Water Heater Temperature & Pressure Relief Valve Discharge Pipe

A few installers have raised a question about the discharge pipe for the water heater Temperature and Pressure (T&P) relief valve. Ultimately, should the water heater T&P discharge pipe be run to the exterior of the crawl space? 

I did a little research and quickly discovered that this issue needs much more discussion than is provided in the manufacturer’s installation instructions, or that we can accomplish here! But we should start thinking about it.

The first thing to understand is the job of the T&P valve. If the temperature in the water heater reaches 210° or the pressure reaches 150 psi, the relief valve will open reducing the pressure or releasing heated water to be replaced with cold water. This is  to prevent a potentially catastrophic failure. Just image the power that can be generated when the water starts to boil inside the tank! There have been reports of water heaters exploding and propelling themselves through the walls causing significant damage to the home.

The amount of heated water that can be expelled through the T&P discharge pipe can range from a trickle to a gusher! Either way it enough to cause severe burns!

The manufacturer’s installation instructions don’t offer much guidance. I reviewed several “Complete Installation Checklists and they all state “Dryer vent, range/cook top exhaust, water heater temperature and pressure relief overflow pipe and AC condensate drain installed to the perimeter of the crawl space”.

If you look a little deeper in the manufacturer’s installation instructions, several state “If the home is to be installed on a basement or enclosed crawlspace, install the drain pipe connecting the discharge from the water heater temperature and pressure relief valve to the outside or to a sump”. I did find a few exceptions that clearly state that the T&P may discharge under the home. But bottom line, there is no consistency in the various manuals, and no direction as how to safely extend this discharge pipe to outside of the crawl space.

In my opinion, the installation of vinyl skirting around the perimeter of the home meets the criteria of a crawl space, and I have yet to see any definitions that say differently. So, I guess they are saying to run the T&P discharge pipe outside the skirting.

T&P & water heater pan pipes discharging under the home.

Finally, I double checked the Manufactured Home Construction and Safety Standards (245 CFR 3280.609(c)(iii). It says the “relief valve outlet which shall be directed downward and discharge beneath the manufactured home”. 

So, what should a professional installer do?

If you should decide to extend the T&P discharge pipe to the exterior of the crawl space, keep these things in mind:

The T&P pipe should have an elbow directing any discharge downward.

Any water being discharged from these T&P valves is extremely hot with the potential to cause damage to people, pets or property in the area. Make sure the pipe points downward to about 6” above grade to reduce the possibility of injury! Make sure it is visible, but away from an area where people or pets might be injured.

Any extension of this pipe must be the same size as the discharge pipe attached to the valve. Keep the piping as short as possible (less than 30’ total length), with as few elbows as needed (four elbows are the maximum permitted).  Any horizontal sections must slope away from the valve. Do NOT install a trap, shut off valve, or a cap on the discharge pipe!

Never connect the T&P discharge pipe to the DWV (drainage) system of the home. Should the valve become defective, the occupant would never know, and in the event of a violent discharge, the DWV system could be damaged and people possibly injured. Not to mention possible contamination of the water supply!

I suggest a discussion with the local code enforcer to get his take on this issue. Also, review the water heater instructions and see what they say. Contact the home manufacturer’s engineering department for their input as well.

Unfortunately, there may not be a simple, straight answer to this question. But maybe this discussion will help us to make the best decision to assure the safe operation of these water heater T&P valves.

I would appreciate hearing your comments on this issue!


One thought on “Water Heater Temperature & Pressure Relief Valve Discharge Pipe

  1. I have asked this same questions many times over the years. I have been told by the manufacturers to discharge beneath the home. I had a couple of years that we did discharge the relief to the outside perimeter, but stopped when I learned that I am not allowed to make that determination. Greta topic Mark, I look for more Clarity in the future.


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