Need Help > Furnace Starts But No Fan (Sometimes!)

[coffee-rhino]

I have looked everywhere and cannot find an exact problem like mine nor a solution that seems suitable. It may be that I overlooked some things, but I cannot figure this one out!

This is a weird one and this may be a long post. I want to make sure anyone helping has as much information as possible.

I've been in this house since 2017 with no issues. The problem started this summer (2023) when we had the condenser replaced. This was due to the line freezing and no cold air coming through the vents. HVAC pros came, replaced the condenser and AC started working again.

Well, within a few weeks, we noticed the condenser would turn on but not the fan to circulate air throughout the house. We would turn off the AC, wait a few minutes, then turn it back on and voila, the fan would turn on. This happens intermittently throughout the months, but now with the colder months this is becoming a bigger problem since now it doesn't consistently work with heat.

Essentially what is happening now is we turn on the heat from the thermostat. We simply move it from 68* to 72*. We hear the furnace turn on, but the fan does not. The furnace will stay on for about 2-3 minutes, then shut off. When I look at my nest I see these errors (attached). Then, within 5-10 minutes, the furnace turns back on AND the fan does. This happens at completely random times. Sometimes we can turn it from 68* to 72* and everything turns on like normal. Sometimes we turn it from 70* to 72* and the furnace turns on but the fan doesn't. But the fan will always eventually turn on after 5-10 minutes AFTER the furnace turns itself off.

Some additional information:

1. Thermostat Mode (from the Google Home app): Thermostat-1.4

2. This is a gas furnace. We also noticed this happened when turning on the AC, though, so I don't think it's an issue with the furnace itself.

3. We believe this happened after the condenser install.

4. We do not use a C-wire and never have. We did not prior to the condenser install when it worked perfectly fine. I couldn't imagine this being the problem, but want your thoughts.

5. We have replaced our thermostat thinking we had some bad connectors. The new thermostat still has the same problem.

6. We have the setting set to turn on fan with heat. I just verified again.

7. The typical issue occurs following this exactly:

   1. Turn on heat.

   2. Furnace turns on (flames start up and begin heating). Fan does not turn on.

   3. Furnace turns off (I'm assuming it senses it's too hot and turns itself off)

   4. 5-10 minutes later, furnace and fan turn on together.

   5. This happens RANDOMLY. Sometimes the fan and furnace turn on together at step 7.1 above.

8. The house will smell of like a mechanical or burning rubber smell while the furnace runs without the fan.

9. We've had the installers come back at least five times to figure it out, but every time they were here it worked fine, of course (fan and furnace turn on together).

When the issue arises the thermostat says:

1. Undetected wires Y-W-G-R
2. No power g.co/nest/m20
3. To learn more, visit the link below. g.co/nest/e298

I don't know if this information helps:

1. Vin: 34.880 V
2. lin: 385
3. PS: Y-385, YP-999

Everywhere I read there are mentions of adding a common wire.  However, why did this JUST start happening after the condenser install?  I never needed one before.

Please let me know if you need more information from me. I'd be more than happy to supply it as this is driving us nuts.