Brainstorming help on controls

C44

Please help me make a list of all the ways to do this: I want to control a damper on my system based on pressure in ductwork. Simple "on" and "off" is fine if the trigger pressure is adjustable.

Background: A 2-stage AC system, where I want to consider a fresh air vent to the return plenum which can only open in 1st stage. When the system is in 2nd stage it is working harder to meet temperature setpoint and I see no sense in feeding it outside air and making its job harder.

Ideas for non-electric ways to control a damper would be particularly welcome. I know there are electrical signals that could get the job done, am looking for all the possible ways. Perhaps someone makes a pneumatic bellows that can move a damper control arm?

Thanks for any ideas -- C44

Yoyizit

What are your pressures, forces and stroke lengths? How fast does it have to respond?
It may not be possible to do this unless you can use an air pump.

beenthere

One way.

Use a barometric damper(like the kind used for zone systems), and install it backwards.

Set the weight that it takes more pressure then first stage air flow to close it.

Drawback. It will also be open when the system is not running.

C44

I will answer what I can. Vacuum in the return plenum is expected to be about 0.15 inch w.c. in low stage, and about 0.30 in high. If it responds in less than 1 minute that is fast enough.

Could you clarify what you said about an air pump?

-- C44

Yoyizit

The system would not be elec., only pneumatic, but it might need an air pump that does run elec.

You maybe can have the damper powered by pressure differences as long as you don't try to get more energy/power out than goes in, or you have the ductwork pressure control an air pump that drives the damper.

newtech

Just my 2c

C44

FWIW I have far less return vacuum than I had thought, in low stage. Just measured it as 0.06 inch w.c.
High stage is the same 0.30 as stated before.

Probably does not change the thread much, but wanted it to be correct.

Regards -- C44

Yoyizit

1" WC = .04 PSI, so 0.3" gives you .01 PSI. If your damper takes 1 pound to operate you'd need a diaphragm or piston with a surface area of 100 sq. in.
http://www.onlineconversion.com/pressure.htm
I'm not sure anyone makes this bulky gadget for this purpose.

C44

This sounds simple and elegant. However I am only vaguely familiar with the device. Would it be possible for you to post a link to a good example of this kind of barometric damper?

I did try to search the web and what I found were for wood stoves and those seem to me for a different application.

Thanks very much -- C44

beenthere

Look at the PRD-RD

Here

C44

Thank you, that is a big help. The wood stove barometric dampers are entirely different!

C44

I appreciate all the ideas. Now I have thought a little more about the application, which is a fresh air (FA) intake on a 2-stage air conditioner. There might not be a *mechanical* way to shut off the FA intake during heating hours, and I don't think FA will be necessary or desirable when heating. So an electric damper may turn out the best way after all.

All you guys in places with 4 actual seasons, could simply make a change at the beginning and end of winter and that would be OK. In Texas we don't have 4 seasons, more like 2 or 2-1/2. Our winter comes and goes, often with air conditioning needs in the same months.

Thanks for all the help! -- C44