So answering this a completely different way: This is the way I would engineer this: Set up an arduino next to the oil furnace, with a 1 wire temperature sensor stuck through an outside wall. I'm assuming finding power near the oil furnace is going to be fairly easy. https://www.sparkfun.com/products/13975 https://www.sparkfun.com/products/11050 For the control of the furnace add a relay shield: https://www.sparkfun.com/products/13769 Just parallel the normally open contacts with the existing thermostat. This relay should be able to run everything just fine. Write a bit of code for the arduino to read the temperature sensor, and once it is below a certain temp, start cycling power on a timed basis. On Wed, Dec 20, 2017 at 7:15 PM, Art wrote: > 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 > > > -- > 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 *Forrest Christian* *CEO**, PacketFlux Technologies, Inc.* Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602 forrestc@imach.com | http://www.packetflux.com --=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 .