Choosing trinary switch pressures

[andrew vanis]

Choosing trinary switch pressures

Since trinary switches are available in different trigger pressures - Some have a high pressure cut our at 490psi, some have the high cut out at 420psi. similar variance for fan trigger PSIs

So - What determines what trigger pressures to use? For both the low/high pressures and the fan pressures.

The question is general and comes from designing a custom 134a system.

Thanks in advance.

[andrew vanis]

bump

nothing on the Google...in fact my question on this forum come up as the 3rd listing when searching "auto air conditioning how to choose switch pressures"

What is baffling is that the switches id'd for 134a come in such a wide variation, even on just the high side cut out psi...

especially baffling considering that 134a systems tend to all operate at similar pressures so the question is ...

why would one system have a 450 cut out and another 290 cut out...

if it doesn't matter, why make the different switches, and if it matters, what causes it to matter?

and from that discussion, what considerations would one take into account if picking the psi on a switch?

[JohnHere]

I'm not an MVAC designer, but I'm guessing that trinary switch settings would depend on the design engineer working on a particular system.

If I recall correctly, trinary switch pressures are typically set to the following limits: Low-pressure cutoff at ~30 PSI; high-pressure cutoff at ~400 PSI; condenser fan engagement at ~250 PSI. You could also wire-in a toggle switch to manually override the trinary switch fan-engagement setting, turning the fan on or off as needed. Don't forget to install a fan relay as well.