Drilling a hole in an immersion tank.

[snowhead]

Is the ball valve on the inlet to the pump open?
And have you tried the Wilo instruction if airlocked of On/Off for 30secs 5 times?

 

[Stevex]

Is the ball valve on the inlet to the pump open?
And have you tried the Wilo instruction if airlocked of On/Off for 30secs 5 times?

Yes to both; weirdly enough I turned the pump off earlier and went back to it a few hours later and turned it on. The lower pipe started to get as hot as the top pipe, but within 5 minutes back to square one with same symptoms...weird.

Hopefully this video uploads, this is what it sounds like.

 

[ShaunCorbs]

Would try removing the nrv if you can

 

[Stevex]

Would try removing the nrv if you can

The NRV is built into the pump and that component is torqued on fit at the factory.
I did wonder if the NRV might not be opening, but I don't want to disassemble the pump until I've had a reply from Wilo in case it invalidates the warranty.

 

[snowhead]

Close both lever valves on the new pump circuit.
Crack the lower compression nut on the pump.
Open the top lever valve slightly and see if water is getting through the pump and NRV.
If it does, close the top lever and open the bottom lever slightly to see if the lower section of pipe is airlocked.

 

[Stevex]

Close both lever valves on the new pump circuit.
Crack the lower compression nut on the pump.
Open the top lever valve slightly and see if water is getting through the pump and NRV.
If it does, close the top lever and open the bottom lever slightly to see if the lower section of pipe is airlocked.

I took the pump out and blew through it to confirm the NRV works, which it does, it opens with very little pressure required. It also blocks with opposite flow. I removed the pump head and ran the motor to check the impeller was working and for any obstructions, all normal.
Refitted the pump and now when it runs it doesn't pulse and has that normal pump running sound.
Just waiting for the solar to heat the top half of the tank and then will see what the pump does.

Edit: After letting the upper tank temp rise with solar power to about 35°C with the lower temp sensor indicating 24°C, I turned the pump on. After about half an hour re checked and the 2 temp sensors are now within 1°C of each other.
So finally the pump seems to be working, although I've no idea what the original fault was.