economizer question

tbart16

I have a model w7213A1016 honeywell economizer. Why won't the A/C go on while the econo. is suppling free cooling? The free cooling is not enough to cool the building ,but the A/c will not kick on. Both units work properly just not together,if I disconnect the S0 wire (i think that is what it was) the A/c kicks right on and the econo shuts down if I reconnect it the A/C shuts down and the econo opens up. outside temp here is in the low 60's temp inside building is high 70's any help appreciated thanks Tom

Techman

Free Cooling

,ain't free. It's cheaper.!What is the outdoor sensor set at? Is there  a minimum O.A. setting? With the dampers in the outdoor air in position , is the indoor air temp  O.K. ? Actually seen in the open position !

Eugene_Silberstein

Enthalpy Control

Sounds like the enthalpy control on the economizer needs to be adjusted. The economizer senses the enthalpy, or total heat, of the outside air. If the control determines that the heat content of the outside air is low enough to provide "free cooling" then the compressor will not operate.

As Terry mentioned, you also need to check your damper settings. If the outside air percentage during "free cooling" is not enough, you will not cool the space. If you are in "free cooling" the economizer dampers should be giving you maximum outside air.

Just a side note on enthalpy... The total heat,or enthalpy, of an air supply is a combination of the sensible heat (temperature) and the latent heat (humidity). Here's an example: Air at 80 degrees F and a relative humidity of 50% has a heat content, or enthlpy, of about 31.6 btu/lb of air. Air at 74 degrees F and a relative humidity of 70% has the same heat content! So does air at 95 degrees F and a relative humidity of 24%! You can find all fun stuff regarding air and its properties on the psychrometeric chart.



Have fun!

tbart16

wide open

the damper is wide open and it still will not cool the building. Thanks for the enthalpy lesson. I did'nt know if the compressor would kick on if you were getting free cooling

Techman

econo

With everything being normal,it seems that the internal heat load is greater than the  "free cooling" can deliver .You will have to make an adjustment to the enthalpy control to bring on the comp a little sooner or eliminate the economizer .Do you have to provide O.A. for the occupents, thru a minimum damper setting on the economizer ?

tbart16

no

no minimum is required. It is actually a subway resturaunt. If I adjust the enthalpy setting will that kick the compressor on? thanks

will smith_4

Min Position

The ASHRAE standard for minimum outdoor air for a restaurant is 20 cfm per person. Sometimes the economizer is also the only way to bring in make-up air to compensate for exhaust fans-I see it quite a lot. I put in 2 roof top units years ago in a restaurant renovation, and another company put in the kitchen hood and exhaust. When they were up and running, the customer called and said my ductwork was sweating, and dripping on the floor (exposed spiral duct). I was out first thing in morning, checked everything out, and everything was textbook-no sweating, drains clear, pressures fine. Later that day, same call came in. I was out again the next morning, and couldn't figure out what the heck the problem was. As I was standing in the doorway by the kitchen staring at the ductwork trying to figure out the problem, the chef came in and fired up the exhaust fan. I happened to be wearing shorts at the time, and noticed that the hair on my leg started moving-no lie. I checked the exhaust fan, and found out it was moving 5000 cfm out of the building, with an outdoor air opening capable of providing 300 cfm. The building was in such a negative that a dollar bill would stick to the front door opening. The reason that my ductwork would sweat was that in the middle of the day when it was hot and humid, the exhaust fan was dragging in outdoor air, and it reacted with my duct like a cold glass on a hot deck in the summer. I explained to the customer that my roof top units were working fine, and that the exhaust fan was behind the problem, but they still shorted me $5000.00. Ironic, when you think about it- $1.00 per cfm. Life's funny that way! 

will smith_4

Min Position

tbart16

here we go again

new problem now the A/c is on and the econo will not kick on the outside temp is 30 degrees any ideas thanks

Techman

The enthalapy

control needs to be re-adjusted.Your compressor need some sort of head pressure control at that low O.A. if they are to stay running. A  "rule of [ big] thumb" was you needed 55* -60* o.a. temps in orded to have the economizer work . For Conn/NY  ,ect

Tim_75

When does this system work properly?

In the middle of summer and the MOAD is at minumum, does it provide enough cooling to meet the load?



If it does, at what supply air temperature?



There are three types of economoizer controls - dry bulb, reference enthalpy and differential enthalpy. When enabled in the cooling mode, the setpoint should be set such that the OA conditions can meet the cooling load at 100% OA. From that point and cooler, the compressor(s) will not run and the economizer will modulate from 100% down to the minimum until heat is required.



IF, the controller senses that the OA conditions have less enthalpy (comparative enthalpy control) than the return air and the cooling load cannot be met, the compressor(s) will cycle as necessary to satisfy the stat. A reference enthalpy controller will compare the conditions of the OA to the setpoint (reference).



As others have mentioned, occupied spaces need outside air (ventilation) and the economizer should have a minimum setting to be maintained.

Brad White

Wide Open OA Damper

That one got my attention. If the damper is wide open, obviously that is all she wrote. I suspect a balancing issue and that a balancing damper in the return air is necessary to add sufficient pressure drop to the return path. This will induce more outside air into the supply path. Of course, the total pressure capacity of the unit must be considered.



It is very common for the return path to have very little resistance (e.g. short path to a ceiling return plenum or local grille), whereas the outside air/economizer path has an outside air louver or hood, plenum, damper and some distance to run. There is no incentive to draw outside air when the return air is practically a free lunch.



Most economizers have two motorized dampers, supply and return, which work in tandem. (Minimum OA may be a separate damper but is often the economizer damper on smaller systems). Even so, a manual locking quadrant balancing damper is needed in the duct with the least resistance.



And yes, as Eugene and others noted, the condensing unit can run when in economizer mode. Typically when the outside air is below 50 to 55, there will be no mechanical cooling, but economizers can allow 100% outside air up to 70 degrees and when using enthalpy control this range is extended upward. The economizer "seeks the better deal", and whichever air source, return or outside air has the lowest total heat content (enthalpy), it gets the nod. THEN mechanical cooling finishes the job.

Cool.

8^)>

John Mills_5

Depends upon the system

Some units, say Rheem for instance, will let you use a 2 stage stat. Stage 1 is economizer assuming conditions are below what the control in the economizer is set for. Stage 2 brings on the compressor. Can't recall if the damper shuts or not. Most economizers are either outdoor or compressor based on the setting in the control. If wide open doesn't do it, probably have to lower the changeover to outside air. We just put in a Bryant 5 ton with econ, it was a 30° day, the mixed air stat was keeping the damper part way open according to the installers. We just left it at the factory setting.



If you run the compressor at 30° out, should have some kind of low ambient control and possibly a crankcase heater .

Empire_2

A few thoughts...

First off Hello Eugene and Terry,.....It's been a while.  Evertone has touched on some basic characteristics of various Econo's.  Carrier, Ruud York, primarily use Honeywell actuators and sensors.  Like most of you stated stg 1 if below enthalpy (Outside) will opern the dampers to different positions and modulate according to what the DAT discharge air sensor is sensing.  It try's to maintain a 50 DAT for this so called free cool. If stage 2 calls,.....and the internal wiring on the RTU not the econo, determins how the additional demand for cooling is satisfied.  I have seen econo damper close when stg 2 calls and I have also seen the damper modulate to a smaller percentage while still in Mechanical cooling.  As stated LO amb is needed.  Go to Honeywell .com and download the wiring diagrams for the actuator you are working on.  In addition you may want to eliminate the mixed air enthalpy control which can be done according to HW.  This will eliminate the R/A imput to the control.



Peace Mike T.