Q: I was hoping to get some insight on which M&V Option would work best for a retrofit project where the ECM involves installing Variable Frequency Drives on the Air Handling Units so as to optimize their operation and control their setpoints. Submeters will be installed and there is an IES energy model completed for the building. Would Option A or B be more fitting?

A: Thank you for the question – apologies for the delay in response.

First, a few questions to help clarify and frame the situation for your VFD retrofit project “Installing Variable Frequency Drives on Air Handling Units”:
• Are the existing AHUs VAV? This is key to understanding the retrofit intent and associated savings.
o If the systems are already VAV, and this is purely replacing the existing fan modulation device with a VFD, resulting in the majority, if not all, of the savings focused on the reduction of fan motor electrical energy.
o If the existing units/systems are not VAV, then this is retrofit may be better characterized as a VAV conversion utilizing VFDs versus just installing VFDs. In this case the savings would be a combination of fan energy and heating and cooling loads/energy reductions.
• Where are the submeters being installed; on what energy streams? Fan electric submeters as well as submeters on the heating and cooling loads or energy sources? This response is directly related to the question above regarding the nature of the retrofit and the associated energy savings streams. Also, are the submeters being installed to capture the existing, baseline energy or only as part of the retrofit?

Understanding that we do not know the answers to the questions above, the following are insights into the use of Option A or B and associated items to consider.

Typically, the VFDs allow a reduction of the electricity consumption from the motors. But if the retrofit involves modifying the air system delivery (conversion to or optimization of VAV), the main benefits can come from the reduction on heating and cooling load (less air).

Key parameters:
• Since we do not have any detail about the fresh air ratio, I consider this is to be a key parameter (to be monitored).
• Airflow (mixed fresh air and recirculated air) is a key parameter too.
• If the air handling units (AHU) do not represent a major part of the heating-cooling load for the building, the heating-cooling plant efficiency may not be affected. The efficiency could be estimated in this situation. Otherwise, it should be part of the key parameters. Which we assume it is, since the question mentioned many AHUs.

The energy model can be of use, but it has to be calibrated before implementation of the ECM. Considering that there is a calibrated energy model, Option A could be considered as follows:

For an Option A: the measurements, before and after implementation, can be done under specific conditions (external temperature and humidity level, specific airflow and % fresh air, etc.) and the result could be extrapolated with the energy model for a 12-month period. This is possible if there is no need for a long-term demonstration of the real savings.
Note: if the heating-cooling plan efficiency is estimated,.

Depending of the configuration, the heating-cooling plant efficiency can be affected significantly by the ECM. Unless it is possible to monitor the efficiency, or maybe simulate it, the precision of the determined savings will be affected under an Option A applied with an estimated efficiency.

Option B : This VFD retrofit could be a great application for Option B, and it will help you optimize savings over time. Be sure and conduct sufficient measurements of the baseline conditions – depending on the air distribution system, there may be variation in the baseline motor power which needs to be documented. Look for a correlation between baseline fan power and key system data from the controls. You may need to functionally test the system to cover all operating conditions during baseline measurements.

Also, may be interesting to rely on the length of the reporting period to provide a recommendation on the use of option B.

Length of the reporting period:
For AHUs, the normal operating cycle (reporting period) should consider a full heating cycle or full cooling cycle, with regard to the normal needs for the area where the project is located. This can be up to a 12-month period. Option B could be difficult, or expensive, to implement for many AHUs over a year.