Hot or Cold Water When Showering

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RLSharp

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Several months ago there was a discussion on why some people experienced a blast of hot water when they turned their shower back on after soaping up. I also think someone mentioned getting a blast of cold water when the shower was turned back on. Several theories were advanced involving check valves on the water heater, difference distances from the water heater and cold water source from the shower, different water line sizes, etc. The Tucson Sunday paper contained the following article which very likely addresses the hot water blast but not the cold water problem.

******************************************************

Faucet spouts don?t have shut-off valves

By Ed del Grande

SCRIPPS HOWARD NEWS SERVICE​
Q? Hi, Ed. Call me old-fashioned, but I still wash my dishes by hand, and I'm tired of wasting the water by keeping the flow running all the time! Once I find the right temperature, I can't keep turning the faucet on and off or I lose my setting. Can a valve be installed on my spout to stop the flow and keep my settings? Why has no one thought of this?
? Jim, Arizona
A? Jim, you bring up a good question, and I'm sure many people have wondered why faucet spouts have no shut-off valves. Believe it or not, there is a very good reason to keep a faucet flowing constantly. The reason is that a plumbing problem known as crossover can occur if faucet flow is stopped at the spout.

I love to explain crossover to people because it's such an interesting phenomenon. Believe it or not, hot- and cold-water lines are two completely different systems in your home. A cold-water line will feed your water heater, but the hot-water lines leaving the water heater are now a separate system and should never make a direct connection to the cold lines. Hot and cold lines meet again only at places such as faucets, where a mix will take place and create usable temperatures as long as a constant flow through the spout is maintained. But if that flow through the spout were completely stopped while the faucet hot and cold handles remained open, you'd have just made a direct cross connection between the hot- and cold-water lines in your plumbing system. That is a big no-no. And it is here where things get really interesting.

Whenever hot- and cold-water lines make a direct connection, the hot water will travel into the cold-water lines throughout the house ? and I mean travel! You can actually get hot water coming out of your cold-water fixtures, creating an unsafe condition. That's why showers and sinks do not use complete shut-off valves on the spouts.

You can, however, buy restricting aerators for your faucet to slow down the flow, and some even have levers for a strong flow and a slow flow setting. That's one option. You also can get special cooking faucets with a spray head that will stop the flow temporarily while you squeeze the lever, but as soon as you let go, the water flow is returned.

As far as completely stopping the water from passing through a faucet spout, most plumbing codes will not allow it. So I'm afraid you'll just have to ... go with the flow!
******************************************************

This crossover effect seems to explain the blast of hot water we always get when our shower is turned back on. Years ago I installed a complete shut-off valve on our shower hose and therefore I created the perfect cross connection between our hot- and cold-water lines. I have noticed that the longer the shower is shut off, the longer the hot water blast lasts.

Now, if somebody could explain to me why physically the hot water will travel into the cold-water lines throughout the house (RV), I would be completely satisfied with the crossover effect as the culprit which has puzzled RVers for years. I have never heard an explanation which I found entirely plausible for this effect -- perhaps this is it.

I don't know what to say to those who get a blast of cold water from their showers!

Richard


 

Ned

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Our shower goes to cold when we use the shutoff on the shower head.
 

Woody

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Ned said:
Our shower goes to cold when we use the shutoff on the shower head.

Mine too Ned. I;ve learned to direct the showerhead toward the side when turning it back on until it gets hot again. When I forget I get a rude awakening.

Woody
 

Karl

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O.k., I'm going to give it a shot - so to speak ;)

Fact 1: Two closed containers of water; one hot, one cold, connected by a tube will eventually mix and the temperatures will become equal in both. (Incidently, this has absolutely nothing to do with the problem; just thought I'd throw it in here ;D)
Fact 2: Everything else being equal, the container at a higher pressure will empty into the one at a lower pressure until the pressures are equal.
Assumptions: 1) A separate but connected closed hot/cold water system with the pressure maintained (more or less) by a pump.
                  2) A regular hot water heater; not the 'instant hot water' in-line type heater.
                  3) The 'head' in the water heater has not been totally absorbed.
                  4) You don't have a pressure tank on the cold water line.

Here's my theory: When you're showering with the water on, water flows in the hot and cold water lines to the shower head in proportion to the faucet settings (temperature) you have chosen. The pump cycles on and off, but the individual hot and cold flow rates maintain that same proportion regardless of pressure. Now you shut the shower head off and lather up (or down -  your choice). If the pump starts or is running now, and from assumption 4 you know that there's only water in the cold water line, and you know you can't compress a liquid, and you know from fact 2 that the pressure will try to equalize, therefore the only place for the pump to pump the cold water is back thru the hot water side of the faucet to build back the head pressure in the water heater.

Oh, you DO have a tank in the cold water line??? Then never mind ::)

Actually, if you do have a pressure tank and you're getting a HOT blast, chances are you've lost the head in the water heater.

 

Ned

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Richard,

it's the typical RV shower head that leaves a small trickle of water running when shut off.

Karl,

It happens on city water, not just with the onboard pump.? There have been many attempts to explain the phenomenon but none that can explain all the different variations.? Some get hot, some get cold, some don't have a problem, and some are inconsistent.
 

John From Detroit

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The root article in this thread is an interesting mixture of fact and ferterlizer.

First, you can indeed buy shut off valves that go on the end of many fauctes (any faucet that takes a standard areator will accept a demand flow valve) I've seen several of these valves installed in rest areas along I-40 in some of the dryer sections of the nation.

Now, if the ONLY load on the water system is your shower, or sink,, ONLY ONE LOAD, then the pressure in the cold line will balance the pressure in the hot line,  If there is a check valve in the hot water's supply line (common in RV type situtations) there may be some slight increase in pressure as the water heats but it won't cause but a small amount of hot to flow into the cold line (Causing that blast ofhot water when the flow resumes that is mentioned here)

However if someone flushes the toilet... There may be a MAJOR flow of hot into cold lines

As for the blast of cold water some folks get... Well water cools as it sits in the line

IN any case. I've added a cut off valve to my shower (The salesman said it came that way... WRONG) this one does not completly shut off the flow.  I have not added them to my sinks
 

Jim Dick

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Richard,

I have had three coaches that gave a blast of cold water. All of them had a trickle of water when the shutoff was operated. One other coach would give me a blast of hot water. I believe that one stopped all flow when the shutoff was operated. Sure goes along with the explanation.
 

Chet18013

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Never happened to us. Our Beaver came with a temp control mixing valve on the shower that has a push/pull on/off function. Water always comes back on at the same temp. If I'm taking a shower and Laurie runs the water in the sink, no change in temp occurs.

Chet
 

Jeff

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I am not sure what National does on the Tradewinds but it works.

We do not get the temp variations we had on our Pace Arrows, the first would give us a burst of very hot water, the 98 pace shower was cold???
 

John From Detroit

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Yup, a temperture controling mixer (which is a bit more expensive than the regular king) is deffently the best, Still, you can get a blast of cooler water from what is in the shower hose/pipe, however one hopes you are not spending so much time soaping up that it gets overly cold.

Best is to direct the shower to a safe area when activating or re-activating.. That way you don't freeze/fry

And for your amusement there is an alleged singer named Tom Smith who has a song titled "The smallest things known to man" if memory serves.... About sharing a shower the song is "the smallest things known to man" by "Lorne Elliott" if I'm not mistaken but I've not been able to google and find the lyrics

Very funy, also, "R" rated if the raters are being kind
 

Terry A. Brewer

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Chet

>> Our Beaver came with a temp control mixing valve on the shower that has a push/pull on/off function.<<

My Magna works the same way...always perfect temp.

 

MrRiddle

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This has certainly been an interesting conversation!? I'm in the apartment maintenance business and have pretty extensive experience with heating and hot water systems.? I think it was Chet that mentioned the mixing valve.? I have not looked at these for trailer applications but the standard "anti-scald" valve is a balancing valve used in showers.? They work very well, simply adjusting the flow of hot and cold water depending on the pressure on each side of the valve.

Normally, the cold water tends to flow to the hot water side under static conditions because the pressure/flow is higher on the cold side.? This seems to be due to the water traveling through heat exchangers/smaller pipe sizes etc. on the hot water side and thus, reduces flow.?

In campers I think there is a different situation because there are no heat exchangers used (as far as I know). The variation in getting hot or cold blasts could be directly related to the water heater.? If the heater is running the hot water expands, of course, if? it shuts down during the lather process the pressure will equalize pretty quickly I think. So, if you turn the water back on using a full shut off valve whether you get burned or frozen will depend on the current status of the water heater.

Well, that's my theory, and I'm sticking to it!? ?;D
 

Chet18013

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One of the things no one has mentioned is the water pump starting and stopping. Our Beaver has a pressure tank so the pump runs intermittently and the pressure at the shower head is constant.  Many RVs do not have a pressure tank, so everytime the water is turned on there is a surge as the pump starts.  It stands to reason the if this is the case, there should be an initial cold surge, since the water goes from the pump to the shower, whereas the hot must go through the water heater and travel farther which means a little more pressure loss, thus a delay in equalizing the pressure of hot and cold at the shower head.

That's my theory on the cold surge.

Chet18013
 

MrRiddle

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Chet18013 said:
One of the things no one has mentioned is the water pump starting and stopping.". . .
That's my theory on the cold surge.

Chet18013

I think that is a good theory.
 

Ron

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If the rig has an AguaJet water pump you don't need the pressure pump to keep the water flow constent.  The Aquajet is a variable speed pump and will supply ample steady water to the shower even when somebody turns on another water faucet. 

 

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