Having received great advice in the past from all the experts in this forum, I am posting here to see if I can get some suggestions on where I/technician should look to trouble-shoot this problem. To give you some background: I have a 4-ton Climate Master two-zone split system (an air handler in the attic with the heat pump in the basement) with an open loop (with the well about 400 feet deep) which was installed in 2007 and which has run without much trouble in both summer and winter for the past ten years. In November 2017, the pressure tank (a 20 gallon tank) seemed to be short cycling and creating a thud when cutting in. The well pump technician confirmed that the pressure tank had leaked into the bladder and needed to be replaced. It was replaced with a 30 gallon tank and the heating system worked fine until about April when the Taco valve in the system burned out. The control unit for the Taco valve was replaced and things appeared to be back on track. I was traveling in June and got back in July and when I run the cooling system, it works really well (meaning cold air is abundant) but seems to shut down (compressor and the water pump) after which only the blower fan on the air handler is operating (i.e no heat exchange happening!) after operating for a while (I estimate about 3 hours). I have the technician coming tomorrow and while I intend to run it while he is here, I can't be sure it will stop while he is here. So what ideas can the experts here suggest I ask him to check? My questions: a) Could the new pressure tank be the cause of this problem, i.e too much pressure (it came set at 38/58 cut-in/cut-off)? Will changing the input flow (by closing the valve a bit instead of leaving it fully open) make a difference? b) Could this be a faulty Taco valve acting up? (I hear the older motorized valves are prone to fail often in these installs. I also have bought a newer ball-valve which I hear is a good substitute.) c) Could this be the condensation in the drain pan getting clogged and tripping a switch? I read a similar suggestion in another topic here. d) Is my submersible pump a likely problem? It has worked fine all this time, but is this a symptom of that? Are there any other suggestions - better ones than these - that he should look at? Thank you for all your suggestions, in advance.

What model control board do you have? Does it have any LEDs on or blinking indicating a fault code?

Have you verified that your condensate drain line is freely draining and not partially plugged up which is causing condensate to flood the drain pan?

Thanks for the suggestions.
The board they used is a Honeywell Board. I do't see any flashing lights on it.

I will be checking the condensate drain pan since I did notice that there was no water coming from the pipe outside after a few hours of operation.
Last night the system ran for at least 4 hours after I reduced the flow a little by closing the input water valve but then the compressor locked out but the water pump and pressure tank kept working.
This can only happen in cooling mode right?

The tech is coming this morning so I will have him check the board and the drain pan.

A Honeywell board is something I would expect to see on a zone damper control unit or thermostat, not inside the Climatemaster geothermal unit. It is the control board inside the geothermal unit that I was referring to.

No water coming out of the condensate drain line with the unit running in cooling mode is a strong indication the line is plugged up unless you live in a very low humidity arid environment.

Don't reduce water flow to the unit unless you know it is getting sufficient flow. That challenges the compressor safety interlocks and risks compressor damage.

If you have had the system installed for 10 years and never cleaned the slime that typically forms in the condensate drain line or added tablets to prevent the slime formation, then that has a high likelihood of being the cause of your problem. The unit protects itself from flooding the condensate pan by initiating a shutdown of the compressor when a switch senses a high water level. The control board on the unit can be used to diagnose this fault. You can also run the unit until it shuts down and then open the side cover to access the drain pan and see if it is full of water which would tell you the line is plugged. The slime plug typically forms right at the point where the condensate line attaches to the pan. Breaking the plug with something stiff but flexible like a plastic drain snake or nylon zip tie is recommended followed by a cup of distilled white vinegar to kill the organism that forms the slime plug. There are commercial products and tables you can put in your drain pan to prevent this from occurring.

Thanks again arkie6 for your suggestions.

Ok - I thought you were referring to the thermostat control board.

Well, it was not the drain pan which was quite dry and without any scum but I like your suggestion of flushing it with some vinegar and maybe even use some tablets as a longer term issue of making sure it does not become the problem. The condensate was not the problem for the compressor lockout.

It was the Taco valve that was causing the compressor to lock out and the tech thinks it is possible that the Taco valve was probably heating up after operating for a while after which it would cause the compressor to shut down, so while the water pump would continue to run, there was no heat exchange and so no condensate to drain.

So we are planning to replace the older orange Taco valve (with the motorized actuator) with a newer sentry Taco valve (ball valve) since the old Taco valve control unit was replaced only in April 2018. The body of the valve is probably in need of replacement (it is 10 years old and looks somewhat corroded). For now, before the replacement is done, the tech has bypassed the Taco valve and the system is running fine for now.

An ancillary issue happened while the tech was working on the problem. the 5A fuse connected to the thermostat and the 24V line blew when we shut off one of the two zones.. He tried to replicate this by systematically turning on and turning off the two zones in sequence but could not replicate it. Not sure why the fuse blew - is it likely caused by the Taco Valve again? He wasn't sure but I know the Taco valve has led to fuses blowing before.

Most units shut down after running for a long time. I think it is to protect against a stuck thermostat. All you need to do is turn it off and back on again.

Most digital thermostats have a max run time setting. Hope is applies to what you are describing.

My CM Tranquility 27 4 ton shuts down after 4 consecutive hours of operation. This is mentioned in the CXM documentation. I've attached the pertinent page.

Hello, this machine in the upper cabinet has a radiator and a water tray and drain outlet. Usually this drain gets clogged and the security bulb (in contact with water) shuts down the compressor to avoid more water spill on the electrical cabinet below. So you have to blow air into the drain outlet to restore the flow. Also maybe need to wash the bulb contact as it has been in contact with water and may now be dirty