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Andy Andy is offline
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Default Gas wall convector thermostat question?


"alexander.keys1" wrote in message
ups.com...
Recently I've been doing some voluntary work at a local church, and
I've come across a strange arrangement with their gas heaters.

The appliances in question are large wall-mounted convectors with
integral thermostats. On the bottom of the heaters there is an
electrical connection, taken from specially installed
timeswitch-controlled circuits (so not just tapped off the socket
circuit). This feeds only a small heating element (actually a small
enclosed wirewound resistor of a few watts rating), attached to the
capillary sensor tube of the thermostat. There are no electrical
controls on the heaters themselves, in fact the resistor and an
associated terminal block seem to have been fitted on installation
rather than in manufacture.

So what's going on here?


I agree with Alexander, this is a 'predictor' and is used to give a 'heads
up' to the thermostat ahead of when the actual heat from the convector
arrives.

If you're into control theory, a full-on control loop consists of a
proportional, an integral, and a differential block. The differential block
looks for input trends, the integrator looks at input history, and the
proportional looks at the current input.

I've got a feeling the resistor acts as the differential block in this case,
though I expect someone will correct me.

Three wire thermostats have this same heating element built in, as opposed
to the basic two wire thermostats, and this feature helps dampen the large
swings that can occur with a two wire thermostat due to the large time lag
between the thermostat demanding the heating goes on and heat actually
reaching the thermostat and turning it off.

Andy.