Domestic water pump won't prime....

[Elemental]

Hi. I have a domestic water pump (about 8 years old) that is connected to two tanks - fore and aft. When a tank runs out (and the pump sucks air) it can be the devil's own job to get the pump to start drawing water. I can usually get it to work by sucking like crazy on the galley tap, but it's hardly ideal and yesterday I failed completely. Eventually, I got it working by disconnecting the pipe from the input side of the pump and sucking water through the pipes from the new tank and quickly plugging it back into the pump,

Subjectively, the issue seems worse when engaging the aft tank. And when running on that tank there's some spitting and aeration that results in the pump running for a second or two after the tap is closed. I suspect therefore that air get's in or there's some kind of blockage in the pipework to the aft tank. But I can't find a leak anywhere. One thing I wondered about is whether there's some kind of inline manual pump I can insert into the pump feeder pipe to manually create a stronger vacuum than the electric pump itself is capable of as a form of priming after a tank change?

 

[pvb]

Most pressurised fresh water pumps will happily self-prime up to a metre or more, so you may need to overhaul or replace your pump. Many pumps have service kits available which include the valves which enable self-priming. Fix the pump first - adding a primer bulb is just papering over the cracks.

 

[Leighb]

Fresh water pumps should self prime as they are positive displacement types. If they don’t there is usually an internal leak.
Our fresh water pump did this a couple of years ago, only way to get it to prime was to pressurise the tank using the dinghy pump on the tank overflow, I replaced the pump and no further problems. It is possible that a complete pump overhaul might have sorted it, but it was about 2/3 the cost of a brand new pump!

 

[Momac]

Check the water tank breathers are not clogged.
Check the filter on the pump inlet is not clogged
Check/replace the O rings on the pump inlet and outlet

 

[rogerthebodger]

There could also be some muck jammed in the inlet valve keeping it open preventing the pump priming.

 

[MoodySabre]

This happens to mine. Thumping the pump very hard for a while usually fixes it.

 

[Graham376]

Ours can be a pig to prime as well but rarely run out of water as tanks have a balance pipe so pump draws from both.

 

[Silvercloud]

Had a similar problem with mine(new pump) and solved it by backflushing the pipes from the tank to the pump with mains pressure fresh water.

 

[Elemental]

Had a similar problem with mine(new pump) and solved it by backflushing the pipes from the tank to the pump with mains pressure fresh water.

That's a thought. I had wondered about trying something like that.
I've ordered a valve set from Jabsco as well so will have a crack at the weekend.

 

[graham]

Could be an airlock forming if the pipe goes up and down along its length?

 

[Elemental]

Could be an airlock forming if the pipe goes up and down along its length?

That was my thought, but the pump is supposed to self prime to a height of 1.8m that should wipe out any airlock surely?

 

[Graham376]

That was my thought, but the pump is supposed to self prime to a height of 1.8m that should wipe out any airlock surely?

Should yes but doesn't always happen, most likely in my case due to restriction in tank breathers. Occasionally, when I open the deck filler, I can hear air being sucked in. Have you tried running the pump with filler cap off? Try a bit of pressure into the filler with dinghy pump, should push it through.

 

[vyv_cox]

You will not be able to backflush through the pump. The valves open in the forward direction and close in the backward. Unless that's the cause of the problem of course, in which case you might get some water through. If you do, the problem is definitely the pump.

 

[pvb]

Had a similar problem with mine(new pump) and solved it by backflushing the pipes from the tank to the pump with mains pressure fresh water.

I can't imagine what that would achieve, unless you had crud in the tank which was blocking the outlet.

 

[Elemental]

Update:

I replaced the pump valves. Sadly the pump is no better I guess I'm going to have to exhaustively check all the pipes and seals. What's puzzling me is that even when the pump is running I have to suck _very_ hard on the outlet tap for quite a long time before the water reaches the pump. Maybe the pump is knackered...

 

[johnalison]

Eight years isn't that old. I replaced ours after about sixteen years regular use but it was mainly the sensor that had gone.

 

[RichardS]

Update:

I replaced the pump valves. Sadly the pump is no better I guess I'm going to have to exhaustively check all the pipes and seals. What's puzzling me is that even when the pump is running I have to suck _very_ hard on the outlet tap for quite a long time before the water reaches the pump. Maybe the pump is knackered...

Disconnect the pump and try sucking to see of the blockage is somewhere other than the pump?

Richard

 

[greeny]

Disconnect the pump and try sucking to see of the blockage is somewhere other than the pump?

Richard

+1
Are you sure there isn't a filter/strainer tucked away somewhere in the feed that may be partially blocked?

 

[Sinbad1951]

The problem and the solution is simple.
Once one of your tanks is empty the pump is pulling air through the empty tanks breather pipe. If you fit a simple on/off valve in your breather pipes (I used plastic twist washing machine valves) and close this valve when one of your tanks has emptied the pump will again suck water instead of air.

Beer coupons accepted.

 

[coopec]

I must be getting careless? Have you stated what sort of pump it is? Some types of pumps won't self prime e.g. a bilge pump. (centrifugal pumps won't self prime)

A positive displacement pump (Jabsco, diaphragm, piston ) will normally self prime.

Now where am I going wrong!!