HVAC Blower Not Working (suddenly) - Ideas?

This is a foreclosure - old A/C condensor trashed, the cheapest possible installed with new tstat,
The furnace is a Lennox Elite natural gas - simple 4 wire Robertshaw 9600 tstat,
The Fan switch has never worked in the “On” mode, but it kicked in (set to “auto”) as needed when the condenser needed it. no more - the condenser is running, I can hear the coolant circulating, but now, even after 20 minutes, no blower. All wires are intact, the fuse is not blown. On a guess, I jumped between the red wire on the tstat and the green (also both yellows) - nothing.
In heat mode, the burner comes on for a minute or two, then shuts down - the blinking LED inducated what take it to mean a (high) limit switch is tripped. Some motor runs, but not the blower
The old (trashed) condensor was made in 07/96.
I’m guessing that, given the display and the proper temp-triggered switching working, it is not the batteries…

mrAru thinks that the new thermostat came out of the box as defective [which is not impossible]

Probably the thermostat is trashed, the reason being that both the blower and the burner are controlled off the thermostat. If the blower lights initially and then drops then it is being told that it is up to temp and not needed. That the blower will operate but will not kick out in the normal control circuit would mean that the thermostat is not sending the signal to the blower to circulate.

You need to get either a diagnostic run on the thing, hopefully you have a tech sheet available, so you can use that to check the test points with a multimeter for function, or replace it and hope the new one functions.

It’s highly unlikely to be the thermostat.

With a gas fired furnace with A/C, that fan is controlled by the thermostat in the cooling mode only. In the heating mode it is controlled by the furnace. (either by a heat sensitive switch or a delay times)

Yet you’re telling us that the blower doesn’t work in either heating or cooling, right? If this is so, it is probably not the t-stat.

It’s much more likely that it is miswired.

R 24V Power
O/B Common
G Fan
Y A/C unit
W Heat

Underneath the cover there is a small sliding button that is not evident with the cover on. (the others all are as they can be manipulated with the cover on)

This is a “secret” button of sorts, and may have “G” on one side and “E” on the other. Is it set to "G’?

Does the motor hum?

Is it hot to the touch?

My SWAG in order of likelness:

  1. Miswired.

  2. Button in mid position. (see above)

  3. Bad capacitor

  4. Bad motor

  5. Bad t-stat

After re-reading the OP it looks like this wasn’t done yesterday and this exact stat has worked for some period of time, and it’s only now that it’s stopped working.

Is that correct?

Has anything else changed? (e.g. someone in the furnace for something…)

If this is true, that I’d be looking at the cap and maybe the motor.

Remember, the thermostat does not control the fan in heating, yet you have no fan in cooling or heating.

So I’d revise my SWAGs to be

  1. Bad cap.
  2. Motor lead come loose off of the circuit board.
  3. Bad motor.

New development: when fan is switched to “On”, I get a hum - either a relay or a motor stopped in a dead spot (do motors get “dead spots” anymore?)
The slider is in “gas” position
Where IS the blower motor? I take it in the metal box at the bottom of the furnace - can it be opened without pulling it out of the closet?
And: what is with the oneumatic (?) tube from the flue blower? What function does it serve?
I take it my guess of jumping the red tstat line to the green SHOULD have turned on the blower, yes?

It’s running
Most likely - cap. Second - motor - I got into the blower and turned it manually, then switched fan t “on” - nothing. Turned it again, and, this time, switched it to on while the squirrle cage was still turniing. Worked. Turned off, waited a couple of minutes, then back on. so far so good.
Of course I now have both the outer and inner panels open, with a clip-type clamp defeating the cabinet interlock switch, but with an interior temp of 77, I’ll take it.
The Q re. the tubing, however, stands - what on earth does it do?

Nice while it lasted…

I get a choice - manually start the blower (which cuts out when I turn on the cool-off-heat switch, or run the condenser and have a cold closet.
There should be a way to jump something-to-something to force the blower on.
OR - I have the old tstat would that be worth a try.
I am broke - calling a pro is our of the question. a box fan in the air intake s about all that I can afford
Thanks again

Meanwhile, back at the ranch -

I am getting NOTHING in cool mode from the blower - no hum, nothing - back to tstat?
(yes, the slider is set to gas. I have also hit the ‘reset’ button several times at this point

Modern furnaces require small blowers (“draft inducer motors/fans”) to pull the flue gasses through the heat exchanger. That tube is connected to a pressure switch, and should the draft inducer not be working, or the chimney/flue be obstructed, (a birds nest at the top of the chimney is one example) the pressure switch will pick up the fact that there is insufficient draft and turn the burners off. IOW, it’s a safety issue.

Without a doubt, however,** that is not your problem.** (or your fan would work in cooling where none of that comes into play)

Between your thermostat and your furnace are wires (duh) and they may be the problem (although highly unlikely because the problem exists in heating also) so the better “jump” is Red to Green in the furnace. By jumping at that point you’re eliminating the stat and stat wires as potential suspects.


If you’re the handy type and want/need to troubleshoot this yourself, do this:
1)Disconnect the low voltage wires in the furnace.** (remember to defeat the door switch) (the wires coming from the thermostat; those wires.)

2) Jump R to G. Thats fan only. Report back (including any noises) Unjump.

3) Jump R to W Thats heat only. Remember, in heating the thermostat does not turn on the blower----the furnace does; either through a heat sensitive switch or a timer. So…the burners will light and after 1-2 minutes or so the blower should start. If it does not------and the burners shut off------ it’s because the furnace overheated and “high limit” switches shut the burners down.

What that tells us is that the problem is in the motor, capacitor or circuit board.

Do that, and we can tell you how to troubleshoot the circuit board.

red-to green at the furnace pcb produces nothing - not even the hum of a stalled motor.
(of course jumping it to the OTHER red produced a breif hum before tripping the c/b (the other red is, I’m guessing, the one which goes to the condensor.
I’ll call around tomorrow and see if anybody will sell a cap to a civilian then to wal-mart for a 20" box fan…

A box fan in the air intake will damage your condenser in cooling and produce a Very Large repair bill.

A box fan in the air intake will simply kill you in heating.

Not just ineffective, but A Really Bad Thing.


What about R to W?

OK, no box fan
c/b - there is a cute alum. panel with a rocker-switch circuit breaker
R-G fan hums, starts with manual assist (as it does (sometimes) with the tstat in off and fan “on”
R-W burner comes on, shut off with led doing it’s 4-blink (hi temp cutoff)
I did not hear a hum, and a manual assist on the blower immediately prior to it’s trip produced no traction
OK to do the obious with the R-Y + R-G?

If you want, but not necessary. R To Y will bring on the condenser (you’ve stated that is no problem) and obviously the R to G thing is not working.

If the motor will start with a manual assist (be careful not to get hurt by the fan blades!) it seems that it is the capacitor, and possibly the motor.

There is a way to bypass/ troubleshoot the circuit board if you’d like. It involves a wires stripper, wire nuts and an extension cord. I don’t know how much more you’d like to do with this.

At this point my guess:

  1. Cap 50%
  2. Motor 50%

Have you looked at the capacitor? (Be careful with caps! Turn the power off, and short out the cap by placing a screwdriver across the terminals before handling a cap. With the power off!)

The capacitor will have ratings like this: 370 VAC / 5 uf, or something like this.

Caps are inexpensive and a safe bet if you don’t want to call a tech.

Can you get this info?

please say R to G and Y is OK? it’s 80 F in here (the furnace test were not helpful), and I finally have both blower and condensor running simultaneously for the first time today…

Oh! Btw…

If you’ve disconnected all the low voltage wires you’d actually need to jump the wires going out to the condenser: One to R, and the other to C.

Don’t ever jump R to C. Ever.