AAON Overheating

[duke of earl]

I do Aaon startups all the time and have met a real brainteaser. Have a 20 ton MAU that is configured for outside air but has a return duct also. The oa damper is being controlled by space pressure in a lab. Problem one is the lab is open to the rest of the building so the hood exhaust fans can not pull the lab into a negative (common ceiling plenum).This is keeping the unit in return mode and therefore the unit heat exchanger is oversized for the load. What happens is that every once in a while the unit has a diagnostic (brief) and shuts down momentarily but then restarts and then heats the space to 90 degrees. My SAT setpoint is 72 degrees with no reset configured. I'm not on the job when this happens so I haven't been able to catch the diagnostic. After the overheating happens, the unit goes back to operating normal and keeps the space between the heating and cooling setpoints. I'm using space temperature for my mode enable since the oa damper is sometimes closed. Anyone ever seen such a thing?

[lwarren]

I would call Mario or Andy or one of those guys at Orion and see if they can offer any insight.

[Pneuma]

I do Aaon startups all the time and have met a real brainteaser. Have a 20 ton MAU that is configured for outside air but has a return duct also. The oa damper is being controlled by space pressure in a lab. Problem one is the lab is open to the rest of the building so the hood exhaust fans can not pull the lab into a negative (common ceiling plenum).This is keeping the unit in return mode and therefore the unit heat exchanger is oversized for the load. What happens is that every once in a while the unit has a diagnostic (brief) and shuts down momentarily but then restarts and then heats the space to 90 degrees. My SAT setpoint is 72 degrees with no reset configured. I'm not on the job when this happens so I haven't been able to catch the diagnostic. After the overheating happens, the unit goes back to operating normal and keeps the space between the heating and cooling setpoints. I'm using space temperature for my mode enable since the oa damper is sometimes closed. Anyone ever seen such a thing?

You can blend return air in after the cooling coil as sort of a reheat to keep the supply registers from sweating, since air leaving the dx is saturated. Maybe that's what you have...

[maxpower]

Sounds like a air flow design problem. Have you checked the RLA of the blower with the return full open,O.A. closed? Then with the OA. full open,return closed? It might be possible to bump up the air flow with adjusting the sheave. Do you have a freq drive?

[cannedheat]

are you trying to heat the space only?

[cannedheat]

can you program for manual damper only .set oa for 100%. you said dat was 75deg. whats room sensor set for?

[duke of earl]

I think my thread is misunderstood here. The unit runs great most of the time and the problem is very intermitten. Please read the original again as the issue is not airflow related, it is an "out of control" issue that happens momentarily and then corrects itself.

[dd98724]

I think my thread is misunderstood here. The unit runs great most of the time and the problem is very intermitten. Please read the original again as the issue is not airflow related, it is an "out of control" issue that happens momentarily and then corrects itself.Is there a way to ensure that the lab door remains mostly closed? There is nothing else you can do unless they're willing to live with a slight positive. In which case you can set o/a damper min position.

[duke of earl]

Even with the door closed the ceiling is open to the rest of the building. The pressure/oa damper is not the root of the problem. It is like Indian Spirits come up from the grave once in a while and take control of this unit. I have watched it for hours at a time and the heat cycles fine without high discharge air temperatures, even with the oa damper closed. The unit just shuts down fan and all and then comes back on and runs full heat without any regard to the DAT setpoint. After a short while of this, the unit goes back to running normal.

[maxpower]

So we can now throw out all the tech stuff and look for a loose connection?....? bad board? strange for sure..Any diagnostics come up?

[monkeyman#1]

I am not familiar with this particular unit, but I think that I would be taking a look at the discharge air sensor and wiring. Also is it mounted securely? Just a couple of thoughts. Very strange.

[duke of earl]

I found the unit wiring was a birdsnest (butchered by the last contractor). I rewired the control panel slowly and methodically with the Aaon/Wattmaster wiring guides. I re-commissioned the unit and verified all functions very thoroughly and tightened all electrical connections. The unit ran for months trouble free and then started acting up. I hate to be a parts changer because of the cost of the boards but I think it is possibly the next step. I've never caught the unit in alarm because it happened so fast when it happened. The alarm clears quickly and the unit goes overboard on the heat and doesn't care about DAT setpoint.

[energy star]

The unit still does exactly what it did from the first post? From post one to post twelve the unit performed or failed in the same manner. Just the duration of up time seemed longer between codes. In the first post you state the system has an OA & return duct. You did mention that the Lab is open to the rest of the building making it almost impossible to achieve a negative pressure. Do you know if any other exhaust system is in the lab or close by? Perhaps your intermittent problem is tied to another piece of equipment that does not come on as regularly as the aaon system, but when it does and the aaon system happens to be running it creates a fault. I would look around the building at other systems or make certain any pressure sensors are working correctly. It seems to me you have checked out your system pretty well, these are Just ideas I had to help you.

[duke of earl]

The labs have constant volume hoods that continually exhaust air from the building. Exhaust is not being used on the rooftop. Just opening the oa damper if the space goes too far into a negative which it never will with the common plenum to the rest of the building. I'm guessing no one has ever seen such an issue before. The customer is aware that a tight lab is necessary to make the unit function as designed.

[DavyB]

Even with the door closed the ceiling is open to the rest of the building. The pressure/oa damper is not the root of the problem. It is like Indian Spirits come up from the grave once in a while and take control of this unit. I have watched it for hours at a time and the heat cycles fine without high discharge air temperatures, even with the oa damper closed. The unit just shuts down fan and all and then comes back on and runs full heat without any regard to the DAT setpoint. After a short while of this, the unit goes back to running normal.

I'm having a hard time understanding as well. I'm not versed in Aaon mua units, but I know that on Sterling units, you have to use shielded control wire for some of the controls to operate correctly. Sorry if I'm no help at all, been drinking;)

[duke of earl]

It's like pulling a vacuum on a system with the caps off and all the schraders removed. You just keep sucking in air cause the system ain't tight. Same with the lab, it's not tight so it won't go negative enough to make space pressure setpoint.

[mrhvacmechanic]

Duke,
Since your using a oa and return duct, do you think it would make sense to use your mode sensor in the mixing plenum?I know you said your SAT setting is 72F. When your in RA mode does the unit overshoot the setting?

You may have better dischage temp control this way due to your damper positions. Do these dampers modulate based on the room static pressure?

Reason being when damper is in RA position, your entering air may be too high from space for the lowest setting on the mod gas valve. Unit cuts out, then resets. When you go to OA its inlet air is low enough to keep from over heating?

Just a thought?

[duke of earl]

This unit has no mixed air sensor. It was originally purchased as a MAU. The customer plans to tighten the lab up so the exhaust fans can pull the space into the desired negative. The unit is set up for dehumidification also. The unit works great most of the time but shuts down momentarily and then heats out of control for a while. This is not just DAT overshoot, it gets out of control!

[mrhvacmechanic]

Any possibilities of electrical noise? Are sensor cables sheilded and grounded at one end only? Does unit have a good ground?

[duke of earl]

These are the next things on my list to check. Wattmaster said the sensor wiring does not need to be shielded.