QSE Logo

LetsTalk2015@QSEng.com
612.308.4716

Commissioning: Heating Hot Water System Trend Analysis

In this column I will describe the “issue” disclosed through analysis of the trend graph included in my March, 2012 Commissioning column. I will also introduce a new system; show its trend graph; and then leave it up to the reader to discover the operational “issue” illustrated in the graph. The answer will be given at the start of my May, 2012 column.

February 2012 Trend Log Analysis

Last month’s trend graph showed about two weeks of 15 minute interval data for selected points on a single zone, constant volume, mixed air handling unit. The system was equipped with air-side economizer, heating hot water coil, and chilled water cooling coil for discharge air temperature control. The discharge air temperature setpoint was reset based on space temperature setpoint.

This trend included days with hot weather, as the first six days peaked above 80°F. The majority of the trend, however, was during cooler weather with outdoor temperatures ranging from 30°F to 60°F. For an air handling system with economizer, this was an ideal, but rare, period for observing all modes of operation.

From the perspective of discharge air temperature setpoint maintenance, this air handling unit appears to have been performing extremely well. This is evidenced by the fact that the actual discharge air temperature reasonably matched its changing setpoint throughout the trend period. However, the way in which the temperature was being maintained is suspect.

First of all, during hot water the outside air damper minimum position appears to have been 0%, or fully closed. About halfway through the graph, after the weather had turned colder, the minimum outside air damper position appears to have been 10%. This raises the question of whether the facility operators were manually adjusting the programmed minimum outside air damper position based on weather conditions or was this programmed into the control sequence? In either case, why? There could be a number of reasons, some of which may be masking fundamental problems with the system, such as:

  1. Is the cooling capacity of the air handler insufficient to properly condition 10% outside air during hot weather?

  2. Are there high humidity problems when introducing outside air during hot weather?

  3. Are the operators trying to save energy?

Second, in the middle third of the graph, during cool weather and air-side economizer operation, there were a few times when the chilled water valve was modulating when the outside air damper was not 100% open. For the most effective air-side economizer, the control programming should be tuned to prevent mechanical (chilled water) cooling from occurring until the outside air “free cooling” is at its maximum (i.e., outside air damper 100% open) and unable to maintain the setpoint discharge air temperature.

Third, at the end of the graph and still during the cold economizer weather, the outside air stayed at a constant 10% while the chilled water valve modulated to provide all of the discharge air temperature control. It appears that someone may have manually overridden the outside air damper to a constant position, but why? Because the system was in cooling mode, this probably was not because the air handler heating capacity was insufficient. Is it possible that too much cold outside air made the space too dry? The absence of active air-side economizing resulted in unnecessary chilled water (electricity) use and expense.

Finally, when the chilled water valve was modulating to maintain a constant discharge air temperature setpoint over an extended period of time, the valve control signal hunted between about 0%-80% resulting in discharge air temperature swings of about ±4°F around setpoint. This is not a major problem from a temperature control or energy perspective, but it does cause unnecessary wear-and-tear on the valve which is likely to shorten its life expectancy.

New System

This month’s trend graph is for a central heating hot water system. The system consists of two high efficiency modulating condensing hot water boilers with primary boiler pumps injecting hot water into a secondary distribution loop serving the building. The secondary loop includes variable speed pumping and hot water supply temperature reset. As the outside air temperature decreases, the hot water supply temperature setpoint increases and vice versa. These boilers are located in a northern climate where winter heating and summer reheat are both critical for maintaining space temperature and humidity criteria.

The boilers are controlled by on-board manufacturer-provided controllers to maintain the secondary loop hot water supply temperature setpoint. The central building automation system sends a temperature setpoint signal to the boilers, and the integral controllers modulate the boiler firing rates and the sequencing of the lead/lag boilers.

Trend

The building automation system was used to trend the following sensor data at 5 minute intervals for one week in August 2010:

  • Secondary hot water supply temperature setpoint (red)

  • Secondary hot water supply temperature (purple)

  • Secondary hot water differential temperature = secondary supply temperature minus secondary return temperature (yellow)

  • Outside air temperature (blue)

Trend Log

 

Commissioning: Getting It Right

Engineered Systems, April 2012

by
Rebecca Ellis, PE, LEED AP BD+C, CCP, CPMP, CxA
President
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318
rteesmag@QSEng.com