Proposed Measure Specification

At the August 31, 2017 ARC (RTUG) subcommittee meeting, we received feedback on the proposed specifications for the ARC-lite (VFD on supply fan) and full ARC measures.  Here are the revised proposed specifications for both measures.  Please provide your feedback, including any of your proposed specification language changes, along with any explanation, in the comments section.

VFD on Supply Fan Measure Specification

  1. Applicability
    1. Measure applies to:
      1. Existing rooftop units,
      2. with 5 tons or greater nominal cooling capacity,
      3. with an existing single speed supply fan motor without a VFD; and
      4. that serve a single zone.
  2. Install a VFD
    1. A variable frequency drive shall be installed and controlled to, at minimum, reduce the supply fan motor to 40% of full speed during ventilation-only operation. 
      1. Exception: Where the volume of outdoor air required to comply with the ventilation requirements of the International Mechanical Code exceeds the volume of outdoor air that would be delivered with the supply fan motor at 40% of full speed and with the outdoor air damper in the maximum open position, the minimum speed shall be selected to provide the required ventilation air. 
    2. The outdoor air damper shall be controlled to maintain proper ventilation rates according to ASHRAE Standard 62.1 under different fan speeds. 

Advanced Rooftop Control Measure Specification

  1. Applicability
    1. Measure applies to:
      1. Existing rooftop units,
      2. with 5 tons or greater nominal cooling capacity,
      3. with an existing single speed supply fan motor without a VFD;
      4. that serve a single zone; and
      5. that have an existing air-side economizer.
  2. Install a VFD
    1. A variable frequency drive shall be installed and controlled to, at minimum, reduce the supply fan motor to 40% of full speed during ventilation-only operation. 
      1. Exception: Where the volume of outdoor air required to comply with the ventilation requirements of the International Mechanical Code exceeds the volume of outdoor air that would be delivered with the supply fan motor at 40% of full speed and with the outdoor air damper in the maximum open position, the minimum speed shall be selected to provide the required ventilation air. 
    2. The outdoor air damper shall be controlled to maintain proper ventilation rates according to ASHRAE Standard 62.1 under different fan speeds. 
  3. Install Economizer Controls 
    1. Economizer controls - controls that automatically allow the cooling system to supply outside air to reduce or eliminate the need for mechanical cooling during mild or cold weather - that have the following characteristics shall be installed:
      1. Economizer operation shall be integrated with the mechanical cooling system and be configured to provide partial cooling even when additional mechanical cooling is required to meet the remainder of the cooling load.  Economizer controls and dampers shall be configured to sequence the dampers with mechanical cooling equipment and shall not be controlled by only mixed air temperature. Economizer controls shall have the mechanical cooling capacity control interlocked with the air economizer controls such that the outdoor air damper is at the 100 percent open position when mechanical cooling is on and the outdoor air damper does not begin to close to prevent coil freezing due to minimum compressor run time until the leaving air temperature is less than 45°F (7°C).
      2. Control shall have not fewer than two stages of cooling.
      3. Economizer shall be configured to automatically reduce outdoor air intake to the design minimum outdoor air quantity when outdoor air intake will no longer reduce cooling energy usage.  High-limit shutoff can be accomplished using:
        1. a fixed dry bulb setting, where the economizer shuts off when outside air is greater than 75°F;
        2. a differential dry bulb setting, where the economizer shuts off when the outside air dry-bulb temperature is greater than return air dry bulb temperature;
        3. a fixed enthalpy with fixed dry-bulb temperature setting, where the economizer shuts off when the outside air enthalpy is greater than 28 Btu/lbdry air or when the outside air dry-bulb temperature is greater than 75°F; or
        4. a differential enthalpy with fixed dry-bulb temperature, where the economizer shuts off when the outside air enthalpy is greater than the return air enthalpy or when the outside air dry-bulb temperature is greater than 75°F.
  4. Install Demand Controlled Ventilation 
    1. A ventilation control system shall be installed that is capable of providing automatic reduction of outdoor air intake below design rates when the actual occupancy of spaces served by the system is less than design occupancy.

Previous Discussion Feedback

Mike Kennedy

Mon, September 11, 2017 11:36 AM

Sorry to miss what looks like an interesting meeting. I've obviously not followed the discussion to this point but have recently been modeling RTU energy use related to upcoming standards which include significant fan energy reductions. From talking with carrier I would wonder whether the flow should be 50% rather than 40%. Getting too low will result in cool delivery temperatures which could result in uncomfortable conditions. At 50% flow the fan savings are very substantial. Savings. I do not understand the mechanism by which the heat pump savings are larger than air conditioner savings. Perhaps just happenstance. Based upon modeling, the fan energy reduction results in an increase in heating energy use. My first impression is that this would substantially offset savings from DCV. These same mechanisms would determine increasing or decreasing gas consumption. Measure. Certainly if incentives pay for the whole cost this will be adopted even in older equipment. At some point in the life cycle we would be much better off offering an incentive for early replacement with a code compliant (WA) or better than code compliant unit with the DCV spec.

Adam Hadley

 RTF CAT

Tue, September 12, 2017 7:50 AM

Thank you. I agree with most of your points, but I want to be sure I understand one of them: We're calling for 40% fan speed during ventilation-only mode. I think you'd rather we call for 50% fan speed, but I don't see how reducing fan speed during ventilation-only mode results in cooler delivery temperatures. I'm likely just misunderstanding something basic.

Mike Kennedy

Tue, September 12, 2017 7:51 AM

The ventilation mode includes a typically fixed portion of outdoor air. At 40% supply flow the OA fraction starts to get pretty high and during winter the unit will be blowing cold air. We have this problem in the local movie theater where the ventilation levels are high and without tempering the system blows cold for awhile and then switches on the heat. With a higher supply flow the vent mode SAT would be more acceptable. At 50% it is still an issue, just less so, and it seemed like what Carrier has settled on. Not a critical issue but a concern. If the controller had some sort of control to elevate fan speed if SAT was below X that would take care of this issue.

Adam Hadley

 RTF CAT

Tue, September 12, 2017 7:52 AM

Thanks - that makes sense. Do you have a feel for what temperature we should specify for X?

Mike Kennedy

Tue, September 12, 2017 7:52 AM

I don't know enough about this measure (purpose, whether there is a standard unit already available) or what design engineers think about SAT during ventilation. Of course the main place design engineers are present is with multizone systems where the heating and cooling are fully modulated rather than RTUs where it comes in blocks. You should ask who ever dreamed this up which hopefully includes a design engineer. The key here is too low a temperature and you hinder acceptability and too high a temperature and the fan speed runs higher much of the year in some cases. probably 65F or so. Example assuming 75 RAT and 30 OAT and 10% OA, at 100% supply SAT is 70.5, at 60%, 50%, and 40% supply the SAT will be 67.5, 66, 63.75 respectively. At higher OA or lower return air the sat would be even lower, at 15% the SAT at 60%, 50%, and 40% would be 63.75, 61.5, and 58.125 respectively which clearly starts to feel like the AC is on. Rather than being constantly adjusted (best for fan energy savings) this could be a design calculation to set the minimum flow value so that at 30F OAT the supply air stays above some temperature. From a measure perspective though, once the idea of modulating fan speed on SAT is introduced it is a slippery slope to circumventing the fan speed.

Adam Hadley

 RTF CAT

Tue, September 12, 2017 7:53 AM

This is helpful. I'll propose we add another exception to the 40% fan speed spec: "Exception: Higher supply fan motor speeds are allowed during periods of cold outdoor air temperatures (e.g. below 30 degrees F) in order to maintain comfortable supply air temperatures (e.g. above 60 degrees F)."

 

Joe Schmutzler

 Transformative Wave

Tue, September 19, 2017 12:28 PM

It is extremely rare for us to have cold complaints based on our sequence alone. We have only ever had these in very cold conditions, and usually with VVT systems. With DCV decreasing the amount of outside air, in ventilation mode you will almost always have less outside air than before the installation unless the customer had shut the dampers.

Adam Hadley

 RTF CAT

Fri, September 15, 2017 5:07 PM

I received a comment from Bob Davis that the fixed drybulb and fixed enthalpy options for the economizer controls should be removed to improve efficiency. I have removed those specs in the version I'll present to the RTF.

 

Adam Hadley

 RTF CAT

Fri, September 15, 2017 5:09 PM

I received a comment from Bob Davis that the DCV portion of the spec needs more specificity, specifically, describing how CO2 control should operate. I have added more detail in the version I'll present to the RTF.