Economizer install on package Geothermal heat pump

[bytenik]

Hello,

In my own home, I had a package 3 ton Waterfurnace Envision unit installed -- 2 speed with ECM. On these units, a call for fan (G wire) calls for the low fan speed only. By calling for stage 1 or stage 2, the fan speeds up, but of course the unit starts heating/cooling as well.

I am now installing a new kitchen exhaust hood that requires significant makeup air due to how tightly my house is constructed. My plan was to run the makeup air through the HVAC system so that the outside air gets filtered, and potentially pre-conditioned depending on outside temperature. As long as I'm putting in a giant outside air intake and damper anyway, I figured I may as well hook up an exhaust damper and install an economizer instead of just the single damper. I find that I end up running the AC during times in the early Spring/late fall when the outside temp is lower than the indoor temperature, because I have severe seasonal allergies and can't just open the window. (But running outside air through the HVAC filter is fine.)

Ordinary economizer panels rely on sitting between the compressor/condenser and the AHU, so that the thermostat can call for stage1/stage2 cooling and the panel can "lie" to the compressor about what stage to actually run, if any. This way, you get the "free" cooling effect of running the system fan but pumping in outdoor air instead of running the compressor. Meanwhile the AHU receives the thermostat's true call and the ECM speeds up to whatever speed it thinks is appropriate for the stage.

In a package system, I can't put the panel between the AHU and compressor, because they're of course one unit. There's only one set of thermostat connections. Is there a way, on a Waterfurnace Envision unit, to get it to speed up the fan without (a) adjusting the low fan speed to always be faster and (b) without turning on the compressor?

[epmiller]

Any way you can control the low voltage to the compressor contactor with small pilot relay(s)? Without seeing the schematic of your unit I can't go much farther than that, I've never worked with a true economizer.

[buford]

You would think companies that make dedicated MAUs would make smaller units like what you need. Seems like there is a market for them. At the least a combo exhaust fan make up set up. Then trying to figure out how to place it in these designer kitchens.

[bytenik]

Any way you can control the low voltage to the compressor contactor with small pilot relay(s)? Without seeing the schematic of your unit I can't go much farther than that, I've never worked with a true economizer.

If you Google for waterfurnace envision installation manual, the first link, page 26-27 is the schematic. I don't have forum privileges to post a direct link. However, I would assume given that these things all have microprocessors in them nowadays that if the unit calls for the compressor to turn on and nothing happens (due to a relay disconnecting the contactor), it will flip out in some manner.

[bytenik]

(Perhaps the pressure switch detecting low pressure due to the compressor not running, causing the system to lockout, or some such behavior.)

[epmiller]

If you Google for waterfurnace envision installation manual, the first link, page 26-27 is the schematic. I don't have forum privileges to post a direct link. However, I would assume given that these things all have microprocessors in them nowadays that if the unit calls for the compressor to turn on and nothing happens (due to a relay disconnecting the contactor), it will flip out in some manner.

I looked at the schematic in the manual and read the relevant sections and yes, you will get a fault and lockout if you interrupt the compressor contactor control voltage. My idea would not work. I didn't read every word in the manual but I didn't see any way to accomplish what you want. You almost would need another thermostat to run the economizer and when that wouldn't do the job it would call the main tstat. Don't go there, no one could repair it.