RV Doctor – Dripping Shower Faucet When Hot Water Heater Is On

October 22, 2009 by Gary Bunzer · 19 Comments  
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Dear RV Doctor,
We have a single lever faucet on our shower that has a continuous drip when the hot water heater is on. I carefully replaced all the parts within the faucet with a kit, but to no avail. I understand that as water heats it expands, but where does the water normally expand to and how does it seek relief? I know that some water leaks through the hot water tank relief valve, but that is not an option that I like either. — Leon Hill, (Belleville, ON, Canada)


Gary BunzerLeon, I too, would suspect a faulty seal inside that faucet, but if you’ve replaced the guts with the proper components we can probably rule that out as a cause. And you are correct that as water heats it expands.

Normally this expansion occurs inside the water heater. A cushion of air must always be present in there to accommodate the expansion. In optimum circumstances this cushion of air inside the tank above the water level acts as an accumulator of sorts. Over time, this air space (oxygen) is eventually fully absorbed by the oxygen molecules in the water. This results in no place for the expanding water to move into since the tank is, at that point, completely full.

Like you mentioned, usually the integral pressure and temperature (P&T) relief valve then performs one of its safety functions and opens, expelling the heated water through the P&T valve. This also allows additional cold water to enter the tank (thus lowering the temperature) and causing the valve portion to close. Perhaps the lesser resistance to this expansion is through the shower faucet instead of the relief valve. It is normal for the P&T valve to drip a little during each heating cycle. I’d try renewing that air cushion inside the water heater. There’s a good chance that air space has been eliminated over time.

To reestablish the cushion of air inside the tank, remove the water source by either turning the demand pump off or by interrupting the flow of city water. Open any two hot water faucets in the coach. Next, open the manual lever on the relief valve until the water flow completely stops. Close the P&T valve and the faucets, and then turn the pressure back on. There is now a cushion of air trapped above the water level inside the water heater once again. Hopefully this will eliminate that pesky leak in the shower faucet. I’d be tempted to install another faucet if the leak persists.

(Please feel free to comment, however, please also note that due to the volume of communications I receive from multiple channels I cannot guarantee a personal response in every instance. However, questions of an overall general interest may be considered and published in an upcoming RV Doctor column.)

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19 Responses to “RV Doctor – Dripping Shower Faucet When Hot Water Heater Is On”

  1. Jim Hunter on October 22nd, 2009 2:07 pm

    Dear RV Doctor

    The drip this person may be experiencing may be coming from the Shower Head.
    Water is often trapped in the line going up to the Shower Head after being shut off before the the water supply is shut off. The water column is suspended in the tube going up to the Shower Head and is being held up only by the seal in the Shower Head fixture. If this water is not drained by opening the shower head valve it will often drip (it seems like forever). It will feel warm if the shower water was warm.

    Just a thought


  2. Carlton Annis on October 22nd, 2009 2:09 pm

    I’m a plumber and am guessing your faucet is a Moen. If so, changing the cartridge with a brass one instead of plastic should fix the problem.

  3. Casey Donovan on October 22nd, 2009 2:55 pm

    Your recommended solution seems like good advice, but I question your explanation of the cause. If “absorption” isn’t the cause, the real problem is being overlooked and is likely to recur.
    Air does dissolve very slightly in water, up to about 0.001%. But when water is heated, it gives up that air, since the solubility in hot water is virtually nil. That would tend to -increase- the air cushion, not reduce it.
    To get the facts straight, air is only about 20% oxygen. The rest is mostly nitrogen, plus small amounts of other gases. There are no oxygen -molecules- in water, except for the relatively few from any dissolved air. There are oxygen -atoms-, bonded to hydrogen atoms, but they aren’t directly involved in the dissolving phenomenon.
    I’d look for mechanical leakage, or maybe turbulence, bleeding off the air cushion.

  4. Rick on October 22nd, 2009 5:11 pm

    I have owned my RV going on 4 years now. My showerhead also leaks. On my showerhead I have an on/off switch. On when in use and off position when not in use. Dealer explained to me the dripping was to release water pressure. I still don’t understand. I do know I have to drain my gray water tank every 2 days. If someone finds a solution to this please let me know.


  5. santa skip on October 22nd, 2009 9:06 pm

    when you have the water off it is a good time to pull the drain plug and flush out the water heater and ,or replace the anode.

  6. Alpenliter on October 23rd, 2009 12:04 am

    I agree with Carlton on the Brass vs Plastic Cartridge, but I also wonder if the Pressure Relief Valve is defective. It seems that the shower head is acting as a relief valve. Just a thought. If the Doctors fix doesn’t work, I would replace the PRV.

  7. Fritz on October 23rd, 2009 4:23 am

    I’m having a similar problem, I’ve developed two leaks, one in the hot water line under the bathroom sink and the hot water side of the washing machine. Both in the same week on a 3 year old motorhome. I’ve replaced the rubber seals and the washing machine still has a minor drip.

  8. Jim Peterson on October 31st, 2009 2:43 pm

    Water should never “leak” from a PRV (Pressure Relief Valve). If the PRV (and the shower head in your case) only leak when the water heater is ON, it’s a clear indication the water is expanding with no better place to go. I suspect this problem is made worse because your RV is hooked up to a pressurized water supply (”city water”) and most of the RV hose supply connections have a check valve built into them. This is to prevent the city water supply from being contaminated by anything in the RV, but it also prevents the increasing pressure (caused by heating water) from equalizing within the larger water supply (which likely includes a pressure tank somewhere in the system). It’s unlikely your onboard water pump generates enough pressure for this to be nearly as much of an issue.
    While the “air cushion” which can exist in an RV water heater (only because the outlet is NOT at the very top of the tank as it is in a conventional residential water heater) might temporarily resolve this problem IF the air cushion is regularly restored, water under pressure tends to absorb pressurized air it is next to. (And draining and refilling your water heater once a week is kind of a pain, eh?) Old-style pressure tanks on well pumps have to be regularly “aired up” because of this. Newer pressure tanks have an internal rubber bladder, so the air quantity is maintained and can’t be absorbed by the water (until the bladder fails, and they all eventually do). One sure cure for the problem you’re experiencing is to install a small pressure tank somewhere in your water supply system. This tank can be installed anywhere on any of the cold water supply lines in your RV — anywhere ‘downstream’ from your water pump that is. The tank location must ALSO be ‘downstream’ from any OEM check valve which prevents pressurized “city water” from flowing backwards towards your water pump.
    Pressure tanks come “pre-charged” but the PSI in the air bladder is easily adjusted through a schrader valve (just like airing up — or letting air out of — a tire). The pre-charge PSI (measured with NO pressure on the water system = water supply turned off and at least one faucet valve open) should equal the lowest pressure in the usual city water pressure range. If city water pressure fluctuates between 30 and 50 PSI (for example), your pre-charge pressure should be 30 PSI. When using your RV water pump, this pre-charge pressure should be changed to equal the pressure at which your RV pump first turns ON. Proper pre-charge pressure keeps the bladder from hyper-extending which leads to premature failure of the bladder.

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  11. bonnie on February 2nd, 2011 6:18 pm

    We have an rv 5th wheel. The weather was extremely cold last night; we left a water drip in the tub and kitchen sink. This morning, nothing will come through the hot water tap in the bathroom sink or shower, but the cold water faucet works. Hot and cold faucets work fine in the kitchen. Have had space heaters in an attempt to thaw out the line all day, but don’t think that is the problem. Any suggestions?

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