2 water heaters, combined TPR and pan drain line

Todays inspection had 2 water heaters. The TPR drain for each meet at a ‘T’ in area between them, then travel 2’ to the rear, 90 degree angle down, 90 degree angle toward the exterior.

Can these lines be joined together? If one were to blow, the discharge would first travel to the other unit, then to the ‘t’.

The drain pan is engineered the same way.

Don’t these units each need a drain for the TPR, and the pan?


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The discharge piping serving a TPR valve should:

  1. Be constructed of an approved material such as CPVC, copper, polyethylene, galvanized steel, polybutylene, polypropylene, or stainless steel.
  2. Not be smaller than the diameter of the outlet of the valve it serves (usually no smaller than 3/4").
  3. Shall not reduce in size from the valve to the air gap (point of discharge).
  4. Be as short and as straight as possible so as to avoid undue stress on the valve.
  5. Be installed so as to drain by flow of gravity.
  6. Not be trapped since standing water may become contaminated and backflow into the potable water.
  7. Discharge to the floor, to an indirect waste receptor, or to the outdoors.
  8. Not be directly connected to the drainage system to prevent backflow of potentially contaminating the potable water.
  9. Discharge through a visible air gap (atmosphere) in the same room as the water heater.
  10. Be first piped to an indirect waste receptor through an air gap located in a heated area when discharging to the outdoors in areas subject to freezing, since freezing water could block the pipe.
  11. Not terminate more than 6 inches (152 mm) above the floor or waste receptor.
  12. Discharge in a manner that could not cause scalding.
  13. Discharge in a manner that could not cause structural or property damage.
  14. Discharge to a termination point that is readily observable by occupants because discharge indicates that something is wrong.
  15. Be piped independent of other equipment drains, water heater pans, or relief valve discharge piping to the point of discharge.
  16. Not have valves anywhere.
  17. Not have tee fittings.
  18. Not have a threaded connection at the end of the pipe so as to avoid capping.
    See 15 and 17


Thanks Gary,
Is there a reference code #?

No it’s fron the NACHI Water Heater TPR Valve Discharge Piping mini-course.Gary

I know that recent IRC requirement do not allow the TPR drain pipe to have a T. Does anyone know how long this has been a requirement?

2003 IRC:

TPR piping should never be PVC. Metal pipe (preferably copper, it doesn’t corrode) and drained to within 6" of the floor or to the exterior.

Just because Home Depot sells it, that doesn’t mean its correct. :mrgreen:

CPVC is allowed by many AHJ around here, even though 3/4" CPVC is undersized for this application. The interior diameter of CPVC is 0.71". The code and TPR valve manufacturer require it to be 0.75" the ID is .04" smaller, so go figure how they came to reason this is OK.

Until the code changes I continue to write them up.

Regardless what Linda has depicted is wrong.

Where does one get a copy of the national plumbing code book? I checked Lock Supply this afternoon and they had a 2006 copy of the International Plumbing Code for $63. They said there is a national code book but they can’t get it.Thanks


Barry, are you an electrician?:mrgreen:
Where did an ID of 0.71" come from???


I won’t even comment on Will’s “copper doesn’t corrode” fairy tale.

The NACHI guidelines have several wrong and or inconsistent remarks. Why try to reinvent he wheel. Just say what the manufacturers say and the codes say. Very simple. IMO, this needs to be removed as it could get inspectors into trouble.