Humidity control in casual dining restaurant, beach location

[jll4065]

In troubleshooting a reported condensation issue in a local restaurant (Myrtle Beach, SC), built less than 1 year ago, I find the following:

- condensation at table tops in dining room, heavy enough to warp/split new wood tables (mainly in summer), and keep salt shakers caked up

- untempered fresh air intakes on (3) RTU's in dining room; total tonnage 21 tons with estimated fresh air at least 2000 CFM, over 1500 CFM thru 1 7.5 ton unit (too much!); total AH flow ~ 8400 CFM

- untempered fresh air intakes on (2) RTU's in kitchen; total tonnage 12 tons with estimated fresh air at least 1100 CFM; AH flow ~ 4800 CFM

- approx. 600 CFM differential between kitchen hood exhaust and dedicated makeup air (MUA ~ 90% of exhaust, total exhaust ~ 5300 CFM)

The client's kitchen hood supplier recommends a DX makeup air conditioning system, but I don;t think the dining room condensation is being caused by kitchen makeup air differentials. I lean towards:
a) balancing dining room units fresh air to distribute OA load
b) install ERV's to temper OA
c) reduce OA flow (my calcs show abt. 1700 CFM required)
d) in lieu of a dedicated MUA DX system, bring in more OA thru the kitchen RTU's and condition there.

This is all preliminary, and I appreciate any other opinions about this kind of situation...the owner wants to avoid spending unnecessary money.

Other data:
Dining room area ~ 2200 SF
Kitchen area ~ 1600 SF
Hood size - 22' x 4.5 '
Occupancy ~ 180 patrons, ~ 20 staff

Thanks for your insights!

 

[chicopee]

May be the place is kept too cold at night when the restaurant is closed. In the morning as the staff arrives condensation will appear when the doors are opened and air ladden with moisture comes in. Moisture will condense on cold objects.

 

[jll4065]

Seems like the condensation is heavier than a once a day kind of occurrence...

What would lead to stratification of colder, moister air at the table level? The supply air is all from diffusers spread around the 10 ft high ceiling.

Thanks,

JLL

 

[IRstuff]

Is there moisture on the floors as well?

TTFN
faq731-376

 

[jll4065]

Not that I know of...the floors are carpeted.

 

[IRstuff]

What exactly happens?
> A/C during the day?
> Circulating OA during the night?

TTFN
faq731-376

 

[jll4065]

As best I can tell, condensation occurs during summer months when heat/himudity load are high and occupancy is highest (vacation months). I was called in at beginning of year, and can only observe what's going on in winter.

A/C runs during day, and is cut back during evening hours (this is a breakfast/lunch only, closes at 2 PM. OA is drawn directly into RTU's at "economizer" interface, only no economizer, just a filter and damper. OA can infiltrate when blower is off, as the duct work allows direct communication with OA intake.

That's all I know right now...

 

[Compositepro]

When the inside temperature is below the outside dew point you will have problems if you also have outside air infiltration. You need to dehumidify the make-up air and keep the dining area at a slight positive pressure. Basically the humidity in the dining area is too high. This is often caused by too much air conditioning capacity, which causes the compressor to turn-off due to cold temperature when it is needed to remove moisture.

 

[Wareagl487]

jl4065 -

I believe you have a fixable issue given the information you have provided. I also think it is can be achieved for very little cost to the owner - mostly just an O.A. balancing issue as I see it.

I calculate O.A. requirement per ASHRAE 60.1 to be roughly 1,750 CFM for the Dining area. 60.1 does list "Kitchen" specifically for O.A. so I would calculate based on 5 CFM per person and 0.06 CFM per SF for roughly 200 CFM.

Obviously by the numbers you listed from observed O.A. installed, infiltration into the space causing condensation is not a factor. So the problem with condensation probably arises from a combination of short cycling of the units combined with the large O.A. values.

Here is what I would suggest:

- Adjust the Dining area 3 RTU's to deliver a total of 1,750 CFM required.

- Adjust 2 Kitchen RTU's to deliver the 200 CFM required.

- Many thermostats presently have inputs/outputs that can control auxiliary equipment such as dampers. I would suggest that the thermostats be capable of closing the O.A. dampers completely on night set back.

- Verify or provide transfer path between Dining and Kitchen to make-up exhausted air by the kitchen hood. This way the kitchen remains negative not allowing kitchen cooking odors into the dining space.

- My guess would be the RTU's serving the dining area are manually set two position type. Suggest to the owner these be replaced with modulating type interfaced with a space mounted carbon dioxide sensor to modulate them based on air quality. Set the minimums on all three combined to be no less than 600 CFM to keep the space positive and allow for kitchen make-up.

Just my first blush opinion Hope it helps.

Andy W.

 

[DRWeig]

I think all the above answers are valid, and they might very well eliminate your moisture issue. I just want to throw in what I learned from a Hawaii job many years ago... Recovering from night setback, even with all the controls above done right, you'll have some high humidity in the space at morning cool-down time. If you allow the supply air temperature to run down below the dew point in the room by about 8° to 10°F or more, you'll create moisture on every surface. If it's bad, you will have rain from the diffusers, maybe even the ceiling tees and tiles.

It's tough to control on a DX system unless you have the ability to mix with return air or induce room air. Maybe even add some reheat. Of course, it's really expensive for the owner to cool the place 24/7. A tough problem, but stick with it. Keep the ideas coming. Let us know what solves it!

Good on ya,

Goober Dave

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[jll4065]

Thanks guys - these are good comments. I had not considered seriously the night setback, but space humidity can change in a matter of an hour in these parts...which I have measured before. (sometimes you have to have extra eyes to see the forest!).

So, does anyone expect the kitchen makeup air to be a serious player in this problem? Personally, unless we take new measurements and show the MUA flow is much lower than the 90% stated, I can't see how it would affect the dining room at the level indicated.

What do you think about using a dehumidication system for the dining room, or using ERV's on the RTU's?

JLL

 

[chicopee]

Your statement about the kitchen being a contributor to moisture can be delt with push-pull ventilation hoods over the cooking appliances.

 

[317069]

1- I didn't understand exactly what do you mean by(untempered fresh air intakes), could you please explain?
2- you said the problem mainly noticed in the summer, and you want to temper the fresh air while the hood contractor wants to cool the make up air?
3- did you talk with the engineer who did this project.
4- did you check the original design with the actual installation.
5- is ther a condensation on the walls
6- did you check the temperature and RH inside dining room.
21 ton over 2200 sqft look like oversized

 

[SAK9]

The Units could be oversized.This would mean inadequate dehumidification of the large ventilation air quantity with resultant high humidity in the conditioned space leading to condensation.One solution is to treat the fresh air seperately before allowing it to flow into the RTU(dining area) so that the extent of dehumdification is not dependent on dining room load.