On Jul 29, 12:37 am, wrote:
On Jul 28, 4:50 pm, "Dig" wrote:

this is the 2nd thermostat to fail in 3 years time its a digital Honeywell
ct8775a. I live in Florida and it fails in the summer time while on cooling.
the thermostat will register the 80 degree setpoint and turn on but not shut
off ( thermostat always read house temp at 80) the house was 74 degrees last
night, replaced the thermostat and all seems fine. I don't know why these
would fail in the same way, should I go with a normal honey well thermostat?
(non digital).
thanks in advance
jeff

Isn't Florida the lightning capital of the US? If you
experience a lot of lightning or power surges in your
area the resulting voltage spikes could be damaging
your thermostat. I would start by measuring the AC
voltage feeding the thermostat because sometimes
the control transformer could be defective, improperly
installed or have the wrong tap from the primary winding
hooked up. Many of the control transformers installed
in HVAC units have a 208 or 240 volt primary winding
selection and if the wrong wire is connected, the voltage
from the secondary winding which is supposed to be 24
volts will be too high or too low. If your thermostat is
properly installed, there should be 24 volts AC between
the red and blue wires. If you're problem is voltage spikes,
you can install a surge arrester on your HVAC unit.

[8~{} Uncle Monster


A lot of critical info is missing. Like were all 3 the same model?
I don't see how 3 different thermostats from diff manufacturers could
all fail in the exact same way (goes on showing 80 and never chgs or
turns off) from surges. If they are all the same, or at least
similar ones from same company, then it sounds more like a weak point
in the design.