Q & A Temperature and Pressure Relief All information that follows are opinions of individuals. The "Authority Having Jurisdiction" (local building department) has the final say in whether an installation is "to code".
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Q & A Temperature and Pressure Relief All information that follows are opinions of individuals. The "Authority Having Jurisdiction" (local building department) has the final say in whether an installation is "to code".
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The setting on the relief valve on a domestic water heater is 210 degrees F. May the discharge pipe be plastic, PVC, CPVC, Etc.? The BOCA mechanical code M605.5 mandates that pipe be rated for the same temperature of system. Is the temperature of the system the temperature of water in the tank(140 degrees F.) or temperature setting of the relief valve? Lavi Dania lavidania@aol.com , Jan .2,1997
Central life safety issues
- Explosive potential of hot water heaters
- Scalding from hot water heater pressure relief blowoff.
Facts
- Water can scald on contact at temperatures as low as 110 degrees F.
- Water, when it changes state to a gas(steam), expands 1700 times.
- At sea level atmospheric pressure, water boils at 212 degrees F.
- Temperature relief valves are set below 210 degrees F.
- Pressure relief valves are set to 150 PSI.
- Under pressure, the boiling temperature of water goes up.
- At 150 PSI water boils at 358 degrees F.
Principal
Without protection, a domestic hot water heater whose thermostat has failed would see a continous rise in temperature and pressure(from the expanding water). This temperature and pressure would continue to rise until the pressure exceeds the pressure capacity of the tank(300 PSI). In the same instant the tank bursts, the superheated water would instantly boil and expand with explosive force. The danger of superheated water like this flashing over into steam inside a steel container is obvious. It is a bomb.
To prevent such catastrophic failures, water heaters are required to be protected for both over temperature and over pressure. Usually this is done with a Temperature and Pressure Relief valve. (for more Code Check information)
Discussion re: T&P Relief Piping
Here is where things get a little confusing. In the city where I am a building inspector, we use IAPMO's Uniform Plumbing Code. Section 608.5 says the material for T & P R piping may be; galvanized steel, hard drawn copper, CPVC(PVC inadequate), PB or listed straight sections of relief valve drain tube. We, therefore, except CPVC or PB(but rarely see it). I can only assume it would be the same for BOCA and SCCBI, but I will look into this.
Only problem with the allowance of CPVC or PB for this application is that their temperature ratings are well under 210 degrees F. (CPVC=165 degrees F., PB=?__?), certainly well under the temperature of steam at 150 PSI.
The consensus amongst inspectors I've talked to is that the acceptance of these materials is adequate because the pressure relief of an over temperature or over pressure condition is so intermitant and short term that CPVC or PB are adequate materials for this application.
Semantics - 'The System' in this case is not the hot water reticulation system or the water in the tank but the system to relieve excessive pressure. This system operates at 210 degrees fahrenheit ( if we can trust the relief valve to be accurate ) so the discharge piping should be rated for 210 degrees.Plastic pipe is pressure rated in terms of temperature so before discarding it as an option we should look at the maximum pressure to which it will be subjected. This can be calculated (Hazen Williams formula and others) and then a detemination made taking into account flow rate and length and route of pipe. My guess is that the pressure would in most cases be low enough to admit CPVC pipe and presumably the code writers guessed the same way or were pressured into it by the plastics lobby!
Check ©1998
by Redwood Kardon