Monday, May 17, 2010

Thermostat + electric improvement

16 May : When David Atkins and Chris Wood visited on 5 May, there was a discussion about the thermostatic trigger for the sunbox pump - it has been using a delta-T of 5 degs C since soon after I worked out how to program the thermostat. So, for all this time the thermostat has turned on the solenoid valve, which has activated the pump in the loft.

We have one problem with that 'differential' setting. The sunboxes do not turn themselves on in the morning unless it gets really hot, early on, or unless we manually turn on the GSHP. If we left the GSHP on 24 hrs, this would not be a problem, but we are not willing to do this. Overnight, the Cold Pipe sensor rises to room temperature,  21ºC or 22ºC. So the Sunbox air sensor would have to rise to 26º or 27º to get started, but at 0900 or 0930 at any time before June, it is still something like 19º or 21º, so the sunbox system will not self start. The moment we turn on the GSHP in the morning, cold glycol flows down the pipe, and the sunboxes come on without hesitation. But we have lost an hour of possible morning charging by this time.

I woke up Sunday morning thinking about the Open Day, but also thinking about this problem. I know that the thermostat has TWO channels, i.e. can perform different actions based on data from the same two sensors. Currently one channel powers the pump, and the other channels set the neon lights and the relay. So I moved all those wires all to one channel, then programmed the spare channel to work on a single trigger Temperature, pure and simple. Then wired both channels to the pump so that either can set it running.

    From now on, if the Sunboxes get to 20º (regardless of what the cold pipe says) they will come on. Over time, I have discovered that the ground temperature never rises much, and certainly not past 15º, so it seems that 20º trigger temperature is a fair guess. In the evening I watched this working, and it is good. If there is the usual 5 degs difference it comes on, and when the temp rises above 20, it comes on...  it is Either-Or.
     An Amber light comes on to indicate 'Cold working' i.e. it is operating below 20º, driven only by temp difference. If both channels are OK, Amber goes off, and a single Green light shows. There is a Red light to show that the ground loop is being pumped, and I plan to add another Red neon to indicate that the underfloor heating circuit is running.

17 May : I swapped the purpose of the Amber light, so that when the sunboxes are working with a cold temperature (based solely on delta-T) only the Green light shows On, and Amber remains Off. When the sunboxes are working with an air temperature higher than 20ºC, the Amber light comes on to symbolize sunshine. Secondly, I have taken a wire from the underfloor heating pump to an additional red neon indicator, so we can see if that loop is circulating.
   I also installed a 2-way switch so that later, I shall be able to swap between the Surya and Savita with a simple touch of a switch.
  A tiny disadvantage of adding in the extra trigger (>20ºC) is that in summer, it will now start up before we even get up in the morning, so I may have some days of not knowing the time of start up on the Hour meter. For the same reason, the only time I can safely do a deep ground temperature measurement is about midnight, not in the morning when it may have had an hour of warming before I get up.

7 June : Two small changes recently (see later in the blog), I added a pushbutton switch that allows me to force the system on even when the thermostat says No! this allows me to read the digital clock. But also, it enables me to kickstart the system when there is warm glycol in the panels, but everytime warmed glycol rushes down from the sunboxes, it changes the delta-T and turns the sunboxes off again. To make the Delta-T less of a hindrance, I reduced it to 4 degsC.

No comments:

Post a Comment

Comments will be moderated before showing. Please make them relevant to the subject of the posting. Comments which advertise commercial products will usually be deleted.

Popular Posts