Simple boiler control for off-grid?


Hi all,
Calling from Greece here. Managed to install an olive kernel boiler last week and need to pick your collective minds regarding temp control. Boiler will be feeding floor heating, undertile, and AFAIU, need to keep the temp between ~ 45-25 centigrade to avoid lifting tiles, cracks etc.
Currently the system has a simple thermostat that controls the leadscrew feeder under the hopper as well as the fan blower (they are wired in parallel). When it reaches temp. the system goes off for a while. Another thermostat controls the water circulator.
Olive kernel boilers here are typically run without much concern given to efficiency because olive kernel is quite cheap, and when they are installed "on-grid", electrical consumption is not a concern. People usually turn them on in November and off in April, and many keep the "windows open.." I would not be surprised if this was how the coal burners were run in the UK...
I live off-grid, and my main problem is that the leadscrew drive system has a gearbox, and sucks down ~ 20 amps (@12v) off my battery bank, with no load, and with the simple on-off control, the leadscrew (burner) is mostly on. The thermostat has only a few degress hysterises, so the boiler comes back on ~42 degrees...
What I'd like to do is have the burner "sleep", and come back on when water in the return line reaches ~ 25 degrees instead. This is not considering lags, etc...
Anyone have any ideas on doing this in an easy way, outside of using an programmable boiler controller..dont need the expense, nor the hassle. I wonder if they make thermostats with adjustable hysterises-differential?
One not so smart option is to use a programmable timer here, the octal plug type, that has adjustable TIMEon, TIMEoff, but this does not take into consideration temperature, the thermostat is simply wired in series with it and the motor/blower...
regards, nikos
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On Tue, 29 Nov 2005 10:54:31 +0200, Nicolas Boretos

You could do this using two thermostats and a relay.
Use one thermostat set to the low temperature to energise the relay. Use the pair of contacts on the thermostat which close below the set temperature to do this.
A spare pair of relay contacts is connected to latch the relay on. THe second thermostat is wired in series with the latching contacts and using the thermostat contacts which open above the set point.
You don't say what type of thermostat is needed for this. Does it need to be submersible or in contact with a surface?
An electronic temperature controller and probe is not going to be that expensive (about 70) e.g. rswww.com part no. 250-0079
--

.andy


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Nicolas Boretos wrote:

I must be missing something, as I don't see the problem.
You set the main thermostat to 25 degrees. You use a second thermostat,connected to the supply *after* the main stat and set to 45 degrees, to power a relay with contacts across the 25 degree stat.
Initially, both stats are closed and the 25 deg stat is shorted by the relay. The temp will rise until the 25 stat opens. No problem, the supply is maintained by the relay contacts. The temperature will rise until the 45 stat opens. This opens the relay, the 25 stat is already open - so no power then goes to the load or the 45deg stat or its relay.
Only when the temp has fallen to 25 deg will the main stat close, restarting the cycle.
HTH Sue
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On Tue, 29 Nov 2005 10:54:31 +0200, Nicolas Boretos

Kalimera !
The problem is of course that even when the leadscrew stops turning,the boiler will still keep generating heat for some time as it isnt a simple on off operation like a gas or oil boiler!. Maybe you need to consider some kind of bypass arrangement on the water distribution circuit to the UF heating,maybe with a mixing valve?
It seems to me that the two main elements which need to be measured are the boiler temp (to start/stop the stoker) and the flow temp to the UF heating to prevent overheating?
Where in Greece are you by the way ?
Joe
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