Blower will only come on when in "ON" position. "Auto" position will not cause it to go on. One can hear the thermostat click (basically saying its time to kick in) but the blower will not commence until I manually set it to "ON". At first, I thought it was thermostat so I replaced it with a programmable one I wanted anyway, but the same issue. After turning off the power, I checked connections of wires from thermostat to the blower and back to furnace. All seem tight. All other zones work fine except for this one blower for my bedroom. I've read in past threads something about a relay being the culprit. I'm no expert, but there seems to be only one relay for all three zones... I may just be missing something, but if it works fine for the other 2, why is my bedroom having issues. Anything else I should check? Thanks much.

I seems that you have a forced hot air system - is this correct?

Do you have more than one blower to this system? (most forced hot air systems have only one blower)

If you have more than one blower and the other two blowers are working then it sounds like the third blower motor is either worn out or needs attention.

Need more info.

Zoned forced air systems are not common in residences if that is what you have. The thermostat has a switch to operate the blower manually, but usually in the heating cycle, the blower is controlled at the furnace by a relay activated by limit switch or in newer furnaces a timer on the control board. If you have 3 blowers, you must have 3 relays to run them individually, and likely 6 to provide 2 different speeds for heating plus manual and A/C. They could be ganged in one housing. Likely the problem is on the control board. It has a timer that should activate the relay controlling the blower a minute or 2 after the gas comes on. Problems on the control board are difficult to find and fix. Unfortunately paying for a control board is difficult too, I paid about $300 for one.

Older furnaces use a limit switch to control the blower, but mostly timers now. There are temperature sensors in the furnace, but they shut the gas off if it fails to ignite or the blower fails to come on.

Yeah, I did steal part of this answer from my last.

Likely you will have to replace the control board if that is where the blower controls are. No big job if you can remove about 10 wires and plugs and get them back where they belong. Note the thermostat wire, red on R, white on W, etc. I hate to say replace an expensive board without being sure it is the problem. The other alternative is to pay a technician another $100 to do it.

Will the blower come on if you select On? If so, you may be able to run the furnace that way, buying you time to order a board over the net.

Zoned Forced Hot air seems to be more common around where I live I guess. Most of the newer houses being built have these put in. If it helps, it is a peerless boiler less than 5 years old. We run on oil. Probably obvious, but the A/C and the Heat run off the same blower per zone (in case there are some instances where it requires that a/c have one blower and heat have another). You mention a control board. Where would this be? Also, the blower will come on if I place the fan switch into the "ON" position.

The control board usually is in the furnace. Follow the thermostat wires to it. It will have logic that when the thermostat switches power to the white wire, closes the relays to start the oil pump, the ingnitor, inducer, etc. to begin the combustion cycle. The timer to start the blower and its relay will be there too. Again it is possible it uses a limit switch to start the blower.

In most systems, heat uses the same blower as ON or A/C, but a different winding giving a slower speed. It is possible the problem is after the relay. To check that out you may need to identify the wire or wires supplying that winding. It could be a bad connection, or that winding is open. If you can locate a plug on the board with a bundle of wires that leads to the blower, you could unplug it and check for conductivity of all the wires to the neutral. If they are all connected to the neutral, the problem is before the plug.

I had the furnace cleaned and the technician checked the blower for me. He said it's the heat relay. Installed, it would cost me 130 bucks. Since he didn't have the part in the truck, he'd have to come out again anyway so there's no money saving there. I've been trying to find a site that sells these relays but no luck. Maybe I'm using the wrong terms. Since I've seen posts of folks installing their own parts, I'd expect they know a site to purchase from. Thanks again for all the help. Hopefully these facts help others figure out the issue if the same happens to them.

Do you know specifically where it is? Could be one of those things you take out, plunk down on the counter of a HVAC supply and the guy goes and grabs the right thing. If that doesn't work, plug the part number from it into your favorite search engine.

Where the tech works may sell it over the counter.

Did you get it fixed?
I have the same problem.
After cleaning battery connection of the control box, it works now.
I wonder it will happen again, so I have to know what could lead to this.
More details:
When fan was set to AUTO, heater went on for around 2 minutes, and then shut off, and I could hear sound in the fan (maybe power is supplied to it at this time), but fan did not turned on.
When fas was set to ON, heater went on until the set temperature was reached, but fan kept running.

After cleaning rusty connection piece of the battery, and plug back control box, it works
Now. But I may not correct the real problem. Some parts may be in GOOD/BAD condition,
So I might experience again.

Yes, I ordered the part off a local electronics supply store. Fairly cheap. I just replaced it and it worked fine. I tested it initially by taking from one blower and trying in another. I looked up the blower manufacturer, called up their 800 number and found out a local distributor: the local electronics supply store.