I doubt it is the transformer, more likely the 120v input is sagging and the 24v output is just following along. Monitor the 120v from an outlet and see if it sags. Do the lights dim?
If your HVAC causes that big of a sag then it seems to me you have bigger problems. I am not an electrician, but if there is a problem with the service to your home, the line into your home is inadequate, the HVAC is defective, or you have some poor connections somewhere in the line it could cause this problem.
If your HVAC causes that big of a sag then it seems to me you have bigger problems. I am not an electrician, but if there is a problem with the service to your home, the line into your home is inadequate, the HVAC is defective, or you have some poor connections somewhere in the line it could cause this problem.