At my parents house, they have a single pole light switch and a duplex outlet in a double gang box. It quit working last week so the landlord came and rewired it. My mom has the microwave plugged into the outlet and the switch turns on the ceiling fan. After their landlord rewired it, when my mom turns on the microwave, it starts up and then everything just browns out on the microwave and it turns on the fan until you push stop on the microwave. It only does it with heavier appliances such as vacuum. The fan works fine without anything plugged in. My mom moved the microwave to a different plug and she doesn't use the fan. Can't get ahold of landlord and he doesn't want us doing anything to the house. I'm just curious of why this would be. Any info would be appreciated.

He somehow wired the receptacle in series with the fan.

I know what wiring in series means but I'm not sure how you would be able to wire in series in this case.

The microwave is a heavy load compared to the fan, so both in series cause the fan to run at almost full voltage and the microwave to run at reduced voltage. I hope your landlord hasn't ruined either the fan or the microwave.

The microwave still works. My mom has it on a different plug. The fan works fine, they just never use the plug. The scary thing is he did all of the electrical in the house and one old plug in the bedroom quit working and there are a lot of circuits that are overloaded. The furnace runs their bill up to almost $400 a month just because it burns so much gas when it runs. The fire in it is yellow and it is big flame. One time it caught the thermostat wires on fire. Thankfully my dad was working on the furnace when it happened so he put it out.

If the power goes to the fan box first and then to the switch box with a three wire, he may have mixed up red and black at the switch. Your dad should not be working on the furnace never mind while it is running. Buy them a CO detector for xmas.

Look on the Web for the proper colors for a gas flame. Yellow = incomplete combustion = CO.
Seems like your landlord is endangering public health, and yours.

They have a co detector right by the hot air vent closest to the furnace and one by the furnace and it has never went off

Maybe no CO, maybe it no workey and it's there to humor you.

I'd have a sensitive CO detector (less than 35 PPM) in the bedroom. As to mounting height, CO weighs less than air
http://www.engineeringtoolbox.com/molecular-weight-gas-vapor-d_1156.html
but the difference may be negligible.

While this may be technically true the difference is so minute that it does not matter. Just breathing will create enough air movement to mix it up.
Conclusion: It's not worth mentioning.