I have fireplace that I have installed a gas log system, where the natural gas burner is controlled by a remote thermostat. It has a remote transmitter that you can set to particular temperature and a receiver box that connects to the gas valve to turn it on and off to keep the room temp constant. Basically the same as any home gas furnace, except the thermostat is not hard wired to the burner valve.

What I am trying to do is place a 120 volt blower in front of the fireplace to help push the heat into the room The issue is that I need a way to have the blower only come on when the burner is on, so I'm not getting cold air blowing when the burner is off.

The way it works now is the receiver gets a signal from the transmitter to turn on and I get continuity between the 2 wires coming out of the receiver to the burner control unit. When the receiver gets the signal to turn off, I get an open connection. So it's acting exactly like a switch, in fact there is a simple switch connected across the wires that you can use to turn the burner on and off manually, in case you don't have a remote.

I would think a simple circuit that would detect the closure and turn on a relay, that can handle the current and voltage needed by blower, is all I need, but there is another thing I need to consider. The burner/pilot control has a millivolt safety system, where the pilot flame has a thermocouple underneath it and if the flame goes out the burner control is disabled. I don't know how the wires from the receiver connect into this system inside burner control, so I don't want to take the chance of introducing any voltage into the wires that might damage the receiver or possibly the burner control unit. Both are rather pricey to replace. Any help, someone can give me with this, is greatly appreciated.

 

What is the nature of the origin of the 2 wires? is it a simple contact closure, or is it electronic.
If you can confirm this it would help, if the former would just need a relay possible unless the contact closure is capable.
Max.

 

I suggest that you start by measuering the voltage across the wire from the thermostat receiver unit when the thermostat is set below the room temperature. (The contacts will be open.) Then measure the current through the wires when the thermostat is set above the room temperature. (Contacts closed) It is possible that it may be AC signals that you are measuring. I think it is more likely to be a DC signal and it will be driving the electronics on the control board rather than driving the gas valve directly so the current will probably be a few milliamps. If these reading give suitable values it may be possible to use the signal to drive an opto isolator in series with the contacts.

Les.

 

The simplest way I can think of tying into the system would be to tap off of the primary gas control solenoid. Odds are it's either 24 VDC or 24 VAC solenoid which if so all you would need to do is put a like powered relay in parallel with its power feed.

 

What he ^ said ...

 

The simplest way I can think of tying into the system would be to tap off of the primary gas control solenoid. Odds are it's either 24 VDC or 24 VAC solenoid which if so all you would need to do is put a like powered relay in parallel with its power feed.

That would definitely solve the problem, but in this case they use something called a millivolt system. As far as I can tell the gas valve is literally powered by a tiny amount of voltage provided by a thermocouple or sometimes referred to as thermopile, that is located under the flame provided by the pilot. I know this has to be the case, because I can totally disconnect the receiver box, which has 3 AA batteries in it, and still turn the gas valve on and off manually with the switch that connects in parallel with the 2 wires from the receiver. When I disconnect the box, there is absolutely no other power source, other than that thermocouple. I really don't know how such a tiny amount of voltage generated by something no larger around than a small pencil can activate something like a solenoid, but it does. It's actually kind of ingenious, in that you don't need any outside voltage source to make the system function. Even the pilot is lit by a spark generated by the press of a piezoelectric type igniter. I've never had it happen, but if I lost power to my home in the dead of winter, at least I would not freeze to death.

 

What is the nature of the origin of the 2 wires? is it a simple contact closure, or is it electronic.
If you can confirm this it would help, if the former would just need a relay possible unless the contact closure is capable.
Max.

I was thinking of attempting to open the box to see if I could determine this, but I would have to pry it open and didn't want to take the chance of damaging the parts inside. I have not tried contacting the company that manufactures it. It's just possible they could provide a schematic of the circuitry.

 

I suggest that you start by measuering the voltage across the wire from the thermostat receiver unit when the thermostat is set below the room temperature. (The contacts will be open.) Then measure the current through the wires when the thermostat is set above the room temperature. (Contacts closed) It is possible that it may be AC signals that you are measuring. I think it is more likely to be a DC signal and it will be driving the electronics on the control board rather than driving the gas valve directly so the current will probably be a few milliamps. If these reading give suitable values it may be possible to use the signal to drive an opto isolator in series with the contacts.

Les.

When I connect my meter across the wires, I see no voltage either way, even set at in the 200 millivolt range. If I disconnect one of the wires, and put the meter in series, I can only detect a very small reading of about 20 mA current flowing when thermostat is calling for heat and 0 If it's not. Since this system is described as the millivolt type, the 20 mA is probably correct. Do you know if there is an opto isolator that can operate at such a low current?

 

I can't say for sure what you have. What I know of gas valves is that they are typically controlled by 24 VAC or VDC (as you've been told). The thermocouple (or thermopile) is a micro volt sensor that keeps gas flowing to the pilot. If the pilot goes out then the TC loses signal and shuts off the pilot gas.

The MAIN gas valve is too big and bulky to be controlled by a micro-voltage. Just not going to happen. The only way I can see it is that there MUST be something else powering the gas valve. Maybe I'm wrong.

Assuming you have a control voltage (not the micro-voltage), if you use that to activate a fan then when the gas comes on your fan will come on too. Immediately blowing air that has not yet been heated. That's going to get real uncomfortable real quick.

Here's how old style furnaces do it: They come on when the 24 volts is applied. As the furnace heats up eventually a thermostatically controlled switch turns the blower on. The fire box is hot and the fan begins moving air to extract the heat. When the gas shuts off - because the fire box is still hot the fan continues to blow until the thermostatically controlled switch clicks off. Hence, you don't get cold blowing air the moment the heater comes on. AND after the gas has turned off you still extract the residual heat in the furnace.

Your fireplace should operate the same way. Provided you can install a thermostatic switch to turn the fan on ONLY when it has warmed up, and off before it starts blowing cold air around.

One word of caution I didn't notice anyone else mention - - - carbon monoxide. If your fireplace is not equipped for a fan then blowing the heat into the room could possibly be blowing exhaust gasses into the room. Another word for those exhaust gasses - carbon monoxide. If you do this - be deadly careful. Not "Very" careful, - DEADLY careful. Failure on this level means "Death."

 

I've never had it happen, but if I lost power to my home in the dead of winter, at least I would not freeze to death.

I hope that's true. However, there MAY be an auxiliary manual valve. If not - then without main power - um - I think it's going to get cold.

 

I really appreciate all the suggestions and advice given, but I just thought of a way that might get around using the fireplace system altogether.
If I could find a standalone, 120 V thermostat control, like the ones used to turn an attic fan off and on in the summer. I could set the temperature differential close to the setting on the fireplace remote and turn the blower on and off that way. The trick is finding one that can be set at a few degree swing between on and off. For example, on at 72 degrees and off at 75. I'm sure a circuit could be designed to do this, but if I could find something I could just buy off the shelf it would make things simpler. Does anyone know if there is a device like this?

 

I can't say for sure what you have. What I know of gas valves is that they are typically controlled by 24 VAC or VDC (as you've been told). The thermocouple (or thermopile) is a micro volt sensor that keeps gas flowing to the pilot. If the pilot goes out then the TC loses signal and shuts off the pilot gas.

The MAIN gas valve is too big and bulky to be controlled by a micro-voltage. Just not going to happen. The only way I can see it is that there MUST be something else powering the gas valve. Maybe I'm wrong.

Assuming you have a control voltage (not the micro-voltage), if you use that to activate a fan then when the gas comes on your fan will come on too. Immediately blowing air that has not yet been heated. That's going to get real uncomfortable real quick.

Here's how old style furnaces do it: They come on when the 24 volts is applied. As the furnace heats up eventually a thermostatically controlled switch turns the blower on. The fire box is hot and the fan begins moving air to extract the heat. When the gas shuts off - because the fire box is still hot the fan continues to blow until the thermostatically controlled switch clicks off. Hence, you don't get cold blowing air the moment the heater comes on. AND after the gas has turned off you still extract the residual heat in the furnace.

Your fireplace should operate the same way. Provided you can install a thermostatic switch to turn the fan on ONLY when it has warmed up, and off before it starts blowing cold air around.

One word of caution I didn't notice anyone else mention - - - carbon monoxide. If your fireplace is not equipped for a fan then blowing the heat into the room could possibly be blowing exhaust gasses into the room. Another word for those exhaust gasses - carbon monoxide. If you do this - be deadly careful. Not "Very" careful, - DEADLY careful. Failure on this level means "Death."

I know it goes against all logic, but I am 100% sure there are no other sources of power connected to the gas valve. I disconnect the gas line, and pull the whole burner/ valve assembly out once a year, in early fall, so I can thoroughly clean the burners, etc., and there are no wires to disconnect, only the natural gas line coming from the meter.

Actually a mllivolt thermocouple powering a solenoid goes way back to the early days of home heating. My father did furnace repair back in the day before all the high efficiency furnaces were developed, so I learned a lot about them when I was a kid. Back then they used what was called a standing pilot, which just meant, the pilot was always lit whether heat was called for by the thermostat or not. For safety you did want the thermostat calling for heat, when there was no pilot, because there would be no flame to ignite the main burner, and raw gas would be released from the burners. So they had a very simple safety system to prevent this. They placed a thermocouple under the pilot flame, and as long as it was lit it generated voltages, in the millivolt range, that held a small solenoid open in the burner control assembly, allowing gas to flow to the burners, if heat was called for. If the pilot went out for some reason, the thermocouple stopped generating voltage and the solenoid closed off the gas to the whole system. To light the pilot again, you had to push in on a button, actually the plunger of the solenoid, and it pushed up against an electromagnet connected to the thermocouple. You had to hold the button in to let gas flow to the pilot as you lit it with a match. Since the solenoid was spring loaded, you had to hold the button in for 20-30 seconds, until the electromagnet could take over, and the pilot would stay lit. The thing is, the return spring on these little solenoids was fairly stiff, so that electromagnet had to be pretty strong to hold the plunger in. To this day, I am still amazed how such a small voltage could do this. Of course I'm just talking about the pilot light. The main burner was controlled by a thermostat that used 24 V AC from a transformer to power it's own solenoid.
Even the newest home water heaters use exactly the same system. They have a pilot, a burner, and a thermostat and they have to depend on some type of thermocouple to make the whole thing work, as there is no other power source connected to them. I just had one installed and can say for a fact there is nothing connected to it but a gas line and water pipes. The only real difference is you no longer need a match to light the pilot, because they have a piezoelectric igniter button, just like my fireplace.

 

I can't say for sure what you have. What I know of gas valves is that they are typically controlled by 24 VAC or VDC (as you've been told). The thermocouple (or thermopile) is a micro volt sensor that keeps gas flowing to the pilot. If the pilot goes out then the TC loses signal and shuts off the pilot gas.

The MAIN gas valve is too big and bulky to be controlled by a micro-voltage. Just not going to happen. The only way I can see it is that there MUST be something else powering the gas valve. Maybe I'm wrong.

Assuming you have a control voltage (not the micro-voltage), if you use that to activate a fan then when the gas comes on your fan will come on too. Immediately blowing air that has not yet been heated. That's going to get real uncomfortable real quick.

Here's how old style furnaces do it: They come on when the 24 volts is applied. As the furnace heats up eventually a thermostatically controlled switch turns the blower on. The fire box is hot and the fan begins moving air to extract the heat. When the gas shuts off - because the fire box is still hot the fan continues to blow until the thermostatically controlled switch clicks off. Hence, you don't get cold blowing air the moment the heater comes on. AND after the gas has turned off you still extract the residual heat in the furnace.

Your fireplace should operate the same way. Provided you can install a thermostatic switch to turn the fan on ONLY when it has warmed up, and off before it starts blowing cold air around.

One word of caution I didn't notice anyone else mention - - - carbon monoxide. If your fireplace is not equipped for a fan then blowing the heat into the room could possibly be blowing exhaust gasses into the room. Another word for those exhaust gasses - carbon monoxide. If you do this - be deadly careful. Not "Very" careful, - DEADLY careful. Failure on this level means "Death."

I can say for a fact there is no power source connected to the system. I pull the whole assembly out once a year to clean it, and all I have to disconnect is the gas line.

I am thinking it may be working just like a water heater. They have a pilot, burner, and gas valve and they function without any outside power source, just like a furnace, but on a smaller scale. Actually not much smaller than my fireplace setup.

 

I know it goes against all logic, but I am 100% sure there are no other sources of power connected to the gas valve. I disconnect the gas line, and pull the whole burner/ valve assembly out once a year, in early fall, so I can thoroughly clean the burners, etc., and there are no wires to disconnect, only the natural gas line coming from the meter.

.
Even the newest home water heaters use exactly the same system. They have a pilot, a burner, and a thermostat and they have to depend on some type of thermocouple to make the whole thing work, as there is no other power source connected to them. I just had one installed and can say for a fact there is nothing connected to it but a gas line and water pipes. The only real difference is you no longer need a match to light the pilot, because they have a piezoelectric igniter button, just like my fireplace.

 

Yeah, I was thinking the same thing - just use a temperature controlled fan. Lots of modules out there. Here's the first one I found but am pretty sure there are cheaper ones out there.

 

My apologies TonyR, I did not mean to reply with that whole explanation of how a gas furnace works, obviously you already knew that. I tried to edit my reply but for some reason it would not let me

 

Yeah, I was thinking the same thing - just use a temperature controlled fan. Lots of modules out there. Here's the first one I found but am pretty sure there are cheaper ones out there.

Thanks for sharing. That might just fit the bill, and I would be willing to pay that much to make this work.

 

I can say for a fact there is no power source connected to the system.

I believe it! My gas log fireplace also uses no mains power. I’d guess that’s pretty standard. Just flip a switch and away it goes. I believe it’s powered by a turbine driven by the pilot flame flow, but that’s little more than a guess. I’ve always been impressed with how reliable it is.

Oh, and I like the idea of a thermostat for the fan. This will automatically take care of the warm-up and cool-down lags. The fan will run only when it’s useful. Any thermostat you buy will have a hysteresis (the spread between off and on temps) of a few degrees and it might even be adjustable.

 

On a slightly different approach method you could conceivably add a flow sensor/flow detection switch to the gas line that detects when the main burner is on.

 

Re post #8 test results. As you could measure no voltage between the wires to the thermosat it must be using a very low voltage. There are opto isolators that will work with less than 20 mA but the need about 1.5 volts before the IR emitter starts to conduct.This would probably not matter for a 12 or 24 volt control but it would not work with your system. I don't understand why you can't see any voltage across the wires. Even if the heater was sensing the voltage across a 1 ohm resistor I would expect to see at least 20 mV. This does not make sense to me as it would mean the the wiring to the thermosat would have to have a very low resistance to work. It would not make sense to design a product that way. I may be that the thermostat is interrupting a pulsed signal with a low duty cycle but again it is unlikely that it would be made this complicated.

Les.