At my weekend retreat I have a floor furnace for heat. I have successfully replaced the old mechanical switch thermostat with a digital unit compatible with the "millivolt" interface the furnace uses. The LCD has an icon showing when the furnace is heating but I would like to add an LED so I can see at a glance from across the room its status.
I understand the millivolt control signal is generated by the pilot light heating a thermocouple so I suppose I need a very low current sensing circuit with external battery for LED current source. Would appreciate some suggestions.
Thermostat hack
Thermostat hack
"if it's not another it's one thing."
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Chris Smith
- Posts: 4325
- Joined: Tue Dec 04, 2001 1:01 am
- Location: Bieber Ca.
Good suggestion, Chris.
I just thought of another possibility. There must be a relay controlling the gas valve solenoid in the new t'stat cause it clicks when switching on/off. If I can just get to the relay pins this might be a simple deal. However I will have to wait until summer for that solution as wife would have a cataclysmic fit if she saw me disassemblying the unit on a cold wintry day.
I just thought of another possibility. There must be a relay controlling the gas valve solenoid in the new t'stat cause it clicks when switching on/off. If I can just get to the relay pins this might be a simple deal. However I will have to wait until summer for that solution as wife would have a cataclysmic fit if she saw me disassemblying the unit on a cold wintry day.
"if it's not another it's one thing."
.
Indeed, the unit is using switched 24V AC for the gas solenoid via a relay in the thermostat, (physically quite small at
that..if the unit uses 2 internal AA's for power) on all that I have run across, and not a SS relay.
So just double pigtail fuse at the solenoid and remote LV grade wiring off that valve to feed your indicator source.
Naturally..........for the benefit of Mama Cass
.......... this minor refinement / ploy, is being done under the guise of
cleaning / pulling maintainenance on the burner unit.
73's de Edd
[email protected] ...........(Interstellar~~~~Warp~~~~Speed)
[email protected].........(Firewalled*Spam*Cookies*Crumbs)
Life is so unsure; always eat your dessert first.
.
Indeed, the unit is using switched 24V AC for the gas solenoid via a relay in the thermostat, (physically quite small at
that..if the unit uses 2 internal AA's for power) on all that I have run across, and not a SS relay.
So just double pigtail fuse at the solenoid and remote LV grade wiring off that valve to feed your indicator source.
Naturally..........for the benefit of Mama Cass
.......... this minor refinement / ploy, is being done under the guise of cleaning / pulling maintainenance on the burner unit.
73's de Edd
[email protected] ...........(Interstellar~~~~Warp~~~~Speed)
[email protected].........(Firewalled*Spam*Cookies*Crumbs)
Life is so unsure; always eat your dessert first.
.
The "millivolt" system operates the gas valve with the output of a thermopile heated by the pilot light. This voltage level is a fraction of a volt, probably not enough to operate an LED. The best solution might be to connect an LED across the relay coil, since the signal is probably at a reasonable voltage at this point.
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Chris Smith
- Posts: 4325
- Joined: Tue Dec 04, 2001 1:01 am
- Location: Bieber Ca.
They make great little hockey puck FETs into a solid state relay.
The solenoid voltage [probably AC] would be just perfect, but possibly needed to be rectified into DC with cap, as most of these relays trip at 3 to 40 volts DC.
Most of the thermocouples, although they do produce a voltage, some merely create a magnet or magnetic effect that cause the gas valve to stick open.
Used commonly on the manual type of start valves.
Each one is different depending on the style of operation.
But it should be easy.
The solenoid voltage [probably AC] would be just perfect, but possibly needed to be rectified into DC with cap, as most of these relays trip at 3 to 40 volts DC.
Most of the thermocouples, although they do produce a voltage, some merely create a magnet or magnetic effect that cause the gas valve to stick open.
Used commonly on the manual type of start valves.
Each one is different depending on the style of operation.
But it should be easy.
I don't think the millivolt thermostat works the way most people are expecting it work. They do not work exactly the same way as a thermosat on let's say a forced air gas (or oil) furnace. Those use 24V solenoid gas valves and relays. If the thermostat is working like I expect my old gas space heater worked, then the it uses a thermal couple for the pilot and a similar thermal couple for the temperature control. There is no need for electicity unless a blower is desired. The space heater was great as it was small but threw out enough heat that the main furnace only ran when it was real cold. During blackouts when the furnace did not work this thing kept putting out heat but without the blower. In any case the millivolt system is one that can run without need of an electrical source in most cases. The thermostats are only controlling voltages that are less than one volt and have limitations as to how long the thermostat wires can even be. You DO NOT want to tie into this system if you can avoid it.
With that being said let me ask this, does the thermostat have a relay click noise and a small relay when it operates? Or does it use a semiconductor switched device like an analog switch? (Trace the inputs for the control wires inside the thermostat to see what is making the contact closure.) Then obtain an analog switch such as this:
http://www.analog.com/UploadedFiles/Dat ... L_702L.pdf
Disconnect the thermostat from the heater. Wire the ground and Vdd of the analog switch to the battery - and + respectively of the thermostat. (These switches draw VERY LITTLE POWER so don't worry about using them in a battery configuration.) You should see either a similar analog swich, IC, or relay that controls the millivolt application inside the thermostat. If not, connect your DMM to battery ground and probe with the other. Probe inside the thermostat until you find a logic high when the thermostat is calling for heat and a logic low when it is not. (Considering that these switches draw so little power you might even be able to find the connection that makes the HEAT symbol on the LCD come on and use that as the control input. Tie the IN of the analog switch to this logic signal. Use the S and D terminals to control an LED with a SEPARATE power source. (But tie the two grounds together.) In other words the two negative terminals of the battery circuits for the thermostat and LED would be common. The analog switch would be powered by the thermostat. The LED would have its own power source.
If you are extremely lucky the thermostat will have a relay in it. Do not mess with the gas valve at the heater. This is a specialty relay/solenoid device working on extremely low voltage.
I hope this helps and is clear.
With that being said let me ask this, does the thermostat have a relay click noise and a small relay when it operates? Or does it use a semiconductor switched device like an analog switch? (Trace the inputs for the control wires inside the thermostat to see what is making the contact closure.) Then obtain an analog switch such as this:
http://www.analog.com/UploadedFiles/Dat ... L_702L.pdf
Disconnect the thermostat from the heater. Wire the ground and Vdd of the analog switch to the battery - and + respectively of the thermostat. (These switches draw VERY LITTLE POWER so don't worry about using them in a battery configuration.) You should see either a similar analog swich, IC, or relay that controls the millivolt application inside the thermostat. If not, connect your DMM to battery ground and probe with the other. Probe inside the thermostat until you find a logic high when the thermostat is calling for heat and a logic low when it is not. (Considering that these switches draw so little power you might even be able to find the connection that makes the HEAT symbol on the LCD come on and use that as the control input. Tie the IN of the analog switch to this logic signal. Use the S and D terminals to control an LED with a SEPARATE power source. (But tie the two grounds together.) In other words the two negative terminals of the battery circuits for the thermostat and LED would be common. The analog switch would be powered by the thermostat. The LED would have its own power source.
If you are extremely lucky the thermostat will have a relay in it. Do not mess with the gas valve at the heater. This is a specialty relay/solenoid device working on extremely low voltage.
I hope this helps and is clear.
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!
Thanks for all the info. I'm pretty sure the relay is not solid state since I hear the clicking of contacts. I was hoping for a "non-invasive" way, avoiding going into the t'stat itself, and I sure won't be crawling under this old shack to find the solenoid valve, as elegant as that solution would be. Ultimately I would like to use the derived signal to log the on/off cycles when we are away (we keep it set low to avoid frozen pipes).
"if it's not another it's one thing."
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Robert Reed
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Jimandy
I have not had gas heat in over 15 years, so forgive my ignorance if I am wrong, but at that time the "thermocouple" served only one purpose and that was safety. Its only function was to keep a small steady current flowing thru it ( and almost no voltage)to a small soleniod valve . This current kept the main valve open as long as their was a pilot flame in contact with the thermocouple . If the pilot flame extinguished for some unknown reason, then no gas could flow to the burners since it wouln't be ignited and there by fill the room with deadlly fumes.This thermocouple had nothing to to with the thermostat. I am sure modern day furnaces are much more efficient now and probably eliminated this part, so I was wondering what are they doing with the "thermocouple " nowadays.
I have not had gas heat in over 15 years, so forgive my ignorance if I am wrong, but at that time the "thermocouple" served only one purpose and that was safety. Its only function was to keep a small steady current flowing thru it ( and almost no voltage)to a small soleniod valve . This current kept the main valve open as long as their was a pilot flame in contact with the thermocouple . If the pilot flame extinguished for some unknown reason, then no gas could flow to the burners since it wouln't be ignited and there by fill the room with deadlly fumes.This thermocouple had nothing to to with the thermostat. I am sure modern day furnaces are much more efficient now and probably eliminated this part, so I was wondering what are they doing with the "thermocouple " nowadays.
I was curious why the interface was designated as "millivolts" so found this in Wikipedia under "thermostat".
This is an old cabin and this is an old floor furnace. Fortunately I found online documentation that specified it was a millivolt interface. In fact, I think this model is still sold.
It seems rather redundant because even if the thermostat would have the gas valve open at an above-set point temp, if the pilot was blown out, the thermocouple could shut off the main gas source anyway.As illustrated in the use of the thermostat above, the power is provided by a thermocouple, heated by the pilot light. This produces little power and so the system must use a low power valve to control the gas. This type of device is generally considered obsolete as pilot lights waste a surprising amount of gas
This is an old cabin and this is an old floor furnace. Fortunately I found online documentation that specified it was a millivolt interface. In fact, I think this model is still sold.
"if it's not another it's one thing."
In the millivolt system, the open circuit voltage on the relay contacts would be about 3/4 volt. This woould drop to a few millivolts when the relay contacts close and the furnace comes on. A crude comparator circuit could easily detect this, but would have to be externally powered.
The current in the control loop is probably quite high. The magnetic field around the wire might be detectable with a hall effect sensor. Again, external power would be required.
The magnetic field might be enough to operate a reed relay, which could switch the LED on. I would try a few turns of heavy wire around the most sensitive reed capsule that was available.
The current in the control loop is probably quite high. The magnetic field around the wire might be detectable with a hall effect sensor. Again, external power would be required.
The magnetic field might be enough to operate a reed relay, which could switch the LED on. I would try a few turns of heavy wire around the most sensitive reed capsule that was available.
Hi again,
How about another thermocouple, or a simple thermistor mounted
away from the main flame but near enough to heat up to 160 deg F
when the flame comes on? This could trigger your comparator
circuit that runs totally independent of the heating system so
does not run the risk of either causing a gas problem or shutting
down the heat system for no good reason.
How about another thermocouple, or a simple thermistor mounted
away from the main flame but near enough to heat up to 160 deg F
when the flame comes on? This could trigger your comparator
circuit that runs totally independent of the heating system so
does not run the risk of either causing a gas problem or shutting
down the heat system for no good reason.
LEDs vs Bulbs, LEDs are winning.
Access to working area is not an option for me. I would either have to pull the furnace out of the floor or crawl under the house, and there are things under there that go boo in the night.
If I understand what I'm reading I should be able to use the millivolt signal to indicate the fact that the pilot is actually lit in the first place by presence of voltage and then use the relay circuit to indicate main gas valve on/off status.
If I understand what I'm reading I should be able to use the millivolt signal to indicate the fact that the pilot is actually lit in the first place by presence of voltage and then use the relay circuit to indicate main gas valve on/off status.
"if it's not another it's one thing."
Are you wanting to know if a pilot light is lit or know if a "millivolt" heater is producing heat. These are two different things. Millivolt systems use no AC power to trigger the valve on and off as in a forced air system; they only use a minute power of a standing pilot to toggle the valve on and off. A standing pilot will keep a valve open as long as their is a flame. But these are two different systems. The millivolt system is used in automatic gas fireplaces where electric power is not available and goggle the main burner on and off while the pilot light uses a thermal couple to keep the pilot lit.jimandy wrote:If I understand what I'm reading I should be able to use the millivolt signal to indicate the fact that the pilot is actually lit in the first place by presence of voltage and then use the relay circuit to indicate main gas valve on/off status.
If all you want to know is if the pilot is lit, and the system is not sealed, how about a little light sensor? Even expanding on this, if the heater has a window to check the flame of the main heater, a light sensor can be used as well.
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!
If my thinking is straight the pilot
1. Heats a thermocouple to keep gas flowing thiggh itself (only if it's lit)
2. Heats a thermopile dohickey to generate a small voltage used to control a stronger current for the main gas valve per the thermostat relay.
If no pilot light, no millivolt signal hence I can detect that pilot is lit or not lit. But since the relay in the t'stat will be on when the main burner is on I can also detect the condition main burner on/off.
In other words I can monitor both conditions of the furnace, pilot condition and main burner condition. I have looked hard and find no glimmer of light from the pilot, even when I can faintly hear it burning.
1. Heats a thermocouple to keep gas flowing thiggh itself (only if it's lit)
2. Heats a thermopile dohickey to generate a small voltage used to control a stronger current for the main gas valve per the thermostat relay.
If no pilot light, no millivolt signal hence I can detect that pilot is lit or not lit. But since the relay in the t'stat will be on when the main burner is on I can also detect the condition main burner on/off.
In other words I can monitor both conditions of the furnace, pilot condition and main burner condition. I have looked hard and find no glimmer of light from the pilot, even when I can faintly hear it burning.
"if it's not another it's one thing."
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