Domestic water pump won't prime....

[Elemental]

Sorted.

It is, indeed, the backpressure between the tap and the pump. The pipe runs through the bilge and I guess that's creating a 'sump' that the pump can't push against. I can just disconnect the outlet pipe at the pump, but I think I might add a diverter neat the outlet and allow a runoff into the bilge to be used during tank switchover.
Thanks all.

 

[Elemental]

I can just disconnect the outlet pipe at the pump, but I think I might add a diverter neat the outlet and allow a runoff into the bilge to be used during tank switchover.

Well, I would if I could find such a thing for my hose (an inline tee and a plastic inline valve for the new runoff). Circa 12mm ID hose

 

[PaulRainbow]

Sorted.

It is, indeed, the backpressure between the tap and the pump. The pipe runs through the bilge and I guess that's creating a 'sump' that the pump can't push against. I can just disconnect the outlet pipe at the pump, but I think I might add a diverter neat the outlet and allow a runoff into the bilge to be used during tank switchover.
Thanks all.

I have a Par Max pump, the pump is about 4 metres from the tank, and almost 1 metre above it. The closest tap is the galley tap, about 3 metres from the pump and maybe 0.5 metres above the pump. The pipe from the pump goes down almost 1 metre, into the bilge, runs forward about 3 metres to a tee, from which it continues forward to the heads sink or up to the galley tap. If i run the tanks empty it always primes, but sometimes takes a while unless i open one of the taps.

So, not sure why yours isn't priming, but it should. You mention a tee and fitting, what sort of pipe is used on Sea Spray?

 

[jimi]

Check the inline filter?

 

[Elemental]

The only inline filter is before the pump (tank-side) and is fine.

The pipework is 16/17mm OD (10mm ID) black with blue stripe PVC (I think). Even with both taps open it won't prime. Here are the steps I took (taps open)...

1. I remove the inline filter cover to induce air into the pipework on the tank side of the pump
2. Placing my finger inside the filter I can feel no suction at all on the pipe to the pump.
3. I removed the outlet pipe from the pump and immediately feel a strong vacuum at the filter.
4. Place the cover back on the filter and the pump primes immediately.
5. If I blow into the (now disconnected) outlet pipework I can feel a strong resistance and upon stopping I feel a water column collapsing and expelling the air I've blown in.

I wonder if there's a vent I can place near the pump that permits air to escape, but not water, as it seems that the pump (for whatever reason) is unable to `push` the air away during the priming process.

@PaulRainbow - I wonder if you have such a thing in your pipework that allows yours to prime?

 

[RupertW]

My setup sounds like similar distances, a bit longer for one of the tanks, to Paul Rainbow’s but the pump is level with the top of the tanks and is also a ParMax. I do usually have to open the nearest tap - the galley one - after changing tanks or the pump will run and run, but with the resistance mostly gone by having the tap open it sputters air and water out of the tap and soon runs just water, at which point I can switch it off while the pump builds up the pressure then stops. I do have an accumulator tank but had to bypass it 3 years ago when it rusted through, and have never got round to plumbing the replacement back in again.

 

[PaulRainbow]

@PaulRainbow - I wonder if you have such a thing in your pipework that allows yours to prime?

Not as far as i know and i have recently had most of the plumbing apart to replace the calorifier etc. One thing i hadn't thought of, all of the measurements i quoted are from the tank under the saloon sole, there is an additional tank in the forepeak, which is higher up. This may be high enough to provide some positive pressure at the pump inlet.

Ref your search for a Tee etc, as your hose is 16/17mm OD, cut it and slip some 16mm ID hose over it, you can then use a 16mm tee.

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[vyv_cox]

I wonder if there's a vent I can place near the pump that permits air to escape, but not water, as it seems that the pump (for whatever reason) is unable to `push` the air away during the priming process.

There is, it's called an air vent valve, Automatic Air Bottle Vent 1/2 I have one in my calorifier circuit because the calorifier is about a metre higher than the engine and would suffer airlocks without one. Very easy to use, just open the vent, air comes out until water lifts the internal float and blocks off the needle valve.

However, I doubt that it is going to solve your problem. Diaphragm pumps don't care what is downstream of them, they are positive displacement pumps that will just keep on pumping until they burst. Any one who has ever run out of water, refilled the tank and started the pump again can tell you that the system refills with no problem at all. We do this every spring, having drained down the previous autumn.

If it won't prime there is a problem on the suction side.

 

[rogerthebodger]

I would look for a small air leak into the suction side of the positive displacement water pump.

That in my view is the only think that will prevent a positive displacement pump from priming.

 

[Elemental]

However, I doubt that it is going to solve your problem. Diaphragm pumps don't care what is downstream of them, they are positive displacement pumps that will just keep on pumping until they burst. Any one who has ever run out of water, refilled the tank and started the pump again can tell you that the system refills with no problem at all. We do this every spring, having drained down the previous autumn.

If it won't prime there is a problem on the suction side.

But with the output side disconnected it primes perfectly - doesn't that mean the vacuum is OK? And once primed, it remains primed - even for weeks/months of non-use. I have two independent pipe runs to two tanks and they both exhibit the same issue. Plus, the inline filter is right next to the pump and if I remove (and replace) the cover to induce a dollop of air there the problem immediately returns, even though the pipes to the tanks are still full...
If I could find a way of draining down the pipes top the taps then I suspect mine would prime OK too.

 

[rogerthebodger]

But with the output side disconnected it primes perfectly - doesn't that mean the vacuum is OK? And once primed, it remains primed - even for weeks/months of non-use. I have two independent pipe runs to two tanks and they both exhibit the same issue. Plus, the inline filter is right next to the pump and if I remove (and replace) the cover to induce a dollop of air there the problem immediately returns, even though the pipes to the tanks are still full...
If I could find a way of draining down the pipes top the taps then I suspect mine would prime OK too.

Reading the above the other possibility is then inlet valve on the pump not sealing properly when there's air in the valve allowing air to pulce backwards and forward through the valve but when water is in the valve this does not happen.

 

[Elemental]

Reading the above the other possibility is then inlet valve on the pump not sealing properly when there's air in the valve allowing air to pulce backwards and forward through the valve but when water is in the valve this does not happen.

I have dismantled the pump and replaced the valve seals with new ones. There was nothing obviously wrong with the old one but I fitted the new one anyway - but to no avail ....

 

[Elemental]

I revisited this issue last week. A couple of weeks ago we had the devils own job getting water out of the taps... SWMBO wasn't impressed. In desperation I bought a new Par Max 3 (the existing Par Max 2.9 is no longer available) and fitted it - there was no discernable difference.

After a whole day of detective work, I finally discovered that the manifold (on the suction side of the pump) that combines two tanks to a single supply had been incorrectly assembled and was missing some O-rings where the tank inlet pipes are attached. This isn't something one could easily see as the manifold itself had to be disassembled to discover their absence. This had evidently been the case since I bought Seaspray some 13 years ago. The water supply has always been heavily aerated but seems to have been getting worse lately. So, three new O-rings and a few new fibre washers on the vacuum side of the pump, and all is good.

As everyone said, and I simply didn't believe, the pump is totally capable of priming from totally air-filled pipes without drama. Plus, I'd estimate that the tap water pressure and flow have been boosted by 25-50%. Huge improvement. There's absolutely no pulsing or fluctuation at all in the flow now.

So, all sorted - many thanks to everyone for all the suggestions.