High Humidity New Tenant Finish Space

[AxisCat]

We are experiencing high humidity levels in a new tenant finish in one space of a mini mall. This is your typical setup glass store front on one side, exterior wall on the back and demising walls separating each space.

At present this space has a Jersey Mikes on one side and an unfinished space on the other. The demising walls are fire rated and extend to the bottom of the roof deck.
The space itself is about 1700 square feet. My load calculations show requirements of 35,948 btu sensible, 8,516 btu latent, 44,465 btu total. The building is located in southwest Kentucky.

We chose to install a single 5-ton packaged rooftop with DX cooling (single stage) A 4-ton may have been a better choice but I don’t think oversizing is the issue here.

The air balance contractor measured total supply air at 2,150 cfm (direct drive motor on lowest speed tap) Outside air is set at 200 cfm.

The new tenants have not moved in yet and the system has been in operation for the last 3 weeks during final stages of finish work. The unit appears to operating perfectly with all the tests we have done so far. All the ducting is inside the building envelope so I can’t see any duct leakage contributing to pressure unbalance. The first thing we did was block off all the outside air which had no measurable effect on the high humidity inside.
The humidity level has been hovering around 80% plus or minus a couple of percent for the last 5 days with high temperatures around the upper 80’s – low 90’s. Needless to say it is effecting the interior of the building; ceiling tiles are warping, doors aren’t shutting properly, etc.

The General Contractor can’t think of any recent activities that would have added moisture to the space. I am at a complete loss of where to go from here, if anyone has any suggestions on what to check next it would be greatly appreciated.

Best regards,
Darrell

 

[dbill74]

1) How much exhaust are you providing to the space? If the exhaust in more than 200 cfm (include toilet exhausts), you have a negative space.
1a) Are EFs running continuously or intermittently?
2) You mention doing finishing work for the last 3 weeks. Are doors being kept open as workers go in/out? This will have a negative affect on your humidity as well.
3) Check ALL penetrations of roof and walls (including tenant partitions). Make sure there are no visible gaps or holes.
4) Oversizing your RTU can be problematic as it will cause the unit to short cycle; as a result it will not run long enough to remove the moisture.
- What type of tenant? Resturaunt or retail?
- What is the T'stat set at? Lower the setpoint to 68 for a cuople days, do this over the weekend when no-one is working, close the OA and disable exhaust fans, this will help dry the space out.
5) Check your calculations, 4 tons sounds low. Even 5 tons sounds a touch low for 1,700 SF.
6) Check RTU condensate drain. Make sure it is flowing, removing moisture.
7) Do you have the ability to shut OA during unoccupied periods (overnight)?
8) What is control sequence for RTU? Is the fan cycling ON/OFF with the compressor? (Are you bringing in unconditioned OA when the compressor is OFF?)

 

[IRstuff]

residential rule of thumb for sizing AC is on the order of 1 ton per 500 to 700 sf. That would make your 5 ton unit considerably oversized, resulting in the aforementioned short cycling.

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[AxisCat]

Hi dbill74,

Thanks for the quick reply.

Here are the answers to your questions:
1) How much exhaust are you providing to the space? If the exhaust in more than 200 cfm (include toilet exhausts), you have a negative space. 150 CFM total restroom exhaust
1a) Are EFs running continuously or intermittently? Exhaust fans run on occupancy sensors
2) You mention doing finishing work for the last 3 weeks. Are doors being kept open as workers go in/out? This will have a negative affect on your humidity as well. Doors were open and shut
3) Check ALL penetrations of roof and walls (including tenant partitions). Make sure there are no visible gaps or holes. There is 'some' air communication between the spaces above the firewalls
4) Oversizing your RTU can be problematic as it will cause the unit to short cycle; as a result it will not run long enough to remove the moisture.
- What type of tenant? Resturaunt or retail? This is retail / office space - Cash advance store
- What is the T'stat set at? Lower the setpoint to 68 for a cuople days, do this over the weekend when no-one is working, close the OA and disable exhaust fans, this will help dry the space out. We did that for 4 days over the weekend with the space vacant
5) Check your calculations, 4 tons sounds low. Even 5 tons sounds a touch low for 1,700 SF. I try to be as honest with load calculations as possible, I did assume fluorescent lights and it got LED
6) Check RTU condensate drain. Make sure it is flowing, removing moisture. Drain is good, no water standing in the pan, makes a lot of water when it runs
7) Do you have the ability to shut OA during unoccupied periods (overnight)? We do now, not before though
8) What is control sequence for RTU? Is the fan cycling ON/OFF with the compressor? (Are you bringing in unconditioned OA when the compressor is OFF?) Original control stradegy is fan runs constantly during occupied mode and cycles at night

Right now, thermostat is set to hold 70 degrees 24/7 with outside air completely blocked off

I appreciate your interest in my problem and eagerly await some more comments.

 

[dbill74]

As the OP clearly indicated, this is for a new tenant in a mini mall, not a residence. IRstuff's rule of thumb is misapplied and will result in a unit that is too small for the space. For the indicated application, a better ROT is 150-350 SF/Ton. Which makes a 5 ton unit on the smallish size.

 

[lilliput1]

You need to reduce the supply air temperature either by reducing the amount of supply air to about 1470 CFM (or minimum allowed to not get frosting on coil) or recirculate about 680 cfm while supplying 1470 CFM to the space and keeping the fan CFM at 2150. To recirculate tap 12" dia flex with volume damper upstream of the cooling coil and fan discharge. Connect the other end of the flex direct to the return duct. Plot on psychrometric chart and compare with table of unit capacity at rated fan CFM.

Add OA sensible and OA latent loads and also add fan heat gain sensible load in your calculation. A 6 ton unit may be more applicable when the space is fully occupied.

 

[lilliput1]

Also check temperature drop across cooling coil. If below rating the refrigerant circuit may not have been properly evacuated then charged with the correct amount of refrigerant. Also confirm there are no significant infiltration of outdoor air into the conditioned space.

 

[AxisCat]

I appreciate everyone’s response to my post. We have done countless projects like this, but this one is a true mystery. Rule of thumb sizing in our area for these retails spaces is 300 to 400 square foot per ton. I have seen the results of oversizing equipment before which can result in higher humidity levels; over 60% and approaching 70% in extreme cases. But with 80% I can't help but to think something else is at work here.

We installed a temporary dehumidifier to bring the area back to normal. At some point in the next day or so we will shut it off and see how the HVAC system does on its own. We will recheck temperature drop across the coil (was 17 degrees with the humid entering air)

I will follow up with a post to let you all know how we come out.

Thanks,
Darrell

 

[dbill74]

8) What is control sequence for RTU? Is the fan cycling ON/OFF with the compressor? (Are you bringing in unconditioned OA when the compressor is OFF?) Original control stradegy is fan runs constantly during occupied mode and cycles at night

Are the controls still setup like this?
If so you are bringing in 200 CFM unconditioned OA whenever the compressor is not running.
Does the RTU have a dehumidification function?

3) Check ALL penetrations of roof and walls (including tenant partitions). Make sure there are no visible gaps or holes. There is 'some' air communication between the spaces above the firewalls

Need to fix this for sure, especially at the partion with Jersy Mike's.

Where are the toilets and how is make-up air for the EF(s) getting in?
Has TAB been in and done their job yet? Total OA/EA results?
What's your anticipated occupancy (how many people)?

I agree with lilliput1 that slowing the air flow across the coil will reduce and LAT and get more moisture out. I don't think you need to go as low has suggested though. Review what the unit can do and go from there.

 

[AxisCat]

The controls are set at present to cycle the indoor fan with the compressor. The outside air is completely blocked off. The space is still vacant at this point.

I always give an option for hot gas reheat on these type jobs but it can be a hard sell due to the increased cost. So like most this does not have any dedicated dehumidification.

My first thoughts as well was Jersey Mikes making this space go negative and increasing infiltration. The workmanship looks good were the firewalls are sealed to the roof deck. All we noticed was a slight bump of the exterior door in this space when a door in an adjacent space was opened and shut.

The two toilets are located in the back portion of the building. Makeup air is being supplied via supply form the RTU. Balance of makeup air by undercut doors.

Yes, the TAB did their work a couple weeks ago. They measured overall air volumes and set outside air at 200. Supply air was set to 2150; the lowest speed tap on the direct drive motor. If it was an easy thing to do we would already have dropped the supply volume down to 1750 cfm or so. And we still may.

I accounted for 10 occupants in my load calculations. Of course being a mini-mall Tenants come and go. And who knows what may be in the space a few years from now. I did want a 'little' extra capacity in the system.

Thanks

 

[dbill74]

The outside air is completely blocked off. The space is still vacant at this point.

Are the EFs still cycling? If so and the OA is blocked off, then the space is going negative at times. What did TAB set EFs to?

My first thoughts as well was Jersey Mikes making this space go negative and increasing infiltration. All we noticed was a slight bump of the exterior door in this space when a door in an adjacent space was opened and shut.

For this to be happening, the Jersey Mike's space would have to be negative AND the two spaces communicating. To have the door of your space move when a Jersey Mike's door opens/closes would appear to indicate communication between the spaces. Does your door operation affect Jersy Mike's door?

With the OA blocked off, your space is neutral to outside at best, bringing in some OA will help fight vapor pressure from forcing moisture in.
Assuming TAB set the EFs to 150 CFM, that gives you only +50 CFM pressureization, have you figured what that works out to in inches w.g? May need to look at increasing the amount of OA.
(I know you have 20 CFM per person, but remember mechanical codes and ASHRAE only specify a minimum, you can always have more OA than what's called for.)

How are you heating the space in winter? Can you fix the controls to use heating elements to give you a dehumidification cycle?

 

[BronYrAur]

What is the discharge air temperature from the evap coil? You may have a refrigeration issue giving you an elevated coil temp and thus little humidity removal. Still might be enough to satisfy space temp since the space is vacant.

 

[Compositepro]

With 80% humidity it seems that you are not removing water from the space. Have you checked the drain line in the evaporator to see the water dripping to the outside?

 

[lilliput1]

Run unit at continuous fan and since the space is currently unoccupied, set the thermostat at 65F to keep the compressor running with the low load. Check location of thermostat. Make sure conditioned supply air is not blowing directly at it. Confirm AC unit has been properly evacuated and charged with refrigerant.

 

[berkshire]

Following up on Lilliput1s remarks , This kind of problem often surfaces when the system is overcharged.
B.E.

You are judged not by what you know, but by what you can do.

 

[AxisCat]

The temporary dehumidifier has somewhat turned into a diagnostic tool. With it inside the problem space it easily dropped the humidity to below 50%. What we didn't expect to see was the adjacent unfinished space (opposite the Jersey Mikes) to drop from 80% to 60%. When we shut off the dehumidifier the humidity level in 'both' of these spaces steadily rose.

A little more information on the unfinished space. The demising wall is finished and fire taped to the deck in the finished area only. The unfinished side is stud wall; fiberglass batt insulation with paper backing vapor barrier. Concrete poured along the perimeter with the center being gravel. No threshold on the doors.

It is difficult for me to wrap my head around this much water vapor migrating through the demising wall by vapor pressure alone. But at this point the problem appears to be this unfinished space.

I will follow up with a post next week when we are sure we have nailed down the problem. This is a great forum and i appreciate everyone's help.

 

[berkshire]

Axis cat,
The obvious question, the gravel center, " Is there a vapor barrier under the gravel ?", water vapor rising from the floor may be contributing to the high humidity in the adjacent room. You said in an earlier post " There is 'some' air communication between the spaces above the firewalls." Can you seal that?
Or re adjust the air balance to provide slight positive pressure to your space?
B.E.

You are judged not by what you know, but by what you can do.

 

[AxisCat]

Berkshire-

I am not sure about the vapor barrier under the gravel. I would expect our fix for this issue will be to go ahead and finish out this space. The owner has a tenant interested in it. And this is the last available space in the building.

Let me ask you guys; do you have any rule of thumb on when you would ask for a unit with hot gas reheat mode? It would seem every building could benefit from it. Or in other words when would I very strongly recommend it and when could we get by without it?

thanks again.

 

[ChasBean1]

Nobody talks about temperature. In any of these posts. Relative humidity is nothing without temperature... meaningless. Talk in dew points or grains of moisture. Is your space 63°F with 80% RH? Or 80% with 72°F? The former is fine; the latter might be an issue.

 

[AxisCat]

ChasBean1,

You are absolutely correct. I assumed everyone understood I was maintaining a typical indoor temperature of around 75 degrees.

Regards,
Axis