Fresh water system gremlins

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samthetramp

samthetramp

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Ok I've got a 2019 Montana 3791RD and twice now I've had to take it back to have the fresh water system looked at. Sometimes I can fill the tank and it works flawlessly. Twice after filling the pump will come on periodically suggesting a leak. 2 different dealers can't figure it out.

The 1st time it happened I drained the tank before towing it and dealer 1 couldn't recreate the issue. 2nd time I left water in the tank. Dealer 2 saw the issue. Opened a low point valve and air came out. System worked fine afterwards.

1st time it happened was on the very first full of the system. The 2nd time was after 3 thanks of water had been run thru it without being drained.

I'm $370 in on this and no one  can tell me why its happening. System is pressure fill if that helps.

Any ideas?
 
An air pocket in the lines will indeed create a varying pressure situation, which is likely to cause the pump's pressure switch to activate.  Depending on how the water lines are routed, air pockets may be easy to create and hard to get rid of.  If you get foamy water from the faucets, you have air trapped somewhere in the lines. Running a faucet for a long time may get rid of it, but maybe you have to run other faucets (including toilet) or even the low-point drains to help flush it out. Once you get it out, though, it should not return as long as you do not drain the lines again.  Leave water in the lines once they are clear and you should only have the bubble problem when you winterize or de-winterize.
 
Always run faucets to expel air, but "air pockets" simply will act as expansion volume in addition to that at the top of the water heater.  My guess is that the check valve in the water pump leaks back sometimes due to debris.  A small leak in the supply line to the pump will allow air to be drawn in when the pump runs.  In any case, a new pump and supply hose would have cost less than you already paid, but you may have only needed a clamp tightened or the pump cleaned.
 
Rena T. Both dealers told me that since they couldn?t recreate the problem warranty wouldn?t cover the labor.

I ran the water until it flowed steady without spitting air. The ark has been refilled when it gets low so I didn?t think I had to purge it every time. The campground I?m at shut down the city water supply for the winter but fills up water tanks 3 times a week. It hasn?t been empty since Nov 8.
 
As was mentioned when purging air from the lines don't forget the low point drains or the outside shower head.  Everyone forgets about the outside shower.
 
How can a low point drain get air in it? If it's truly the low ponit that should be the first thing to fill up.
 
samthetramp said:
How can a low point drain get air in it? If it's truly the low ponit that should be the first thing to fill up.
Click to expand...

I believe what they meant was to run water out of the low point drains with the pump on just to verify there is no air trapped in the lines.
 
lynnmor said:
My guess is that the check valve in the water pump leaks back sometimes due to debris.  A small leak in the supply line to the pump will allow air to be drawn in when the pump runs.  In any case, a new pump and supply hose would have cost less than you already paid, but you may have only needed a clamp tightened or the pump cleaned.
Click to expand...

This was my first thought too.  A leak sucking in air anywhere between the tank and the pump.  If you have a screen filter on the pump check it for an air bubble.
 
In retrospect the 1st time I had issues was the first time I filled the tank. The 2nd time was after I ran the tank low enough that the pump was having trouble picking up a constant flow. Guessing that I didn't get the trapped air completely out.

Thanks for the input guys.
 
Your water pump uses small rubber poppet valves that run on an eccentric.  All it takes is for one of those to get a small piece of crud stuck on one of the valve seats and it will allow the pressure in your system to bleed back towards the fresh tank which will be sensed by the pressure switch on the pump causing it to run and pressurize the system again.  Usually this can be cured by running enough water through the pump to flush out the offending grain of sand or whatever it is.  Worst case is you pull the pump apart and manually rinse the valves which I've had to do exactly once before I learned to filter all water going into that fresh tank.

Personally I use a 5 micron filter on the rig an fill using a low point drain that I have a hose bib installed on but I'm probably over doing it a bit.
 
Water expansion/contraction in the hot water tank can also make the pump "burp" occasionally.  You heat the water in the tank, which makes it expand.  If it expands enough the excess pressure will weep out the blowout valve on the outside of the tank.  Or you use some hot water which also relieves the excess pressure in the tank. 

Then you turn the water heater off.  As the water in the tank cools, it contracts and admits a little bit of water from the supply line to make up the difference, reducing the line pressure enough to make the pump come on momentarily.

If this is the cause of your pump burping, an accumulator tank on the cold water side will eliminate the pump cycling, as it provides enough expansion space to ride out the water heater expansion and contraction cycles.
 
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