Well today was as bad as yesterday, air wise, but we also had a strong northerly wind, and it got to 44C on my little thermometer. However, despite this, at 2pm the greenhouse was still a comfortable 26C.
I hooked up the second thermo fan and measured 4.4kW of cooling (well, 4.4kW of water heating). The fans seem to work quite well, although when the second one is running the first one drops in speed due to the resistance of the wire I'm connecting to my powersupply (the powersupply in question is 300VA, so I doubt that's the problem).
MDS and I discussed the best way to make a PIC a slave. Bit banging is certainly possible, but my fear is that the overhead will make doing anything else interesting too hard. I had a strange recollection that there was a dual ported memory that could be read and written either from 1-wire or something like i2c or or an 8 bit port. After digging through all the docs I found the DS2408. In the datasheet it describes how to use this 8 bit PIO chip as a parallel port interface to a micro. It should be very straightforward to port pypicn to work over usb -> 1-wire->ds2408->pic providing a nice low speed controller bus for various controllers such as my ever complexifying greenhouse controller.
I will need something like this to control the logic on the radiators (decide how many fans and how fast to drive them.
We went to Kuranga with Tif today, and admired her garden on the way. She's using greywater whereever possible, and the garden is looking very lush indeed. There's some nice species of lily doing very well, and not a sign of Erharta erecta -- very impressive!
When we got home Lynne did her thing with Dolly, and I started propagation of all the new plants we got at Kuranga. After sucking all that heat out of the air during the day, the wootank had heated to 25C. Now it is busying keeping the greenhouse warm (mind you, I left the door open to try and get some coolth in).
I had a chat with dnh about this too, and after hearing my results he's now thinking about building his own to cool his cottage. I'm pondering whether to run 13mm poly pipe into our house and have our own set of radiators for cooling in summer (and possibly heating in winter). If any of my super low cost water heater ideas work out, I should be able to keep the wootank warm (say 23C) and pump that around.
This brings up an as yet unsolved problem - how to stop debris from clogging the radiators. A filter would obviously work, but the problem is then how to keep the filter clean. I'm currently thinking about using a tee with a cylindrical mesh filter inside - the tap off for the cooling/heating water would take off from the tee, the majority of the water going straight through, but the cooling water would run through the mesh down to the radiators.
I'm also concerned about corrosion. David and Andrew assure me that it will live long enough to pay itself off, but I'm not so sure - there's a lot of dissolved oxygen in the wootank.