Condensate trap causing humidity issues?
Condensate trap causing humidity issues?
(OP)
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.
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.





RE: Condensate trap causing humidity issues?
1" of water column can lift one 1" of water. the same effect is used for measuring pressure with a tube filled with water.
RE: Condensate trap causing humidity issues?
RE: Condensate trap causing humidity issues?
Attached is a great article on the subject.
RE: Condensate trap causing humidity issues?
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.
RE: Condensate trap causing humidity issues?
RE: Condensate trap causing humidity issues?
RE: Condensate trap causing humidity issues?
Of course, humidity also can come from the DX cycling you mentioned. but those problems are not mutually exclusive.
RE: Condensate trap causing humidity issues?
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.
RE: Condensate trap causing humidity issues?
RE: Condensate trap causing humidity issues?
RE: Condensate trap causing humidity issues?
RE: Condensate trap causing humidity issues?
RE: Condensate trap causing humidity issues?
RE: Condensate trap causing humidity issues?
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.
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.