Tuesday, January 31, 2012

What a Load... Room Load, That Is!

I was in Chicago for a week at the ASHRAE winter meeting. One of my tasks, as a member of a Project Monitoring Committee, is to participate in an ASHRAE Research Project (RP). ASHRAE RP 1515 is a comprehensive study of energy and occupant satisfaction at the Yahoo campus in California. The study is being conducted as a joint exercise between Berkeley, Taylor Engineering, and a manufacturer. It should be completed before the San Antonio meeting this summer.

The data so far is very interesting. In both summer and winter (this is California, so it is a mild climate), interior loads are very low. Interior loads are pretty much independent of outside conditions. The system has VAV boxes and plaque diffusers. The assumption (going in) was that at these low loads there was a potential for poor diffuser performance. What was found was the opposite. The plaque diffuser operates well at low flows and there were (little if any) complaints of “dumping” (where cold supply air falls into the space). The complaints came at full flow when the diffusers collided; it was a bit drafty at the mid-point between diffusers.

The real surprise was how low the load in the space was. The design was the “traditional” 1 CFM/SF in the interior, with the boxes set for a 30% minimum, or 0.3 CFM/SF. It was discovered that the boxes were going into reheat at that low flow! They were reset to 10% minimum with a heating setpoint at 50% (per the latest 90.1 addendum on reheat). Analysis after the reset shows that they were averaging about 0.26 CFM/SF in these interior spaces. I assume that, being Yahoo, these spaces had at least a full complement of computers in the work stations, and likely dual monitors. Yet the loads were very low.

This confirms what I have been seeing for some time. The DuPage City courthouse, that I was an observer for back in 1991, had an interior load of 0.4 CFM/SF and had issues with diffuser performance at these low loads. Nonetheless, we are seeing designs based on much higher loads. 0.5 CFM/SF is about 11 BTUH/SF with 55┬║F supply air. The Yahoo buildings are likely close to 4 BTUH/SF in the interior spaces. The load is also pretty well matched to the minimum ventilation rate.

It is apparent that we need to reevaluate the way we are calculating loads and operating buildings with these low loads. Humidity control, minimum ventilation rates, and minimum effective control for VAV boxes are all tied together. The same is true for other systems, especially chilled beams. At low loads, an oversized chilled beam (which reduces first costs) is just a very expensive diffuser. The same is true for VRV, fan coil and displacement ventilation systems. Failure to understand what is happening in today’s offices can result in wasted energy as all these systems start to go into reheat to maintain acceptable space temperatures. What a load!

Authored by: Dan Int-Hout, Chief Engineer Krueger