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Old November 8th 06, 05:29 PM posted to uk.d-i-y
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Default Gas wall convector thermostat question?

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?


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Old November 8th 06, 07:11 PM posted to uk.d-i-y
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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?


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)


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Old November 8th 06, 07:42 PM posted to uk.d-i-y
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Default Gas wall convector thermostat question?

In article . com,
"alexander.keys1" writes:
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?


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.

--
Andrew Gabriel
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Old November 8th 06, 08:08 PM posted to uk.d-i-y
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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.


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Old November 9th 06, 08:28 PM posted to uk.d-i-y
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Default Gas wall convector thermostat question?

On Wed, 08 Nov 2006 09:29:14 -0800, alexander.keys1 wrote:

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?


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.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards



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Old November 10th 06, 07:22 AM posted to uk.d-i-y
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Default Gas wall convector thermostat question?

In article .uk,
Ed Sirett writes:
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.


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.

--
Andrew Gabriel


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