Art
2017-12-21 02:15:27 UTC
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
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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