[Bldg-rate] ASHRAE 62.1 Compliance with Underfloor-Air Distribution

Dan Russell danr at engineeringinc.com
Wed Nov 5 07:50:39 PST 2008


Thanks for the response.  I suppose when designing a UFAD project one needs to utilize controllable diffusers with adjustable minimum settings or determine acceptable leakage rates through access-floors and diffusers.  The latter, I'm sure, will take a greater degree of coordination with the architectural team, and would likely need to be documentable to satisfy the USGBC.  Do your colleagues have any common resources for determining the expected leakage through access-floor assemblies?  The diffuser leakage should be attainable, I would imagine, from the diffuser manufacturer.

This whole idea of using leakage to meet ventilation requirements is sort of funny when I think of at least one premier LEED Platinum building that was constructed in the downtown area here in Boise, ID.  When the air balance was being performed for the air handlers they determined that they were losing a significant amount of air thru the floor system.  So, the construction/design (I'm not sure which) decided this was a problem that needed to be solved because leakage was unacceptable in their minds.  The story goes that they proceeded to hire a bunch of high-school kids to come in and tape every access-floor joint in the building...floor-by-floor, wing-by-wing until it was completely "sealed up."

I have no idea what the actual leakage rate was before or after this process, but I would think for it to be a good design one would need some documentation or accepted calculation methodology for accurately determining the real leakage rate thru the floor system.  Also, this leakage rate, though small, should probably be considered in a space heating load calculation.  Furthermore, leakage if present should be accounted for in energy simulations either as re-heat load, or reduction in cooling load - which I don't recall reading in the UFAD modeling resources I have used.

Lastly, with leakage at 0.25 CFM/SF that will rarely be sufficient to meet minimum ventilation requirements unless the OA% at the air handler is near 100%.  With that in mind, it seems almost imperative to have controllable minimum setpoints on the UFAD diffusers so that compliance with ASHRAE 62.1 can be calculated, documented, and realized.

Quick recap of questions I posed above:

1.       Are there any commonly accepted calculation methodologies for determining the expected leakage through access-floor assemblies?

2.       Should UFAD designers consider leakage as increased space heating load or reduced space cooling load?

3.       How are energy simulations accounting for expected leakage?

Thanks again for your response,

 Dan Russell, EIT

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From: bldg-rate-bounces at lists.onebuilding.org [mailto:bldg-rate-bounces at lists.onebuilding.org] On Behalf Of John E. Beeson
Sent: Wednesday, November 05, 2008 7:14 AM
To: bldg-rate at lists.onebuilding.org
Subject: Re: [Bldg-rate] ASHRAE 62.1 Compliance with Underfloor-Air Distribution


I passed your note over to some experienced UFAD mechanical engineers (they have done many projects with UFAD that met LEED and ASHRAE 62.1).

The response is below.  I hope this helps some!


 Sent: Tuesday, November 04, 2008 4:15 PM
Subject: RE: ASHRAE 62.1 Compliance with Underfloor-Air Distribution
A combination of factors and options must be considered. Most UFAD systems include large open office areas, for which C02 monitoring can be applied, and there is a non-trivial amount of general leakage of supply air through the floor and the diffusers, even in minimum settings. Depending on the UF plenum pressure and the floor and carpet systems used, this may be 0.25 CFM per SF, beyond what the diffusers provide at their minimum. Some UFAD diffusers have definable minimum stops, and some (e.g. the Titus TAF-LV system) can be designed and controlled to provide a minimum aperture area, which, in conjunction with known underfloor plenum pressures, can provide defined amounts of supply air.

From: bldg-rate-bounces at lists.onebuilding.org [mailto:bldg-rate-bounces at lists.onebuilding.org] On Behalf Of Dan Russell
Sent: Tuesday, November 04, 2008 11:07 AM
To: bldg-rate at lists.onebuilding.org
Subject: [Bldg-rate] ASHRAE 62.1 Compliance with Underfloor-Air Distribution
Hello All,

I am struggling with determining how the common UFAD systems today are meeting ASHRAE 62.1.  The systems which I have seen utilize floor grilles with either a motorized damper controlled via wall-mounted t-stat or a manual damper.  When the space is satisfied the dampers close and all of a sudden there is no supply air entering the space.  This is also the case during all heating modes I have observed in UFAD systems.  During heating mode for these systems the primary supply air is completely shut-off and the space is heated using 100% re-circulated air from the space.

I recently attended a seminar by a UFAD manufacturer where one of the speakers was a chief engineer for the company.  After the seminar I posed this question to him, only to have him respond by saying that it's a tough issue to tackle and he's not sure how compliance would be met.  I've also been told that since heating only takes place on the perimeter that ventilation is drawn from interior zones.  I've also been told, "Well we generally only see corridors on the perimeter of buildings with UFAD."  To my knowledge ASHRAE 62.1 doesn't allow ventilation from interior zones to compensate for perimeter zones that have little or no primary air flow.  Also, ASHRAE 62.1 requires ventilation in corridors anyways so this argument seems to be just as weaselly.  ASHRAE 62.1 requires zones to be adequately ventilated during all load conditions, which includes no load or heating load

UFAD seems to be touted as the Messiah of multi-story office HVAC, but the lack of ventilation air appears to be a glaring omission.  Am I missing something about these systems?

Does anyone know how to design a UFAD system that complies with ASHRAE 62.1?  Surely there have been several successful LEED certifications for projects utilizing UFAD technology;  does anyone know if the USGBC has given guidelines for this issue?


 Dan Russell, EIT

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