high humidity at night from OA?

asked 2017-07-28 09:52:57 -0600

Matt Koch
1998 ●3 ●17

I am looking at a VAV air loop driven by chilled water cooling coils and electric heating coils. This is in Climate Zone 2A. I noticed that the relative humidity in almost all my zones increases at night to 80% to 90% and then has a hard time coming back down during the day. As a result (I believe), the OpenStudio Results - Zone Conditions report shows more than 6,000 h in excess of 80% RH, while the temperature setpoints are met just fine (80 oF at night, 75 oF during the day).

So, I thought this might be caused by outdoor air being allowed into the building even at night, and I tried to turn it off by creating an Outdoor Air schedule (FBCCTX Outdoor Air) that is 0 at night and 1 during the day, then assigning it to the air loop's outdoor air component (see images below - "Minimum Outdoor Air Schedule Name"). However, I see no change, and the system outside air flow in the EnergyPlus Results Viewer is still not 0 at night.

image description

Is there another trick to turning off outdoor air at night?

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What is your HVAC Operational schedule for the air loop, what is the Nightcycle Setting, and are there any schedules on the design specification outdoor air objects in the zones?

David Goldwasser ( 2017-07-28 11:50:40 -0600 )edit

Well, that is a great question. I have never actually found a spot to stick a HVAC Operations schedule. Where does one specify that? My individual cooling coil, heating coil and fan are on the "Always On Discrete" availability schedule, as are my VAV reheating coils. As far as nightcycle setting, I presume you are referring to the nighttime setback in the thermostat setpoints? My cooling goes from 75 oF to 80 oF and my heating goes from 68 oF to 60 oF between the ours of 10 PM and 6 AM, and back to 75 oF and 68 oF, respectively from 6 AM to 10 PM.

Matt Koch ( 2017-07-28 16:18:10 -0600 )edit

And finally, the Spaces - Properties - Airflow has Design Specification Outdoor Air Objects for each space, and each object has the same FBCCTX Outdoor Air schedule (not sure how to attach an image here). Anyway, wouldn't the system level FBCCTX Outdoor Air call override any space/zone FBCCTX Outdoor Air call. And if it did not, why do I still see outdoor air at night even though I have this schedule at the space/zone level?

Matt Koch ( 2017-07-28 16:22:44 -0600 )edit

NightCycle is an AvailabilityManager. The default is StayOff. Options include CycleOnAny, CycleOnControlZone, CycleOnZoneFansOnly, etc.

Chris Jones ( 2017-07-28 16:44:00 -0600 )edit

What OpenStudio tab would I find this on? It does not seem to be in the air loop?

Matt Koch ( 2017-07-28 22:18:23 -0600 )edit

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2 Answers

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answered 2017-07-29 16:16:30 -0600

Matt Koch
1998 ●3 ●17

OK, so I do not know if this counts as a proper answer, but I am getting plausible figures now. Basically, at first I thought I had too much infiltration and/or too much outdoor air in a very humid climate, so that nighttime relative humidity in the zones came out to be very high (100% during many nights). So I implemented Molly Curtz' recommendations from above (Maximum Fraction of Outdoor Air Schedule and 0.04 cfm/ft2 at 25%/100% infiltration schedule). However, this only resulted in minor improvements of the situation.

As it turns out, I also have two restrooms on each of my floors. I had given each a zone exhaust fan at 400 cfm and had left the Balanced Exhaust Fraction Schedule Name for each fan empty. I cannot claim I understand the balance concept, but this had evidently and in a very reproducible way led to the restrooms having extremely low relative humidity and the other spaces having extremely high relative humidity. Simply selecting the Always On Discrete schedule for Balanced Exhaust Fraction Schedule Name seems to have now resolved these implausible results.

I know this is the monkey pushing the keys on the typewriter, without real understanding, but I thought this description of symptoms and their cure might help other people. Perhaps there is someone out there who can truly explain how this exhaust fan balancing phenomenon would have led to the observed results, though.

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Hi @Matt Koch. If the control intent is that the building "shuts down" at night when unoccupied, then the zone exhaust fans presumably should also turn off on the building operation schedule. Assuming the "Always On Discrete" schedule is a schedule with the value of one (1) for all hours,then it sounds like your system fan, system CC, system HC, zone exhaust fan all operate 24/7. Depending on the control intent, you may want to change that so it shuts off at night (schedule value = 0) but enable night cycle operation on a call from the T-stat or RH sensor. Typically zone EF would not cycle on.

Molly Curtz ( 2017-08-01 16:37:48 -0600 )edit

Yes, I have the exhaust fans and the outdoor air systems on a schedule that turns them off between 10 PM and 6 AM. However, the air handlers are operating at a minimum flow rate. I think that is so the nighttime setbacks can be maintained,

Matt Koch ( 2017-08-01 18:39:36 -0600 )edit

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answered 2017-07-28 15:01:14 -0600

Molly Curtz
1348 ●3 ●13

updated 2017-07-28 21:49:50 -0600

I am editing my original answer based on realizing some more things and reading comments added to the orginal question.... In short, I still have the same suggestion that you should try assigning your schedule to the "maximum fraction of outside air schedule".

The schedule you used applied as you applied it should act as a multiplier on the minimum outside air requirement. ...so, you also asked in your comment: "why do I still see outdoor air at night even though I have this schedule at the space/zone level?"

It might be economizer. From the I/O Reference for Controller:MechanicalVentilation: "The actual outdoor air flow rate may be higher than the minimum if free cooling is available. Regardless, the outdoor air flow rate will not exceed the Maximum Outdoor Air Flow Rate specified in the associated Controller:OutdoorAir object or the Maximum Fraction of Outdoor Air Schedule (if specified) times the current system supply air flow rate."

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I can confirm that applying my schedule to Maximum Outdoor Air Flow Rate does indeed eliminate nighttime outdoor air. Obviously, I will keep it that way, because it also saves energy. It also does have some small effect in terms of reducing nighttime zone relative humidity, although there are still many days where it reaches 100 % (!!!). I also did a quick run with San Antonio rather than Corpus Christi as the weather file, and that seemed to make major difference. The only other source I can see is infiltration - at 0.2 cfm/ft2 per exterior area. I have no idea if that is a good number.

Matt Koch ( 2017-07-28 22:17:26 -0600 )edit

OK, so changing the infiltration schedule from the default "100% night/25% day" to almost 0% at all times does reduce the night time infiltration and hence the nighttime relative humidity. So, I suppose the problem as changed from finding a way to turn off nighttime outdoor air to finding appropriate infiltration rates. (I find it implausible that a zone should see 100% relative humidity at night). Should I change from Flow/Exterior Area to Flow/ExteriorWallArea? WHat number should I use?

Matt Koch ( 2017-07-28 22:33:34 -0600 )edit

This document seems helpful (http://citeseerx.ist.psu.edu/viewdoc/...) in that it does seem to mention about 0.2 cfm/ft2 of exterior wall area, not just exterior area, but I may be misinterpreting it. IECC 2015 in C402.5 mentions 0.4 cfm/ft2, but it is not clear to me what area is being referenced. And then in C402.5.1.2.1 it mentions 0.004 cfm/ft2 for materials and in C402.5.1.2.2 it mentions 0.04 cf/ft2 for assemblies. Again, not sure what area is meant here. Plus, C402.5.2 mentions about 0.2 cfm/ft2 of window area - confusing.

Matt Koch ( 2017-07-28 22:49:52 -0600 )edit

Okay, glad to hear that the Max fraction of OA schedule worked for you. Regarding infiltration, you can find more advice reading other questions here, such as this one. The IECC 2015 value of 0.4cfm/ft2 is at a positive pressure of 75Pa (this is not a normal operating pressure - think blower door test.) and it is what code expects new constructions commercial buildings to achieve. Also, I believe that value is per SF of exterior envelope....

Molly Curtz ( 2017-07-29 00:27:47 -0600 )edit

...the recommendations in the PNNL18898 report that you provide a link to, I frankly find a bit hard to follow when reading the report, but they have been incorporated into modeling requirements in the Seattle Energy Code as follows:

Convert the leakage rate at 75Pa, to a leakage rate for normal operating conditions by multiplying by a factor of 0.112. (In the report they have a pretty high leakage @75Pa of 1.8cfm/SF, converts to 0.2 cfm/SF under normal operation. IECC 2015 requires much lower leakage of 0.4cfm/SF, converts to 0.045 cfm/SF under normal op.)

Molly Curtz ( 2017-07-29 00:43:57 -0600 )edit

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