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. 

  On 6/16/2018 at 4:10 AM, 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.  

  On 6/16/2018 at 7:16 PM, 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.

  On 6/16/2018 at 7:35 PM, 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.

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,

  On 6/16/2018 at 8:02 PM, 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.

                              

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.

  On 6/16/2018 at 9:03 PM, 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 June 16, 2018 by Kirk Wood

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.

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 June 16, 2018 by Dutch_12078

typo

  On 6/16/2018 at 7:35 PM, 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. .

  On 6/16/2018 at 9:25 PM, 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 June 17, 2018 by Kirk Wood