Need help testing a presure switch?
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  1. #1
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    Need help testing a presure switch?

    Will I'm still new to the hvac world and I was wondering if anyone could help me out. I'm not sure how to go about testing my pressure switches. Does anyone have any idea on how to do this? Do I have to evacuate the system to replace one? Are they a common problem? Any advise is helpful thanks.

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  3. #3
    You can read continuity through the switch if it is ok. If pressure falls for a low pressure, or rises for a high pressure then you would not read continuity through the switch. This also goes with all safety's, which break the circuit to shut off whatever component it is ran to.
    As with voltage: You will read 0 through a switch if it is ok, due to it being the same circuit and a voltage reading <24, 120 etc> since the switch is broke and it is being tied back to a neutral on the opposite side of your switch.
    I have seen wiring jobs that have been a maze but for 99.9 this will get you through on your pressure switch.
    Hope this helps.

  4. #4
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    Let's say you have normal pressures, meaning above low pressure opening value for a low pressure switch, or below the high pressure opening value for a high pressure switch.

    In that case, you would be able to read zero volts across the individual switch contacts (or a very small amount, like .2 volts) when those contacts are closed.

    These switches are not designed to have a long service life. I found a Carrier rooftop unit last week that had a weak 2nd condenser fan, and the unit was near laundromat dryer vents on the same roof. The coils were packed, and the fan was weak, because the superheated air the fan was moving was not cooling the fan motor.

    What this came down to is the first stage of cooling was opening the high pressure switch as it should, and it had done just that, over and over again. This wore out the switch. A good cleaning with splitting the coils, and a temporary jump out until a new motor and switch could be ordered kept the store cool.

    Switches DO go bad.

    An easy easy to confirm switch continuity is to connect guages and look at pressures, so you know if the switches should indeed be closed, and look at where the voltage is being lost, if the contactor is not getting the voltage at its terminals.
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  5. #5
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    Quote Originally Posted by jjmarquez84 View Post
    Will I'm still new to the hvac world and I was wondering if anyone could help me out. I'm not sure how to go about testing my pressure switches. Does anyone have any idea on how to do thits? Do I have to evacuate the system to replace one? Are they a common problem? Any advise is helpful thanks.
    Most of the time there is a valve core under the switch so you can just unscrew without pulling charge. If you are uncertain, you could always put a swivell tee on the port where you are checking pressures and add your new safety there.

  6. #6
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    Do you mean testing switches in a working system? If so, you can easily test the HP switch by elevating the head pressure via blocking the discharge or disconnect the condensor fan. To test low pressure you could disconnect the power to evap blower or a safer way would be to pump the unit down.

    Most residential systems would have to be evacuated to replace one. You could also bypass it and install a shcraeder type without evacuating using those brazed on CD saddles.

  7. #7
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    That’s awesome glade to finally find a place where we Techs can chat about real on the job problems. I appreciate all of your advice and will give it a shot out in the field thanks you ya'll.

  8. #8
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    If the pressure switch is open, It's usually doing it's job. Find out why it's open before you start tearing things apart. Thats what seperates parts changers from people that know what they are doing. At least that's what the boss tells me.

  9. #9
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    Quote Originally Posted by upant View Post
    If the pressure switch is open, It's usually doing it's job. Find out why it's open before you start tearing things apart. Thats what seperates parts changers from people that know what they are doing. At least that's what the boss tells me.
    Don't assume that when a switch is open, that it is open because it is doing it's job. I had one last week that was NOT doing its job, precisely because it had been forced to do its job too many times.
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  10. #10
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    Why don't they engineer the system with a latching relay so that a "power cycle" is required to reset the limit control? The same could be done with compressor internal overloads. A customer would call for service after manually resetting the breaker so many times, vs. having no idea that the unit was cycling on the limit switches.

  11. #11
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    Quote Originally Posted by 54regcab View Post
    Why don't they engineer the system with a latching relay so that a "power cycle" is required to reset the limit control? The same could be done with compressor internal overloads. A customer would call for service after manually resetting the breaker so many times, vs. having no idea that the unit was cycling on the limit switches.
    A lot of older units did have lockout relays. Carrier naturally went nuts with the most complicated POS I ever saw with mechanical timers, amp measuring doodads and everything else under the sun. That round unit with the plastic cover on the door that concealed mission control.

  12. #12
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    Quote Originally Posted by Glennhvac View Post
    A lot of older units did have lockout relays. Carrier naturally went nuts with the most complicated POS I ever saw with mechanical timers, amp measuring doodads and everything else under the sun. That round unit with the plastic cover on the door that concealed mission control.
    With today's technology it would be simple just to have a board to monitor the functions. It could even have error codes for easy troubleshooting.

  13. #13
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    Quote Originally Posted by 54regcab View Post
    With today's technology it would be simple just to have a board to monitor the functions. It could even have error codes for easy troubleshooting.
    already out there

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