Mitsubishi WP Type Pump Issue

[Mattd]

I have a Mitsubishi WP355Q5 water pump that services the house, this is connected to a 2,000L above ground tank, the pump is about 3 years old.

 

Lately it has been short cycling, which I put down to the air in the pressure tank getting low, to resolve this I was going to empty the tank of water to recharge the air, the pump was working OK and would shut off, the short cycling was only when a small tap was in use, when more that one tap was open it was fine.

Last Sunday whilst washing my car the pump stopped pumping water altogether, the pump itself was not running, so I unplugged it, waited a few minutes, plugged it back in and it worked OK again.

I then had to go to Bangkok for work, on Wednesday I had a call from one of my sons to tell me that there was no water in the house, I asked him to check the tank had water, which it had, he then said the pump was constantly running and was hot.

He unplugged the pump, waited till the following morning and plugged it back in, since then it has worked.

 

I'm headed back to there this afternoon and do not really want to buy a new pump unless I have to, could this intermittent pumping of water be related to the short cycling and probable lack of air in the pressure tank?

 

What can cause this loss of air in the pressure tank, I know how to recharge it, but interested to know what could be causing the issue in the first place, is there anything on the pump that regulates this air that if is not working well can cause this?

[Pib]

 

Quote

Lately it has been short cycling, which I put down to the air in the pressure tank getting low, to resolve this I was going to empty the tank of water to recharge the air, the pump was working OK and would shut off, the short cycling was only when a small tap was in use, when more that one tap was open it was fine.

The pump should not short cycle with only a small use of water.   And by "short cycle" I mean when using a small amount of water the pump is "on" for a significantly longer time than it is off.  Say you have one tap turned ''half-way" on, the pump should have "approximately" a 50-50 on-off cycle....like approx 5 seconds on, 5 seconds off....or 8 seconds on, 8 seconds off....or 5 second on, 10 seconds off.    If  you don't have approximately a 50-50 cycle rate with a small use of water then the pump pressure tank is waterlogged (not enough air in the tank).   

 

And just how did you drain the pressure tank?   Unless you drained the tank until practically no more water was coming out the drain hole, the water didn't drain low enough in the tank to allow air to enter the water drain hole.    I say this because just a few months ago I had a neighbor who had a waterlogged Mitsubishi WP255 due to a defective air balance control valve (that six side valve screwed in the top of the tank with the small black/grey PVC hose coming out the top).   He thought removing the drain plug and letting the water drain for around 10 seconds was good enough--wrong!  He didn't drain it long enough for enough of the water to come out to where air could enter the water drain hole.  Once he did drain it properly...turned it back on...his short cycling was gone...but he still had to replace the air balance control valve to keep the short cycling from coming back in a week or two.

 

Once draining the tank and allowing air to flow into the water drain hole, screwing back on the drain plug, and then restarting the pump it should run approx 20 seconds in refilling its pressure tank with the right mixture of water and air before it turns off.    The water-air mixture is now correct and the pump should not short cycle with small uses of water...remember, short cycling means the pump is on a lot longer than it is off with a continuous draw of water.

 

Even if you have a defective air balance control valve the proper draining of the tank to get a proper recharge will allow the pump to work properly for a week or so...no short cycling.  But as the air in the tank is naturally absorbed into the water, the air must be replenished and that's what the air balance control valve does.

 

And if your pump is short cycling a lot it will overheat and a built-in thermal relay protection device will turn off the pump. It will take approx 15 to 30 minutes for the pump motor to cool enough for the thermal relay to reset itself and allow the pump to run again...until it overheats again.

 

 

 

[lopburi3]

Any local pump shop with have someone they can send to check your pump at a very low cost - and the parts to fix if required.  Would recommend doing as they can be quite nasty (especially if not wired correctly) - shock/death hazard if you do not know what you are doing - very easy to have live wire still connected with switch in off position as live and neutral lines are often incorrectly connected.