Preheat coil on AHU - cold climate
Preheat coil on AHU - cold climate
(OP)
Im doing a job where the outsdie design temperature is -5 deg C. Obviously to protect the AHU cooling coil, the design drawings show a medium temp hot waer preheat coil on the air handling unit and it is sized to bring the air temperature up to +5 deg C. The design document also mention something about a frost thermostat. What is it that should control the valve on the preheat coil.?? A temperature sensor down stream of the coil within the unit? Should the stat actually be a temperature sensor??





RE: Preheat coil on AHU - cold climate
As far as t'stat vs. temperature sensor, I'd say that we are mainly talking about semantics. While there is a difference between the two, when you say t'stat or sensor most people know what you are getting at. In reality a temperature sensor is just that, something that senses temperature. A t'stat will then provide a signal to the controlled item.
This may sound obvious, but what is protecting the preheat coil? Is there a strategy in place to protect the coil if the fluid stops circulating, or is there glycol in the loop?
RE: Preheat coil on AHU - cold climate
The main function of the preheat coil is to prevent freeze-up.if it’s hydronic you will need a glycol solution or you can use steam. It also provides heat for proper humidification when required and a thermostat, will have its bulb located after the preheat coil, which controls the valve in the preheat coil supply line.
Roger(
RE: Preheat coil on AHU - cold climate
The "frost thermostat" you mention is commonly called a freezestat. It is normally an averaging sensor installed downstream of the preheat coil, and it is wired into the fan control circuit to shut down the fan if the temperature approaches freezing. This is your final protection against freezing the coils, and should be separate from your controlling sensor/stat.
Rod
RE: Preheat coil on AHU - cold climate
Regards
RE: Preheat coil on AHU - cold climate
To provide freeze protection on the hot-water coil you could provide a dedicated coil pump that circulates the fluid when the outside air temperature is below +5 C. The freezestat would shut down the air handling unit if the temperature downstream of the coil is too low.
Just a comment, depending on the seasonal operation of the unit, the cooling coil may be drained during the heating season (if there is sufficient lag time where the unit does not require mechanical cooling). Some of the maintenance people on some projects prefer to drain the cooling coil and dry it out to prevent damage.