If you use an old fashioned mercury switch and bimetal thermostat you can invoke an offset from the calibrated temperature by simply rotating the device out of plumb. Allen -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Art Sent: Saturday, December 23, 2017 5:10 PM To: Microcontroller Public Subject: Re: [EE] simple low voltage switch for ac On 12/21/2017 12:03 AM, Art wrote: > The problem with a second tstat is that the kick space heaters aren't > accessible because they're embedded into the existing walls and built in > upgrades done by the previous owner. So, using a modified tstat for the > kick space heater control is problematic. Without good access to the > kick space heater guts, it would be difficult or impossible to find the > lower temperature within the box, so where to mount the modified tsat > temp sensor? > > I called the heating inspector, who came and looked at what he could > actually see. It's very improper to run hot water heating pipes inside > walls, but it happens because the homeowner has the pipes installed and > then later on hires a carpenter to finish up the home improvement > project. The carpenter of course doesn't have a clue about what's proper > with regard to the heating system plumbing! > > A similar problem exists with the shut off valves for the kick space > heater. They are in the basement and are proper....but, the previous > owner made the basement into livable space and sheetrocked the ceiling, > so no access to the shut off valves. So, I'm in a bad position if the > kick space heaters do freeze and start leaking water. > > Sure wish I was living in Hawaii now! We've got single digit low temps > coming on Friday! > > Aloha and TY. > > Art > > On 12/20/2017 11:33 PM, Bob Blick wrote: >> Hi Art, >> >> If you just use another thermostat in parallel with your existing one, it would work the way you want, right? Except you can't set a standard thermostat below 55 degrees. All the home thermostats I've seen use a thermistor. You could get a second thermostat for the remote area, replace its thermistor with a different thermistor having a lower base resistance. It would think everything was hotter than it really was. Then experiment until you find out the temperature setting that gets you where you need to be. >> >> Best regards, >> >> Bob >> >> >> ________________________________________ >> From: piclist-bounces@mit.edu on behalf of Art >> Sent: Wednesday, December 20, 2017 6:15 PM >> To: Microcontroller Public >> Subject: [EE] simple low voltage switch for ac >> >> I recently discovered a vulnerability in my house heating system during >> the latest cold snap. It has to do with hot water lines that go to 2 >> kick space heaters in the 'breezeway'. The kickspace heaters were added >> by the previous owners, who also enclosed the breezeway to make a mud >> room like entrance. And, custom built cabinets, drawers, raised floor, >> built in coat racks and bench seats with hinged lids for storage were >> added. But, they built the pipes for the kickspace heaters underneath >> all the built in niceties, it's horribly expensive to tear all the add >> ons out just to get to the pipes and the kick space heaters to insulate >> the pipes. >> >> We discovered the frozen pipes during the recent cold snap, there's no >> telling how many times its frozen without our knowledge. It was just >> luck that the frozen pipes were discovered last week. >> >> I need a small low power timer circuit that will turn the oil burner for >> that zone on when the outdoor temp gets below 10 or 15 degrees F. The >> timer would run for a few minutes every hour or so, just enough to make >> sure warm water circulates though the pipes and both kick space heaters >> so they don't freeze. >> >> The timer is not an issue. I want to make a parallel connection to the >> existing zone thermostat for the house and the breezeway. When the timer >> expires, it would short the thermostat terminals which would start the >> oil burner every once in awhile. >> >> The heating system is a modern 2 wire system, so I need a non >> relay/triac method for shorting the thermostat wires together, which is >> how the oil burner/tsat combo operates normally. The tstat is fed by a >> small 24 volt ac transformer in the oil burner. When the tsat calls for >> heat, it closes a relay which draws current from the 24 volt ac source. >> But, all the relays I can find draw big coil currents, which is not >> conducive to a simple low power timer circuit that runs on battery >> power. Latching relays might work, but they're expensive and need extra >> driving electronics. >> >> I need some other means to short the 2 thermostat wires together, >> without using much power-which will enable the use of a small battery >> powered circuit. >> >> If transistors or mosfets can be used, it would be simple and cheap. >> Bipolar transistors as a switch would be ideal, since driving the base >> above .7volts makes battery power practical. >> >> I have not managed to find much technical information about how the >> current is sensed inside the oil burner. Several web resources explain >> that the power transformer is shorted out by the relay closure in the >> tsat, but say nothing about the current limiting/current flow sensor >> that actually makes the oil burner turn on. >> >> What type of switching circuit do I need? >> >> Any ideas? >> >> TY >> >> Art >> >> > --=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 --=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 .