HVAC Freezestats - A Coil's Best Friend

A freezestat is a safety device that protects a water coil from freezing (DX coils do not require them).  Another name for a freezestat is a low-temperature cut-out, limit, or detector, take your pick.  In my years of inspecting these simple devices, the installations have been like snowflakes, no two were alike.  This seems odd because they all come with installation instructions that are pretty clear, to me at least. Here's how they should be laid out.

1. Mount the body of the unit outside of the ductwork and near the top.  The rule of thumb is that each sensor is about 20 feet long (exact lengths may vary) so at a ratio of 1 linear foot of element per 1 square foot of coil, one sensor should cover about 20 square feet.  And at a standard coil velocity of 500 feet per minute, one sensor should cover the coil area for a unit supplying approximately 10,000 cfm.  Now this doesn't mean that if you have a 11,000 cfm unit you should use two sensors to cover a coil, but if you have a bigger unit, realize you may have multiple sensors (I saw one the other day with five).

2. Where the sensing element enters the case you should have some sort of rubber grommet or bushing.  Airflows within the unit's casing can cause the element to vibrate and damage itself.

3. Don't crimp, pinch or crush the sensing element.  This capillary tube is delicate and care should be taken to install in properly with the approved turning and mounting devices.  Failure to do so may render the element ineffective. Resulting in a burst coil.

4. Install the element mostly in the horizontal plane and in a downward direction from the body of the device.  You should first layout how you are going to install it on the coil.  Then starting at the bottom, you should work your way back to the body of the unit.  A lot of times I have seen the opposite type of installation where you work from the body to the end of the tail.  Often this results in ending up with a foot or so of extra element and the tendency is to just wrap the tubing back on itself.  This is not correct.

5. The bottom of the coil will freeze first.  The row with the end of the sensing element should be six inches from the bottom of the coil with the next row 12 inches above the last.  The elements cover six inches above and below, that's where you get the one linear foot to one square foot ratio.  Also, if the bottom is most likely to freeze and the top is the least likely to freeze if you don't cover the very top of the coil its really not that critical (unless the designer says so).

6. Freezestats should be installed on the downstream side of the heating coil.  This assumes that you have a heating coil or that you heating coil is in the pre-heat position (i.e. before the cooling coil as seen by the flow of outside air). If you cooling coil is the only coil or if you have a heating coil in the re-heat position (after the cooling coil) then I would say put in on the downstream side of the cooling coil.

Once installation is complete, the operation of the freezestat is pretty simple.  If a portion of the element (typically 12-18 inches depending on the manufacturer) falls below the temperature set on the sensor body (which typically is adjustable from 15-55 deg F) then two sets of contacts will open.  The first line voltage contact shuts the unit down and should return all components to the unoccupied conditions (i.e. outside air damper 100% closed, etc) and the second low-voltage contact will send the alarm to the building automation system.  Once the freezestat is tripped it requires a manual reset from the building staff.  This ensures that someone visually inspects the unit prior to returning it into service.  This reset button also typically doubles as the test button to make sure that the freezestat is working properly.

This manual reset is really important, but it can also be the bane of the building operators.  Because if the freezestat is not properly installed it can cause nuisance tripping that may cause the operators to start a series of corrective measures that could result in poor system performance or safety violations. So it all starts with proper installation.