Water pump when on battery

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Gregg Lawson

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Sep 9, 2020
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Hello, the water pump in my 5th wheel works well, it is a new to me unit the pump is quiet, give good pressure and when i am plugged into hydro works well. However, when i am on battery (i have 2 6 volt battery's) the water pump still works but when all the taps are closed it goes into a low setting and just keeps running. Is it possible that on battery i don't have enough voltage to fully bring the system up to pressure, is there a way to fix that?
 
Doubtful it has anything to do with power. Most likely a valve is leaking just enough to keep the pump from pressurizing, or the pump head itself may have a worn out diaphragm inside. Get the model number off of the pump and look on line for a parts diagram. It should show where the pressure diaphragm is.
 
It's possible..  Low voltage or inability to deliver enough steady current (amps) would prevent the pump from building enough pressure.  A bad cell in one of the batteries could drag the voltage down to about 10 and that is probably enough to do it.  A quick check with a voltmeter would tell you how much voltage is present.

Are you sure the same thing isn't happening on you have shore power available? That's a key data point here.
 
I am going to share my $0.02, Theory or Opinion with the Forum. 

Gary may come in with a mathematical formula and 4 real world stories and blow this Theory of mine right out of the water into the Fake News Category of reply, Who Cares, Here goes?. 

I have a 32 ft RV, 2 sinks, toilet, Shower, Outside Faucet.

Being a new RVr and from a temperate climate I had blown the air out of the lines carrying water to the faucets, toilet a couple times due to a slight Quick overnight freeze right at 32F for a brief period of time, didn?t want the freeze to expand those plastic elbows and Coombs clamps on those plastic water tubes cracking something with the expansion of freezing water and plastic joints that will pop. 

Now the next follow on trips where I air voided the lines (Only) I was able to re-pick Prime on the pump just by refilling to 100% capacity the fresh tank.  Having a full Fresh tank and the Hydro-static Pressure Weight from the Height of the full tank plus the 8.3 LB per gal weight of the water to press / void the air out of the pump and lines from being ?just slightly? empty there was enough in the system to reprime easily with this tiny pump.

Doing this slight purge of the system pre freeze the Pump would immediately pick up next use and shut off not continually running as I always leave with a full tank of water just for good luck. 

After a few Blissful Adventures, a DEEP SUSTAINED RARE FREEZE was forecast by Mr. Weatherman.
 
I had become a little smarter and I learned I need to pull the the nylon plug on the Atwood h20 heater in addition to void out the system as much as possible with air also to avoid ice expansion within the day tank on demand, but this dropped at least another 4 gallons of water totally voiding the vast majority of water for sure and blowing 50 LB PSI pretty empty the lines of the system. With minimal water retention, if any remaining.

I did note that location of the H20 Heater plug seemed to be rather high mount location when pulling the nylon plug, "Up Hill" so to speak......due to gravity and the path water would naturally take if left to flow at will from source tank.  Tank seems higher that the height of the fresh water tanks ability to push and totally press out the air to full prime the entire system or pretty even at best.

Remedy I feel Works: The two times I pulled the Atwood Water Heater Nylon Plug, Not sure what size 4 or 6 gal without researching but with that additional void of water creating a Air pocket to purge in the system possibly a slight air leak on the diaphragm on the suction side of pump to air out and Prime the motor would NOT cease running trying to achieve prime.  Pumps don't like air in the system.

The thing that worked for me twice to stop the Constant Run of the Pump was to fill the tank, attempt to air out the system as before. Once seeing the motor was continual run mode.

Reconnected the city fill City Fill and Turn on RV Elec water pump while the water valve is seeking the city source, I feel flooding / pressuring the system, with the PSI from the city pushing / assisting the Running Pump airing out the system by playing with the valves letting the areas clear the air out the lines at each disto point.

It worked to keep the prime and motor operating and shutting off as it should twice when I was eXperiencing this.  Luck.  If so I am batting a 1000% 2/2.
 
The motor is only running to supply water, regain the prime and shut off. But with the partially primed system and lots of air to press out for this Tiny, Plastic Pump I don?t see it, self correct when dropping all H20 from System.  Too much volume too small pump. 

I would Try forcing City source H20 In Concert with a Running H20 House Pump Elec Motor Engaged.  Go to all sources Airing the valves Outside (H/C), Shower (H/C), etc?..get them all toilet like with City tag teaming the pumps action. 

Then once the flow seems not to sputter........take off the city pressure augmenting the pump and see if it will quiet down. I think the system needs to purge the air. 

Also adhering to the KISS principle of life: Did it correctly work before? What happened since then?  What did I possibly do to screw this up? 

John

 
There's not much to priming these pumps and the shurflo in mine says on the data sheet it can dry-prime up to 6ft.  Odds are the pumps in most RVs are at or below the water tank level so as long as you run a faucet long enough to get the air out of whatever tubing is between the tank and the pump, no amount of air on the output side is going to make a difference in how the pump shuts off.  It will reach the setpoint pressure and stop, whether it's pushing up against air or water.

So in my view the key test would be to let the water flow until one is confident there's no air anymore on the inlet side.  If it's still running once the flow has stopped it's implied that there's either a slow water leak on the output side, or a compromised diaphragm.  The pressure switches in these are usually binary with a bit of hysteresis so even if the pump was running slowly from low voltage it would still eventually reach pressure and shut off unless the voltage is so low the motor stalls and would then likely blow a fuse.  Monitoring the pump voltage at the pump and not the batteries may reveal a voltage drop or other anomaly which may provide additional clues.

Mark B.
Albuquerque, NM
 
Gregg Lawson said:
definitely only happening on battery, I will check the voltage tonight. Thanks

When checking the battery, make sure you?re not plugged into shore power. If you are plugged in, you will be checking the voltage from the converter.
 
To carry on with Roughneck's theory.

1 - When on battery and you do open any faucets do any of them spit or do you always have positive flow? Spitting would indicate there is in fact air in the lines.

2 - If the system works fine on shore power and symptoms only occur on battery power, then that is a major clue. Volt meter on shore vs. battery is definitely something to look at.

3 - How old are the 2 house batteries and what is the total amp/hrs available?

Finally there could be a real weep/leak somewhere in the water system. My pump was cycling about every 30 minutes for like 3 seconds. For a long time I tried to convince myself it was "normal" although a few folks here told me I had a leak. Turns out I did at the 5/8" inlet. Not enough to be real noticeable. I installed a garden tap "shut off" adapter on the 5/8" inlet and it stopped. Some day I guess I will have to change the RV inlet fitting...
 
Ex-Calif said:
To carry on with Roughneck's theory.

1 - When on battery and you do open any faucets do any of them spit or do you always have positive flow? Spitting would indicate there is in fact air in the lines.

2 - If the system works fine on shore power and symptoms only occur on battery power, then that is a major clue. Volt meter on shore vs. battery is definitely something to look at.

3 - How old are the 2 house batteries and what is the total amp/hrs available?

Finally there could be a real weep/leak somewhere in the water system. My pump was cycling about every 30 minutes for like 3 seconds. For a long time I tried to convince myself it was "normal" although a few folks here told me I had a leak. Turns out I did at the 5/8" inlet. Not enough to be real noticeable. I installed a garden tap "shut off" adapter on the 5/8" inlet and it stopped. Some day I guess I will have to change the RV inlet fitting...

Cali,

A quick come back reply.  I ask

On the Suction side of the pump? Did you see a dribble or proof of water damage in the past?  My guess is  your pump was sucking air or does this diaphragm type pump differ from a centrifugal type?  I still think the constant run is from a system that is not aired out fully.  You kind of prove my theory...I say.

JD
 
X-Roughneck said:
Cali,

A quick come back reply.  I ask

On the Suction side of the pump? Did you see a dribble or proof of water damage in the past?  My guess is  your pump was sucking air or does this diaphragm type pump differ from a centrifugal type?  I still think the constant run is from a system that is not aired out fully.  You kind of prove my theory...I say.

JD

The "theory" I am working with is that air is compressible and it is probably  more likely to seep past tap valves than water. So that might explain a cycling. It doesn't really explain a continuous run at low pressure.

For that one I am in agreement with Gary that most probable cause is low voltage. A higher voltage (as when on converter power) is more likely to pump the system to a higher pressure and it will shut off.

Hence my questions to explore the voltages, battery condition etc.
 

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