What is a Btu Meter and How to Calculate Energy

A Btu meter is typically a system consisting of four components (Btu Meter, Flow meter, and two temperature sensors) to make a dynamic calculation using the sensor inputs to calculate instantaneous energy usage in a heating hot water or chilled water loop.

A typical building will be required to have both heating and cooling via water or glycol/water mixture in a closed loop. The energy consumption in a heating hot water or chilled water system will employ a flow meter to measure volumetric flow in the loop, two temperature sensors with one installed in the feed line and one in the return line, and a Btu meter to take all three sensor inputs to calculate the energy consumption. This calculation is made by taking the dynamic temperature of the feed and return lines to calculate the density and enthalpy of media (water or water glycol mixture), and then multiplying by the instantaneous volumetric flow rate.

P= V x ρ x (hTV –hTR)

Where  P = Power (BTU/Hr)

V = Volumetric flow rate (Gal/Hr)

ρ = Density (Lb/Gal)

hTV = Specific Enthalpy (BTU/Lb) – Feed Temperature

hTR = Specific Enthalpy (BTU/Lb) – Return Temperature

For a chilled water system the energy consumption can be calculated in BTU’s or TON’s, where the conversion calculated inside the Btu meter is as follows:

1 Ton x Hour (Refrigeration) = 12,000 BTU’s

Considering the Volumetric flow measurement occurs in either the Feed or Return line the installation location must be identified so that the proper Density (ρ) value is applied in the power calculation for the highest system accuracy.

System accuracy of the Btu measurement is dependent on the RMSE (Root Mean Square Error) of all the components that make up the system to complete the measurement. In this instance it would be the (+/-) % accuracy of the Btu meter, flow meter, and temperature sensors all combined in the calculation. Ideally the RMSE for a Btu meter and its components should be +/- 1.0 percent for any billing application or to qualify as a LEED certified building measurement. Without going into great detail about specific accuracies, we will leave that for another discussion, the system components break down as follows. The Btu meter typically has a very high accuracy (+/- 0.1% or better) depending on how the internal calculations are made. The temperature sensors have the next best accuracy (+/- 0.1% to 0.5%) with it being close to or slightly worse than the Btu meter. This leaves the flow meter and the technology employed to measure volumetric flow. Depending on the technology and the flow velocity range, the accuracy (+/- 0.2% up to 50.0%) of the flow meter will vary significantly. Bottom line, the flow meter employed will make the overall measurement very accurate or essentially worthless.

With Btu meter accuracy in mind, a typical building will require both the Heating and Cooling. In order to measure total building Heating/Cooling energy consumption it has traditionally required two Btu meters, two flow meters, and four temperature sensors to achieve both measurements. With the development of a new Btu meter (HEATX2) this is no longer the case. It is now possible to measure both heating and cooling energy loads within one Btu meter. This provides the capability of making dual calculations simultaneously and providing a single point for data extraction via communications (Modbus-IP and BACnet-IP). This allows the end user to eliminate one Btu meter, and it may be possible to eliminate a flow measurement and a temperature measurement, while providing better accuracy and lower installation costs.