conventional thermostat failure modes
conventional thermostat failure modes
(OP)
I've had (wax pellet?) thermostats stick partially open, and one whose metal frame broke, leaving it open.
Is there a "stuck closed" failure mode to worry about, too?
Dan T
Is there a "stuck closed" failure mode to worry about, too?
Dan T





RE: conventional thermostat failure modes
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RE: conventional thermostat failure modes
The stuck closed failure for the wax pellet actuator would be due to diaphragm rupture and loss of the wax fill. Not a common failure mode, but can happen, most especially if the diaphragm gets overheated (but it would take temperatures over 250 to 300 F for that to happen), or lots of cycles at middling-high temperatures.
RE: conventional thermostat failure modes
Rod
RE: conventional thermostat failure modes
I've discovered a few stuck open or broken open, mostly during teardowns for other reasons.
Mike Halloran
Pembroke Pines, FL, USA
RE: conventional thermostat failure modes
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thanks,
Dan T
RE: conventional thermostat failure modes
Regarding stuck closed failure mode, and diaphragm ruptures...it's a tricky problem. The best seal for hot water is EPDM. But EPDM is attacked by paraffins (e.g. hot molten wax). So the diaphragm is usually a nitrile rubber, sometimes a Viton or similar. It's basically then a race as to whether the diaphragm rots away due to hydrolysis attack from hot water, or gets dissolved/softened by hot molten wax.
There are some more "modern" designs with a viton diaphragm, and a seperate rod seal or wiper of EPDM.
Oh, another failure mode is a stuck-open one, and happens when the piston is allowed to stroke a little too far, and the diaphragm inverts. Reverting the diaphragm is sometimes possible, but takes a lot more force than the return spring usually has in it. This mode is usually an infant mortality failure (one cycle), unless something else breaks or deforms first.
RE: conventional thermostat failure modes
Regards
Pat
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RE: conventional thermostat failure modes
Was an 80's car.
Regards, Ian
RE: conventional thermostat failure modes
As Pat remarked, fail closed does tend to get immediate attention!
Regards
Nick
RE: conventional thermostat failure modes
Ya got that right!