I have been doing this for over 13 years and my company for over 35 years and we have never had a tstat in the coil on a WIC. But let me clarify what I am calling off cycle defrost. We always recomend and install pump down systems and install defrost time clocks. As I stated earlier the defrost is 4 x 30min, sometimes we might go to 6 x 30min if it is a high traffic box or they are putting hot product in it. We set and mark the tstat to cut out near 32 degrees on return air temp. If your super heat is set properly 10-12 degrees, and you can go lower as long as your superheat at the compressor isn't below 20 degreees during the busy time(this ensures that during the low load at night you still have superheat at your compressor). This will prevent the coil from freezing up. This all being said you need to be checking the size of the system and the box size. I always do tis when it is a job site we don't normally service or if we have had multiple issues on the system.
Back to the bulb in the coil, none of the manufactures put their bulbs in their coils on WIC's. Their bulbs are always in the return air, Their is a reason the factories don't put them in the coils.
I believe a manufacturer like ICS for example who drops their boxes complete, minus plumbing and power, off a trailer won't put a bulb in a coil for a number of reasons. The first on being someones going to mess with it. Improper startup procedures, lack of knowledgeable techs, possible breakage during shipping to name a few. If I set my CI between 38-40 and set my CO appropiately it will not freeze up. Fast food restaurants are notorious for proping doors open. With the bulb in the coil as soon as the coil temp reaches the CO setpoint the compressor shuts down and will not come back on until the coil temp reaches the CI setpoint not allowing the coil to freeze. With a door propped open and the bulb in the return air the compressor will run forever.
Neither way is incorrect. There is just more than one way to skin a cat.
Bridge ..... lets take a refrigerator with the bulb hanging in the airflow , and its suddenly packed with warm product and the box temperature shoots up to 60°F. The compressor starts, but it has a long journey before the box temperature drops all of that warm product to 40°F. The coil begins to frost, blocking airflow and heat transfer. Because the frost is insulating the coil from the warm boxes temperature , the evap temp continues to fall eventually forming ice. This Air sensing stat keeps the compressor running because it still senses 60 degree product, adding to the frost problems .... however if the bulb was indeed mounted in the coil fins, the stat would shut down the compressor when the evap dropped to the cutout temp of 25. Then while the compressor was off the Fans would be steadily running circulating the warm box temp air over the evap melting the frost ,and when the coil defrosts to the point of ultimately raising the coil temp to 40 deg cut in , the compressor comes back on. This cycle continues back and fourth until both product and box are at same temp.
Like 2sac said , both work , but the air sensing one needs a time clock which is alot more wiring and more crap to go bad.
In my world units are undersized Boxes are poorly insulate with leaks,loaded with hot steaming food etc. On Indoor condensing unit temp was frequently controlled by the LP switch. I have placed t'stat bulbin the evaporator coil on outdoor units too allow proper defrost as it mimics the action of the LP switch. If you set the cut in to box cut in and adjust cut out according to appropriate coil temp drop box maintains temp based on coil temp.
out of all the coolers I service maybe 1% have a defrost clock....... you usually don't require one.....
I set most of my coolers around 5 psi cut out , and 40 cut in .....
Knucklehead maybe , snapperhead for sure
I am so used to having high cut ins and outs (being where I'm at for back up) that I forgot there are people in cold climates. 2sac corrected me rightly.
Cut ins he's exactly right. For cut out, I set a med temp box no lower than I have to. Maybe 10* coil if possible. If the clock fails and/or the tstat/sol. is stuck , the coil ices and loses airflow, the suction will drop quickly.
If the cut out is set real low , there will be a long deice job soon. I set cut in/out as high as possible to prevent Ice block on coil.
What I am saying is, if the cut out is set real low...(3-5-10# r22 example), the evap is getting unnecessarily too cold.
If the system needs to run at the 5* evap range (for a medium temp box) there are design and sizing issues..more than likely the system is a problem child already. You gotta do what you gotta do to make the box run, but I like high as possible settings..That's all I'm saying.
5 days not going into defrost will not hurt it but it should get the new clock. you will always need a defrost on any system.