BMS Basics – Frost Protection

Written by HVAC Guy

This blog will give you an honest insight into the day-to-day process of being a BMS Engineer. I will be talking about all different aspects of the job from software strategy planning to on-site hiccups and frustrations.

17 July 2021

An essential part of any Building Management System is to provide automatic functions such as frost protection to the building. The BMS should be configured to protect the building and its contents against all forms of potential frost damage.

Building Management Systems usually provide a 3-stage frost protection routine. This ensures that the building fabric and its contents are best protected against frost damage. The first stage circulates water around the heating pipes. Second stage adds heat into the circuit when required and the third stage heats the building when the internal temperatures start to drop.

Typically, a 3-stage protection routine is enough to properly protect the building. Having said that, each building is different and may require a unique approach to properly implement frost protection.

Often the frost protection routine will be outlined in the design specification. All setpoints, delays and operations will be predefined by the designer/consultant. The BMS engineer will configure these into the software and demonstrate its operation during the commissioning and handover period of the project.

Frost Protection Stage 1

The outside air temperature is always monitored by the Building Management System. Frost protection stage 1 is activated if, during unoccupied hours, the outside air temperature drops below the frost stage 1 set point (usually 1°C). When activated the BMS will automatically enable the duty pumps for all heating circuits to circulate water around the systems and reduce the risk of freezing up. It is also worth noting that any valves will be driven to an open position so that water can circulate around the full system.

The reason that the BMS only operates frost stage 1 out of hours is so that we do not unintentionally start to heat any areas which do not require heating. Obviously, this can change depending on a job by job basis but typically this is how it would be set to work.

Frost protection stage 1 would be disabled when the outside air temperature increases again by a degree or so.

Frost Protection Stage 2

Stage 2 frost protection is only activated if stage 1 is already enabled. Think about it, there is no reason to add heat to the pipes out of hours if a) there is no circulation and b) there is no risk of them freezing.

Stage 2 is activated when stage 1 is enabled and the pipework sensors detect a drop in temperature below the frost set point (typically around 10°C). Stage 2 will remain activated until the pipework temperature is sufficiently warm enough for a set period of time. Again, the setpoints and time delays are often detailed in the design specification. Typically stage 2 will be enabled when the pipework drops below 10°C and disabled when it has reached above 35°C for 5 minutes or so.

The boiler return temperature sensor is often used as the measuring device for stage 2 frost protection, but this does not always have to be the case.

Frost Protection Stage 3

Stage 3 frost protection is also known as internal frost protection. The Building Management System will be configured to monitor the internal conditions during unoccupied hours. If the internal temperature sensor detects a drop in temperature below the frost set point, then stage 3 frost protection will be activated. This essentially enables the heating as if it were required for normal operation, in some cases, it boosts the heating to increase the temperature as fast as possible.

Depending on the project, there can be many instances of stage 3 protection on the job. Each heating zone can have its own independent frost routine depending on how it is heated. For example, a VT radiator circuit may have a frost 3 set point of 12°C but an underfloor zone may have a frost set point of 16°C. This is because of the time it would take for underfloor heating to bring it back up to temperature, typically underfloor heated zones have a higher “set back” value.

It is worth noting that each job is unique and the frost 3 setpoints should be considered individually and set accordingly.

Other means of frost protection

There are also other forms of frost protection that could be applicable to the job. These are often less obvious things but still very important. Examples of these are:

  • Heating coil frost protection
  • Trace heating

Heating coils are susceptible to frost damage if they are not correctly protected. A capillary thermostat is often installed downstream from the coil which will trigger if the air temperature is measured at 5°C or lower. In this case the BMS should be configured to switch off the fan and set the heating valve to 100% open.

Trace heating is often used to protect exposed pipework running in cold areas of the building or even outside. Trace heating controls are often packaged units with sensors that electrically heat the pipe just enough to prevent any frost damage from occurring.

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