Any electrical engineers here?

[FanMan]

I know this is feasible, and they gave us enough EE in the mechanical engineering program that I can figure it out, but picking the specific components a good EE could probably do off the top of his head... and probably somebody's done it before (or maybe there's something off the shelf?) Anyway...

I have a gas heating stove in my living room. Does a great job, overkill even, of heating the living room and kitchen. I want to move heat from there to a bedroom that's on the other side of the wall from the stove. Mechanically that's simple, an air inlet high on the wall above the stove, and a fan blowing into the bedroom near the floor, the space between two adjacent studs forming the duct. A thermostat in the bedroom controls the fan. So far, simple. However, even if the bedroom is cold and calling for heat, I naturally don't want the fan to come on unless the air at the inlet of the duct is warmer than the bedroom.

My thought is this: A J-type thermocouple near the duct inlet in the living room, connected to a transistor in the bedroom. The output of the transistor switches a small relay, which is wired in series with the t-stat in the bedroom. Then whenever the the thermocouple in the LR is warmer than the cold junction in the BR, the voltage is positive and switches the transistor on, which in turn turns the relay on, and the fan runs. Simple in theory, but the choice of the specific transistor (and any related components?) is less clear to me. I'm also not clear on what happens when the temperature difference (and thus the B-E voltage) is below the saturation level, i.e. would the C-E current flow, with the voltage drop, be too much power for the transistor to dissapate?. Or is the thermocouple output voltage (say 150μV for a 5°F teperature difference) even enough to switch the transistor?

 

[jimbom]

Two thermostats in series?

 

[FanMan]

Two thermostats in series?

I thought of that, but it would only work at the set temperatures... I want it to work at a temperature differential, regardless of what the actual temperature in the warmer room is.

 

[raybonz]

I know this is feasible, and they gave us enough EE in the mechanical engineering program that I can figure it out, but picking the specific components a good EE could probably do off the top of his head... and probably somebody's done it before (or maybe there's something off the shelf?) Anyway...

I have a gas heating stove in my living room. Does a great job, overkill even, of heating the living room and kitchen. I want to move heat from there to a bedroom that's on the other side of the wall from the stove. Mechanically that's simple, an air inlet high on the wall above the stove, and a fan blowing into the bedroom near the floor, the space between two adjacent studs forming the duct. A thermostat in the bedroom controls the fan. So far, simple. However, even if the bedroom is cold and calling for heat, I naturally don't want the fan to come on unless the air at the inlet of the duct is warmer than the bedroom.

My thought is this: A J-type thermocouple near the duct inlet in the living room, connected to a transistor in the bedroom. The output of the transistor switches a small relay, which is wired in series with the t-stat in the bedroom. Then whenever the the thermocouple in the LR is warmer than the cold junction in the BR, the voltage is positive and switches the transistor on, which in turn turns the relay on, and the fan runs. Simple in theory, but the choice of the specific transistor (and any related components?) is less clear to me. I'm also not clear on what happens when the temperature difference (and thus the B-E voltage) is below the saturation level, i.e. would the C-E current flow, with the voltage drop, be too much power for the transistor to dissapate?. Or is the thermocouple output voltage (say 150μV for a 5°F teperature difference) even enough to switch the transistor?

Install a bimetallic thermostat in the ductwork in series with the T-stat (N.O. close on temp rise) that says do not run the blower unless the bimetallic T-stat is satisfied? Simple and essentially how a hot air furnace works..

Ray

 

[WES999]

Sounds like you need a differential controller.
Here are 2.
http://www.lightobject.com/Solar-Heater-Temperature-Controller-with-dual-input-P214.aspx

http://www.mydtcstore.com/ I am using one from here for my solar DHW system.

 

[woodgeek]

A thermocouple won't generate enough voltage to bias a transistor....go need an electronic controller.

 

[FanMan]

Install a bimetallic thermostat in the ductwork in series with the T-stat (N.O. close on temp rise) that says do not run the blower unless the bimetallic T-stat is satisfied? Simple and essentially how a hot air furnace works..

I thought of that, though the thermostat would have to be near the heater, not in the duct, because hot air won't be drawn into the duct until the fan runs. I want the blower to run whether the heater is on or not, as long as the air near the duct inlet is warmer than the air in the bedroom. The idea is that if the bedroom is calling for heat and the living room is warmer, the fan runs, drawing cooler air into the living room and eventually the heater starts.

Basing it on the heater temperature means that nothing happens until both the bedroom and the living room are calling for heat. If the bedroom is set cooler and doesn't need as much heater "on" time as the living room, this could work. Or just run the blower whenever the heater is on, no bedroom thermostat, and control the heat balance with the fan speed (which is essentially what you do with a single zone furnace with dampers on the registers or radiators. I' probably just making this overcomplicated... I'm just playing with ideas now but I'll start simple when I actually build it.

Wes999, an industrial thermostat like you linked to would definitely do the job, but it's more than I'd want to spend until I'm reasonably certain that the operating mode I'm describing is actually what works best. I'll spend a few hours and a few bucks playing with discrete components though, no great loss if it doesn't work and I learn something new regardless.

 

[FanMan]

A thermocouple won't generate enough voltage to bias a transistor....go need an electronic controller.

I was afraid of that... well, as I said above I guess I'll start simple and see how it works.

 

[slim]

I was afraid of that... well, as I said above I guess I'll start simple and see how it works.

Considered using thermostat controlled vents, somehow. Check Ebay & good luck.

 

[Don2222]

Hello

I am and EE. I put a Honeywell Line voltage thermostat with cooling contacts above my stove and set the Temp for 75 Deg F The cooling contacts are connected to the ductwork fan, so when the air above the stove goes above 75 Deg F the fan turns on and pulls the hot air into the room. Simple and easy to wire up! If you want a wire diagram, let me know.