I think I am onto something simple.
Use an air sensing pressure switch like this
SUPCO Pressure Sensing Switch,1-1/2" D, NS2000031
It is adjustable from .1 to 1 WC setting.
It ought to sense the pressure from the pump.
It can control a contactor.
Being plastic, salt water wont corrode the internal parts.
I do not know if water would fill the interior eventually or drain out when off if it did.
I think a washing machine water level uses a similar design.

Sample Line Connector: Push-On Electrical Rating: 1/10 HP @ 120 to 277 VAC, 28 VA Pilot Duty @ 24 VAC, 125 VA Pilot
Mounting Holes O.C. (In.): 3-5/32 Type: HVAC/General Purpose
Item: Pressure Sensing Switch Agency Compliance: UL, CUL, CSA, CE
Depth (In.): 1-1/2 Function: Sense Positive, Negative or Differential Air Pressure
Application: Switching Based On Positive, Negative or Differential Air Pressure Mounting Location: Vertical or Horizontal
Package Quantity: 1 Pressure Setting Range (In. WC): 0.10 to 10.0
Height (In.): 3-1/4 Switch Type: SPDT
MPN:
NS2000031
Features: Normally Closed Terminal, Normally Open Terminal and Common Termi
Operating Temp. Range (F): -40 to 190 Width (In.): 3-1/4

 

If you use that method I think it would be better to look for a difference between the pump input pressure and it's output pressure rather the difference between the pump output pressure and atmospheric pressure.

Les.

 

If you use that method I think it would be better to look for a difference between the pump input pressure and it's output pressure rather the difference between the pump output pressure and atmospheric pressure.

Les.

I got one for $15 so cheap to find out, and its adjustable
I can create a 1/4" hose connection down by the pump.
And run it up 2 foot higher.

I was wondering if a 70 amp 12v DC relay can switch on and off a 13 amp AC compressor load.
I have been using the same kind relay for switching on remotely an 8 amp pool pump with DC on the control coil for couple years now, and I took it apart and it looks like new inside. Those amp numbers are after they are running, not the startup surge.
All these AC circuits are GFCI-AFCI breaker protected.
Be a lot smaller package and easier to setup than a contactor.
If it fails, wont be like a huge problem.

 

I returned that switch, cost me $3.74 for return shipping. I found out they work on suction not pressure, measuring the suction created by a draft inducer motor.
But I got a free washer machine water level pressure switch off eBay as I had some eBay bucks which was listed at $5.
Has 3 built in levels by turning a knob, works on pressure and I read a blog saying they can be adjusted by turning a screw when used as a level switch in other apps. This one has both NO and NC, not all do..
GE PART# WH12X10065

Idea is, water pump comes on pressure goes up clicks on switch clicks on relay turns on compressor. If pump pressure gone, reverse that.

 

Updating, that switch works GREAT. Turns on and off as the water flows off and on. I have it set tot he middle position, so far no need to adjust anything, I drilled a small hole in the 3/4" heater hose near the pump and shoved the polyethylene tube in. It does not leak, and grips it tightly.

BUT the small 70 amp DC 12c relay arcs when the compressor tries to start, too much current flows for the size of the points. Surprisingly, the relay was not destroyed. So the project got more complicated. I will use a Nema 3 outdoor covered enclosure box, (has a swing up lid) and put some kind of contactor relay in the box. The box will be big enough to hold the water pressure switch too.

Somewhere I have these things in the garage. I bought two contactors 60 amp rated 3 phase with 120vac coils a few years ago for another project, but never used them.

Only thing I dont like, is will the contactor coil suck up a lot of power by itself? They seem to slam down with a lot of force when they activate.

 

I found the pieces.
This contactor I can also use screws for the ring terminals.
I plan to make and break the neutral and the hot, so I pull those 2 wires to the contactor, then run a new short cable into the heat pump.

It is kinda bad using a contactor, the pull in coil measures 94 ohms, it runs on 120vac, so it ought to consume 1.2 amps extra now. I will measure this and see.

It is a very robust contactor, but lost the label. I figure at least 60 amps. The contacts are huge inside, anyone ever seen this one, know what model it is?

The other one I have is listed for 85 amps, and its terminal screw points are much smaller.

I took the set screws out, and I was wondering, do they make these out of aluminum? How do they prevent it from galling? I greased them with some marine grease. Aluminum threads on aluminum, if you ever tighten that up, it welds together, must be they do something to the metal to prevent that?

 

If the meter is to be believed, it maxes at 0.04 amps
Does that make sense ?

 

But I got a free washer machine water level pressure switch off eBay as I had some eBay bucks which was listed at $5.

Really hope your going to be lucky using that switch the way your using it. Most of those don't come into contact with water in their use. They measure air pressure instead of water pressure. They usually fail when the diaphragm springs an air leak and they fill with water.

"You can distinguish the water level switch from other round switches inside the washer cabinet by its rubber tube that runs from the switch down the side of the tub. As the washer fills, water enters the tube from the bottom and increases pressure in the airspace within the tube. When the pressure reaches the critical point, the switch shuts off water flow to the tub." From - https://www.thespruce.com/repair-washing-machine-water-level-fill-switch-1824607

 

Really hope your going to be lucky using that switch the way your using it. Most of those don't come into contact with water in their use. They measure air pressure instead of water pressure. They usually fail when the diaphragm springs an air leak and they fill with water.

"You can distinguish the water level switch from other round switches inside the washer cabinet by its rubber tube that runs from the switch down the side of the tub. As the washer fills, water enters the tube from the bottom and increases pressure in the airspace within the tube. When the pressure reaches the critical point, the switch shuts off water flow to the tub." From - https://www.thespruce.com/repair-washing-machine-water-level-fill-switch-1824607

It wont get wet, the tube is about 5 feet long. One end of tube is stuck into the rubber heater hose, other is on the switch. The tube came off my old fridge, it is a lot thicker version of cheap white polyethylene tubing, that fridge tube ran from the water solenoid to the fridge door. The entire working parts are all plastic, some have metal sides, and I avoided looking at those.

The switch has been working great so far when tested. Off and on with water flow. To make it easy you need the analog switch, NO, NC idea, some of these are electronic.

 

It wont get wet, the tube is about 5 feet long. One end of tube is stuck into the rubber heater hose, other is on the switch.

You missed the part about when the diaphragm gets a leak. When the air isn't able to "compress" in the tube, water will flow up the tube and the switch will fail. This is the most common failure mode in a washing mach level switch.

 

You missed the part about when the diaphragm gets a leak. When the air isn't able to "compress" in the tube, water will flow up the tube and the switch will fail. This is the most common failure mode in a washing mach level switch.

I though wonder because I have never had to replace one, and I am talking decades of working on things.
Yes, it certainly would leak water if it.failed. The heat pump circuit is GFCI - AFCI breaker protected, so it would trip off the power. The water would flow into the bilge and the bilge pump pumps it out. But I am thinking the water would cause a ground fault and the pump is going to turn off. I plan to put the switch in the electric box next to the contactor. I suppose I could put an inverted plastic cup.over top the water level switch to keep water away from contactor.

 

Today a hot day, I got it all hooked up and working perfectly.
If water flow quits, the compressor turns off.
I have the water level switch set to high water level.

I have a little 12vdc computer fan for the water pump, I thought it might keep it cooler.
I used a wall-wart to power the fan.
Small red wire is the water pump on wire. It connects to the water level switch, wall-wart and the water pump.
Small blue wire is for the heat pump reversing valve.
Large purple wire is compressor hot
Large white wire is compressor neutral.

The water pump is actually a dishwasher drain pump.

The entire circuit is gfci-afci breaker protected.
For these setups, there is no ground wire back on the compressor, the copper tubing is the ground wire, on the evaporator there is a ground wire, as that is where the on -off and control switches are located. It is electrically grounded through the copper tubing. I used to wonder about that till I realized it.

I am not totally done with this. I want to put a plastic cup over the water switch just in case it leaks. Was very nice that contactor had extra connections for joining wires.
And nice that if you pry the metal edge, you can make the cover removeable on the electrical box.

The main compressor wires, its just a yellow ring terminal secured in with the big set screw.