This is Version 2 of a house monitoring system. Version 1 is currently running in a seasonal home in Canada, monitoring an empty house which I would like to keep from freezing up. The house is 160 years old, and I have been told that in old plaster and lathe houses, freeze/thaw cycles are liable to cause the plaster to separate from the lathe (tho I'm sure that at that age, it has suffered some of those cycles). Still, I want to do what is recommended. Since I want to keep it from freezing, I also want to monitor it to assure that the heating equipment doesn't malfunction and run the oil tank dry, as happened winter of 07/08 when a zone valve stuck.
Version 1 uses a PICAXE 28X1 with wired sensors which monitor the outside temperature, the inside temperature in various places, the boiler run time, and whether the power goes out. The 28X1 is on a breadboard, and a SimpleLan module which connects to the internet is also on board. This module allows the system to present a web page (which has errors and deficiencies which I can't fix remotely), and to send me a status email twice a day, and emergency emails if the temperature drops below the desired level or the power goes off (assuming the internet stays up--breadboard, router, and cable modem are on a UPS). I am pleased with this system, but it is limited.
Version 2 will use wireless sensors in the 315mHz range. The baseboard hot water system has three zones. I figure I need at least three temperature sensors per zone to be able to know exactly what is happening--one for the air, one for the water feed pipe, and one for the water return pipe. I also want to monitor other conditions, like the presence of water in the basement.
The wireless sensors will be powered by PICAXE 08Ms and 14Ms, but I have strayed off the reservation for the control hardware. I have used the NSLU2 NAS device with reflashed operating system for various purposes for nearly four years. It has 8 megabytes of flash memory and 32 megabytes of ram, 2 usb ports and an ethernet port. I am using the slim openWrt Linux operating system (still very much a work in progress). All the programs that I need can fit in the flash. Secure access can be had over the internet, so the programs can be updated. One of the USB ports has a USB->Serial cable to talk to (or rather, listen to) an 08M which receives transmissions from the sensors (use of a message qualifier cleans up a lot of junk). The other connects to a GSM phone which can make an emergency call if the internet goes down. No active control is planned at this point, but it could be added later.
Thanks to all who post on this board--those who answer questions and those who ask the questions that provoke discussion which we non-pros can learn from.