Tuesday, March 16, 2010

Thermostat puzzles

15 Mar: My most prolific writing day yet - perhaps because the system has now proven that it works (based on observation....), and the next step has to be to verify this for certain, and to consider whether it can be replicated or costed.
 I am getting to grips with the thermostat, to get it to activate the pump instead of the timeclock (allowing it to work all night on summer nights). It has a number of parameters, such as set temperature or differential temperature, cold or hot working,  and these can be selected and set.
16 Mar: With a little help from David Atkins to work out the details of these parameters - whether minus or positive, etc. and the help of an Ikea table lamp to test it out, I managed to get the thermostat working. If the air temp. in the boxes is 3ºC warmer than the glycol circuit, it will operate the pump. I hope this is a big enough gap to stop it 'hunting' (i.e. alternating on and off). There is a hysteresis of 1.0 º to deter 'hunting'.

19 Mar: I fastened the Cold sensor to the heatpump's pipe right down close to the ground, but with the HP off overnight even this position comes up to room temperature after a night. So I have lowered the Triggering temperature difference to 1.0 deg C, as first recommended by David of Ice Energy.

  Another thing I don't seem to have is an override. At the moment, in the evening, the only way I know to activate the pump manually is to pull the probe from the glassy box, put the end under my tongue and wait a while! I might devise a parallel light switch next to thermostat, if David can't think of a way to command the thermostat to turn on the relay manually.

June Postscript: Now the system is working well, the activating temperature difference is 5 degs, and also if the sunbox air temperature is 20º or above, they come on, regardless of delta-T. I have an Override switch to force the sunboxes on, and another Override to force them off. I have coloured neon indicator lights to display the condition of the system.

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