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See here, in a standard home water heater, how there's an overflow pipe which would just dump the extra water on the floor if activated.

Why is this? Why does it not go the same way as the drain on a sink or bathtub, or a toilet? (I am not sure what those output pipes for drains are actually called....waste pipes? drain pipes?)

Why would you ever want it to pour out onto the floor?

enter image description here

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  • You wouldn't. But an exploding water tank is even worse. The overflow valve prevents catastrophic failure and limits how much water spills (just enough to bring the pressure down). If you didn't have the valve and the tank ruptured, you would have an unending stream of water spilling continuously. – bib Jul 13 '16 at 21:45
  • But even so, shouldn't that spilt water go into the same place as water down the sink? – temporary_user_name Jul 13 '16 at 22:11
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    Obviously, if you have a floor drain, it could go down there. But this is for emergency only, and you don't want it draining without you knowing about it. When it does trip, you have a problem and need service. – bib Jul 13 '16 at 22:35
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    To clarify: a drain can be piped to beneath this outlet (complete with an air gap), but you should not pipe this pipe to a drain. This is a safety device and you do not want to add any restriction to it. – pdd Jul 14 '16 at 2:21
  • If the overpressure valve kicks in, you do not want very hot water spraying you in the face. – Wayfaring Stranger Jul 23 '16 at 17:21
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A- This is not an "overflow" pipe.

B- It is not connected to an "overflow" valve.

C- It is a drain pipe connected to a Temperature & Pressure Relief Valve which is not only a legal requirement, but a critical safety device to prevent an over-pressure event (boiler explosion- google it)

For years and years, it was common to see no pipe at all. The pipe is a great idea whether or not it is plumbed to a drain or alternative location because if the valve activates it is less of a hazard to humans when scalding steam and water are directed downward and away from hapless victims. Some (if not all) localities require that they be plumbed to the exterior of the building or to a floor drain if the appliance is located in the home. Appliances outside of the home (as yours appears to be) may not be subject to the same requirement. Check with your local building code authority office.

P.S.- there should not be water coming from that drain pipe. If there is you may have a defective TP Valve (or worse, a malfunctioning boiler). Call a plumber.

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    To clarify: a drain can be piped to beneath this outlet (complete with an air gap), but you should not pipe this pipe to a drain as the added fittings and distance will add resistance which will limit it's ability to relieve the pressure in time. – pdd Jul 14 '16 at 2:26
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    @pdd I concur: TP Valve drain line into catch pan, catch pan drain plumbed to other location. Another problem with this installation is that it is a best practice (also maybe required by the code enforcement officials) to install gas fired water heaters on a pedestal of some sort. Most flammable vapors are heavier than air and the burner on your unit is a potential source of ignition. – Jimmy Fix-it Jul 14 '16 at 3:52
  • @JimmyFix-it: Um, no, natural gas (essentially 100% methane) is lighter than air. – Vikki - formerly Sean Feb 11 '20 at 3:10
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    @Sean... um yes, the water heater burner is a potential source of ignition for heavier than air vapors from other hydrocarbons commonly stored in garages and outdoor storage rooms (e.g. gas cans). In my locale it is required by law that the heater burner is elevated because fires caused by this issue are historically so common. – Jimmy Fix-it Feb 11 '20 at 5:51
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    @Sean it says most, not all. I think the list of common lighter-than-air flammable gases is pretty short: methane, ethane, hydrogen, acetylene, ammonia, carbon monoxide... – Jimmy Fix-it Feb 11 '20 at 5:55
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That "overflow" pipe is supposed to go into a drained overflow pan, or barring that, to a drained floor. This goes to the same place as all the other drains, but can't be plumbed "hard" due to the need to provide an air gap. This air gap keeps your drains from backing up and proceeding to contaminate your water supply with nasty stuff.

P.S. It's really not an "overflow" -- but the outlet from a Temperature and Pressure relief valve that keeps your water heater from rocketing its way through your roof and into your neighbor's yard should the control valve or thermostat fail. If water is coming out of it, call a plumber, as that indicates one of three possibilities:

  1. The valve itself won't seat properly because of crud (sediment/CaCO3) deposits in it -- it's not terribly hard for a plumber to replace, although they will have to turn off and partially drain the heater to do so.
  2. The control valve or thermostat has failed in the open position/short-circuit -- this is a serious problem in that your heater is now producing utterly scalding hot water in addition to being possibly damaged further on the inside. If steam or scalding -- i.e. hotter than your normal hot water supply -- water is coming out of the downpipe, CALL A PLUMBER IMMEDIATELY.
  3. There is something (backflow preventer, pressure regulator valve) in the cold water supply line preventing expansion back into the city water supply and causing the hot water tank to over-pressurize as a result. A plumber can address this by installing an expansion tank in the system.
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  • Oh....and the open air nature of a sink/tub is the difference. They inherently have air gaps. – temporary_user_name Jul 13 '16 at 22:48
  • Wouldn't a check valve suffice for that though? – temporary_user_name Jul 13 '16 at 22:48
  • @Aerovistae -- not at the hazard levels posed by overflowing sewage – ThreePhaseEel Jul 13 '16 at 22:51
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The reason that the pipe from the pressure and temperature relief valve does not "go to a drain like any other pipe" is that you are Supposed to notice and take action if it is operating There is a problem that needs to be resolved if it is operating.

It is specifically forbidden in the plumbing code (IPC 504.6) to run it directly to a drainage system. If the floor will not be damaged by water (looks like a concrete slab garage floor) then no further "drain" is required. If the floor will be damaged by water then it should drain into an open catchment (which can run to a drain) which cannot fill up and allow waste water to siphon back into the potable water system, and which is visually obvious to the building occupants.

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It drains to a place that is adequate for a very very rare discharge of water. It's about cost. A garage floor has a perfectly good drain, the door. Installing an additional drain under the floor would be a waste of money. If this were in a finished space the whole boiler would (or should) be in a pan that drains to somewhere appropriate.

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