Under Floor Air Distribution Systems & Humidification

[belz]

I am considering a underfloor air distribution system in a new single story library. The library requires humidification. Has anyone designed a system like this? If so, how did you ensure that condensation or mold growth in the plenum did not occur?

 

[facmgr]

What type of HVAC do you have and what is your humidification spec? I use air washers on incoming air and use dew point discharge to maintain humidification in the manufacturing area.

 

[friartuck]

Depends on the supply air condition and the slab temperature. On some jobs, we lay the floor with water resistant hardboard to give a bit of insulation or add a thin layer of insulation perhaps 25mm polystyrene with boarding on top (old buildings). Modern buildings in the UK are insulated now anyway due to building regs (50mm minimum generally)

The raised floor sits on top of the boarding or if insulated as well, the stands are cut round using a jigsaw or router and the stands sit on the concrete base but are surrounded by the cut out boarding and insulation.

Cold surfaces might cause some condensation (i.e. if the building was left unheated over a long weekend etc during winter), but this would soon evaporate during use anyway.

You might consider monitoring the slab temp and assessing the supply dew point to avoid problems but I have never had to do that.

Which part of the World are you in?



Friar Tuck of Sherwood

 

[ChasBean1]

Belz,

Surfaces in the plenum should be at equilibrium temperature with the supply air from the air handler because it is a constant flow, positive pressure environment. Unless humidification is added mechanically, you should not experience plenum surface temperatures below the dew point of the air. This should inhibit biological growth. HOWEVER, if you serve the underfloor plenum with an air handling system with varying discharge temperature (e.g., a single or dual stage DX system), the on cycle could cause reduction in plenum surface temperature while the off cycle brings in air with a higher dew point. This could be a problem. You need a system with a steady, 62-64°F discharge temperature. A chilled water cooled air handler is therefore much preferred for this application.

I favor these systems but here is the primary concern:

With an underfloor system your supply air temperature is limited to about 62-64°F for thermal comfort reasons (drafts on feet/legs) versus the normal 52-55°F discharge. This limits dehumidification by cooling. If the humidity in the space becomes too high, the system should fully open chilled water to the coil and bypass a portion of air around the coil to regain supply temperature back to the 62°F setpoint. Cooling coil bypass of up to about 35-40% (while maintaining the same discharge temperature) will allow further dehumidification. This is not 'street' HVAC knowledge and you have to be careful about people wanting to apply duct system common practice to underfloor systems.

Feel free to repost any questions, although I haven't been in this forum too much lately (I'll try to look back at it). Just got through a commissioning job where getting an underfloor system to work right was an interesting challenge...

CB

 

[imok2]

Belz,

In addition to the previous comments I believe that condensation can be a concern in under floor systems if the cool plenum is suddenly exposed to warm, moist air. The dew point of the air entering the plenum must always be greater than the lowest temperature of any exposed surface within the plenum, otherwise condensation will occur. This phenomenon can be avoided if the plenum is well sealed against outside air infiltration, and if sudden step-changes in supply air conditions are avoided. For example, when the fan shuts down at the end of the day, warm and moist air should be blocked from rushing into the cool plenum. Similarly, during occupied hours, if the mechanical cooling abruptly shuts down and the dew point of the supply air suddenly rises, the fan may need to be shut off, or the outside air dampers adjusted. In addition, if your intention is to use the slab thermal reservoir to cool the space, for example, after the slab is “charged” with cool air at night, the mechanical system must be controlled to ensure that it does not supply air that exceeds the temperature of the coolest surface in the plenum. This can be a problem with systems installed over a slab on grade, since the slab temperature will be difficult to control, and condensation can occur.

 

[ChasBean1]

Belz,

I re-read your post and realize humidification IS to be added mechanically. You should not worry about humidification so long as high limit humidistat works right. Sentence 1 of Paragraph 1 applies. With a humidified environment and if the high limit humidisats work right and is placed right (in the supply, downstream of the humidifier vapor trail), you should never be above about 85% RH under the floor during seasons in which you need the humidifier to operate. With surface temperatures equal to supply temperature under the floor, there should not be worry of condensation. Only in the Dehumidification season (summer) should you be concerned (see my previous post).

Imok2 has a good point, however, regarding occupied/unoccupied thermal difference issues. I would recommend running the system around the clock, but reducing air handler speed during unoccupied times for energy savings. If you can maintain a constant, conditioned under-floor temperature (even with humidification in the winter), you should be in the clear.