Lights dim when furnace turns on

I just bought a home that was built in 79. The previous owner renovated a good portion of the basement, and unfortunately, he was a do-it-yourselfer with low standards. Since it is now winter, we had to crank-up the furnace, and I've been noticing that when the furnace kicks on certain lights in the house slightly dim and then return to their original brightness.

I have a 200-amp front-end, and the furnace is on a 20-amp breaker. My furnace room contains a metal junction box that the previous owner used to power additional stuff like outlets, recess lighting, track lighting and a fireplace fan. This box is crammed with romex. In fact, I'm afraid to remove the cover for fear that my head will be ripped off.

So far I've check the power at the box, and got a reading of approximately 123.4 volts. This reading is also consistent throughout the house. Should I replace the metal connection box with a larger plastic one, and check all of the connections contained within? Or is there a better solution? Can anyone suggest others things for me to try?

 

I really know nothing about this, but would just like to point out that my house has also done that ever since I've lived here. I've never thought anything of it. Is it really something to be concerned about?

 

I don't know a lot about what causes these types of problems, but I do know in some cases it can be bad. My first guess was that I had a problem with my service. So I checked the power at the breaker box, and everything seems to be all right. Today I decided to ask for help.

 

Slight dimming of lights is usually not a serious problem. More significant dimming often indicates a poor electrical design and/or loose connections.

 

Are the lights that are dimming on the same circuit as the furnace? Normally a furnace would be on its own circuit without any other loads to contend with.

 

Thanks for the response. To answer your question, the lights that are dimming are on two circuits. One being the furnace circuit and the other the upstairs bedroom circuit. I also thought that the furnace should have a dedicated circuit, but as I said the previous owner did some work, which drew from the furnace's circuit. I didn't think that a furnace (pump plus igniter) drew much current. But now I'm not so sure. Maybe I should dedicate a circuit for the furnace. What do you think?

 

Yes the furnace should be on a dedicated circuit. If you have other things on it and they trip the breaker when no one is home your libel to come home to frozen water pipes .
As for the dimming of lights slight dimming is not a major thing to be concerned about but major dimming may mean it is time to tighten connections around the house if you experience the problem over two or more circuits then I'd seriously look for loose connections in a neutral somewhere.

 

How difficult would it be to add a new circuit from the main panel to the furnace? Dedicated circuit. That's the way it should be done anyway. That should take care of the dimming.

 

It would be quite easy. It is a straight shot through the joists and I can access every inch through the ceiling panels.

 

Electric motors pull significantly more current at start up than after coming up to speed. The motor is operating at stall (called 'locked rotor') when power is first applied untill motion starts and the current falls to the running level. The heater should be on its own circuit for the reasons already stated, but dimming of lights on a circuit shared with a motor is pretty much normal when the motor starts up.

 

Thanks for the responses. This helps me a great deal.