I've installed a 24v transformer in my furnace. Old one out new one in. Simple. NOT!
The heater kicks in, heats to the target temp. when done turns off then the funny thing:
About 30-45 seconds later the blower comes back on and runs about 30-45 seconds then turns off. Does this 2 or 3 times.
Anys tips?

A few questions:
Why did you change the transformer?
What did the furnace do before you changed the transformer?
What makes you think this is a transformer problem?

Thank you for responding. I changed the transformer because I accidentally pulled out a wire thus ruining the old transformer. It started when I decided to rewire the garage and update old fluorescent lamps to new more efficient ones. In doing so I learned that someone before me had pulled electricity from the box that the heater was hard wired into. (The heater is hard wired into a box with its own switch to cut the power to the heater as needed). There were so many lines being pulled from this one box that they had completely overloaded the circuit. I decided to add another circuit to rectify the problem. In doing so, I learned the heater/furnace was not grounded directly. It had a MC cable tied into the box and running back to the heater. I suppose the Metal Clad cable was the ground but did not want to worry about any loose connections, so I pulled out the (2) 12-gauge wires and installed Romex 12-2 inside the existing flex conduit and found a screw in the back of the heater panel to attach to, thereby connecting the furnace to ground. As I was doing this I accidentally pulled the wire out of the 24v transformer. Ooops! I replaced it, but not after a day or two. I am not 100% sure about the way I wired it back up. I replaced a Primary 120v Secondary 24v 10va with a Primary 120v Secondary 24v 40va.

It seems straight forward 120v side had black (hot) & white (neutral) so I connected those same as before. The 24v side had Blue & Yellow & they connect to the thermostat which has only 2 wires. Forming a loop in the electrical connection I connected by memory as follows: I connected the blue 24v side to the white thermostat; then the red thermostat comes back and connect to the gas valve; then a second red from the gas valve I connected to the yellow 24v side completes the loop. See attached pic.
Before all this began the heater worked as it should: the target temperature was set and the gas valve would come on igniting the heat exchange and after about 30 – 45 seconds the blower would come on. When the target temperature on the thermostat was reached the gas valve would shut off, the heat exchange would shut off, and the blower would remain on say another 90 seconds then power down. Worked fine.

Now everything powers up as described, the blower will start up and run a while, then the blower will turn off during the heating cycle for around 30 seconds then come back on, then do it again.
Sometimes it does stay running the entire cycle until the target temperature is reached.

The second thing it does is after everything powers down, the blower will come back on by itself for 15 seconds, then turn off for about 30 – 45 seconds then come back on repeating 4-5 times until it finally stops.

This sounds like a problem with the limit control. There are two units of the brand name CamStat see (2) attached pics.

I never touched either of these EXCEPT for trying the RESET switch on the one on the left.
The switch is on a lever and is “pressed” (as instructed on the unit) – either up, or down. If I let go of the lever, the lever falls on its own and stays in the down position. My train of thought is that this is the RESET position so it should not be down. Right? So I added a plastic plug to hold the lever in the up-right position and test this theory. It does the same thing. Either way with or without the plastic plug it does the same thing. The plastic plug was an experiment trying to eliminate one variable at a time.
I am at a stand still for now, we are getting heat but only under supervised conditions – never at night (I shut off all the power to the heater). Any suggestions? Here are my questions:

· Did the grounding affect the way the transformer works?
· Does the transformer seemed wired correctly?
· Did the new 40VA transformer affect the limit controls?
· Should the lever stay in the reset position?

Thanks ahead of time for helping me. I appreciate it.