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Preheat coil on AHU - cold climate

Preheat coil on AHU - cold climate

Preheat coil on AHU - cold climate

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

The preheat coil could either be controlled by temperature sensor downstream, or alternately with an outdoor air sensor. Generally the rule of thumb with control is to measure what you are trying to control directly whenever possible. So I'd recommend the sensor between the preheat coil and the coil you want to protect.

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.

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.

RE: Preheat coil on AHU - cold climate

I'll add if permitted an advice, besides the above Show's technical description. Let the contact state of the thermostat  controlling the following sequence: stop the fan, close fresh air damper, close the valve on the protected coil, start a small recirculation pump installed on the preheat coil (if any).Also, if the temp probe senses, again more than +5degC and the conditions are fulfilled, is there an automatically restart of the AHU , or do you have to reset manually (more secure)?

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.

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