep 2000
Location: United States
Posts: 18,497
Could be a poor connection of your service wires, an undersized utility transformer, undersized service wires, or a poor connection in your panel. The first three of these are the responsibility of the power company. You could complain to them, but your complaint would sound better if you had an electrician check out the fourth item first. You also might have more success with the power company if you said that you were planning to upgrade to 200 amp service, but that this issue concerned you. The utility is more likely to upgrade their lines and equipment if it is in support of selling more power.
Old 02-15-05, 10:35 AM
Join Date: Sep 2000
Location: Fayetteville, NY, USA
Posts: 1,052
I would generally consider the dimming to be normal. When the furnace kicks on the fan motor starts, and motors can draw up to 400% of their rated running current, or full load amps (FLA). That produces a voltage dip but, as you described, it lasts only about a second. I would only start to worry if your furnace circuit breaker trips, but that is apparently not happening.

For most of the gas furnaces I've seen, they normally require either a 15 amp or 20 amp circuit. 12-2 is rated 20 amps, so you're covered. I would be more comfortable knowing what the nameplate on your furnace says in terms of breaker sizing. There should be a box on the nameplate that tells you. Sometimes it says "max OCD", or over current device (breaker or fuse). If you knew the fan motor size, in horsepower, I could tell you what breaker size would ordinarily be required for that motor. But the NEC says manufacturer's nameplate rules. Just a quick check for peace of mind. (That won't help your dimming problem, though.)

Old 02-16-05, 08:35 AM
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Join Date: Sep 2003
Location: The Colony, Texas
Posts: 917
Unless other things are causing the same problem or lights are getting dimmer AND brighter at times, I wouldn't even address this as an electrical issue yet. I'd address it as a furnace issue. A 47 year old furnace motor is probably going to pull a bunch of extra power at start-up. It may have a bad capacitor or be full of "gook". Either way, I'd be saying thanks every time it comes on at all.

If the furnace is original to the house, it's probably around 60% efficient. A new one would be 80 - 90% efficient and save at least 20% on your gas bill and, from the sound of things, some on the electric bill too. Do the math and look into financing options. Replacement may not cost you a thing. If the lights dim when the new furnace starts, come back here and we'll look at electrical problems.

Doug M.
Old 02-16-05, 09:58 AM
ElectricalMan's Avatar
Join Date: Feb 2005
Location: Harrisonburg
Posts: 744
I saw this happen on one situation on a service call and I tightened the main lugs on the panel and it stopped ......might have been to this case only but it worked...lol...loose connections in the panel can cause this sometimes also as they draw more powier and arc....

I always suggest tightening of the main lugs and terminals if AL every 1-2 years.....hey I charge for it so stop laughing...lol...they do work loose over time...

BUt I would also say yes it is time to think about a new furnace in my opinion..but you can start by tightening all connections in the panel.....

Also for the heck of it...if you had a AMP Meter you could measure the draw on start up....but as someone said I bet you would be way more efficient in replacing it...but only if that is a financial option otherwise the breif flicker is not going harm much....

John has some good points.....and so does doug.....if it was me ( a electrician ) I would tighten all my terminals in the panel, if that did not work I would clamp a AMP Probe on the furnace line...and they try it on the main lines.....and do a start up.....to see....

Then think about the furnace upgrade...would save you money to pay for itself probably over time
Old 02-16-05, 03:50 PM
sparks n arcs
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Hello and thanks,

A furnace replacement is definately in the works, just not immediately. At the moment the Mrs. has me doing a complete kitchen remodel and the funds are running low. I suspected that the age and gunk build up inside the furnace to be the problem, but just a guess. I agree that a furnace replacement will be worth the $$$ when I have 'em.

The connection from the furnace to the panel is good and snug. Tightening up the main lugs seems worthwhile, although I don't think that I'm supposed to go out back and remove my meter to do it. I'm definately not touching them with the meter on. I'm guessing that power companies frown on homeowners that pull thier own meter for any reason. Should I give them a call to see if it can be breifly removed, if not by me then maybe by them?
Old 02-16-05, 03:54 PM
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Join Date: Feb 2005
Location: Harrisonburg
Posts: 744
ahhh...dont go into the meter cab.....just the main lugs on your service panel is what we are refering to not in the meter cab itself...usually they tag that with a clip and you can't get into it.

It is always a good to check the main lugs and breakers screws every few years to keep them snug....but only at the service panel and not the meter cab...yes I can see the utility company not wanting that...
Old 02-16-05, 04:08 PM
sparks n arcs
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Also the plate on the blower motor reads - v 115, a 5.4, hertz 60, 1/3 HP, 1725 RPM.

Electrical Man, the only reason I thought the meter had to be pulled is because I thought that the main lugs in the service panel (breaker box) are always hot. Am I incorrect? I was able to check tightness for all the connections for the branch circuits at the breakers and the neutral and ground busses, but I have not touched the main lugs.
Old 02-16-05, 05:46 PM
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Join Date: Sep 2003
Location: New Hampshire
Posts: 174
Is it possible the furnace breaker and the light circuit are on the same leg? It might help to move the furnace breaker to the other leg (up or down one slot in most panels).

Old 02-16-05, 05:55 PM
Join Date: Sep 2004
Posts: 83

Don't attempt to tighten anything energized in the panel. You have done your job by tightening all terminations that could possibly be done by turning off the main breaker. Yes, the wires on the main are HOT from the meter.

I would NOT hesitate calling the utility company at this point. My neighbor had the same situation for a week (lights dimming with dryer, range, and furnace). He called me on a Sunday morning because he lost a phase completely in the meter socket, the utility company disconnected him at the pole and told him to get an electrician and replace meter socket and drop. It was quite a mess in there....Thank goodness for Home Depot!
Old 02-17-05, 02:39 PM
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Join Date: Jul 2002
Location: USA
Posts: 1,839
My neighbor had a simular problem in that some lights/outlets worked, some didn't, but none of the breakers were tripped. With the handy voltmeter I saw that one side of the line was dead.

I went outside to the meter box and checked again, lines coming in are good, so I test the breaker.. odd, doesn't want to turn off.. yep, one half of the breaker went bad, so only 1 'phase' was going through, something internal went bad on it, probably a power surge got it. Took the breaker out and replaced it with a new one and all is well.
Old 02-18-05, 06:25 AM
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Join Date: Feb 2005
Location: Harrisonburg
Posts: 744

I got ya....I sometimes forget not everyone here is a Electrician..lol.....Yes, We do check main lugs....but we are trained to do this....sad thing is I have found ALOT of loose ones that over years the AL works a loose arc connection and causing issues....

BY no means do I want ANYONE tightening the main lugs unless you are a licensed Electrician and understand how to do this......tightening the breakers is fine.......

When we wire a house we offer free follow up every year for 5 years....to tighten lugs and so on....part of our package...

Yes, you did right......don't mess with the main lugs.....Even when WE do this we have a long insulated alllen wrench that is 1 foot long so that we contact nothing in the panel but the lug...and no other contact...just for safety even after doing this 17 years.....you can NEVER be too safe....

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