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Can you gravity feed a RV water heater?


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I'm building a small toyhauler from a 16' narrow track enclosed trailer and have been thinking about what bothered me most about the water system on my old 24' toyhauler..  Mainly, it didn't have enough fresh water storage at 45 gallons.   When I had company, we ran out in 3 days and weren't about to pack up our beautiful campsite just for more water so we pumped stream water into the tank for showers only which worked great! Second, was the noise the water pump made.  Yes, I unscrewed it and set it on bubble wrap.  But it was loud and annoying, period..

I want to pump stream water again next time I'm in colorado because I usually camp in the same spot all week.  And since I'm already planning on buying a better Honda WX10 water pump with allot more head in case the stream is allot lower, why not pump the water to a elevated tank so that you could have enough pressure without needing to run that annoying RV water pump??  I know, sort of silly hoisting a tank from the tongue of my trailer into a tree and then filling it.  Or not?  

So if I did that..  How much pressure would be the minimum needed to push the water through the water heater and to the shower?  You get .43 psi per foot of rise.  Or does it have more to do with the pipe size when dealing with gravity fed water? 

Thanks!

 

Edited by rebar
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1 hour ago, chirakawa said:

Water will exit the water heater at the same pressure it enters the water heater.  If you raise the tank to a height of 12' then you'll have about 5 lbs of pressure going in and leaving the water heater.

Thanks.  I'll assume your saying 5 psi would be the minimum pressure needed?  12' sounds higher than its worth to not need the rv water pump

Edited by rebar
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I don't know what the minimum pressure NEEDED by the water heater is, but I remind you that when you hook up to city water, most people use a pressure regulator that cuts the city water back to 40 psi to prevent it from blowing out the pex plumbing line.  Looking at the specs on a few rv water pumps, I've found a few of them that operate up to 50 psi.  At 5 psi, I suspect the water is going to trickle out of your shower in a most unsatisfactory way.  But I could be wrong.

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20 minutes ago, rebar said:

Thanks.  I'll assume your saying 5 psi would be the minimum pressure needed?  12' sounds higher than its worth to not need the rv water pump

No.  The water heater is probably about a foot tall.  Water goes in at the bottom and comes out at the top.  So, you'd need about 0.5 psi of head pressure to force water out of the heater.  As already mentioned, I doubt you could take much of a shower on 5 psi anyway.

Just buy a quieter water pump.

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It wouldn't be minimum pressure needed, but minimum gpm for the shower head. You could stop water supply with 0 psi and heater would shut off at its set temperature. 

At 40 psi you only need a 1/2 pipe to get the needed gpm.  You can get the same gpm using 5 psi, but the pipe has to be bigger with no elbow or choke points.

Yeah waste of time

 

Edited by rebar
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19 minutes ago, Optimistic Paranoid said:

At 5 psi, I suspect the water is going to trickle out of your shower in a most unsatisfactory way. 

That's about right.

Most RV plumbing can easily handle 100psi, however, "recommended" is around 60psi on newer and larger rigs and 50psi on older and smaller rigs. Myself... and many that I know get by just fine with 40psi. I don't imagine you could go a whole heck of a lot lower though and still maintain "normal" operations. 

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If you add a 2-5 gallon accumulator bladder tank to the system, the pump would run much less often, albeit longer at each cycle. Turning the pump on long enough to pressurize the tank and system would be enough to run the system at pressure for normal flushing and hand washing several times before the pump would need to run again. For showers, the pump would need to be left on, but does it really matter then that it's noisy? I mounted our pump in a basement storage bin with flexible hoses connecting it to the input and output piping. It's quiet enough that we can't tell it's running with the TV on.

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6 hours ago, rebar said:

I know, sort of silly hoisting a tank from the tongue of my trailer into a tree and then filling it.  Or not?  

Water weighs 8.34 pounds per gallon. So to fill a 45 gallon tank you would need 375.3 pounds of water. How big is that tree limb? And how wide is the limb on which you plan to balance that tank? And what kind of parking spot are you in that will even let you do that? I'm having trouble with my vision of your idea.

Linda Sand

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11 hours ago, Yarome said:

Most RV plumbing can easily handle 100psi, however, "recommended" is around 60psi on newer and larger rigs and 50psi on older and smaller rigs.

Unless it has recently changed, the RVIA standard for pressure testing of an RV water system while in construction is 100#. The reason for the statement of 60# is for a safety margin to allow for occasional pressure increases that could take place. An example is when the water heater has lost the air bubble and you fill it with cold water, then turn it on. 

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15 minutes ago, Kirk Wood said:

Unless it has recently changed, the RVIA standard for pressure testing of an RV water system while in construction is 100#. The reason for the statement of 60# is for a safety margin to allow for occasional pressure increases that could take place. An example is when the water heater has lost the air bubble and you fill it with cold water, then turn it on. 

It doesn't matter if there is an air bubble or not, when you heat water it expands and the pressure increases.  The purpose of the air bubble is so the air can compress and absorb the water expansion instead of bleeding the pressure through the relief valve.

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9 hours ago, sandsys said:

Water weighs 8.34 pounds per gallon. So to fill a 45 gallon tank you would need 375.3 pounds of water. How big is that tree limb? And how wide is the limb on which you plan to balance that tank? And what kind of parking spot are you in that will even let you do that? I'm having trouble with my vision of your idea.

Linda Sand

I don't pay for campsites Linda and it would take maybe a 5" branch to hold 375 pounds.  A smaller tree?  Maybe pump the tank only half full?

You may not be able to envision it, but I can..

Maybe its not if its practical, but more the challenge.  Like my wood fired hot tub. 

Edited by rebar
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34 minutes ago, chirakawa said:

The purpose of the air bubble is so the air can compress and absorb the water expansion instead of bleeding the pressure through the relief valve.

It also mitigates the pressure increase, since air will compress and water does not. Most water heater pressure relief valves are set to relieve at 210° or 150#, well above the water system test pressure. 

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3 hours ago, chirakawa said:

It doesn't matter if there is an air bubble or not, when you heat water it expands and the pressure increases.  The purpose of the air bubble is so the air can compress and absorb the water expansion instead of bleeding the pressure through the relief valve.

With regards to the pressure increase, it absolutely does matter if there is an air bubble or not. The volume of water is going to increase either way, but the easily compressible air in the bubble will accommodate that expansion with very little pressure increase. Without the air bubble (or an accumulator which includes a captive "air bubble") the expansion can result in dramatic pressure increases in the water system.  Hence the relief valve opening.

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7 hours ago, Kirk Wood said:

Unless it has recently changed, the RVIA standard for pressure testing of an RV water system while in construction is 100#. The reason for the statement of 60# is for a safety margin to allow for occasional pressure increases that could take place. An example is when the water heater has lost the air bubble and you fill it with cold water, then turn it on. 

6 hours ago, Kirk Wood said:

 

 

3 hours ago, mptjelgin said:

With regards to the pressure increase, it absolutely does matter if there is an air bubble or not. The volume of water is going to increase either way, but the easily compressible air in the bubble will accommodate that expansion with very little pressure increase. Without the air bubble (or an accumulator which includes a captive "air bubble") the expansion can result in dramatic pressure increases in the water system.  Hence the relief valve opening.

I agree with you.  It's very important to keep an air bubble in the water heater.  Read Kirk's first post, then read my response.  I was simply stating that the water pressure rises when water is heated, whether or not there is an air bubble present.  His post made it sound like there was only  a pressure increase when the air bubble wasn't present.  I have an air bubble in my water heater right now, yet when the water heats up, the pressure rises by at least 40 psi.

 

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But water pressure does not necessarily rise when water is heated. If you heat water in an open container is there a pressure rise?? No. Pressure only rises if the expansion of the water is constrained. And the amount of constraint is the key. With an adequate air bubble the pressure rise is minimal. 

The volume change of water from 20C to 90C (68F to 194F) is approximately 3.4%, and that is a significantly larger range than you'll see in a water heater. So we're probably talking about a volume change on the order of a couple of percent.

It you are getting "at least 40 psi" increase when the water heats you've either got an inadequately small air bubble in your water heater or another issue. 

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14 hours ago, chirakawa said:

His post made it sound like there was only  a pressure increase when the air bubble wasn't present.  I have an air bubble in my water heater right now, yet when the water heats up, the pressure rises by at least 40 psi.

Really? Have you actually measured that much as I have a pressure gauge on my water system and while it does change noticeably from cold to heated, it isn't nearly that much? I have observed mine but haven't ever recorded the amount of change so am not sure.  

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13 minutes ago, Kirk Wood said:

Really? Have you actually measured that much as I have a pressure gauge on my water system and while it does change noticeably from cold to heated, it isn't nearly that much? I have observed mine but haven't ever recorded the amount of change so am not sure.  

No, I was just guessing.  My gauges are both on the inlet side of the check valve.  I'd be interested to know what your readings are.  I'd appreciate if you'd share that.

I do know that after showering and depleting the hot water, thirty minutes later when I open a faucet, there is a momentary burst of higher pressure probably lasting 1 or 2 seconds.  It's been that way with every RV I've ever owned.  That's why I stand by my statement that heating water raises the pressure in the system, whether the air pocket is present or not.

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6 minutes ago, chirakawa said:

I'd be interested to know what your readings are.  I'd appreciate if you'd share that.

I need to replace the one that I have used in the past as it died so I don't have one currently. When we were fulltime I used a Watts, whole house pressure regulator that was adjustable and it had an output pressure gauge on it.  This is a picture of one nearly identical to what I used more recently and although this one is from Valterra and mine was from Camco I do not remember ever seeing mine more than barely into the low side of the red band. My regulator was kept at 50# when cold and my current, non-adjustable regulator is preset to 45#. The one that I had was 6 years old and I pitched it last spring.

                              51pNdGOLHnL._AC_US200_.jpg

In reading about water volume versus pressures, I can't find anything specific to actual pressures, but it does state that water volume can increase by about 3% when heated from supply temperatures to set temperatures. If BarbOK is reading this, I think that she as a chemist may have better information on this,

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49 minutes ago, Kirk Wood said:

I need to replace the one that I have used in the past as it died so I don't have one currently. When we were fulltime I used a Watts, whole house pressure regulator that was adjustable and it had an output pressure gauge on it.  This is a picture of one nearly identical to what I used more recently and although this one is from Valterra and mine was from Camco I do not remember ever seeing mine more than barely into the low side of the red band. My regulator was kept at 50# when cold and my current, non-adjustable regulator is preset to 45#. The one that I had was 6 years old and I pitched it last spring.

                              51pNdGOLHnL._AC_US200_.jpg

In reading about water volume versus pressures, I can't find anything specific to actual pressures, but it does state that water volume can increase by about 3% when heated from supply temperatures to set temperatures. If BarbOK is reading this, I think that she as a chemist may have better information on this,

I use a watts regulator on my supply hose also.  However, any increase in house pressure from the water heater won't be shown on that gauge because there is a check valve between the gauge and the house plumbing.  Pressure increases can't back up from the house plumbing to the gauge.  The gauge would have to be installed in the house plumbing itself.

If volume increases 3%, that would be an increase in the 10 gallon water heater of .3 gallons, more than a quart.  mptjelgin suggested that the number was closer to 2%.  I don't know what size the air pocket is so can't say what the compression ratio is.  I don't have any accurate numbers, just know pressure increases in the system when water is heated.

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12 minutes ago, chirakawa said:

However, any increase in house pressure from the water heater won't be shown on that gauge because there is a check valve between the gauge and the house plumbing.

My water heater's check valve is on the output side of the tank, which is more common today. Thus water heater pressure does reflect back to the supply hose on ours. 

Edited by Kirk Wood
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I checked the pressure gauge at my accumulator tank line this morning before and after bringing the water heater up from ambient 62 deg.F to shutoff temp, and the pressure increased by ~2 PSI. I'll try it again later today with the accumulator valve shut off so only the heater tank bubble is in play.

Edited by Dutch_12078
typo
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3 hours ago, chirakawa said:

I do know that after showering and depleting the hot water, thirty minutes later when I open a faucet, there is a momentary burst of higher pressure probably lasting 1 or 2 seconds. 

I get that surge only after I've been stationary for a while and that's when I know that I need to restore the air cushion in the water heater. Once I've done that  (added air) there is no perceptible surge for weeks or even months.

I know that my water heater takes a substantial amount of air to finally get sputtering out of a hot water faucet indicating max air cushion. Something like the volume of a 25' hose plus both canisters on my water filter. Perhaps different models have different volumes of air trapped. .

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21 hours ago, Darryl&Rita said:

There's also a check where the hose connects to the trailer, so the gauge on the regulator output is isolated from the house pressure rise.

There is a backflow preventer there, but in my experience, it doesn't prevent that from showing. Since it is a static pressure condition with no actual water movement, I don't think that it would, but I suppose it could.

Edited by Kirk Wood
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