I have a 24 year old furnace and A/C unit combined. I have a 4 wire thermostat for the unit. I have interruptable service, where there is a box outside my house, in conjunction with the electric meter for the whole house. The interruptable box ONLY works for the A/C, and sometimes under heavy DTE Energy use, they interrupt it for 15 minutes each hour. When they do, a little RED LED comes on in the front of the box.

Now, last week, my wife and I heard a terrible rattling noise coming from the furnace, when we turned the A/C on. It was one of those sounds that rattles you, as your thinking, fire, smoke, A/C unit going out, etc. Anyway, it did this rattling for about 3 days, and the A/C would NOT work, as the little Red LED was on. However, since then, we have NOT heard the rattling anymore, (fingers crossed), and I can get the heat and fan, and the A/C and fan to work like they are supposed to.

Two caveots: One is the A/C will come on IF the red led is off, BUT it takes about 7 minutes before the compressor outside comes on. The second caveot is: The fan runs continuously, no matter what I do. It works like it always has, just that it won't shut off, no matter what setting I put the thermostat on. I even went and bought another thermostat, and hooked it up, and it does the same thing - won't shut off.

Lastly, I used my meter, and checked voltage between the green and blue wire, (I think it's supposed to be 24 volts, BUT there is no voltage) and checking the other terminals in every combination, there is no voltage? How can that be, I don't know.
Any input would be most appreciated! Joe R.

You probably need one of these.
United Refrigeration, Inc.

The relay contacts that control the blower have stuck together. Common problem. You need to make sure you got the right one and there are a number of wires to connect. Be careful!

To understand measuring the voltage at the thermostat, go to the sticky at the top of the forum here and work down to thermostat wiring. If you don't have 24 volts between the red and the other wires, you have a problem, bad transformer, blown fuse for the transformer, or the red is shorted to something, maybe the green at the furnace which would cause the fan to run. It is also possible you have a transformer problem and a stuck relay.

Is this furnace a Carrier or Bryant by chance? If the system is still functioning for heating and/or cooling, then I do not suspect a problem with your transformer. It may be the circuit board on the furnace. Most older residential Carrier and Bryant furnaces were unique in terms of blower control; that is, the heating-speed blower relay coil was actually powered up to turn the blower OFF, and any failure of the high-limit circuit, low-voltage power supply, or the circuit board itself would result in continuous blower operation in the heating speed. Try this: if you move the thermostat fan selector switch from "auto" to "on", do you notice a change in the blower speed? If so, the circuit board is likely the culprit. Check & post back.

You were right - the relay contacts were shorted. Bought a new one, and the same thing happened. When powered up, the NEW relay chattered, and I had to shut down. I then bought a new thermostat and installed. Things got better. I could start and stop the fan - yeah! The heat worked fine, BUT when I turned on the A/C, the condenser didn't come on? However, I waited about 6 minutes, and the compressor and fan came on, and I ran it all day and was COOL - another yeah!

Okay, now that part of the problem was fixed, I still had the interruptable portion from DTE to contend with, and they came last night and checked the box for that. The tech said she could see a faint, (very faint) red light coming on, so she agreed to change the box.
While she was changing the box, I told her that this problem had cost me having someone come out, and had to replace the relay, AND a new thermostat, and she told me that their interruptable had nothing to do with those problems whatsoever, and they were not liable. However, as we talked, she said that is the interruptable box wasn't allowing 24 volts fully, it may have made the relay chatter? Whatcha think?
Lastly, after she changed the meter box, (Interruptable), the new box hummed. At first she said she heard it, and said its coming from inside. I convinced her to put her ear to the new box, and she said, YUOP, it's definitely coming from the box, and told me that I needed to call an electricial. OH, by the way, the Same hum, is coming from the furnace, even when it is running fine. Whew, that's a lot, can anyone help? Thanks in advance, Joe R.

The 6 minute thing is normal.

Relay chattering will damage contacts. More so, he fan contacts than the t-stat.

HUM:
Some causes:
Transformer is overloaded.
Transformer is mechanically broken.
Transformer is not properly mechanically fastened.
Transformer is attached to a flimsy piece of metal.
Sometimes it's normal.

Listen to how loud it is and whether the transformer is hot.