Condensate trap causing humidity issues? 1

[dbill74]

I am looking for some feedback on a situation and not sure I am being fed a load of BS.

The problem is a recently completed ~3,000 SF 1-story office building experiencing high indoor RH (70%+) levels.
Building is in Charleston, SC, USA.
HVAC consists of 2 DX split heat pumps, OA is ducted direct to an OA louver into RA duct of each HP, motorized damper connected to DDC, interlocked with HP compressor.
HP indoor units are in an indoor mechanical room. Walls between mechanical room and occupied space is CMU up to about 10', then gyp-board on metal studs to roof. These walls are "sealed" to create an air barrier. AHUs are off-the-shelf draw-thru units, =< 5 tons, installed vertically air flow moving up thru unit.
Ventilation for mechanical rooms is via an exhaust fan. Make-up for this fan is a louver with motorized damper interlocked with fan. Make-up air for mechanical room is not filtered or conditioned. This fan is controlled via thermostat and when on.
Condensate trap for the indoor units are ~6" deep, with 1/2" drop between trap inlet and trap outlet. (This is per a detail on the design documents.)

We recently did a walk through of the building with the designer of record and a rep for the installed heat pump and discussed the findings afterwards. Aside from making improvements to the controls strategy, there was one other item that I am not sure about, the construction of the condensate traps. The manufacturer rep made the claim that with the trap inlet/outlet being so close in elevation, that when the unit is blowing air, the condensate in the trap is being sucked back into the unit and the trap is unable to allow condensate to drain properly. As a result, the condensate is being pulled out of the drain pan and back into the air stream.

Thoughts? Experience?
Thanks in advance.

 

[EnergyProfessional]

I'm too lazy to google for a reference, but yes the trap design depends on the maximum pressure the fan can produce and depends on if it is on suction side (like your draw-through) fan or discharge side of fan.

1" of water column can lift one 1" of water. the same effect is used for measuring pressure with a tube filled with water.

 

[Drazen]

Manufacturers rep should give you specification for trap seal height. You can find generic data, but as you are already in sort of dispute, he should give you their official data.

 

[BronYrAur]

The easy check is to see of the pan is overflowing or otherwise not draining properly. Open the AHU door to relieve pressure and see if a rush of water starts coming out the drain. I see this so much that I have actually laminated diagrams of the various recommended trap depths for draw-thru and blow-thru. I carry it in my bag of tricks.

Attached is a great article on the subject.

 

[dbill74]

Thanks for the responses guys.
BronYrAur, thanks for the file, great information there.
Figure 4 is exactly the setup I'm dealing with, except the unit is draw thru.

This is something that has been identified as a problem and we are in the process of getting it fixed. I am curious to see what impact it will have on building RH.

 

[BronYrAur]

I think having a trap like Figure 4 on a draw-thru unit is not allowing proper drainage.

 

[ChasBean1]

The two aren't related. Meaning indoor humidity issues and improper trap configuration. The indoor humidity is likely related to DX cycles providing too much cooling in short bursts while bringing in unconditioned air during off-cycles. The trap height difference won't burn you until later, when you have drain pan mushrooms and airborne aspergillus issues.

 

[EnergyProfessional]

ChasBean1: the two could be related. If water doesn't leave the pan, it will be picked up again. there also is the " geysyring" effect mentioned in the paper attached by BronYrAur.
Of course, humidity also can come from the DX cycling you mentioned. but those problems are not mutually exclusive.

 

[dbill74]

ChasBean1, what is your basis for claiming the two aren't related?
I want to agree with you based on the assumption that the air coming off the coil and being blown through the ductwork is fully saturated (well nearly saturated since fan motor will add heat to the air, but absolute humidity wouldn't change).

However, I'm not so ignorant to think that my assumption may be erroneous. Recalling what I've often seen in calculations that coil leaving air point is rarely on the saturation curve, I suppose it's possible with some geysyring that the absolute humidity entering the ductwork is greater than is desired and adding to the relative humidity of the space. But to raise the absolute humidity to a point that the building is experiencing 75-80% RH at 73°F is a lot of extra humidity. For the equipment rep and designer to attribute all (or most) of this humidity to the trap?

I know short cycling is an issue, the units are 30-50% oversized for cooling.

 

[SAK9]

A choked drain raising RH to 70% is bollocks.Chasebean is spot on.Try reducing air flow if you have a CV system.

 

[Compositepro]

Mmmm, how can they not be related? You are cooling indoor air but not removing any moisture through a drain. In Charleston the outside humidity is often over 70%.

 

[BronYrAur]

I think we're arguing back and forth about whether or not the condensate drains from the pan. It probably doesn't with the incorrect trap which may lead to the pan overflowing and possibly even some water carry over into the ductwork. But since the air comes off the cooling coil basically at situation regardless of the pan condition and drain condition, folks are making the point that an incorrect trap is probably not causing overall High room humidity. So we're kind of going back and forth on two different issues. A bad drain configuration can lead to condensate not actually getting out of the unit correctly, but will that actually result in a high room humidity or just a very messy rusted air handler and possibly supply duct. Will it actually result in a higher room humidity? Probably not much unless the air is being somehow heated and then re-picking up the moisture but that doesn't sound like the case. Sounds like the incorrect traps are being repaired so perhaps we can get some feedback on the effect it had on room humidity.

 

[SAK9]

Dumping humid fresh air onto back of a recirculating DX coil is a bad idea in the first place. On those overcast days humidity will be well over 80% with not enough sensible load to keep the DX unit running. This kind of a system works all right in dry weather but would struggle in humid weather.

 

[dynamo78]

Beginner here chiming in. Hi dbill. Did you plot the coil performance on the Psychrometric chart? after shutting down the AHU, does the condensate flow freely to the drain? I mean while the AHU is operating does it hold the water in the pan then flows freely if it s off?

 

[dbill74]

Well, this is proving to be an interesting conversation, and I thank all who have contributed to date.

The this unit is basically the unit that is installed, the model number might be off, but the configuration of the filter, coil and fan are correct. The units are installed with air flowing vertically up through the unit.

Dumping humid fresh air onto back of a recirculating DX coil is a bad idea in the first place. On those overcast days humidity will be well over 80% with not enough sensible load to keep the DX unit running. This kind of a system works all right in dry weather but would struggle in humid weather.

Not IF it's managed correctly. The problem comes when the compressors cycles off and the unit continues to cycle the mixed air.
Several options exist to avoid this.
1) Close fresh air dampers. - Is this even permitted while the building is occupied? I gotta go through ASHRAE 62.1 again.
2) Continue to run the compressor and add reheat to introduce neutral air to the space. - Energy managers despise this.
3) Install DOAS dumping fresh air direct to the space. - Not always viable due to space and/or budget. For this particular project, space is very much NOT available.