is this the same as?

[newbeetech]

just wondering , reach in cooler has a temp ctr that has the bulb on the evp coil, what would happen if i were to install a temp ctr that has pig tail bulb, in the box instead of evp coil, or the other way around would it work the same? after all both cut when it reaches it's temp setting.

[r404a]

The tstat you are refering to is a "constant cut" in tstat. It is designed so that you select lets say, 5 on the dial. The box runs and the embedded bulb senses evap temp. At (example) 15 degrees of actual evap temp, the tstat opens up. IT WILL THEN STAY OPEN UNTIL ABOUT 37 OR 38 DEGREES, IN MOST CASES, ACTUAL EVAP TEMP. This ensures a positive defrost. Do not inter chang these types of tstats.

r404a

[k-fridge]

^^^^what he said^^^^

You'll probably have lots of iced up coils.

[william smith]

The differents is one is reading evap temp and the other is reading air temp you can put the bulb in the return air and set it at 35 is the samething as setting it at a coil temp of 15 on say 409a but if a system is set up to have the cold control in the evap it is better to stay factory

[hvac wiz 79]

if u do change it up .. you might expect some nuisance calls for ice buildup .. so like they said ... keep it

[newbeetech]

well guy's that was some very good answers, ding,ding,ding, guess what i did get a call back, frosted evep coils, i was thinking if i set a t-stat at 38d it will cut out at 38d it did not accure to me that the t-stat bulb was reading the coils temp, .... but i must admit that i have seen this done before, chainging to a box temp ctr , and it worked how can one explaine that?

[KB Cool]

I run into that all the time. Air bulb stat. Instead of a constant cut-in cold control. Same problem every time. Coil Iced up. I tell the customer somebody put the wrong stat in and now needs a defrost timer. Problem solved.

[rayr]

I run into that all the time. Air bulb stat. Instead of a constant cut-in cold control. Same problem every time. Coil Iced up. I tell the customer somebody put the wrong stat in and now needs a defrost timer. Problem solved.

Exactly!!

[newbeetech]

hey MARKETTECH, whats your oppenion on this, constant cut in ctr or air ctr t-stat, i mean they both are shooting for the same goal, a 38d temp. if it's switch would work ? or is the COIL made diffrant.

[markettech]

hey MARKETTECH, whats your oppenion on this, constant cut in ctr or air ctr t-stat, i mean they both are shooting for the same goal, a 38d temp. if it's switch would work ? or is the COIL made diffrant.

Oh boy......I don't often give my opinion.........:rolleyes:

If I'm understanding your question correctly, it's not a matter of is one better than the other, nor is it a matter of if one works they should be interchangeable.

A coil sensing, constant cut-in thermostat works very well when used as it is designed, which is to sense coil temperature, cycle the system off when the desired coil temperature is met, and to allow for a defrost by not letting the compressor start again until a predetermined temperature has been met.

This type of thermostat is used on small, self-contained refrigeration systems. Personally, I believe that the reason these stats are in such wide use is because of manufacturing costs......One thermostat takes the place of a thermostat and a defrost timer.

When the thermostat is satisfied, it shuts the compressor off. No matter what, the compressor will not come on again until this predetermined temperature has been met (+37F is typical).

As you know, +37F is warmer than freezing - so we can feel reasonably comfortable that there will be no frost/ice build up on the coil by the time the next refrigerating cycle begins.

On the other hand, an air sensing thermostat with adjustable differential - while it also cycles a compressor based on temperature - does so in a different manner.

Generally, the air sensing stat is placed in the return air stream of a unit and cycles the compressor as needed to maintain a rather tight ambient temperature in the fixture. It is not uncommon to see a thermostat such as this set to a 3 degree differential.

This type of thermostat is typically used on larger refrigeration systems - and used, most times, in conjunction with a defrost clock.

Why do we need a defrost clock? Well, if we are trying to maintain, let's say a +35F walk-in temperature, and we set the thermostat to shut the compressor off at (or around) +35F, what happens?

We know that typically there is a 10F TD across our evap coil, right? So, at +35F air temp......the coil temp is roughly +25F - well below freezing. Ok, so we are controlling the temperature with a thermostat that has a +3F differential, and we have a coil temp that is (at it's warmest) 7 degrees away from even BEGINNING to start to melt ice.

.....That t-stat will call for cooling long before the evap has a chance to get rid of its frost and it will ice over rather quickly - UNLESS we install a defrost timer that shuts the compressor off - no matter what - to allow the evaporator coil to rid itself of the frost/ice build up.

So, as you can clearly see - while both of these thermostats have their place in the refrigeration field, and while both work well at their designed purpose........they are not interchangeable.

..........Make sense?