I am beginning to think that the black wire you have coming from the motor to "constant hot" might be the problem. What happens if you take that wire off and just put a wire nut on the end of it ?
usually they do that when the motor load or initial startup load is higher than what the timer control switch can handle. That also means the timer might not be good enough to handle the current of both fans. Just not sure where the current came from to kept it powered since OP mention this scenario and other.
1) He powered up the WHF fine, then after it powers up, and he plug the gable fan (tapping into the red wire from the timer output, that is 120VAC not sure about the amp), and they both works in this sequence, probably at the brink of the circuit capacity... never the less it works. but when the timer is off, both fan won't stop spinning that was his original posting, without any details of the WHF fan unit, it will be hard to tell... He'll either have to call the company hopefully reach some one that knew the issue or does a lot of black box testing by experimenting.
2) if he left the gable fan plugged in, (at the timer output red wire), starts the timer, both fans won't start. either indicating the Fans draw too much initial power and the current supply by the timer is not good enough to power the gable fan and the whf "control wires?" maybe ?
Op will need to find out how much current the timer can handle, they are usually not high, especially for electronic ones....
He will needed to find out what is the current draw of that red timer wire without the gable fan at high/lo start up and running.
He will need to find out the current draw of the gable fan at startup and running, this he can do by just plug that into another wall socket and take measurements.
if the timer capacity doesn't add up to the numbers of both fans, that might be the problem. and experiment with swapping in a regular toggle switch in place of the timer and see if that fix the problem, still need to call the manufacture and figure out the control wires, if they are electronic control at the fan or directly tap on the motor like a normal fan motor without any fancy things. that might be bad, if it is... the WHF might die early death.
***** ah yes, try to replicate the problem, put a AC volt meter on that timer red output wire, and check if there is voltage there at the red wire after the fans started, and timer is off, when the fans remain powered, where is that voltage came from?
**** this says switch shouldn't be a problem unless you have higher load than that combining both fans...
"Leviton LTB12-1LZ Decora 1800W Incandescent/20A Resistive-Inductive 1HP Preset 2-4-8-12 Hour Countdown Timer Switch, White/Ivory/Light Almond faceplates included"
Or...
some one suggest a Current sensing switch for OP and he can place the sensor at the timer output to sense power, that will in turn switch on the gable fan. ... Not too sure what is out there for these things. but at least this way he won't alter any thing with the WHF... that add complexity and don't know what the current practice is.
Not sure what breaker the OP have for these fans, he should also check that if it is good enough to power both fans at once on the startup current. 14ga usually is only 15AMP breaker.
