After 3 years living here, the flexible tube in the output of the boiler burst. That is when I noticed that the hot water tube (blue and red stripes) was installed in the cold input, and the cold water tube (just blue stripes) was installed in the hot output (see attached image).

The hot water stopped spraying after I closed the water supply.

How do I replace hot water tube without dumping 100 l of (dangerously hot) water on the floor?

If I keep the water input closed, will it leak when I remove the broken tube?

Should I drain the tank first? If so, how? And how do I fill it up again afterwards?

water heater input and output pipes

EDIT: added full body picture full body picture

  • 6
    Quality electrical outlet placement, there!
    – Dannie
    Dec 15, 2022 at 10:54
  • 5
    @Dannie Yes, and that's not the only issue. Looks like the builder had in general a special sort of attitude towards applicable regs. For example both of those gas valves should absolutely be capped (the top one certainly isn't, can't see the bottom one clearly); if the house is actually connected to gas, this is a disaster waiting to happen.
    – TooTea
    Dec 15, 2022 at 13:19
  • I have no gas supply.
    – lvella
    Dec 16, 2022 at 11:49

7 Answers 7


You actually don't need to drain the tank much.

Not sure if the other answerers are familiar with the design of European-style hot water cylinders like this one, but it's basically like this:

  • Cold water inlet goes right to the very bottom of the tank
  • Hot water outlet is connected to a vertical riser pipe inside the tank, going all the way to the top

Hot water is less dense and rises up, which is why it has to be collected from the top. This approach ensures that as you consume hot water in the tank, cold water is refilled from the bottom (where the heating element is) and does not mix with the hot water at the top. The temperature of the hot water thus stays constant until the tank is used up completely.

A consequence of this design is that as soon as you drain a little bit of water from the tank, the water level will drop below the top end of the riser pipe and the hot water outlet will go dry.

This draining might have already happened, but it is very possible it hasn't. Get a bucket (or two), hold it under the hot water outlet, and carefully loosen the flex hose. Most likely, water will start coming out unevenly with a glug-glug-glug (there's no other way for air to enter the tank, so air bubbles will have to take turns with water in the hot riser pipe). After perhaps 10 litres, it should be done draining and you can then safely replace the hose.

You can help it drain by operating the red lever on the pressure relief valve after undoing the hot water hose (so that water can come out the bottom port and the hot water riser will be for air only).

  • 1
    So the pipe design inside is the same as North American with the cold water going to the bottom, just the side with the long pipe is changed.
    – crip659
    Dec 15, 2022 at 12:33
  • 1
    @crip659 Yeah, I think it might be that European houses are on average a little short on floor space, which is why these wall-hung tanks are so popular around here (you can then stuff the washing machine or something underneath and save a square meter).
    – TooTea
    Dec 15, 2022 at 13:15
  • What about replacing the cold water inlet hose (that also looks damaged)?
    – lvella
    Dec 15, 2022 at 17:15
  • @Ivella That one is unfortunately much trickier and most likely requires draining the tank completely (or doing it as Jasen says if you're a little brave). If you decide to go that way,you should consider replacing the flex hoses with corrugated stainless to solve the issue once and for all.
    – TooTea
    Dec 15, 2022 at 22:07
  • @lvella If the cold water flex looks damaged, you probably should replace it ASAP, because if that one bursts, you will end up with a tankfull of water on the floor (no fun).
    – TooTea
    Dec 16, 2022 at 11:12

enter image description here


  1. The plastic tube coming from the red handle circled in green is the drain. Investigate where that goes. It hopefully goes outside or into a drain. Make sure!.
  2. Close the cold supply valve (in the red circle), leave the hot water valve open and open the highest hot water faucet in the house, or at least one that is higher than the top of this tank. This will admit air while the tank drains.
  3. Open the red drain valve circled in green. The tank should now drain through the clear plastic tube in the picture.
  4. Check again at the other end of the clear tube that the water is draining to where you want it to drain, that the tube isn't cracked or leaking and the water is not collecting anywhere.
  5. When the water stops draining disconnect the two hoses from the valves in the red circle. Expect a little water out of them so have a bucket ready.


After reconnecting all the hoses:

  1. Close the drain valve, open both of the other valves.
  2. Turn on any hot water faucet and wait for cold water to come out then close the faucet. This fills the tank with cold water.
  3. Power up the boiler and wait for it to heat up.
  4. Test from a few faucets. Expect air out of every faucet the first time you use it.


You must connect the cold supply to the correct connection on the tank. These must not be reversed. It probably doesn't matter if the hoses are reversed, IE you use the wrong color hose for hot and cold, as long as the tank is connected the right way around. The hoses are probably identical and colored differently just to help you get it right.

  • 1
    "Make sure!" : to make sure, open & close intermittently and check surroundings, other side of wall, a floor lower etc... if there is an issue with the drain, a bit of water in the wrong place is a nuisance but usually not a problem
    – P2000
    Dec 14, 2022 at 21:32
  • 2
    @P2000 call me paranoid but that drain valve looks like it has a garden hose spout, and probably not meant to be permanently plumbed. A clear flexible vinyl tube disappearing into a wall? That's, um, unconventional? With a patch made of what looks like cardboard? Who knows where it goes. Hopefully it doesn't just terminate inside the wall! I think the design is to attach an actual garden hose to that valve, temporarily, and run it outside or into a bathtub.
    – jay613
    Dec 14, 2022 at 21:47
  • 2
    yes agree, what I meant was that OP should proceed cautiously, and if unsure it's ok to try it and see, as a small spill is still better than a big spill
    – P2000
    Dec 14, 2022 at 23:23
  • 3
    @jay613 It's not a garden hose spout, but a standard European-style pressure relief valve. The handle is mostly for testing. The white plastic cover is a fairly common way to hide the actual connection to the drain.
    – TooTea
    Dec 15, 2022 at 10:02
  • @TooTea ah I see. Is it normal for a PRV to be connected to a relatively small tube like that? Where I live it needs to be bigger, straight, not very long and there needs to be an air gap within some distance, I think about 2m.
    – jay613
    Dec 15, 2022 at 13:29

Based on this statement:

the flexible tube in the output of the boiler burst. ... The hot water stopped spraying after I closed the water supply.

I suspect the pressure inside the tank has run down through the crack in the hot water flex pipe. I believe it may be possible to replace the flex pipe without draining the tank (if the tank hasn't drained itself already).

Water heaters are sealed units, so if the cold water is kept off, the water should tend to stay inside the tank, similar to how one can lift a straw-full of liquid out of a cup by plugging the top of the straw.

I would also suggest a suitable quarter-turn valve (if your local code allows it) on the hot water side, just like the cold water side, so this sort of repair will be easier next time. Then you could merely turn the hot and or cold valves off on both ends of the flex pipes, then replace the flex pipes easily.

Stand by with a bucket capable of holding the contents of the water heater, in case the water is not held inside. Even if it is held inside, a small steady drip could happen.

If you can use another valve, remove the flex pipe, then attach the valve according to instructions, with suitable sealant if instructed. Have the valve open while attaching it, so the leaking water (if any) flows through the valve and doesn't get in the way too much while threading the valve on. Once the valve is on, close it, then attach the new flex pipe at your convenience.

If you cannot use the extra valve, replace the flex pipe according to instructions.

Filling afterwards: Remove aerators from the house faucets, so dislodged gunk in the pipes doesn't block the faucets. Turn on all valves at the heater, and and a hot water faucet (perhaps all the hot water faucets in the house). Let the water run, and let air bubbles clear through the plumbing. Once no air bubbles appear, start the heating system in the heater.

  • Was also thinking the vacuum might hold enough, but think the bets are against it. Two ends to loosen and one end does not look like it will be easy as expected.
    – crip659
    Dec 14, 2022 at 20:20
  • 3
    Stand by with a 100 liter bucket? 8O
    – jay613
    Dec 14, 2022 at 22:10
  • A big bucket, true, @jay613! 26-ish gallons. A 32-gallon trash can could cover this in the US. Grab the dustbin just after the lorry empties it, and look out below! Dec 14, 2022 at 22:32
  • @crip569, I'm figuring it'll dribble some, maybe a lot, but possibly getting the valve on quick enough might not be too messy. Draining the thing would be a wiser step for little mess as possible. Dec 14, 2022 at 22:33
  • There are already quarter-turn valves on both sides (on the wall), but those are useless for the purpose of replacing the hoses. (Reasonable installers use corrugated stainless tubing instead of these rubber flex hoses for this particular reason.) I guess you might be mistaking the thing with the red handle on the inlet for a quarter-turn valve, but that's a pressure relief valve instead.
    – TooTea
    Dec 15, 2022 at 13:12

OP has stated the picture is showing the correct placement of the tank. Making this answer useless for this question.

Seeing it stopped leaking guessing the picture is upside down.

Should be a drain near the bottom of the tank.

Might not need to drain any, but usually good to drain some(couple litres/half gallon) every couple of months to remove sediment.

With power and shut off valves off, replacement/moving of the tubes should be easy. Drain a bit to remove pressure or leave a hot faucet open.

  • It is not upside down. I am just guessing that for some reason air can not enter from the output, that is why it doesn't leak. Maybe it has a valve that would make the replacing safe? Maybe the hole on the tube is too small? I am just guessing, I don't know.
    – lvella
    Dec 14, 2022 at 18:28
  • I was really hoping it was upside down. This makes what I said useless. If those valves are on the bottom, then the tank does need complete draining. Can you post another picture showing the whole tank/boiler so we might see something else that might be handy.
    – crip659
    Dec 14, 2022 at 18:36
  • Done. I also opened the output valve shown in the picture and a hot water tap. It makes bubbling noise and the faucet drips a little, but there is no serious stream of water.
    – lvella
    Dec 14, 2022 at 18:47
  • so why keep it up
    – Traveler
    Dec 15, 2022 at 7:49

Based on the design (input on the bottom) you will need to drain the tank.

You can simply (after shutting off the power) turn off the input and turn on the drain. Once it's drained and you do your work, turn on the water again, and restore power once it has water flowing.


There is no way around it you must drain the boiler.

The water is under pressure in the boiler at least 30 or more psi.

Close the cold water supply and open hot water faucet in your home.

You also have a drain hose with valve that you should open to assist draining.

The drain hose goes somewhere true the wall, hopefully to the outside,


  • Closing cold water supply and opening a hot water faucet does nothing, no water comes out. And I don't think it is empty because as much water there was on my floor, it was nowhere near 70 liters.
    – lvella
    Dec 14, 2022 at 19:07
  • @lvella do not forget to open the drain valve
    – Traveler
    Dec 14, 2022 at 19:13

You can probably get away without draining the tank much if you work fast and only have one connection open at at time.

Disconnect one hose cap that outlet, the cap will prevent siphoning wwhen you disconnect the other outlet. disconnect the other outlet reconnect that outlet. uncap and reconnect the other.

You will be working with hot water so, wear thick rubber gloves and have plenty of cold water on hand in-case you burn yourself.

  • 2
    Or just turn power off to the tank and let it cool for a day or so.
    – Mark
    Dec 16, 2022 at 0:03

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