Relative Humidity Oscillation

[HVACnewguy15]

Hello

We're currently in charge of two Lennox RTUs (Models LCA102H and LCA120H). Both units service two labs and by design they're always online. The units have been installed and operational since at least 2010.

For some time now, the Relative Humidity in both areas is continuously oscillating. Even under high temperature and relative low ambient humidity, inside the areas RH still oscillates far more than it should (verified by placing data loggers in the areas). Also, this erratic behavior has been verified under almost all weather conditions.

After establishing trends, we've begun to identify the pattern: High RH occurs simultaneously with an increase in the discharge air temperature. As if the units are heating the air.



Per manufacturer service personnel recommendations, compressors were replaced, to no avail. Maintenance is conducted 3 times yearly (coil cleaning, pressures verification, etc...) and air filters are replaced monthly.

In your experience, what could be a possible cause of this erratic control?

I appreciate any recommendations that you guys can offer.

[HVACnewguy15]

Forgot to post this information here, we´re located in Monterrey, northern Mexico. The Units, are equipped with Humiditrol and Economizer, with 2 compressors each and running in ODE DIFF Control Mode. In addition, there is no return ductwork, the return air intake is open to the ambient.

Code 92 is always displayed in the board, been like that since before we started servicing this equipment.

[teddy bear]

What are the specs for %RH/^F temp?
Look at your data on a good day/bad day basis. Too much info in a small space. Any chance for outdoor temp/^F dew point instead of %RH?
The same for the a/c supply, temp/^F dew point.
Get the feel for dew point at the outside/in the space/at the supply. The supply dew point should be <10^F than the return minimum.
Looking forward to info.
Regards Teddy Bear

[HVACnewguy15]

Hello!

Thanks for your reply. I'll download and prepare the data today.

Regards!

[HVACnewguy15]

Hello again,

For both RTUs, specs are 70°F temperature and 60 % RH.

Now, I downloaded data from both the system (both lennox units parameters are monitored through Metasys interface) and an external sensor for the past days. The only parameter I could not measure was supply dew point. Space dew point was calculated via the August Roche Magnus estimation. A "bad day" was chosen.

First graph



ST (Space temperature), SDP (Space Dew Point), ADP (Ambient Dew Point), AT (Ambient temperature), RAT (Return Air Temperature).

Second graph



DAT (discharge air temperature), AmT (Ambient temperature), ST (space temperature).

Third Graph



We're mostly concerned because this particular Unit was working fine, and until recently started exhibiting this erratic, oscillating pattern. Take a look at the following data from October:




Around mid October, intermittent (1-2/day) peaks were registered and after a couple of days, the discharge air temperature just went all crazy. The past days, there were a couple of thunderstorms and both discharge air T and HR were controlled at times, and some other days, all sunny and low humidity, both parameters register peaks. Hence, our bewilderment.

After reviewing the data a service person said the following "maybe the humidity rises because the outside temperature is lower and the compressors stop ... because the system requests the startup of the economizer". However, the other unit displays this behaviour even under high ambient temperature (all year round, from what we're told, it has been like that for almost two years).

Any advice you can offer is much appreciated.

Regards!

[Poodle Head Mikey]

Does either or both compressors' amp draws fall at the same time that the high RH and discharge air temps occur?

PHM
-------

Hello

We're currently in charge of two Lennox RTUs (Models LCA102H and LCA120H). Both units service two labs and by design they're always online. The units have been installed and operational since at least 2010.

For some time now, the Relative Humidity in both areas is continuously oscillating. Even under high temperature and relative low ambient humidity, inside the areas RH still oscillates far more than it should (verified by placing data loggers in the areas). Also, this erratic behavior has been verified under almost all weather conditions.

After establishing trends, we've begun to identify the pattern: High RH occurs simultaneously with an increase in the discharge air temperature. As if the units are heating the air.



Per manufacturer service personnel recommendations, compressors were replaced, to no avail. Maintenance is conducted 3 times yearly (coil cleaning, pressures verification, etc...) and air filters are replaced monthly.

In your experience, what could be a possible cause of this erratic control?

I appreciate any recommendations that you guys can offer.

[teddy bear]

Outdoor %RH means little. It is the outdoor dew point that represents your moisture load. Also the occupants in the space are a moisture loan. Your specs, 70^F, 50% RH is a 50^F dew point. To maintain a 50^F indoor dew point, <40^F a/c coil temperature is needed. Slow the air flow through the cooling coil until the coil temperature (suction pressure/temp} indicates <40^F. Set the reheat for 68^F minimum.
You can go lower on the coil temperature but must avoid freezing.
You data is confusing. Ask any question that you are confused by.
When the system reaches the set points, the moisture on the cooling coils will evaporate back to the space. This will trigger cooling and reheat, nothing you can do. Your settings dead band should control the ranges. These reheat coils on large cooling cools will have some swings. Some of graphs look good.
Keep us posted.
Regards Teddy Bear

[HVACnewguy15]

I'll look into that @Poodle Head Mikey, in previous services that hasn't been reported, but I'll check it as soon as the weather clears...it's raining and weather is expected to continue like this all week long.

Thanks for the advice!

[HVACnewguy15]

Thanks for the reply @teddy bear,

The RH spec is 60 %, that would be 55-56°F dew point, right? How would this change the coil and reheat temperature? <45°F coil?

Now, regarding Reheat, currently the control option is as follows: Humiditrol Reheat. Conditions: blower must be energized. Must be occupied with a Reheat Setpoint of 60 % (system shows %RH on this parameter). Wouldn't a higher setting increase the zone humidity?

Regarding the data, comparing between the data from october and november, those peaks in the discharge air temperature/RH are the ones causing us headaches... What could be causing that pattern, considering that no changes have been made to the RTU settings and that environmental conditions haven't changed that much?

Verifying settings deadbands, this was found:
- Reheat RH : 3 %
- Heating Deadband: 1 °F
- Cooling Deadband: 1 °F
- Autochangeover deadband: 3 °F
- Discharge air control heating stage deadband: 5°F
- Discharge air control cooling stage deadband: 5°F

Other parameters that may be relevant are these:
- Discharge Air Heating Setpoint Occupied: 110 °F
- Discharge Air Setpoint Unoccupied: 100 °F
- Discharge Air heating Deadband: 5°F
- Discharge Air Cooling Setpoint: 55°F
- Discharge Air Cooling Setpoint Unoccupied: 65°F

All these parameters haven't been modified (at least for 1 year and a half), thus our concern about the RTH recent behavior. We were wondering if it's not related to a mechanical/electrical component failure, but so far we haven't been able to identify the possible culprit.

On the other hand we don't have the technical know-how for these systems. We go as far as band, fuses, capacitors, relays, filters and fan replacements (Hence the AOP Commercial Section ). Proper maintenance and settings tuning must be forwarded to contractors. Thus, any information that may guide us, and them, to respond to this issue is much welcomed.

[Poodle Head Mikey]

You graph a lot of conditions - so can you also graph the amp draws for the compressors?

PHM
--------

Thanks for the reply @teddy bear,

The RH spec is 60 %, that would be 55-56°F dew point, right? How would this change the coil and reheat temperature? <45°F coil?

Now, regarding Reheat, currently the control option is as follows: Humiditrol Reheat. Conditions: blower must be energized. Must be occupied with a Reheat Setpoint of 60 % (system shows %RH on this parameter). Wouldn't a higher setting increase the zone humidity?

Regarding the data, comparing between the data from october and november, those peaks in the discharge air temperature/RH are the ones causing us headaches... What could be causing that pattern, considering that no changes have been made to the RTU settings and that environmental conditions haven't changed that much?

Verifying settings deadbands, this was found:
- Reheat RH : 3 %
- Heating Deadband: 1 °F
- Cooling Deadband: 1 °F
- Autochangeover deadband: 3 °F
- Discharge air control heating stage deadband: 5°F
- Discharge air control cooling stage deadband: 5°F

Other parameters that may be relevant are these:
- Discharge Air Heating Setpoint Occupied: 110 °F
- Discharge Air Setpoint Unoccupied: 100 °F
- Discharge Air heating Deadband: 5°F
- Discharge Air Cooling Setpoint: 55°F
- Discharge Air Cooling Setpoint Unoccupied: 65°F

All these parameters haven't been modified (at least for 1 year and a half), thus our concern about the RTH recent behavior. We were wondering if it's not related to a mechanical/electrical component failure, but so far we haven't been able to identify the possible culprit.

On the other hand we don't have the technical know-how for these systems. We go as far as band, fuses, capacitors, relays, filters and fan replacements (Hence the AOP Commercial Section ). Proper maintenance and settings tuning must be forwarded to contractors. Thus, any information that may guide us, and them, to respond to this issue is much welcomed.

[HVACnewguy15]

Hello again,

I'll try to get that data @Poodle Head Mikey, just need to get the proper loggers.

New developments, today we were able to test the units via Lennox's L-connection software and one of the condenser fans (1/2) wasn't responding, tomorrow we'll verify the relays and the motors, to see what's malfunctioning.

In one of the units, we suspect that one of the RH valves isn't working, we'll also look into that.

I'll keep you guys informed

Regards!

[HVACnewguy15]

Hello Guys,

So, we identified that in both units the No 1 Condenser Fan was not working, problem is that we replaced motors, relays, capacitors and even the main board (A55) in one of the units and still we haven't been able to start those fans... verified by trying to start them via the test mode.

We discarded the possibility of a malfunction in the compressor 2 module board (A57) because it starts the compressor fine.

Any ideas?

Reading the manual "Low ambient switch S11 closes to energize condenser fan relay K10" how do you verify a switch?

[Poodle Head Mikey]

Check for voltage across it.

Is control voltage ever present at the K10 coil?

PHM
-------

Hello Guys,

So, we identified that in both units the No 1 Condenser Fan was not working, problem is that we replaced motors, relays, capacitors and even the main board (A55) in one of the units and still we haven't been able to start those fans... verified by trying to start them via the test mode.

We discarded the possibility of a malfunction in the compressor 2 module board (A57) because it starts the compressor fine.

Any ideas?

Reading the manual "Low ambient switch S11 closes to energize condenser fan relay K10" how do you verify a switch?

[HVACnewguy15]

Hello again,

So far, nothing. No, no voltage on the K10 cable (from the board to the relay), however the cable was good, as well as the connections in the board (we even replaced the board to no avail).

It works, momentarily ~10 seconds, by switching the terminals from relay to relay (cable that runs to each motor), but then we hear the click from the mini-relay located on the main board and off it goes. We haven't been able to start it in test mode.

Any ideas?

[Poodle Head Mikey]

Yes; I have ideas about your project. But I also have the idea that I am dangerously close helping you DIY - which I am not allowed to do in this area of this forum Please apply to the management here to move this thread to the Professional Side of this forum and to admit you as a Professional Member.

Si.

Hello again,

So far, nothing. No, no voltage on the K10 cable (from the board to the relay), however the cable was good, as well as the connections in the board (we even replaced the board to no avail).

It works, momentarily ~10 seconds, by switching the terminals from relay to relay (cable that runs to each motor), but then we hear the click from the mini-relay located on the main board and off it goes. We haven't been able to start it in test mode.

Any ideas?

[HVACnewguy15]

Will do @Poodle Head Mikey

Thanks!