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Question DetailsAsked on 9/8/2015

Why would water come out of a hot water facucet when the hot water heater is blocked in and been drained?

I blocked in the gas hot water heater, turned off the gas, opened a hot water faucet to break the vacuum, drained the hot water heater, and water has continuously poured out of the opened faucet at a moderate flow for several days now. I have tried tightening down more on the inlet valve to the hot water heater, and drained an additional 1 gallon out (a couple of days after the initial draining of the tank). If I open another sinks hot water valve nothing happens. No flow. But, if I open the adjacent cold water valve and let it flow for a minute or so, and then close the cold water off, the opened hot water valve will drain water out for about a minute. Is this ghosts in the plumbing or what?

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Not ghosts - Scandanavian Water Gnomes as shown in following link - live in your pipes and during the night they work industriously creating water out of thin air using gnome magic - that is where the water in your pipes come from. The babies live in household piping to keep your pipes full, the 600 pound couch potato old ones live in the main water supply system to provide the bulk of the water supply. Good friends with the dwarves, who are the ones who tunnel to install new water lines when they are not mining for gems and gold or making Hobbit movies. Also cousins of the household Gremlins, the analog type that cause house wiring and house and car mechanical glitches, and the new generation digital gremlins that eat only digital device innards in computers and electronically controlled appliances and such.

Seriously - ten likely sources for this inflow that I can think of offhand, the last two have stumped me for awhile in the past - when I wrote this I did not prioritize, just stream of thought - so if your water flow is ONLY after using another faucet, not all the time, #4 is by far the most likely cause:

1) the inlet valve to the hot water tank is leaking - if (when all other water uses in house are off) you put your ear to the tank you would hear water running or hissing, plus likely a splashing or gurgling sound as it falls or drains into the tank (assuming its drain valve is open). Stethoscope or piece of pipe or even cardboard tube as a "hearing horn" held against the valve would tell you for sure if it is leaking - would be a hissing or flowing sound. NOTE - when listening, the sound travels well in the water in the pipes and in metal pipes, so you have to compare sound at the "source" and at a pipe a distance away to be sure you are not just hearing the sound transmitted from another location in the house. Would flow regardless of any other use going on in house. Definitive proof would be (while tank drain valve is wide open still so tank is basically empty) under (slowly) the fitting where hot water pipe leaves the tank - if no water, then has to be coming from the cold side through the shutoff valve. If water comes out when tank is basically empty, then has to be backflowing from elsewhere in the house, so one of the other possible causes is your culprit.

2) you have a large expansion tank (only if located on the tank side of the valve of the hot water system or incorrectly mounted on the hot water outlet side) which was mostly full of water rather than air as it should be, that is slowly draining out - look like this first image in some older houses (though that one was goofily plastered into the ceiling, presumably as a fireblock measure), current ones like the white and blue tank in the second image in the second link -

Many of these are single-piped so there is no air inlet so takes a long time to drain out. The newer variety have no drain plug (screw-on cap on top is to pressurize tank with air, NOT an outlet or inlet) - some older large ones do, some not. Again, hold stethoscope or pipe to the pipe connection to it and you should hear water dripping/flowing. Would flow regardless of any other use going on in house.

3) leaking backflow prevention valves in toilet mixing valve - a mixing device that commonly looks like this which blends hot and cold water to feed to the toilets so the tanks don't sweat. If the built-in backflow prevention valves don't seat fully or are corroded/eroded, cold or hot water can backflow into the other's part of the line -

Again, stethoscope or listening tube. Would flow regardless of any other use going on in house, or at least when toilet is not refilling.

4) If leaks into tank (and drains out of it) ONLY when another faucet is being used, then you are getting cold water backflow from the faucet into the hot water pipes. Would happen with a two-valve faucet only if the hot faucet was open while the cold was being used. Would happen with a single-handle faucet (like a typical kitchen sink) because unless set ONLY fully to the cold side (and assuming it is not leaking between the two sides internally due to worn seals) the internal chamber of the faucet is open to both hot and cold - which is how you control the temperature. If open to both hot and cold, the cold will partly come out of the faucet into the sink, and partly flow into the empty hot water lines. Ditto to a single-handle tub/shower faucet that is turned on. Would fill the hot lines and flow as long as the faucet is turned on, but hot lines would drain out when off. Again, stethoscope or listening tube to hear if that is the source.

5) Some of the shower/bath single handle faucets with automatic temperature control will also allow crossover from hot to cold and vice versa EVEN if the faucet is turned off - the metering valve inside it is still set to allow part of each temperature into the faucet. No solution to that kind of backflow source.

6) Tankless hot water heater - whole house, under sink, even a permanently plumbed hot water maker/coffee pot, where the incoming cold water is permanently connected to the hot water outlet line, would provide flow continuously to the hot water lines. Would presumably be a goodly flow, not just a trickle, and would flow except probably less when "hot water" is being used somewhere else in the house.

7) You have a whole-house combined domestic hot water/baseboard or in-floor heating loop system, so you are getting backflow from the heating side of the system into the holding tank. Of course turn off the power/gas to the heater, then there should be an isolation valve to shut off access to/from the heating side.

8) Draining out from the house piping, very slowly - you said you opened a hot water faucet to drain the system but you actually may have to open most or ALL of them to get all the various hot water pipe runs to drain out. For instance, open bathroom faucet may drain a bathroom set of pipes, but other bathroom or kitchen runs may not drain out immediately because there is a vacuum formed in that branch of the piping - the one open facuet open in another room is on the "wet" rather than "dry" or "high" side of those lines. Washer and dishwasher should NOT need draining because they sit low enough unlikely to pull the water up in the pipes from them to their shutoff faucet. Reefer water/icemaker/water tubing to coffee maker has very little water in it so no significant contribution there IF it even pulls that water out. BTW - shut off icemaker if you have one. I have seen water drain out of unvented pipes like this for several hours - though total quantity, as long as not draining a tank of some kind, only maybe 2-10 gallons depending on your piping configuration.

9) hot water trickle feed connection to well system - some have a low-flow hot water feed to the well or pump system near the well to heat the water so it does not freeze in the lines or pressure tank in the wintertime - so would backflow continuously if does not have an operating backflow preventer, which would rarely be put on such a system. Again - track by sound, and would flow continuously.

10) Backflow from a hot tub or swimming pool that draws hot water from the water heater rather than making its own hot water, which does not have the required (or inoperational) backflow preventer, so tub/pool water is backflowing into the hot water lines. Again - track by sound, and would flow continuously.


IF you are not able to cut off the flow, then you probably have to shut of the water to the entire house while replacing the hot water tank (if that is what you are doing), or if taking out of service permanently or for long vacation put in a valve on the hot water side also so you can isolate the tank on both sides.

Answered 4 years ago by LCD

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