Protecting compressor if well pump fails

Discussion in 'General Discussions' started by Jimemitchellsr, Oct 10, 2010.

  1. Jimemitchellsr

    Jimemitchellsr New Member

    On an open source (well water or pump & dump) system, What happens if the well pump fails? water thru the heat exchanger. I would assume the high pressure switch would protect the compressor during the cooling cycle, but what about the heating cycle? Unit freeze up, etc
  2. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Most heat pumps have a flow switch to protect the unit in case of this.
  3. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    WF's (and probably others) have a temp switch that dumps the compressor at 30 F loop temperature or 15F loop temp if antifreeze option is selected.

    For an open loop system leave the selction at 30 F.

    In cooling mode a high pressure cutout is typically included; compressors typically have an internal heat / current overload cutout as well
  4. Jimemitchellsr

    Jimemitchellsr New Member

    I would have thought this would be true, but I found out that Fla Heat Pump does not have one. The high pressure switch should protect it in cooling mode, but they are unprotected during the heating mode.
    I will add, they were nice enough to coach me as to the proper way to connect one.(in the compressor contactor line (CC).
    I think you will be doing the public a valuable service to verify which manufacturers include protection (flow switch) for this.
    Also, why wouldn't a pressure switch, say 20psi, do this?
  5. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Measuring pressure alone does not ensure or imply flow unless pressure is measured at two locations and the differential analyzed.

    You could have a system pressurized to 10, 100 or 1000 psi with no flow and in all cases it would freeze in heating mode or trip the compressor in cooling mode

    Proper flow of air and loop fluid is crucial to the success of a geo system
  6. Jimemitchellsr

    Jimemitchellsr New Member

    That may be true, but I have a submersible pump in my well with a pressure switch set at 30-50psi to control it. If a breaker kicks or the pump fails the pressure should remain on the system until the heat pump kicks on, at which time the solonoid water switch opens releasing the water at the given flow rate. The water is being dumped out at 0 psi pressure, so where would the pressure come from that would affect the unit?
    I would suspect the water would continue to fow until the tank is empty and the pressure should continue to drop until it is 0 or very close to 0. If the safety pressure switch were set at, say, 20psi, that should stop the heat pump before it ran completely out of water, shouldn't it?
  7. Looby

    Looby Member Forum Leader

    You need enough pressure in the tank to maintain the desired flow
    rate through the heat pump (typically, 1.5 to 2.0 GPM per ton, for an
    open loop system).

    That includes the (manufacturer-specified) delta-P across the heat
    pump's source-side heat exchanger, plus dynamic head (i.e., piping
    losses), plus any static head between the pressure tank and the
    "0 psi" discharge point, plus the minimum operating delta-P for the
    control valve that "releases water at a given rate."

    As engineer said, flow requires a pressure difference between two
    points in the system. You neglected to mention that one of those
    points (in your system) has a fixed pressure of 0 psig.
  8. Jimemitchellsr

    Jimemitchellsr New Member

    Boy, I'm having a difficult time making my question clear. The origional question was "could a Low pressure switch installed in the water line be used to protect the compressor from a power failure of the well pump"? If a low pressure switch breaks the circuit when it's limit is breached, then why could't it break the circuit to the compressor contactor when the well pump stops pumping and the water valve is open and discharging i.e. the pressure in the water line drops as the water continues to flow out of the system. I ask this as an alternative to the use of a flow switch to perform the same function. (A flow switch cost about $125 and a low pressure switch is about $10.00)
  9. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I see no harm in it - interrupt the 24 Vac to the contactor if pump discharge pressure is below some reasonable value. i would expect existing freeze protection to protect system from a no flow condition, but the additional protection of a pressure switch is OK.
  10. Hydro Pete

    Hydro Pete New Member

    engineer is should have a low pressure switch in the unit. Adding an extra wont hurt. You should have the return pipe in the well below the water level in the well...
  11. teetech

    teetech Member Forum Leader

    Not all GSHP have freeze protection as stated. A flow switch is recommended in most cases...often commercial applications. There are kits available using a temperature control on the water out line. If you want to use a pressure switch i would apply after the coax. For $10 your going to get a preset switch so choose carefully.
    I would suggest wiring into the safety circuit to prevent the compressor from short cycling.
  12. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Not true-
    FHPs have an optional "freeze-stat" which is to be installed on any units used in open loop design per manufacturer installation instructions.
    If you were to have an FHP that was shipped without one, I'm sure it could be added, it is simply a thermistor with a connector that matches the PC board.
  13. Low pressure cut off

    Most geo units shut down on hi or low temp, rather than pressure. That is, they use thermisters to detect water temp. I don't think any use flow switches, to prove flow, or actual water pressure stats, but one could be easily added. Of course, the error code would not show up in the unit itself, so the added element should be tagged and labeled openly so as not to confuse a future service person. All said, I would rely on the unit's built in safeguards, and compressor internal thermodisc, before adding more stuff.
  14. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Just a side note, Hydron and GeoComfort units do have flow sensors
  15. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Interesting - I learn something every day.

    I'm suprised to hear of flow switches on resi equipment - not that they aren't a bad idea, just that they would seem to be vulnerable to failure owing to both having moving parts as well as the possibility of scaling up.

    There are some industrial flow technologies that don't incorporate sails or similar moving parts, but I don't think they'd be cost effective for residential units...perhaps that is no longer true.
  16. Gearhead

    Gearhead New Member

    For my closed-loop lake system, I built a controller that monitors flow rate with a Gems Sensors paddle-wheel sensor. The sensor has pulse output to a small PLC that I use to calculate the flow rate and trigger an e-stop in the event that the flow drops below a set rate.

    In addition to the flow rate, I also monitor inlet and outlet temp of the heat pump. This allows me to provide a winter recovery cycle for the loop in the event that the exit temperature falls too low. By measuring both flow and temp differential, the heat transfer rate to/from the water loop can be determined. And, CoP can be estimated.

    The PLC also serves up a simple web-page so I can easily monitor the system and even make set-point changes on the fly.
  17. dirtmover

    dirtmover New Member

    This will work fine for lack of pressure on the supply side due to pump failure but I think you're being too blinkered looking at only a single failure mode. Other failures that cause lack of flow e.g. blocked discharge, valve failure or associated control wiring, blocked flow control valve etc will not be detected. Flow, pressure delta or discharge temperature (freeze stat) are the only way to cover you for all failure modes and will also protect you against your pump failure.
  18. ralphd

    ralphd New Member

    I have a carrier heat pump with a coaxial heat exchanger I'm using in an open-loop configuration. The factory-installed freeze stat was cutting out at ~40F, so I disconnected it (my well water starts out at ~50F in the fall, but gets down to 45F in the late winter). Without the freeze stat I was able to accidentally freeze the unit up (no water flow, and compressor running for over an hour). Surprisingly there was no damage to the heat exchanger; the unit ran fine after I thawed it out.

    Nevertheless, I agree that it's a good idea to have some way of monitoring flow so you can cut out the compressor.
  19. GregG

    GregG New Member

    I like the idea of a pressure switch

    We put a thermostat on our geo in addition to the geo's internal safeguards. I'm a huge fan of Ranco's digital solid state thermostats for virtually every one of my heating and refrigeration needs but I opted for a standard dial mechanical thermostat in this application so I don't have to worry about power failure to the thermostat or a thermocouple issue.
    I like the idea of a pressure switch for an open loop system. I'm assuming you would place the switch after the solenoid? And I'd assume it might not be a bad idea to plumb in a pressure guage so you can monitor the head loss through the heat exchanger and adjust the pressure switch?
  20. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Holy void warranty bat-man.
    Kids don't try this at home.
    You are lucky you caused no harm when unit froze.
    I would find a temp control right now that can monitor LWT and interupt unit if it gets too low.

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