More old Honeywell thermostat fun

Previously, and thermionic valve based.

One of the sites I work on has two old Honeywell thermostats on the wall that are like nothing I’ve seen elsewhere and it kinda intrigues me. These have a small bellows inside and act upon temperature changing the pressure of gas sealed within it. I wonder if they have to thus be calibrated for altitude?

I’d guess circa 1960s-1970s based on the date of other gear at this site.

First, the cooling thermostat – it’s a pretty straightforward mercury switch two-stage type. BIG mercury switches, though – much bigger than Honeywell would have put in their standard home HVAC controls…

The setpoint is adjusted using the hex screw on the side.

Next, the heating thermostat, which really had me scratching my head:


The contact arrangement is curious. This thermostat does not appear to just switch on and off, rather, its output appears to be a variable wirewound resistor! Said resistor is also mounted far from the bellows, so it’s definitely not just an anticipator resistor. This makes me wonder if it was actually more of a remote sensor to something fancier—? Apparently it’s a “proportional control” and I wonder what the original heating system up there was. The original HVAC system has been mostly removed and replaced with a boringly modern one.

 

Don’t let this happen to you.

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In your own home, this would be merely annoying.

In an installation where the thermostats are hard to get to, this would be very annoying. (I once had to change batteries on some that were 30 feet in the air!)

In a transmitter site, this is EPIC FAIL!!!

I was alerted to this problem by hearing the cooling fans in a large shared equipment room (the same one that awful Sprint equipment is in!) spooling up for takeoff.

There are two HVAC units in there, both had the same digital thermostats with the same dead batteries. Room temperature was at 98 and quickly rising.

Always insist on line powered thermostats for installations like this!!!