10ton Aquasnap - won't start

[HeatXfer]

Hello all, I've just installed a hot gas bypass to a 10 ton 30RAN010 (Aquasnap) that previously had no start or run problems, other than requiring HGBP. The problem I'm experiencing is the unit won't start. There are no alarms, the pump starts, everything is ok to go. I've reconfigured for MLV & "Close Control", and returned configuration to default to no avail. All components function properly in "test", but the compressor won't start in normal run mode, even with LWT 15dF above set point.
The only clue I've got is Mode 26: Short Loop Override. The compressor is not short-cycling, it won't even start. I starting to think this might be a 'firmware' problem.

Anybody else experience this one? I Haven't talked to my Carrier rep yet. Any ideas or experience with this is greatly appreciated.

Thanks

[TRGunner]

Alarm or mode "26" on a Aquasnap with the comfortlink software means remote off. Is there a relay from the EMS in the control panel?

Question: Why did you install HGBP on a chiller barrell? I might be missing the picture.

[HeatXfer]

TR: Thanks for the reply. The mode 26 is 'Short Loop Override' on the scrolling display. It's used to limit short-cycling in systems with low water loop volume.
The problem I'm having is that 'Mode 26' prevents the compressor from starting at all when I power up the chiller.

I've also got some problems with the PID control affecting set point control: With a 40° set point the system drives water temp to 50° and then cycles off. No alarms, no Modes, no indication of why, very aggravating. Last night I had to babysit the system for 16hrs to keep it online.

I've got 2-10 ton chillers for a process cooling load that varies from approx 3tons to 15tons depending on how many loads are running. I'm installing HGBP (Carrier calls it Minimum Load Valve) on both units so either can handle the lower loads. The HGBP can add up to 6tons of load to the evaporator.

[TRGunner]

TR: Thanks for the reply. The mode 26 is 'Short Loop Override' on the scrolling display. It's used to limit short-cycling in systems with low water loop volume.
The problem I'm having is that 'Mode 26' prevents the compressor from starting at all when I power up the chiller.

I've also got some problems with the PID control affecting set point control: With a 40° set point the system drives water temp to 50° and then cycles off. No alarms, no Modes, no indication of why, very aggravating. Last night I had to babysit the system for 16hrs to keep it online.

I've got 2-10 ton chillers for a process cooling load that varies from approx 3tons to 15tons depending on how many loads are running. I'm installing HGBP (Carrier calls it Minimum Load Valve) on both units so either can handle the lower loads. The HGBP can add up to 6tons of load to the evaporator.


I was picturing a HGBP loop to a barrell !! LOL:D
The software in this chiller may be a little different with the application you are using. (Talking about 26 code) The display is the way to be sure like you have indicated. The chiller is trying to reset on return water temperature from your EMS. Can you change the return water setpoint in the EMS PID temporarily or turn off reset all together since you need a constant cold water supply temp.

[jrbenny]

Do you have a glycol in the loop?

If memory servers me correctly, Mode 26 occurs after 6 low temp shutdowns in 30 minutes.

I've found this to happen when glycol (or other inhibitor) is in the loop without the software being configured properly.

Additionally, if you don't have have a decently sized loop, then you will simply be screwed. In this case, I always recommend a storage tank or ten. ;)

[IFRZAIR]

What is your capacity load set on?

[gyhooyabat]

Jrbenny is close. The machine will go into this mode if it starts more than 5 times in 60 continuous seconds. This mode will prevent the machine from short cycling.

MD26 – Short Loop Override
This mode will be displayed when the machine is being held off to limit the number compressor starts. The control tracks the number of starts of the compressors to maintain the desired leaving water temperature. The number of starts will be limited ensure proper oil return and prevent premature compressor failures. If this mode encountered, check the chilled water loop for adequate volume. Low loop volume contributes to compressor cycling. Check the chilled water loop load. Low load conditions, below the minimum capacity stage will result in compressor cycling.

This algorithm is to protect the compressor. We receive many reports of very warm water temperatures and the machine stuck in Mode 26. To start the machine, increase the Z.GN Deadband Multiplier (Configuration Mode, Sub-Mode SLCT) by a value of 1.

This mode can be disabled as well, but would require access to the software via service tool or comfortview software. Under Configurations, Options 2, you can configure the "short loop gain" as enabled or disabled. It is enabled as the factory default. Keep in mind, if you disable this safety you risk short cycling your compressors and causing more damage.

Make sure your machine is receiving an adequate load for it's size. Remember, the minimum load does not work on machines smaller than 15 tons.

[HeatXfer]

First: Thank you guys for your direct answers; not a red herring in the bunch.:D

Since my last post I've returned to the job site and observed the following:

1) When I start the chiller (remote-off-enable switch set to enable) the chiller immediately goes into MD26, even though it has not been running for 24hrs. There is no mention of this in any of the documentation for this chiller, either included or available on line.

2) After approx 5 - 10mins MD26 status went to 'off' and the compressor started.

3) With a 40°F set point the first run cycle drives the CHW temp down to 48° (from 55°F) and cycles the compressor off. With each subsequent compressor cycle the CHW temp drops closer to the set point, until after 3 -4 cycles, about 1½hrs, the CHW temp reached set point. (I assume this is just the PID studying the operation and adjusting the run cycles and loading) It's now controlling from 42°F down to 38°F.

4) With MLV and Close Control enabled the display shows a 70% cap. Since it has been running (approx 60hrs) I have yet to see it load to 100%. If I understand this correctly, the minimum load the 10ton TXV can handle is approx 50%; 5tons. The MLV will supplement up to approx 6.9tons (70% of 10tons) So, if my TXV is at its minimum flow, the MLV will suppliment approx 3 -3½tons. That leaves a 2.5 - 3ton actual capacity. Which, coincidently, is my lowest load on this system.

5) At this point it appears to be running well, even though the load is very low: my best supply/return CHW temp diff is 3° (higher CHW loop flow than secondary process flow, water-water HX). The compressor duty cycle is about 50% (25min run time/ 25min off time). System volume is approx 250gpm

Instead of waiting it out and just monitoring the operation (or non-operation) of the unit, I was jumping-gun and making inappropriate control changes (there are a lot of control options on this machine) that probably caused more delays and poor performance.

Thanks again to everyone. I hope others can get something from this thread.