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Old 10-14-2018, 03:21 PM   #1
Scott in Michigan
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Water system design

Once again, a new owner question. My wife and I have a Cougar 32 RLI.

I'ld like to talk tech for a moment about the onboard fresh water tank and pump. I gather that a couple of the gentlemen on this board have some fair degree of technical expertise.

We can turn on a faucet and run it for a period of time before the pump kicks on. The water continued longer than what I would have expected from residual pressure in the piping. Is PEX tubing elastic enough that it acts like a pressure bladder in the system?

Also, is there a check valve in the discharge of the pump to isolate it when on city water?

Thanks, from Scott & Gin
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Old 10-14-2018, 03:30 PM   #2
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Fresh water system 101: you fill the fresh tank using either a gravity fill port or a pressurized port.

When you turn on the water pump, it pressurized the water system to a preset pressure. Then when you open a faucet, the pump will start at a predetermined pressure and continue to run until the “set” pressure is reached. That’s the reason you hear a “lag” when you start the faucet and it runs a little bit after you shut it off.

There is a check valve inside the pump that stops “city water” from flowing backwards through the pump to the fresh tank.
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Old 10-14-2018, 03:40 PM   #3
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And there you go!
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Old 10-14-2018, 04:03 PM   #4
Scott in Michigan
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Thanks. Yes, I follow the operation of the system.

My question was, I'm surprised there is that much residual pressure in the trailer's piping system. We ran the kitchen faucet and simultaneously flushed the toilet and it was several seconds after that the pump kicked on. The pump coming on wasn't a surprise at all. The delay in it starting, was

I'm wondering if there's some headspace in the water heater that acts as a pressure reservoir, once the system is up to pressure.

The delay in the pump coming on tells me there is a reservoir of some type. The city water system I run in my day job uses a 125 ft water tower to develop static head. Trying to figure out where and how the head originates in my trailer. Mainly curious.

All in all, it's a pretty ingeniously designed system.

Scott
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Old 10-14-2018, 04:10 PM   #5
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There is an air gap in the water heater, but it’s designed to allow for expansion as the water heats up. Some people have installed “accumulators”, I have really never seen the need as our mantra is “if it doesn’t have full hook ups, we don’t go there” LOL
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Old 10-14-2018, 04:24 PM   #6
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I just always assumed after pump is off the lines just hold some pressure for a little bit of time
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Old 10-14-2018, 04:24 PM   #7
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Ha! Full hookups - well said, sir!
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Old 10-14-2018, 04:28 PM   #8
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All in all, this is mainly an attempt to understand the things I come in contact with. I think it was Richard Feynman who who said he lived his life that way. The engineer in me makes me want to do that also, I guess.

Chuckster, you really made me laugh with your full hookups comment. The little platinum blonde I live with says we aren't camping we are glamping. I had to look that one up.

Have a good evening gentlemen.
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Old 10-14-2018, 04:28 PM   #9
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Quote:
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I just always assumed after pump is off the lines just hold some pressure for a little bit of time
As long as the pump diaphragm is intact, the pex lines should hold pressure. One of the first steps of the PDI is to fill the fresh tank, purge the system of air and leave th pump on for about 20 minutes. If it DOESNT cycle then it’s all good. If it does then I have a mental checklist to follow.
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Old 10-14-2018, 07:27 PM   #10
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Quote:
Originally Posted by chuckster57 View Post
There is an air gap in the water heater, but it’s designed to allow for expansion as the water heats up. Some people have installed “accumulators”, I have really never seen the need as our mantra is “if it doesn’t have full hook ups, we don’t go there” LOL
My wife says she has 3 minimum requirements, city water, 50 amps & sewer hook up, "we're living in this thing, not camping" she says.
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Old 10-14-2018, 07:32 PM   #11
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Quote:
Originally Posted by chuckster57 View Post
There is an air gap in the water heater, but it’s designed to allow for expansion as the water heats up. Some people have installed “accumulators”, I have really never seen the need as our mantra is “if it doesn’t have full hook ups, we don’t go there” LOL
My wife says she has 3 minimum requirements, city water, 50 amps & sewer hook up, "we're living in this thing, not camping" she says.
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Old 10-14-2018, 07:56 PM   #12
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Good one, if momma ain't happy nobody's happy
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Old 10-14-2018, 08:06 PM   #13
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Scott:
Something to consider is using a water pressure regulator between the city water and the trailer input. The best ones are the adjustable ones with a gauge. There are two main manufacturers that I know of, Valterra and Camco. The Camco one is cheaper and available from Walmart online.


To answer one of your questions, there is a non return valve inside the water pump that stops the city water flowing backwards into the FW tank. This valve can get jammed by excessive city water pressure. That will show up as the pump not pumping water from the FW tank when dry camping. The fix is simple but involves some disassembly of the water pump.


I personally keep my water pressure regulator set for about 35 PSI so that it doesn't jam the pump's non return valve. Others on the forums say they keep theirs at 50 PSI. I had the NRV jam on my water pump even with a regulator in the line but with the regulator set for 45 PSI.
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Old 10-15-2018, 07:31 AM   #14
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Quote:
Originally Posted by travelin texans View Post
My wife says she has 3 minimum requirements, city water, 50 amps & sewer hook up, "we're living in this thing, not camping" she says.
Other than cable, I always thought those 3 things were "full hook-ups".
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Old 10-15-2018, 07:51 AM   #15
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Quote:
Originally Posted by chuckster57 View Post
There is an air gap in the water heater, but it’s designed to allow for expansion as the water heats up. Some people have installed “accumulators”, I have really never seen the need as our mantra is “if it doesn’t have full hook ups, we don’t go there” LOL
Next thing ya know she'll expect chocolates on the pillow after the "turn-down" service
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Old 10-15-2018, 08:12 AM   #16
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Good one, if momma ain't happy nobody's happy
Words to live by.
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Old 10-15-2018, 02:48 PM   #17
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Old 10-21-2018, 06:08 PM   #18
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Scott, I think the answer to your question about the lag in the time before the pump turns on has to do with the shear amount of relatively flexible piping in the modern RV. There is a natural pressure reserve. The pump comes on after the 'reserve' is depleted.
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Old 10-21-2018, 06:53 PM   #19
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Scott, I think the answer to your question about the lag in the time before the pump turns on has to do with the shear amount of relatively flexible piping in the modern RV. There is a natural pressure reserve. The pump comes on after the 'reserve' is depleted.
Think you are correct
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Old 10-22-2018, 05:40 AM   #20
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I've put a pressure accumulator in all of camper's to cut down cycling of the pump. There is a noticable delay between drawing water and the pump starting. Sounds like you have one too or an air pocket in your water lines. PEX tubing while flexible does not expand any noticeable amount.
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