With longer days and warmer temperatures, the chill of winter is behind us this time of year. While your HVAC system heating performance is likely not on your mind right now; this is the time to consider any changes your heating coils need in order to work next winter.
Like the people who use them, buildings must breathe properly to function. There are numerous reasons that outdoor air must be brought into a building; the two most common reasons are fresh air for ventilation and make up for exhausted air. The HVAC system needs to be sized and designed to properly handle this incoming outdoor air. Cold winter air brought into the HVAC system has the capability of freezing cooling and heating coils that are filled with water or steam.
A frozen coil can cause an avalanche of problems in the building, from water damage to downtime and cost associated with replacement of the coil. In addition to having enough capacity to condition the outdoor air, the system must be designed to prevent the coils from freezing. A common misconception is that steam coils can’t freeze; however a cold spot on the coil or a failed steam trap can cause condensate to back up and freeze within a coil. Freeze protection means should be checked over the summer to ensure proper operation before the first hard cold spell of the winter season.
In order 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 running 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.
A number of methods can be utilized in the design of the HVAC system to allow it to safely and effectively provide the necessary fresh air to the building during the cold months. Among these solutions are the following:
- Glycol mixture – a mixture of glycol and water in the hot or chilled water system can lower the freezing point of the solution.
- Face and bypass dampers – the steam coil is full open and the air is either directed across the coil or around the coil. This ensures the steam condensate is forced to leave the coil and does not freeze.
- Steam distribution coils – these coils distribute steam from an inner tube to an outer tube containing the condensate. The inner distribution tube reduces the likelihood of the outer condensate tube freezing.
- Coil placement – coils without freeze protection should be located downstream of heating coils.
- Air blenders – these devices are installed upstream of the heating coil to mix the outdoor air with the return air, minimizing stratification and cold spots on the coil.
- Freezestat – a sensor to indicate if sub-freezing air is being encountered by an unprotected coil, this can indicate a heating failure and will shut down the system to protect the coil.
- Bypass pumps – the pump turns on when the outdoor air is near freezing, so that constantly moving water flows through a coil to reduce the likelihood of freezing.
Properly designed and installed coils provide fresh air during all conditions without freezing, ensuring the HVAC system can provide ventilation and proper exhaust for occupants. As stated earlier, the best time to check your coils for freeze protection is during the warmer months so any modifications can be made before the next winter.