AAON head pressure trips

[Ashlexall]

Out of ANY units I have ever dealt with AAON units are by far the hardest coils to wash and keep clean.

I have run into this exact issue. It took a fire hose to get those extra thick coils cleaned out properly and I was amazed at the ammount of dirt that we got out compared to a regular garden hose and pressure washer.

I would bet a paycheck that your coils are plugged in the center. They may look clean, it's very deceiving.

I second the motion. You will be surprised aaons and trane screw machines are the worst.

[koolkahuna]

I put a stubby gauge on it and disconnected the condenser fan contactor and watched them trip.
It doesn't seem to have a relationship with the out side air temp. Obviously they run more when it is hotter, but it just tripped with an osa of 70F.
As far as the delta are you talking about air in and out?

Exactly. Air temp out minus air temp in = Delta T or "difference in temperatures"
A dirty coil "slows down airflow" causing the heat from the refrigerant to heat the air longer which would increase the delta T, which if exceeding 30deg would indicate a dirty coil.

[AiResearch]

While a delta t measurement can be useful for evaporator coils , IMO for condenser coils , a TD or temperature difference measurement would be more telling.

TD = SCT (saturated condensing temperature) - Ambient

TD could also be called approach when comparing refrigerant to water heat exchangers.

The smaller the number , the more efficient the heat exchanger.

For a/c duty , anything more than 30 degrees is not up to par.

Have measured as good as 12* on modern equipment.

[koolkahuna]

While a delta t measurement can be useful for evaporator coils , IMO for condenser coils , a TD or temperature difference measurement would be more telling.

TD = SCT (saturated condensing temperature) - Ambient

TD could also be called approach when comparing refrigerant to water heat exchangers.

The smaller the number , the more efficient the heat exchanger.

For a/c duty , anything more than 30 degrees is not up to par.
Have measured as good as 12* on modern equipment.

I agree wholeheartedly. Except you can't measure SCT at different places on the coil.
A delta T will show you where on the coil the airflow is restricted. With some condenser with more than one "layer" debris can accumulate between them unevenly.

Good discussion though AiResearch!

[AiResearch]

I agree wholeheartedly. Except you can't measure SCT at different places on the coil.
A delta T will show you where on the coil the airflow is restricted. With some condenser with more than one "layer" debris can accumulate between them unevenly.

Good discussion though AiResearch!

Excellent point!

Never thought of that particular diagnostic , and will have to try that sometime.

[allstar08]

Condenser approach and condenser delta. I will use them both, sometimes you forget about the easy stuff and get tunnel vision. I do wonder why it doesn't happen when I run it, while I am in front of it.

[AiResearch]

I wonder too.

Which points me in the direction of reheat.

Usually a dirty coil will be present and accounted for.

Think about how a modulating valve in the discharge line could cause a high pressure trip.....

[Bklyntek]

Hi guys, bklyntek, if the coil split it and clean both. I would call aaonn and ask for motor part #'s. then you can verify of what u have r the right Motors. Years ago, would have used nitrogen to clean coils,(no reg.)2guys one on tank valve,next one at w/hose forcing nitro.upwards along fins. With unit running,wouldn't believe all that dirt coil can hold.

[koolkahuna]

Hi guys, bklyntek, if the coil split it and clean both. I would call aaonn and ask for motor part #'s. then you can verify of what u have r the right Motors. Years ago, would have used nitrogen to clean coils,(no reg.)2guys one on tank valve,next one at w/hose forcing nitro.upwards along fins. With unit running,wouldn't believe all that dirt coil can hold.

With no regulator? With 2500psi I'll bet dirt was coming off.
Did you have safety glasses on? And a 1/4" steel plate to hide behind?

[Bklyntek]

Lets just say, you WANT to have BOTH HANDS as close to the NECK of that hose!!!

[Bklyntek]

Lets just say, you WANT to have BOTH HANDS as close to the NECK of that hose!!!

[allstar08]

Generally an outside air unit for make up will provide room neutral air temp (70*) and not actually contribute to cooling or heating the space. The cooling and heating would be staged to maintain supply air set point by the DDC controller. There are times when dehumidification is needed but running enough refrigeration to meet the %RH supply set point would overshoot the supply temp set point. At that time the hot gas reheat coil becomes active to bring the air back up to the neutral set point.

The Aaon modulating hot gas valve controllers will receive a 24v command from the DDC controller or humidistat to the "reh enab" terminal. The hot gas controller will then begin to close the modulating valve feeding the condenser and open the modulating valve feeding the reheat coil as needed to maintain the supply air set point as dictated by the dip switches on the reheat controller. The setting should be similar to the DDC set point for supply air temp. The hot gas controller takes over compressor control generally running 2 stages of refrigeration. The first stage will have the modulating valves , if additional reheat is needed a 3 way valve will divert 100% discharge gas to the reheat coil for second stage.

In your case it could be the cond mod valve closing down with the reheat valve not opening , or a problem with the 3 way heat reclaim valve on the other circuit.

You'll have to go back and see exactly what you have and how it's wired on the controls.

But that should be after all the simple fixes have been checked or confirmed.

Hope that helps you.

Hot gas is normal pressure control reg valve. Pressure set point is 55#

[AiResearch]

Hot gas is normal pressure control reg valve. Pressure set point is 55#

Okay well that sounds like the hot gas bypass valve to the evaporator and not the reheat.

Did you get the unit fixed?