#1361 90.1 PRM return/relief fan ratios

[Lukas Hovee]

Regarding Return/Relief Fan Ratios:

There is not a 1 to 1 correspondance between proposed and baseline systems, i.e. a baseline system may be made up of thermal zones which were originally served by multiple proposed systems. This necessitates tracking Supply, Return Relief (and exhaust) fan ratios at the Thermal Zone level, and then building them back up via weight average to the baseline system level.

One potential issue is the case where a baseline system serves some thermal zones were orignially served by a proposed system with return fans, and others originally served by a proposed system with relief fans -> i.e. the return fan ratio AND relief fan ratio for the new baseline system will both be greater than 0.

Due to current CBECC/OS limitations, the baseline system cannot have both relief and return fans, so the fan power associated with both the return and relief fans must be assigned to either return OR relief.

Our proposal (for these cases only) is to assign both the return and relief fan power to relief fans in the baseline model. The relief fans will be sized and operated to only exhaust the excess outdoor air, so they will generally use less energy than return fans. This is a conservative assumption, since it will generally reduce the performance rating of the proposed design.

[David Reddy]

Supriya, I forgot to bring this new issue up in our call. I think we will just wait until next Wed's call to discuss the PD credit and this issue further.

[Supriya Goel]

Hopefully we can get to it sooner than that. I'll finish writing up the VRP discussion and then get to this

[Michael Rosenberg]

If you combine relief and return fans into a single relief fan that will be the same as if you combine it into a single return fan from a peak fan power perspective, correct? But what about operation? Will the relief fan only run when there is OA > minimum? Or does the relief fan run whenever the SF runs? If it is the former, then I understand why your approach is more conservative. If it is the latter, there should be no difference between assigning it to a return or relief fan.

[Lukas Hovee]

Yes to the former - my understanding is that relief fans only run when Supply-Return-Exhaust > 0, and then only at the relief CFM, so in most cases would use less energy than a return fan.

[Michael Rosenberg]

Is it possible to do a calculation to see what the breakdown of return verses relief fan power is in the proposed? Then assign baseline to whichever is greater in the proposed?

[Nikhil Kapur]

Issue assigned to ASHRAE 90.1. Issue can be reopened when development on ASHRAE ruleset begins.