12V pressure water pump problems

[basewindow]

Hi all.

Having issues with pumping water from my tank to my shack with a 12v 35psi water pump. I've only recently put one them in (previously gravity fed) so I can run a hot water unit.
Problem is I seem to have gone through 2 in 3 months. The first one died and I assumed it was just a faulty unit and threw it away and bought another (Ebay item, Shurflow knockoff i think). However the next one did the same within a few weeks,so I started to think it was my set up. Checked all the electrics etc for voltage drop and that wasnt the problem. Checked inlet and strainer for blockages, all ok. Increased inlet pipe to 3/4 inch for the 20 foot length between the tankand pump. Eventually decided to pull the pump apart and see what was going on. Removed pressure switch etc and eventually tracked down a faulty micro switch (The motor still worked fine). I pulled it apart and noticed that the constant cycling of the pump on and off had worn away part of a plastic stop keeping the switch permanantly open. Now my questions.
(1) Have I just got two faulty pumps with faulty micro switches? (They are Ebay Chineese copies but I would still expect more than a month each).
(2) Do the more costly Shurflo or other Brand pumps use the same system ie: pressure swith with spring and micro switch?
(3) I noted that both thses pumps constantly drew their startup Amps of 8amps instead of dropping back to their rated power draw of 3.9amps when I used my mixer taps(They have a reduced flow), but dropped back to the normal power draw when I opened a normal tap(Higher flow). I assume then that using the mixer taps causes the pump to constantly cycle, reaching off pressure temporarily, then opening again when the pressure drops. Whilst using the normal tap the pump just contunually runs. Is this causing the wear on the microswitch?
(4) The hot water unit has a capacity of 8L (Sorry not sure what that is in gallons) per minute and the pump 12.5L per minute. Could this reduced flow through the Hot water unit be causing the same issue, ie: The constant cycling of the pump on and off causing wear?
(5) Should I buy a pump with the same capacity Ie: 8L per minute to match the Hot water unit or even lower?
(6) Can I use an impeller pump which will operate only when I open a tap?

Any hints would be useful. I am happy to buy a Shurflow pump but don't want to destroy it to with the same problem.
Cheers,
Cam.

[vtmaps]

Do you have a pressure (accumulator) tank? If not, that is probably your problem. If so, then buy an external pressure switch to replace the chinese junk.
--vtMaps

[basewindow]

No accumulator tank. Pump instructions recommend not to have a tank. How would an accumulator tank fix the problem? How does an external pressure switch work?

[vtmaps]

I pulled it apart and noticed that the constant cycling of the pump on and off had worn away part of a plastic stop keeping the switch permanantly open.

There should not be constant cycling. Your lucky that the switch was damaged before the motor itself. The solution is to use a pressure (accumulator) tank.

Pump instructions recommend not to have a tank.

That makes no sense. Shurflo does make a very sophisticated pump (the extreme series) that does not use a pressure tank, but I doubt that your chinese knockoff is that type.

How does an external pressure switch work?

It works the same as an internal switch, but it is external to the pump. Sometimes it is mounted on the pressure tank. It senses the water pressure and turns the pump on when the pressure gets to a certain low pressure limit and turns the pump off when the pressure gets to a certain high pressure limit. The pressure tank allows the pump to turn on for awhile, until the high limit is reached, and then relax for awhile while the pressure tank decompresses. This means that the pump is not constantly cycling on and off. The larger the tank, the less frequent the cycling.
--vtMaps

[basewindow]

Instructions definitely say no tank.

Im only assuming its constantly cycling. If the tap is only say half open, does the pump momentarily reach pressure and turn off. Then when it drops turn on again? Would this constantly cycle the switch?

Might buy another microswitch and see how long that lasts.

[Blackcherry04]

Two things cause them to cycle. 1) A leak on the pressure side. 2) A bad check Valve on the suction side. Most of the 12V ones have a simple ball or use one of the diaphragms as the check valve. You can fix the leak ( dripping faucet, etc ) and / or put in a real check valve behind the pump.

[mike95490]

AC pump switches will not last very long if they are used to switch DC

[icarus]

AC pump switches will not last very long if they are used to switch DC

Actually, Square D AC pressure switches work fine for small pump applications,, as long as you don't try to switch too much current. I have been using a Square D for 5 years straight without problem. (come to think of it,Sq D might be rated AC/DC, i'll have to look!)

Tony

PS get a tank!, reduce cycling!

[westbranch]

since this is a bush shack , you can make yourself a 'water hammer' out of a vertically mounted 1 meter piece of 3 inch Schedule 40 or 80 PVC pipe. Install a tire valve near the top so you can add ~ 30 psi air pressure once there is water in the system. Any excess air will go down the water line once you use enough water to cycle the pump.cheap, simple.

HTH

[Blackcherry04]

The 5+ GPM computer controlled soft start pumps with pressure transducers are now around $150.00. It's almost not worth buying one of the old switch type pumps any more. My luck with the older ones has not been that won't start, it's the hang up running and at about 50 PSI they blow apart.