Heat Recovery and Airside Economizer

[refer madness]

We're in the process of putting together a proposal to retrofit 3 rooftop mounted 35 ton DX AHUs with economizers. The system will be converted to operate with a new air cooled heat recovery chiller and chilled water coils will replace the DX coils in the AHUs.

This is a VAV with HW reheat system, so the recovered heat will be used throughout much of the year in place of or as a supplement to the boiler. Now what I'm wondering is if in lower ambients there's any point at which it would make sense to shut down the mechanical cooling in favor of free cooling (but no heat recovery).

I did some looking online and found some mention of heat recovery and economizer coordination but no specifics of how this would be implemented.

Anyone have experience with this?

Let me know if more detail is needed regarding the existing and proposed system arrangement.

[brewidus]

Are you talking about a water cooled heat recovery chiller that uses condenser water to supplement the reheat system while simultaneously creating chilled water?

Running the mechanical cooling while the ambient temp is low enough to provide full cooling to the building with the economizers is a waste of energy and violates every energy code that I've seen (may not be relevant in your state or city).

The heat recovery chiller uses electricity to supplement the reheat. If you have electric boilers, it would be more efficient to let them pick up the reheat load during economizer hours. If you have fossil fuel fired boilers, it is more cost effective to run the boilers with gas or oil vs electricity to supplement the reheat system.

[refer madness]

Thanks for the input brewidus.

Yes, this is an air cooled chiller (Carrier30RB) capable of full heat recovery with its auxillary coaxial condenser. So the hot water reheat loop would be piped into this auxillary condenser which receives hot gas whenever reheat loop is below setpoint.

[Dallas Duster]

Thanks for the input brewidus.

Yes, this is an air cooled chiller (Carrier30RB) capable of full heat recovery with its auxillary coaxial condenser. So the hot water reheat loop would be piped into this auxillary condenser which receives hot gas whenever reheat loop is below setpoint.

So there is a main boiler? What kind of building is this?

[refer madness]

So there is a main boiler? What kind of building is this?

Yes, there's a 70% efficiency boiler (1800MBH) with 3 way mixing valve which resets HW loop temp off of OAT . It's a 3 story office building. Right now the boiler supplies 100% of the heating requirements of the building at VAV unit HW reheat coils throughout the building.