Freeze Protection – Engineered Air Balance

Freeze protection is nothing new in the HVAC industry; however, if blatantly disregarded or not properly tested, the effects could be disastrous. With the 2020/2021 Winter season taken the south by storm, this was a true test to the safety measures put in place in hopes of avoiding the effects of freezing temperatures. We were able to witness firsthand the effects of lingering freezing temperatures and the damages left behind.

On February 13, 2021, the cold weather hit South Texas relatively colder than usual and lasted for about three days. During this time, a local school district decided to ignore Freeze Protection protocol and essentially shut down their Central Plant and all other related HVAC equipment. This decision had a disastrous effect on the hydronic system and campus property.

The maintenance department turned off boilers, HVAC pumps, chillers, cooling towers and disabled the building automation system (BAS). It is still unclear why these decisions were made. The general practices described below when tested and implemented could drastically reduce the chances of system failure and property damages. Below are images from the related water damage and equipment failure.

Freezestat Control Method

The first line of defense is a quality BAS that has been tested, proven reliable and has quality end devices installed. Quality end devices include low-limited temperature sensors, manual freezestats, temperature sensors, hydronic valve actuators and air damper actuators. While certain freeze protection setpoints vary from project to project the basic principle remains common: You will want a manual reset freezestat mounted in the discharge air stream of the pre-heating coil or entering the chilled water coils. See images below for proper low-limit device manufacturer installation recommendations. Freezestats and low-temperature cutout controls are installed to protect coils. Testing is not as simple as hitting a button because during install the copper tubing holding the temperature-sensitive gas can be compromised.

Here are some tips on how to test controls:

  • Make sure they are installed in the correct location – directly downstream of the hot water coil (upstream of the cooling coil) in a horizontal serpentine fashion, spaced evenly across the coil. This location will prevent false trips and provide full coil coverage.
  • If the freezestat comes with an adjustable temperature setting, set the temperature as low as you feel comfortable without dropping below freezing (35°F is typical).
  • Buy freeze spray or ice and physically apply to 12 ̋ to 18 ̋ of the serpentine copper tubing that holds the temperature-sensitive gas vapor. The gas vapor pressure will drop and cause the freezestat to trip.
  • Once complete, you will need to reset the freezestat if it does not do so automatically. In some cases, you may need to physically reset the freezestat as well as clear any front-end alarms that have been programmed to prevent the unit from automatically restarting after a freeze condition.

ow temperature detection thermostat

thermostat mounting in walk-in duct

thermostat mounting for limited access ducts

 

If the freezestat trips the BAS shall respond as follows until the low ambient temperature condition ceases to exist:

  • Open the hot water and chilled water coil control valves to a minimum of 20% (adjustable) for flow through the coils.
  • Close the outdoor air dampers on air handling units, fan and coil units and outside air handling units.
  • De-energize the outdoor air handling unit supply fans.
  • Open chiller isolation valves.
  • Energize the primary and/or secondary chilled water pumps.
  • Energize the primary and/or secondary hot water pumps.
  • Energize the boilers and maintain a minimum of 85°F.
  • Alarm the central site if the leaving air temperature drops to 34°F (adjustable).

Low-limit Temperature Switch Installation

A manual reset low-limit temperature switch shall be mounted in the discharge air stream of the heating coil or before the cooling coil per manufacturer suggested installation protocol. These low-limit switches are most commonly mounted in a serpentine configuration or can be wrapped around a pole that extends the length of the duct in a sloping, angled fashion. The normally closed contacts of the switch shall be wired in series with the variable frequency drive (VFD) safety interlock circuit so that when activated, the VFD will shut down the fan. Upon sensing a low limit condition, the BAS shall open the hot water and chilled water control valves, close the outdoor air dampers, open chiller isolation valves, energize the chilled water pumps, energize the hot water pumps, activate the boilers and alarm at the central site for freeze conditions.

To limit the possibility of freezing a coil, or to reduce the energy costs associated with conditioning cold fresh air, building operators sometimes shut down the fresh air intake. While this can certainly prevent HVAC coils from freezing, it has some negative unintended consequences, starting with the building occupants not getting the fresh air they need. While the fresh air intakes are shut down, the building exhaust systems are still operating to pull air out of the building. Lacking sufficient make up air, the exhaust systems may not be able to exhaust enough air to perform their functions and they pull fresh air into the building through whatever paths they can. This is often at doorways and windows, creating cold spots, and in some cases even freezing coils in other equipment in the building.

While most facilities have taken precautions to prevent low temperature related damages, there are most certainly other factors involved that are unavoidable. In other school districts, power loss played a big role in making freeze protection obsolete.  Things to consider in the future for these districts is possibly have more HVAC equipment tied into the backup generator to help aid the BAS in controlling freeze protection. In the end a proven sequence of operation, when tested and implemented, can save HVAC equipment, prevent facility down time, prevent property damage and avoid expensive repairs.

Written by Travis Campbell



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