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OT - Looking for cheap thermostat that can control an economizer

I'm trying to stave off some PG&E butthurt this summer by getting the economizer to work on the air conditioner.

Briefly, the air conditioner is like most in that it has a fan, heat, and cold control wires along with a 28vac line to energize the appropriate functions. It also has a control line that will move vanes and allow the fan to draw air from the outside into the building. This is the economizer.

I discovered it's existence while changing the filters. I verified that it works, but it has never been hooked up to a controller.

The plan is to run fan+economizer at 5am to 7am to cool the building down to 65 degrees. This will hold off having to start the air conditioner in the morning.

My plan was to install a second thermostat to control the fan+economizer control wires and program it to come on between 5 and 7. The problem is that all the inexpensive thermostats I can find hard connect the fan and aircon wires together all the time when in cool mode. If I tried to use two of these thermostats, I'd get fan+aircon+economizer on whenever either would come on.

Now I know I could build a box to fix the problem. The problem is that the thermostat is in a very public location and there's not a way to reroute the wires. I'd like something that is inexpensive and looks nice.

Reply to
Jim Stewart
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Why not just hook it up as it should be?

The you would have no visible extra wiring, enthalpy control (will be overridden when it is cool outside but the humidity is high) & it would come on anytime needed (not just in the morning)

MikeB

Reply to
BQ340

Don't change it at the thermostat, put your modifications adjacent to the other end of the wires down at the furnace.

you don't need a thermostat, you need a timer, and a relay to kick the A/C off and the economiser on when the timer says to. Sounds like a DPDT relay fired by a 24 hour timer will get you there.

Stuart

Reply to
Stuart Wheaton

And how would I do that? All I have to work with is what I described.

Reply to
Jim Stewart

I realized that I could hack a second thermostat to do just that. I opened it up, made some cuts and jumps so that the three contacts of the double throw relay come out to the terminals. I connected the fan to the main thermostat fan, the compressor to the normally closed contact of the economizer thermostat, the economizer to the normally open contact and ran the common back to the compressor terminal of the main thermostat.

Everything works normally unless the economizer thermostat is active. Then the economizer is energized instead of the compressor.

Reply to
Jim Stewart

The 'mizers I've worked on have the control as part of it. What is the make & model of what you have? The manufacturer should have documentation online or can fax it to you.

MikeB

Reply to
BQ340

The aircon portion was replaced several years ago and the old economizer remains.

I don't want to go there.

Reply to
Jim Stewart

IIRC the Honeywell T7200xx T7300xx is a programmable commercial /industrial thermostat that incorporates an economizer function with outdoor temp sensing ....they're slightly obsolete and so usually available on Ebay.

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Variations exist within the model lineup so be sure and do your research--you probably just want a basic model instead of one with a bunch of central control and data logging options.

Reply to
PrecisionmachinisT

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