Wednesday, October 10, 2012

Talk to the Occupants!

Last week I spent 4 days in Southern Ontario, Canada. I spoke at three different ASHRAE chapters (London, Hamilton, and Toronto) and called on 15 design engineering firms with our local reps and the regional sales manager. In total, I saw nearly 200 folks. I asked all the same question. “What is ASHRAE’s recommended maximum delta-t when heating from the ceiling”. In spite of Standard 62’s Ventilation Rate Procedure being code in Ontario Province (they have the IMC 2009 in their code, which references the 2007 VRP), there were few who knew the answer. Sadly, several of those who knew the “rule” said they ignore it because “no one is checking”. It is a major problem if there are things in codes that no one checks.

One firm did have a design leaving air of no more than 90°F, but no one in the room knew why. They were glad to understand why this rule was in place.

Checking is surprisingly easy. Every project has a schedule that lists the design discharge temperature of all devices with heating coils. All a code official has to do is compare this data with the stated design outdoor air delivery rate. ASHRAE 62.1 (Table 6.2) clearly states that if the delivered air is more than 15°F above room temperature, they must divide the room ventilation rate by 1.2, which is a 25% increase in the outdoor air quantity, when heating. In Canada, this is likely very expensive in their cold climate. If the outdoor air damper is fixed, this increases the dehumidification and cooling demand in the summer.

In addition to Standard 62.1 compliance, when the discharge temperature is high, it is extremely unlikely that the space will meet ASHRAE Standard 55’s 5.4°F vertical temperature stratification requirement. The ASHRAE Fundamentals chapter 20 on Air Distribution states: “when the room to discharge differential exceeds 15°F, it is unlikely that the vertical temperature limitation of ASHRAE Standard 55 will be met.” When we did the overhead heating analysis in the late 70’s, we ran over 900 different perimeter conditions, and in none of the cases was there compliance to Standard 55 when the delta-t exceed 15°F.

Again, however, many in the design engineering community seem to be unaware of the fact that hot air rises and that releasing it at the ceiling will result in occupant dissatisfaction. When I bring this to the attention of many engineers, too often their response is “I’ve been discharging hot air into spaces for years and no one is complaining”. I question whether they ever actually asked anyone living in such an environment. As BOMA continues to report that the #1 reason for tenants not renewing their lease is “occupant dissatisfaction with their thermal environment,” I would conclude no one is bothering to ask the occupants.

It also appears that there is poor understanding of the need to adjust diffusers prior to balancing. One Canadian air balancer told me that they had to call the engineer to find out how to adjust the linear slots on his project, as he could find nothing in the design documentation. At least one balancer is taking the time to do it right. Sadly, he is in a minority.

It appears we have a ways to go before we manage to design comfortable spaces.

Authored by: Dan Int-Hout, Chief Engineer Krueger

Thursday, October 4, 2012

School Acoustics

I was recently asked about the issues with meeting classroom acoustics requirements, both for the upcoming LEED V4 and many local code requirements.

The Acoustical Society Standard ANSI S12.60 is the base requirement. For most classrooms, it requires a dBA no greater than 35, which is about 26 NC, depending, of course, on the shape of the sound spectrum. This is a pretty tight requirement, especially because it doesn’t state from where the sound comes. Measurements in many schools exceed 35dBA from outside noise alone.

LEED for Schools presently has a 45 dBA limit for getting a point, again not specifying from where the sound comes.

The upcoming LEED V4, however, has a prerequisite of 40 dBA, but specifies that it is for predicted sound from the HVAC system alone; AHRI 885 is referenced as a calculation method. This is a major improvement, I believe, as one no longer needs to hire an Acoustician for this value.

Reverberation time is also specified, but in V4, it is covered by room construction details.

Now, what to do about the HVAC system? One thing is clear, if the 40 dBA requirement is to be met, there will be no mechanical equipment in the classroom and likely none above the ceiling if it contains a fan. 40 dBA is about NC 31. Putting equipment in the corridor with lined duct will be the likely solution. Fire dampers are often required at the wall penetration. There are also space issues that need to be worked out.

Displacement Ventilation diffusers are the quiet solution for air outlets, but there are challenges:

--- The air should not be any colder than 65F to avoid discomfort, but the dew point has to be much lower. The mix of ventilation air and humidity control will be a challenge. Some codes now require that the ventilation be shut off in unoccupied classrooms. This implies a separate ventilation supply, which likely has to be pressure independent VAV that allows easy demand controlled ventilation.

---  The “near zone” is likely about 4 ft at 250 cfm. This means a clear space around the diffusers, which is no problem for the ones often placed on each side of the white board, but it may be a challenge in the rear of the ever more crowded classrooms.

---  Displacement diffusers do not heat as well as other methods. Higher velocity air at the floor is likely required, or at least a baseboard or some other alternate heating solution.

---  The thermostat location in a stratified environment is problematic. Some trial and error is likely, as the ADA height requirement may not be ideal; some offset will be inevitable.

---  Finally, with many classrooms in perimeter zones, the internal heat gain in the winter will offset the heating demand. Many times, discharge delta-t’s are very low or neutral once the classroom is in use. This adds to the problem of controlling humidity without reheat.

This means that the engineer will have to look at a number of solutions. It seems that a dual duct system is a pretty good one, with ventilation air in one duct and recycled air in the other. The dual duct unit may require a heating coil in some areas, but it could be located above the classroom and still be able to meet acoustical requirements. Locate the two supply ducts one above the other, as they will be supplying air from each side.

The DOAS fan unit may be another solution, given its ability to vary the ventilation rate while separately managing heating and cooling. The unit of course, will likely have to be in the corridor.

The chilled beam has also been considered, due to its quiet nature, but openable windows offer a challenge in controlling condensation.

In any case, classroom HVAC design won’t ever be the same.

Authored by: Dan Int-Hout, Chief Engineer Krueger