Stove draft control logic

[cardamon]

The could arguably go under the heat section but I think it is more applicable to controls.  I have a woodstove that has a little box on the bottom with a damper. The damper is actuated by a mechanical thermostat (or was, mine is broken) on the stove.  My plan is to keep this damper closed and instead connect a pipe to it and route it outside.  In this pipe, or at the end would be a butterfly valve or just a flap that will be controlled by a solenoid.  I would like to control this valve with a programmable electronic battery powered thermostat.  Thus I can automate temperature control of the space, and have it programmed for a lower termperature overnight, and just have a more constant temperature.  I am looking for ideas on the control logic, here is what I have so far:

Of course the thermostat's contact will be closed when calling for heat and open when not calling for heat.  I can get a latching solenoid to actuate the valve.  This means that a brief standard voltage will push the solenoid and a magnet holds it pushed when power is removed.  A brief opposite voltage will overcome the magnet and pull it back.  So i need to interface the thermostat to the solenoid.  I have come up with various schemes using multi pole latching relays and capacitors, but they seem excessively complex and my electronics knowledge isnt strong enough to come up with a solid state solution on my own.  I guess it goes without saying that I dont want to just using a regular spring return solenoid and keep it powered 50% of the time, becuase my system is very small and even 4 watts at 50% duty is too much of a phantom load.  Ideas?

Thanks

[wdyasq]

My first try would be a small cylinder and piston actuated by a closed cylinder of oil (or other liquid of your desire). This requires no electricity and gives a variable opening. Various ways can be used to control day/night temperatures.

Ron

[scottsAI]

Cardamon,

Good you understand about the latching solenoid. Sounds like a Good solution.

Use DC to power the solenoid, placing a capacitor in series with the solenoid, limits current to flow for a short time, must be sized such that the solenoid is properly activated.

This could work with your battery powered thermostat.

Energy stored in cap can be used to switch back the solenoid.

Bunch of details are missing to give a specific solution.

Details:

what is the model number of the controller your looking at. (work on DC?)

What voltages do you have to power the solenoid and any additional relays etc.

Have fun,

Scott Beversdorf.

[Ungrounded Lightning Rod]

Have you considered using a position sensor to turn off the power once the actuator has moved to the correct position?

(Downside:  Power stays on if the hardware sticks...)

[cardamon]

wdyasq,

I considered some sort of self regulating solution.  In fact the stock thermostat does this mechanically, however I would like something more refined - I could save a lot of wood at night...

scottsAI,

I was just planning on using your run of the mill digital thermostat.  They usually run off a few AA batts.  Every one I have ever hooked up or messed with just had two wires which become common when it calls for heat and breaks when satisfied.  That said, they have a few other terminals and I dont know what their logic is, I assume they are for more complex system with A/C?  As for your other question, Mine is a 12 v system so I would like to use that directly.  The latching solenoid I was looking at is 12v so thats good.  I had considered using a multipole relay which would switch the polarity around for the unlatching and in the process break the power to the solenoid.  A capacitor could hold a bit of juice for the solenoid so it can complete its motion before the relays cuts off power to it.  But then how do you reverse the operation when the tstat opens the contatcs?

ULR,

Well see above, that was my non solid state solution. Another one would be to run the solenoid coil through a microswitch and have the solenoid roll over ita tthe end of its travel.  Were you thinking of a more electronic way?

[cardamon]

I think the key is to understand what the other terminals do on the thermostat.  Typically there is a W,Y,G,R, and Rc.  For basic operation, two of these open on temperature rise, it would be great if some of the others opened on temperature fall.  An HVAC guy would know....

[Kwazai]

http://highperformancehvac.com/thermostat-wiring-colors.html

hope this helps.

Mike

[ghurd]

All the "thermostats" I saw like that were analog.

Sounds like a solenoid would be digital.  Full roar, or going out?

Have you considered replacing the factory part?  (Lehman's)

G-

[cardamon]

I am ok with all or nothing, I am not trying to modulated the valve. Actually I can put an adjustable stop on the closed position and adjust it for minimum heat.  The factory part isnt programable and references the temp on the surface of the stove, not the other side of the room.

[(unknown)]

Our wood burner has a small blower on the front. When the thermostat calls

for heat, it simply turns on. Other wise it's off. There is a vane that

covers the inlet of the blower. This is how you set the heat output of

the furnace: Close the vane up (1/4 inch gap) when it's 30F out, open it

up (1/2 inch gap) when its 0F out.

It works great and only requires on-off control, just like your thermostat

provides. Your going to need a relay to drive it. The great thing about

this setup is that it "puffs" up the fire, then lets it die off. This

creates vast quantities of charcoal which don't go out, and tend to dry

any new wood added.

The blower is power-fail safe. If you do lose power, you can drive it

with a puny inverter, or do what we do, just open the vane more and

go to manual mode.

Vanes tend to stick open which MIGHT over fire your furnace (depending

on the built in restriction). You can reduce the life of your furnace

by a decade in 1 hour !

Drawing air in form the outside sounds like a good idea, and should

work if you live in a single level house. If you have the furnace in

the lower level of a multi-story house and you draw your air in from

the outside through a sealed pipe, you could run into serious thermal

run away. You have, say, 2000 square feet of heated rooms above

your wood burner, creating a vacuum as it exits the roof-line. It could

force an order of magnitude more air into the intake depending on the

temperature difference between the inside/outside of the house, outside

wind speed, opening up the attic access door, turning on the bathroom

vent, etc.... Bad Karma.

Scott

P.S:

You need a blower, not a fan. If you block half the intake of a blower

you get close to 1/2 the air flow. If you block a fan half way off,

its completely non linear.

The blower is about as powerful as an old (metal) heat gun. It's a

1/25 HP shaded pole motor (which you can control with a lamp dimmer

if you wish to experiment). It's intake is 3.5 inches in diameter, uses

a 4 inch diameter squirrel cage impeller. It pumps the air through 4

1/2 inch diameter holes into the otherwise sealed wood stove (a 1976

WoodChuck 2800).

[alcul8r]

When I was looking for comments on a stove I built, someone suggested using a carburetor, which already has the bimetal spring and butterfly valve.

http://www.fieldlines.com/story/2005/5/12/172658/242

[birdhouse]

is the stove a vermont castings "aspen"  i've seen mechanical thermostats like what you describe on the bottom of them.  they are in place to keep the stove from getting too hot, prevent damage, not to control room temp.  they restrict air from entering the firebox when the stove gets up to a certain max temp.  would hate to see your stove crack/melt from your mod.  just my 2 cents.  good luck!

[(unknown)]

It might be hard to do this with an electronic thermostat or any thermostat for that matter since they are simply on/off control. Ideally, you want to modulate the incoming air so that you create a "more even heat" instead of over firing the stove on full air. This is actually old technology, but you can do this with a resistive type thermostat and actuator. I know that Honeywell still make models that use 135 ohm resistance to control the actuator. That way you aren't using any AC or DC voltage to control a motor.