I have been working on a similar problem here. I kinda poked around several solutions and found something that I think optimized bang for the buck rather than just lowest cost. I will be using an esp8266 module with a 1-wire temperature sensor(s). Wifi, et al, is built into it and programmable in an Arduino IDE. Right now I have a Pi Zero doing it. In your situation, I would put one sensor outside the wall and one in nearest the pipe (drill a 1/4" hole and place inside wall cavity in the back of a cabinet?). Basically you get a number back when you query it as a web server. I poll them for temperature and take action based on what it sees. Wifi insures isolation, I can characterize the thermal conductivity of the installation, it allows me access to the data from anywhere, I have a record, I can make nice graphs in gnuplot (and it txts them to me), it can contact me if there's a problem it can't handle, and I can update what happens from the data anytime the situation may present new news. So you might put a small system (ie, a Pi, beaglebone, etc) at the boiler and use a relay for the system, or you might also sense water temperature and (I am not familiar with your type system...) maybe just run the circulation pump (another relay?) until it warrants firing up the boiler. If that is something that might work, you can also sense temperature on a pipe near the boiler and manage in-wall temp, outside temp and water temp and program various strategies based on those variables. All of the systems are micro-usb powered (esp8266, Pi, etc) and the esp8266 is about $7 on Amazon and it's simple enough that a Pi Zero W could be used for the collecting system at $5 ea local price here. You get to keep historical data for analysis and you can trigger various failsafe mechanisms like txting you if some event is out of bounds or it forecasts it won't keep up. For instance, you might be out and for whatever reason the boiler isn't coming on (boiler pipe temp not increasing while boiler relay on)... It could alert you and you could get home to look into it. Way more functionality for just a few more dollars... Dr Skip On Thu, Dec 28, 2017 at 10:09 PM, Art wrote: > After lots of planning for a microamp switching solution to make a timer > function to keep pipes from freezing during cold snaps.... > > It looks like a timer wasn't really what I needed. > > Two event's (both happened yesterday) caused me to realize I was on the > wrong path. > > First, I chatted with my boiler repair person, who also knows his way > around electronics. I was told that both lines that carry the 24 volts > AC to the thermostat MUST be isolated. Using two N type mosfets in > series for enabling the oil burner causes the mosfets to violate the > heating code. It explains why tstats are almost always battery powered > and why they use latching relays. My next best choice for low power > switching devices is an opto type device, but they require milliamp > supply currents-not what I had in mind. > > And, the second event was my characterization of the oil burner run time > during the coldest night we had here in years (-11F). I discovered that > heating thermostats use 1 degree increments, and since the main zone in > the house is large, the time to change it's temperature up or down one > degree is well over an hour. When the boiler shuts off, it takes about > an hour and 20 minutes before the house cools 1 degree. During this > period, the poorly installed pipes can freeze. > > The answer of course is to use a tstat that does runs on a smaller > temperature differential than 1 degree F, thus the heating and cooling > cycles happen in less time-so the pipes don't have time enough to > freeze. I did find a general purpose tstat on ebay that allows the > temperature differential to be programmed, with values in .1 degree C > steps. I have one on order. The cost is $3, and it's a bargain. It > doesn't include a power supply, but it runs on any 12 volt wall wart-my > junkbox runneth over. > > SO-no timer is needed! > > Thanks to all who offered comments and assistance. > > Art > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .