Gas and Oil Home Heating Furnaces - No fan /no ac/no heater at all

It was working till the owner turned it off at the thermistat. Then when she turned it on. Nothing happened. I seem to have traced the problem back to the furnace red-white wire.I had to leave for work before I could read some voltages there. Is this going to be the location for control power? Is there a "general" wire schematic/block diagram of a "split" system anywhere on the net I could look at?. I found a box in the attic (voltage transformer??) with red and white thermistat wires (with brown covering) . There was 24vac there. All breakers inside and out have been checked.I have voltage at L1&L2 but no power to control relay.Should I be able to read voltage to "C' inside the thermistat? Does the furnace box have fuses or breakers in it? What should I do next?

Open the thermostat to get to the stat wire connections. (You may have to take it apart some.) Use a jumper wire to temporarily connect R to G. (This should make the indoor fan run.) Then jumper from R to Y. (This should make the compressor run.) Connecting all 3 together will make it run in cooling. (Unless you have a heat pump...but you said furnace.)

DO NOT JUMPER FROM C TO ANY OTHER TERMINAL AT ANY TIME.

Starting with the stat makes sense to me, since the problem seemed to have started there.

Ok I jumped th t-stat out and everything worked as you said it would. I cleaned some contacts with an eraser and put it back together. It ran all night and into late today. I wanted to clean it better so I cleaned it with a small wire brush. I put it back together and turned it to cool and everything worked fine. A few moments later(10 min) it quit again and wouldn't do anything. I pulled the t-stat back off and jumped the wires again but this time NO fan / No AC/ no heater at all---What gives???

Hi,

I would add to what Lynn said to first, after taking off covers to expose wiring at t-stat, turn t-stat Heat/Off/Cool switch to Off and Fan/Auto switch to Auto. Then set voltage meter to the closest setting above 24vac. Then place your Red meter lead on the Red (RC) terminal and the Black meter lead on Green (G). You should read approx. 24vac. Then leave the Red lead on RC and change the Black lead to Yellow (Y). You should again read approx. 24vac. If you have 24vac on both of these, then proceed as Lynn stated. If not, your problem will be in the 24vac low voltage circuit between the transformer and your t-stat. You'll have to trace back from t-stat backwards toward transformer due to no power at t-stat. Hope this helps. Take Care and keep us posted please.

CoolBreeze

Read my edit to the No fan/noAC/No heat post down below. I need more help!!!

Hi,

If you mean you cleaned the t-stat contacts with a small wire brush, you most probably shorted out your 24v circuit and blew the transformer. Test for 24v power, as in my reply and then post back with results. Do not jumper anything, just test for power following the instructions listed. Take Care and I'll monitor your reply.

CoolBreeze

Hi,

To keep down any confusion on your posts, just scroll to bottom right of page and click "Post a reply", type a subject name and your message and scroll to bottom and click "Submit reply".

This will keep you from having to edit your posts and will keep ALL posts on board in the order posted. This helps us to help you by being able to re-read and study what has been posted. Thanks and Best Wishes. Awaiting your reply.

CoolBreeze

10-4 , I was not familiar with this forum. I will try again and post my results later---Thanks again

Hi,

I'll monitor for your reply when you're ready. Take Care.

CoolBreeze

Well I had my friend call the AC repair man cause I was experiencing multiple symptoms in the system (frozen pipe at the unit outside). The T-stat was replaced (you all figured that ) and 3 lbs freon was added to a 20+ year olds system.I think it was $175 dollars. ---Thank you all for your help. If the freon is low will that make the outside unit work too hard and kill control power? I wish I knew of a site where I could read the theory of operation of the A/C system (and what each part plays in the system and how they relate to each other------Thanks again all who posted a response--

http://www.howstuffworks.com/ac.htm

http://howthingswork.virginia.edu/home.html