Greetings, Salutations, and Economizers

Greetings, Salutations, and Economizers

By Alex Rippere

Each month Alex Rippere will continue highlighting best practices for complying with the New York City Energy Conservation Code 2016.

Last year we wrote extensively on the 2016 update to the New York City Energy Conservation Code (2016 NYCECC) and its impact on new projects. The code has been with us for a few months now, and some pain points are emerging. These days, engineers’ single biggest gripe comes from the economizer requirement. To be fair, economizers were required for some projects under the 2014 NYCECC, but now they apply to far more project types.

What’s an economizer?

An economizer uses outside air to cool interior spaces when the outside temperature drops below the desired indoor temperature. Some occupancies, like computer server rooms, can generate a ton of heat, and economizers can save owners a lot of money when deployed properly. Why spend money to pump heat out of a server room when opening an outside damper lets Mother Nature handle the job? Unfortunately, economizers can create a serious hassle for many occupancies across New York City.

VRFs: When is a fan not a fan?

Consider multifamily residential as an example. Most of the projects that cross my desk use variable refrigerant flow (VRF) direct expansion (DX) heat pumps, the latest version of those split air-conditioning units seen all over New York City. Like most split systems, a VRF has a condenser (outside unit) and an evaporator (inside unit). The evaporator is basically a fan that moves air over a coil, through which refrigerant flows, moving the heat outside to the condenser. The key difference between VRF and regular split-system heat pumps: VRF systems can finely vary the flow of refrigerant, making them more efficient.

Here’s where we get into the brain damage.

Even though every VRF unit has an evaporator fan, VRF systems were not considered “systems with a fan” under the 2014 NYCECC and the old state energy code. No economizer was required since you only need an economizer for a “system with a fan,” but this interpretation changed with the new energy code. Now, any system with a fan that kicks out more than 54,000 BTU (4.5 tons) requires an economizer. If a project calls for a VRF unit with an output of 60,000 BTU, we have to run outside air to the evaporator—not an option for most projects.

A tale of two codes . . .

Remember, the NYCECC is two codes in one, based on the International Energy Conservation Code (IECC 2015) and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Standard 90.1-2013 (with local modifications to both codes). Both codes let you out of the economizer requirement, provided the output is less than 54,000 BTU. However, the NYCECC limits how many times you can exploit that exception. If we select NYCECC 2016 as our code route, then no more than 20% of the total cooling equipment load can be excluded from the economizer requirement. On a small job with, say, five 54,000 BTU units, only one gets out of the economizer requirement [C403.3 Exception 2 last paragraph].

By contrast, ASHRAE places no limit on how many times you can invoke the less-than-54,000 BTU exception. Of the five units from the previous example, none require an economizer under ASHRAE 90.1-2013 [6.5.1.1, Table 6.5.1-1]. Yet another good reason to go with ASHRAE.

Perhaps we’re reaching the point where it only makes sense to use NYCECC for a new building in the outer boroughs, which can have a long north-south axis. ASHRAE limits the amount of glazing on east and west façades without an energy model, and the Manhattan grid is laid out in such a way that few buildings have primary east and west façade orientations (per energy code definition). Consequently, I can see a lot of Manhattan condos going with ASHRAE in the future.

Stay tuned each month for all things New York City energy code. In the meantime, please email any questions to alexr@metropolisny.com.