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'[EE] simple low voltage switch for ac'
2017\12\20@211532 by Art

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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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2017\12\20@224854 by RussellMc

face picon face
Suggestion:

"Spec":  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.

I need some other means to short the 2 thermostat wires together,

ie Short circuit 24AC source with limited current capability.


An opto isolator with TRIAC output should meet the need.

Voperate, Ioperate, Vout and Iout varry with model.

This selection from Digikey may help


https://www.digikey.com/products/en/isolators/optoisolators-triac-scr-output/904?FV=ffe00388%2Ca402d0&mnonly=0&ColumnSort=-2108&page=1&stock=0&pbfree=0&rohs=0&cad=0&datasheet=0&nstock=0&photo=0&nonrohs=0&newproducts=0&k=opto+isolator+triac&quantity=&ptm=0&fid=0&pageSize=25&pkeyword=opto+isolator+triac

These for $1.52/1 in stock Digikey may suit.
https://www.digikey.com/product-detail/en/vishay-semiconductor-opto-division/VO2223A-X001/VO2223A-X001-ND/4074392

Datasheet
http://www.vishay.com/docs/81924/vo2223a.pdf

1A, 600V switching
Trigger 10 mA max at 1.4V max.

Russell



On 21 December 2017 at 15:15, Art <spam_OUTky1kTakeThisOuTspammyfairpoint.net> wrote:

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2017\12\20@233334 by Bob Blick

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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-bouncesKILLspamspam@spam@mit.edu <piclist-bouncesspamKILLspammit.edu> on behalf of Art <.....ky1kKILLspamspam.....myfairpoint.net>
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


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2017\12\21@000533 by Art

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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:
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2017\12\21@000820 by Art

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Hi Russell and thanks.

I haven't measured the current that is required before the furnace
starts putting out heat. But, it's probably in the low milliamp range-a
guess.

When I started this project, I was looking at eliminating the batteries
completely from the tstat, thinking that a low power switching supply
could charge a super cap when the relay in the tstat was open circuited.
So long as the current drawn by the super cap was low enough to avoid
drawing the threshold current to start the burner, the tstat wouldn't
need any batteries at all.

And, I had hoped to use the super cap to run my flea power timer and its
switching component(s).

I've never heard of a self powered tstat for a 2 wire boiler, although
the latest wi-fi enabled smart tstats actually have an additional wire
to power the thermostat and radio transceiver.

I had seen the 10 ma input current on SSR's and opto's before, but I was
hoping for a lot less power needed to turn on the electronic switch.
Some opto's and SSR's need 15 to 40 ma to enable the output
switches-which is way more power than I want to use since I need to
worry about a battery staying viable.

I was hoping for a switching solution that would use .5 ma max to enable
the switches. I'm not sure that's gonna happen in todays opto/ssr
technology.

Again, thanks!

Art

I'll have a look  On 12/20/2017 10:48 PM, RussellMc wrote:
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2017\12\21@023417 by RussellMc

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If you don't want isolation you can use a pair of series MOSFETs or a
single MOSFET in a bridge.
A little playing gets you an AC switching circiut iwith current drain only
in any biasing resistors - in the uA range with care.


   Russell
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2017\12\21@033915 by Forrest Christian (List Account) n/a

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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 <RemoveMEky1kTakeThisOuTspammyfairpoint.net> wrote:

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2017\12\21@075046 by alan.b.pearce

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Why not use a mains powered thermostat - after all if you have no mains the furnace isn't going to run anyway because the 24v won't be there to turn it on.

Is it really that much hassle to run a mains supply into the affected area if you don't already have one there (wangle a supply from the light fitting?).



> {Original Message removed}

2017\12\21@130254 by Christopher Head

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part 1 1389 bytes content-type:text/plain; charset="utf-8" (decoded base64)

On December 20, 2017 9:08:15 PM PST, Art <TakeThisOuTky1kEraseMEspamspam_OUTmyfairpoint.net> wrote:
>I haven't measured the current that is required before the furnace
>starts putting out heat. But, it's probably in the low milliamp range-a
>guess.
>
>When I started this project, I was looking at eliminating the batteries
>completely from the tstat, thinking that a low power switching supply
>could charge a super cap when the relay in the tstat was open
>circuited.
>So long as the current drawn by the super cap was low enough to avoid
>drawing the threshold current to start the burner, the tstat wouldn't
>need any batteries at all.

May I ask what turned you off from this idea in the first place? I don’t see why it’s unworkable. Although, I would start by measuring the current threshold. I wonder if you may be able to turn on the heater with a resistor in series with a switching element at a lower current level. The advantage of this would be that a voltage drop will develop across the resistor-switch system, and you can feed that voltage drop to your timer circuit’s power supply. This will keep your circuit powered while the heater is on, rather than mucking about with a supercap. Then you don’t need to care how much power it takes to operate the switching element, because that power draw is actually helping you!

--
Christopher Head
part 2 197 bytes content-type:text/plain; name="ATT00001.txt"
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2017\12\21@132019 by Van Horn, David

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Holding current for a relay can be a lot less than the pull-in current.  Something to be aware of. I'd use a relay here.


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2017\12\22@191147 by Art

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Thanks to all who offered assistance, I appreciate it very much.

Russell, isolating is probably not needed, it’s low voltage and both
conductors from/to the tstat are isolated to begin with. Mosfets in
series or a single one in a bridge sounds a little dicey, in my internet
research, several pointed out technical issues that might be
significant. But, I gotta admit, I love the microamp current consumption
though.

Forrest, it’s not about temperature sensing, it’s about a timer that
forces the zone pump to circulate just enough hot water to keep the
pipes and kick space heaters from freezing. If I had access to the
pipes, I’d surely do a thermostat in close proximity to them though. My
timer will go in parallel with the existing indoor tstat, but no
temperature sensing will be associated with the timer needed to protect
the pipes hidden in the walls.

Alan, I need a low budget fix and the most basic programmable battery
powered tstat costs $60 to $90. Line powered tstats just cost to much,
especially when battery powered ones are much cheaper.

Christopher, I’ve long wondered WHY tstats aren’t self powered. When I
realized I needed a timer to piggyback on the existing tstat, the self
powered tstat project was reborn. But, it complicates the timer design
and I already don’t have enough time to get the timer in place. There is
new information based on my internet research and measurements taken on
my existing battery powered tstats. I hope to get an update sent to the
list later this evening.

David, I know about conventional relay pull-in vs holding current. I
have quite a few of them that run 24/7 already and all of them use an RC
combo in the coil driver circuits, which significantly reduces coil
heating and power consumption. Even so, for a low power battery powered
timer, the conventional relay uses to much power. I have an update
regarding relays however, see my next email.

Art

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2017\12\22@210003 by Art

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Here’s an update on my timer project.

When I first realized I needed a very low power timer to piggyback on an
existing tstat to burner connection, I imagined a low end PIC with a
very low frequency clock and some sort of solid state ac switching to
run the heating system a little each hour, just to keep those pipes safe
from freezing. I originally thought of a very small PCB, stashed inside
of the existing tstat, possibly running on the existing tstats
batteries. My hope was for 20 to 50 ua current consumption and no extra
wires outside the tstat enclosure.

However, the solid state ac switch, which shorts the two lines from the
boiler together to turn on the heating system turned out to be more of
an issue than I had previously thought.

Yesterday, I took both my programmable wall mounted tstats apart to
measure the current consumption. Both of them run on 3 volts from AA
cells and the batteries last a very long time. When I realized that low
power conventional relays consume 35 ma at best, I wondered how the
batteries could last so long in them.

My tstat lobotomy confirmed that both units had a very low current
consumption, both units had nearly identical current consumption.

At rest (without calling for heat) they used 30uA. During the short
interval when the uP checked the thermistor, the current draw increased
to 300uA very briefly. When the relays closed or opened, the current
consumption jumped to 1mA, but only briefly. The biggest surprise came
when I looked at the relays. Both units had single coil latching relays,
which are horribly expensive to buy (compared to conventional relays).
They were Omron G6SU, 35 ma coil, 3VDC DPDT
(http://omronfs.omron.com/en_US/ecb/products/pdf/en-g6s.pdf). I’m sure
the brief 1mA in the supply current was due to my slow sampling meter,
the 1mA pulses were actually brief 35 mA pulses.

============

The most surprising observation was the generous ac current availability
from the boiler...this 24V ac is shorted by the relay in the tstat, to
turn the boiler on when heat is needed. I found the open circuit voltage
at the tstat relay terminals was 28 volts, with the relay terminals
open. However, a variable resistor placed across the open terminals
showed that the oil burner didn’t begin heating water until the current
drawn by the resistor was 70 milliamps (at 12 volts). So, a properly
designed switching supply could easily produce enough current at 3 volts
to power the entire tstat, WITHOUT signaling the oil burner that it
needs heat! It seems very probable that a home heating tstat could
easily be self powered-without any need for batteries.

Note that I did not test whether higher current could be harvested in
short pulses, with enough time between each pulse (without starting the
oil burner). I do know that the internal transformer in the boiler that
produces the 28 volts ac is a 10 VA transformer, so even more power
might be available if it was drawn in pulses (as needed).

I might try to produce a working prototype for a self powered thermostat
in the Spring.

I’m on the lookout for some more of these latching relays and someone
who could program an ultra power stingy 3v PIC timer.

Art


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2017\12\22@212932 by Allen Mulvey

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Maybe I'm drifting off a bit here but whenever I think of a low power thermostat I think of a mercury switch on the end of a bimetal coil. No power supply whatever. We tend to get in a rut expecting electronics to do everything but sometimes mechanics work just as well or even better.

Allen

{Original Message removed}

2017\12\22@220120 by Christopher Head

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part 1 1544 bytes content-type:text/plain; charset="utf-8" (decoded base64)

On December 22, 2017 4:11:44 PM PST, Art <RemoveMEky1kspamTakeThisOuTmyfairpoint.net> wrote:
>Christopher, I’ve long wondered WHY tstats aren’t self powered. When I
>realized I needed a timer to piggyback on the existing tstat, the self
>powered tstat project was reborn. But, it complicates the timer design
>and I already don’t have enough time to get the timer in place. There
>is
>new information based on my internet research and measurements taken on
>my existing battery powered tstats. I hope to get an update sent to the
>list later this evening.

Your most recent update does indeed sound like a practical solution. I’ll point out that harvesting power from the line wasn’t my own random idea, though! I have an off-the-shelf digital programmable thermostat which does exactly that. It’s controlling 240VAC electric heat, not an oil burning signalling line, but it has only two terminals and lives in series with the heater. It has no batteries. Presumably it has a wide-input SMPS that can handle 240V when the heat is off and probably a fraction of a volt when it’s on. I vaguely remember reading in the manual during installation something about it being usable to control fuel-burning furnaces (I think there was an option to select PWM periods for that purpose, ~20 seconds for electric or a few minutes for fuel). No idea about the signalling interface or compatibility or anything though.

So that’s why I thought thermostats with batteries inside were weird!

--
Christopher Head
part 2 197 bytes content-type:text/plain; name="ATT00001.txt"
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2017\12\23@042809 by Forrest Christian (List Account) n/a

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I thought you wanted to do temperature sensing to arm the timer?

I'm still kinda confused why you can't put the timer where there is power.
Do you not have access to the boiler/pumps/etc?   Surely there is power
there, kinda hard to run pumps without it.

On Dec 22, 2017 5:20 PM, "Art" <ky1kEraseMEspam.....myfairpoint.net> wrote:

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2017\12\23@051441 by RussellMc

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On 23 December 2017 at 13:11, Art <EraseMEky1kspammyfairpoint.net> wrote:

> Thanks to all who offered assistance, I appreciate it very much.
>
> Mosfets in
> ​ ​
> series or a single one in a bridge sounds a little dicey,


​I don't see it that way.
In the two series opposed FETs you need a DC source as high as or higher
than the max switched voltage.
This is easily derived from the available 24 V voltage.

For the FET in bridge (make it an N Channel FET) you need a source to
charge a cap relative to the FET source to use as a gate drive voltage.
As the FET cct has many off cycles and proportionally fewer on cycles (by
your design) the drive cap can be charged with probably 2 diodes and a
resistor during the FET off periods.
As the gate current when on is minimal and the on time is probably tens of
seconds a modest sized cap will suffice.

Russell
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2017\12\23@124350 by alan.b.pearce

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> The most surprising observation was the generous ac current availability from the boiler...this
> 24V ac is shorted by the relay in the tstat, to turn the boiler on when heat is needed. I found the
> open circuit voltage at the tstat relay terminals was 28 volts, with the relay terminals open.
> However, a variable resistor placed across the open terminals showed that the oil burner didn’t
> begin heating water until the current drawn by the resistor was 70 milliamps (at 12 volts).

I suspect that the power supply is operating either another relay in the boiler, or some form of solenoid.

So from your investigation you could use a self powered device that operates in a similar manner to a 4-20mA sensor, but using a SMPS converter (probably use a chip designed for automotive 40V input) to power your circuit, and you probably don't even need to worry about the relay coil current (it becomes part of the 70mA). Worked right you could even do it without the relay, just use a large chunky FET to connect the suitable sized resistor. That is providing the current increase.



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2017\12\23@131402 by Alan

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Prog. thermostat www.ebay.com/itm/Honeywell-5-2-Day-Programmable-Thermostat-with-Backlight-RTH2300B/162812181932?epid=1111778091&hash=item25e85cbdac:g:8SEAAOSwvc1ZZVG5
~$9.00

Looking forward,
Al Shinn (Tinker)

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2017\12\23@141732 by Dwayne Reid

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I'm coming to this thread late but do have a couple of opinions.

I am assuming that you can float the entire circuit.  It will be battery-powered, so it doesn't need any other outside connections.

1) Use a pair of series-connected N-channel MOSFETs for control.  Tie the source and gates of both MOSFETs together.  Sources go to PIC Vss rail.  Gates go to an output pin.  I'd also include a 1M resistor or so between gates and sources to ensure that the FETs stay OFF should the circuit lose power (should happen automatically but you never know).

The drains connect across the thermostat wires.

For bonus points, a pair of diodes connected so that the anodes go to the FET drains and cathodes tied together can be used for as the power supply for a very-low current charger for the batteries.  Keep the current to less than 1 mA.

2) Use an outside-mounted thermistor if you want to make this fully automatic.  Again: the thermistor must be completely floating and can't electrically-connect to ground (or anywhere else).  Use another port pin to supply power to the thermistor pull-up resistor.  Read the outside temperature at a rate that is suitable- a few times per hour is probably good.

3) Power the entire circuit from a decent-sized LiPo or Li-Ion cell.

More later - gotta run.

dwayne


At 07:15 PM 12/20/2017, Art wrote:
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2017\12\23@170258 by Art

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Hi Tinker,

https://www.ebay.com/itm/Honeywell-RTH221B1039-Digital-7-day-Programmable-Thermostat/182967873347?epid=18007595085&hash=item2a99bc2f43:g:erMAAOSwFyhaLHNq

Currently at $5, free shipping, auction and with 1 hour left before
bidding ends.

Just between you and me, $5 is less than I can buy the latching tstat
for-I've priced them already!

TY for thinking of me and offering the results of your search!

Art



On 12/23/2017 01:13 PM, Alan wrote:
> Prog. thermostat
> www.ebay.com/itm/Honeywell-5-2-Day-Programmable-Thermostat-with-Backlight-RTH2300B/162812181932?epid=1111778091&hash=item25e85cbdac:g:8SEAAOSwvc1ZZVG5
> ~$9.00
>  
>
> Looking forward,
> Al Shinn (Tinker)
>

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2017\12\23@170822 by Art

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On 12/23/2017 04:39 PM, Art wrote:
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2017\12\23@170858 by Art

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On 12/23/2017 04:12 PM, Art wrote:
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2017\12\23@170935 by Art

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On 12/22/2017 10:53 PM, Art wrote:
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2017\12\23@171038 by Art

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On 12/21/2017 12:03 AM, Art wrote:
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2017\12\23@185752 by Allen Mulvey

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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 removed}

2017\12\28@154503 by Christopher Head

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part 1 717 bytes content-type:text/plain; charset="utf-8" (decoded base64)

On Fri, 22 Dec 2017 22:53:59 -0500
Art <KILLspamky1kspamBeGonespammyfairpoint.net> wrote:

> Do you happen to have a model number for your line power scavenging
> battery?? I saw on several years back that didn't have many features,
> and I think it was around $270! Needless to say I didn't buy it::>

It’s a Noma 7-day programmable thermostat, model number 52-2544-0.
Sadly it seems it’s *not* compatible with 24 V systems, so not much
use to you; rather, the slower PWM frequency is for fan-assisted heating
systems. Also it says it has a rechargeable battery inside, though
apparently that’s only used for keeping the clock running during power
outages.
--
Christopher Head

part 2 197 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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