Gas wall convector thermostat question?

[alexander.keys1]

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?

[John]

Be careful.

I reckon it will be an accelerator heater to the thermostat bulb - which will operate the gas valve. The heater is to help the thermostat respond quicker to the slightly increased air temperature and reduce the hysterisis (backlash)

[Andrew Gabriel]

This is amazing -- this is exactly what I made to provide timed operation of a thermostatic gas wall heater at home. The electric heater provides a set-back operation. It might be enough to keep the heater permanently off, or on frost protection, depending by how much it raises the temperature of the capillary sensor. I used a voltage switchable wall-wart to drive the resistor, so I could adjust the effective temperature set-back. I decommissioned it all a few years back when I installed central heating.

[Andy]

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.

[Ed Sirett]

Are these units made by Drugasar (Drugasol?) If so, that make had the peculiar 'feature' of requiring power to be applied to keep the heaters off. Which means they come on in a power cut and are generally not liked. This might be the mechanism they work by, as I've only ever taken them out.

Furthermore with infrequent use they get in churches and other public buildings, the flue gases tend to condense on startup and rot the heatX.

[Andrew Gabriel]

My wall mounted one was a Drugasar. The proportional control on the thermostat was amazing -- my thermometer was 0.1C resolution, and it kept the room temperature abolutely spot on within that 0.1C resolution.

I knew they did electric remote controlled versions too (mine wasn't), but I didn't realise it used the same crude (but very effective) mechanism that I had added to mine. And yes, this did mean it came on during power cuts! I did look into replacing it with a newer Drugasar at one point, but they no longer did a 4kW one, and largest domestic one had dropped to something like 2.5kW at that time, which wasn't enough to heat half the house that the previous one did, or the large commercial ones for churches and the like.