From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Cold shower
Date: Wed, 08 Oct 2003 02:06:27 -0400
Message-ID: <mf97ovopgi9hteoocci7mv573le5stbrb5@4ax.com>

On Tue, 07 Oct 2003 15:46:31 -0700, Alan Balmer <albalmer@att.net> wrote:


>Others in the thread thought that leaving a trickle running helped,
>others didn't.
>
>It was also pointed out that the check valves make it difficult to
>drain the system.
>
>BTW, I found this by searching for "cold shower" in google groups.

The check valves would have no effect on this problem.  All the check valve
will do is prevent hot water from backing down the cold water supply line as
the heater heats up.

The root cause of the problem is the difference in restriction of the hot vs
cold water plumbing.  The added restriction of the water heater and plumbing
causes the pressure in the hot water line to drop with flow faster than the
cold water line.  By definition the knobs are adjusted for prevailing
conditions at the faucet to produce a comfortable shower.  When the flow
stops, the pressure rises further in the hot leg than the cold one to get back
to nearly the pump pressure and so the shower leg gets filled with hot water.

Added to this is the fact that most RV valves are plastic that both expand and
contract a lot with temperature changes and store very little heat compared to
metal.  The result is the hot water valve quickly cools and effectively
changes its setting.  We've all seen the flow slow to a trickle as the water
gets hot through a partially opened valve.   The reverse happens when the
nozzle is closed.

The first effect is by far the most predominant one and the easiest to fix.
The best fix is to install a temperature compensated valve that will maintain
the temperature regardless of the flow.

Assuming one doesn't want to do that, the next best solution is to install a
throttling valve or orifice in the cold water line.  This is adjusted to make
the pressure vs flow curves of the hot and cold lines coincide.  The valve is
adjusted until the temperature doesn't change as the flow is throttled.

Even with this setup there may be some momentary fluctuation when the shower
head valve is opened because the hot water tank's air bubble will maintain the
pressure in the hot leg for a second or two.  Turns out not to be a terribly
bad problem and an air bladder reservoir on the cold water line pretty much
cancels the effect.  I've installed this solution on my mom's MH and enjoy
very uniform temperature showers.

Another part of the solution is to make sure your water pump runs all the time
during a shower to keep the pressure constant.  Assuming you don't want to
spend the $$$ for one of those variable speed pumps, the second best thing is
to install a little resistance in the pump power lead so that the pump loads
down enough not hit the cutout pressure.  In my mom's MH the crappy wiring
introduced enough resistance.  In my rig I had to install a low ohm rheostat
(0-2ohm, 50 watt, Ohmite, surplus) in the pump power lead and tune it to
prevent pump turn off during the shower.

John