Wednesday, September 3, 2014

Engineers Push Back

I recently received a nice letter from an engineer in California who said she had heard me speak a few years ago about overhead heating and had been following my advice (and that in 62.1 and the Handbook) about limiting discharge delta-t to 15oF. She then went on to say how surprised she has been at the push back from engineers on this recommendation. Sadly, I had to tell her that my experience has been similar.

In my role as an ASHRAE Distinguished Lecturer and in making engineering sales calls with Krueger Reps, I see an average of 1000 engineers a year. I almost always manage to bring this topic up in conversations or lectures and nearly every time, there’s an engineer that tells me, “I’ve been doing this for XX years and no one has complained”.

Apparently, the concept of hot air rising (and cold air falling) isn’t a universally accepted paradigm when it comes to air distribution. This person (California engineer) mentioned that there is disbelief in my stated rule on heating a room, where “the hotter the air delivered at the ceiling, the longer it takes to heat a room.” Sorry folks, but it’s true - overly hot air dispensed at the ceiling will most certainly want to stick to it. The hotter it is, the longer it will stay stratified, which means the longer it will take to cool enough to blend with the air below in the occupant space.

Every forced air project with a heating coil likely has an equipment schedule. Somewhere on that schedule is a column headed ‘Leaving Air Temperature’. If any values in that column are 15°F greater than the designed zone temperature (usually between 73°F and 75°F), the code official has the right to ask how the engineer intends to comply with the Standard 62.1 requirement for increased ventilation air and if that requirement was included in the building load calculations. If you recall, ASHRAE Standard 62.1 requires an additional 20% outside air if heating from the ceiling with a ceiling return, as is the case in most commercial buildings. The 62.1 Ventilation Rate Procedure has been adopted by many, if not most building codes.

It’s also worth noting that any space with a discharge to room differential exceeding 15°F will not comply with the vertical stratification limitation of ASHRAE Standard 55, which is often included in many codes, if not a minimum in building design assumptions. Still, engineers are pushing back. It’s no wonder the level of discomfort in commercial spaces is as high as it is. But as they say, “who’s checking?”

Do you work in a stuffy building? Are you using a space heater year round? Tweet us @KruegerHVAC and share your comments with the HVAC engineering community. Let them know that regardless of #whoschecking, proper air distribution matters.

Authored by: Dan Int-Hout, Chief Engineer Krueger