Puzzling Plumbing Problem

[wguss]

A few years back I installed a regulator in the city water system intake to control water pressure. I also installed a pressure gauge to set the regulator and to keep an eye on the pressure to see if the regulator was holding. I set the pressure to the pump shut-off pressure which was about 45 pounds. The regulator has no effect on the pump. The pressure gauge also shows the electric pump pressure and I've noticed that often when driving with the pump on that at the end of the day the pressure often pegs the gauge at 100 pounds. At first I thought my pump had malfunctioned and was not shutting off correctly. I rebuilt it but also decided to get a new pump and keep the old one as a spare. The new pump pressurized the system to about 45 pounds but again, the gauge will show much higher pressures than the cut-off pressure of the pump after driving for a period of time.
The only way to insure that the pressure will not jump is to shut-off the pump and open a faucet. I never hear the pump running or cycling except when we open a faucet. I'm wondering if somehow air is being forced into the system but really can't figure out how.

Thanks for any suggestions.
Bill

[dougrainer]

One thing that MAY cause a problem even if the water heater is OFF. Does the OP have a Motoraid water heater (It would be an Atwood). Motoraid Water heaters when OFF will still have 180 plus engine coolant flowing thru the heat exchanger pipe on the back of the water heater. That may cause a rise in pressure(it will heat the water up to the 180), even with the air gap. To the OP, NOT DRIVING, you don't have the problem? Doug

[Dutch_12078]

I'm with Doug on the gauge being suspect. The water heater should have little effect on the pressure since the normal air bubble should handle it for the most part. In many years of RV'ing, including about 15 with a pressure gauge on the water system, I've never seen the pressure vary by much more than 4-5 PSI while underway with the pump on. I can't speak for other brands, but the common Shurflo pumps used in many RV's do not use magnets for the pump action. It's purely the mechanical function of a motor driven eccentric cam pressing against several small diaphragms in conjunction with check valves that creates the pumping action. There's no possible way for road vibration to cause the pump to raise the pressure in any meaningful way. Bouncing contacts in a weak pressure switch could conceivably cause the pump to operate momentarily, but in the OP's case, the pump was replaced with no change. Again, the suspicion goes back the gauge.

[BruceMc]

My thoughts are aligned with Doug's thread, regarding the hot water heater. That would certainly be a good test - travel with the water heater off, then check the pressure. (Tho one would expect the same issue to happen while parked)

Also, as Doug stated, try a different pressure gauge.

It's a good puzzle!

(Hi naturist - from one to another!)

[wguss]

Lou, yes there is a burst so I'm worried that the pressure is pretty high. Fortunately the pump does not cycle when we're parked so I don't believe there is a leak. If what Naturist said is the case then how many pumps are doing this and not known to the occupant? I wouldn't have had a clue if it wasn't for installing the gauge.

[dougrainer]

1. In 43 years as an RV tech, I have NEVER seen the 45 psi RV water pump go OVER 50 or 60 psi. NEVER
2. To have 2 pumps cause a 100 psi is out of the statistical possibility.
3. I have NVER paid attention to what a Water heater full HOT tank does to the psi. So, is the Water Heater ON when you travel? Possible missing or bad check valve on the water heater, or just the hot water increased pressure backfeeding thru the cold water input. Water Heaters usually do not have a check valve on the input but do on the output.
4. I would also suspect an inaccurate or bad Pressure gauge. Doug

[LouLawrence]

When it's at 100 pounds and you open a faucet, is there a huge burst of water under extreme pressure? Most plumbing systems are not even designed to handle that much pressure so you would also likely have leaking connections somewhere if the pressure is truly that high.

[dodge_guy]

It very well could be the driveline heat, heating up the water which is then increasing pressure.
Just a thought!

[wguss]

Thanks Naturist, that makes sense as I believe the pressure even goes up if the pump is turned off and the faucets are all closed. If that is the case as you suggest I guess a little poppet valve installed to relieve the pressure back into the tank is in order. I'm not sure just how high the pressure gets but it would be a disaster if some tubing broke inside a wall or any of the million places hidden and unreachable in a motor home.

[naturist]

This is just a guess, but if when you hear the pump, it makes a kind of machine-gun sound rather than a whir, it is a diaphragm pump, like most RV pumps. This works by throwing a magnet attached to the center of that diaphragm back and forth. If the pump is oriented just right, the bouncing of the rig going down the road could cause the weight of that magnet to bounce up and down just a little bit, so that over the course of a trip, the pressure is built up a wee bit on every bump. I would expect this to happen whether the pump was on or not, and only be prevented by leaving a faucet cracked. Minimal water would be moved, since it does not take much water added to a pipe to run the pressure up. Just a guess, just a guess.