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Relief Valves:

icesailor Member Posts: 7,265
(( <a href="mailto:rramsour2@Message:hi">Message:

hi</a> wallie ;

thank you for your great explantion  of the  t/p vs the standard relief valves..

also  I  think another reason is that a boiler  is not considered a pressure

vessel  whereas  a   storage   tank  is  a pressure vessel.?

iam still  trying to get my head around this concept.

I understand that at high  pressure  it is harder for water to come to a boil.. 

and I think I understand why  t/p's are not used on boilers because the water

under pressure could get over  240 degees at 12 psi

but dosent a water heater operate at 40 psi ? that is street pressure from the

city. and wouldn't  the water heater get over 240 to 300 degrees? iam a hvac

parts supplier and iam attempting  my customer understand what the state boiler

inspector  is  writing him up for installing  boiler relief valves in a

horizontal postion..  also can you explan  why  the cold water  requires more

btu than the heat souece can provide ?

and I have always wondered why the btu rating on a  relief  valve must be

higher than the boiler name plate . within the last 1.5 years  the local

inspectors are writing people up on this too

thank you ))


This is an E-mail sent to me for a response and I thought it might be of interest to other Wallies. I'm in no way any kind of an expert on these subject and there are far more smarter folks here than I. And if the administrators feel that this is out of line, feel free to delete it.


Any container that can hold a liquid under pressure is considered a "Pressure Vessel". The issue of what happens when a pressure vessel fails is the issue. Does it explode under pressure and can cause harm? A gallon of milk in a plastic jug is a pressure vessel. It isn't designed to hold pressure with out cracking or the top blowing off. That's the key. The top blows off. A hot water tank is designed as a pressure vessel. It is required to have a maximum working pressure and a maximum test pressure. Which must be some percentage higher than the working pressure. I don't remember the amount but it is specified. 66% higher than the working pressure? Water boils at 212 degrees at seal level, Higher or lower depending on elevation above or below. When you boil water in a pot on a stove, the bubbles signify that the water is 212 degrees. If the water was 213, it would vaporize instantly and cause an explosion. If you have a heating boiler that was running at 12#, PSI Gauge, the boiling point of the water is about 244 degrees F. Once the water reaches 244 degrees, it takes extra energy to convert the water into steam. Hence the sounds of bubbling. The pressure will rise when the steam tries to occupy a larger space that the condensed water. The rise in pressure will cause the PRESSURE relief valve to open at its set relief pressure. If boiler is rated as a 30# pressure boiler, the pressure relief valve must not be any greater that 30#. The boiler is tested to contain 30#. If the boiler is rated for 30# and you put a 40# relief valve on it, and the boiler fails at 30#, the boiler was actually unprotected because it needed to have the pressure relieved at 30#Because the heating boiler is used only for using recycled fluids/water, it is not considered a danger to human skin and scalding. It is my minority opinion because of how and why Temperature/Pressure relief valves are rated for potable water heaters that the cold water feed should be left open on boilers. For this one reasons. Other reasons to the contrary. If you accept that 1(one) BTU is the amount of heat energy it takes to raise 1# of water, one degree in one hour, then water flowing into a boiler at 1 gallon per minute is adding 10,000 BTU's of cooling water per hour. That's only with a 20 degree difference between the hot water and the cooler water coming in. Therefore, if the water is 100 degrees colder, there is 100,000 BTU's of cooling being added to the pressure vessel. That is 100,000 BTU's per hour that the input/burner must overcome to keep the water boiling. The relief valve is rated to relieve more not water than the burner can replace. Providing the cold water is entering the tank at the stated flow. IMO.

When it comes to a potable water heating tank, the working pressure is whatever the pressure is at the moment. If the street pressure is 50# and you open a faucet, the pressure will drop momentarily until it is closed. If the tank is cold, and it is at 50#, it will rise from thermal expansion until a faucet is opened and the pressure can equalize. If for some reason, a faucet isn't opened, and thermal expansion is too great, the water pressure can go above the maximum working pressure of the tank. But the water won't boil unless the temperature of the water in the tank gets really hot and goes above the pressure in the tank at the moment. The working pressures of T&P Relief valves are marked on a metal tag on the valve, There is one that is listed as a ASME rating. Another is AGA Rated. The AGA rating is supposed to be within some percentage of the input of the burner. The ASME is a higher number. But for water heater purposes, most use the AGA rating. Problems occur when water is overheated in a water heater pressure vessel, and the water temperature os over 212 degrees but kept from boiling because of the pressure. If or when the pressure drops (like opening a faucet or tank failure), the water will IMMEDIATELY convert to steam and try to occupy a space 1200 times larger than the tank it is in.

So it gate back to the pressure relief valve on a heating boiler standing up in a vertical position. It is my opinion that the designers and code officials want the entire seat area to be exposed to the expanding water vapors/steam for the maximum amount of pressure being relieved by the STEAM. The steam has already gathered the extra BTU's required to convert to steam. The liquid water gas not. "They" don't want that water that can absorbe more heat energy to be wasted out of a relief valve. A valve installed in a horizontal position, will only relieve the steam/gas at the very top, and also relieve water.

Potable water heater T& P Relief valves are set to start relieving at 210 degrees, below the boiling point. Once the 210 degree temperature is reached, cooling water enters the tank like a faucet being opened. The valve is required to be located in the top 1/3 of the tank, with the sensing probe inserted into the top 1/3 of the tank. Where the hottest water is. The valve will operate with high temperature or high pressure. And both. The boiler relief valve will only operate with pressure. The intention with both is to get rid of excessive heat energy if needed.

I personally (I might have been wrong to do this, forgive me) installed all Pressure Relief valves in a vertical position on boilers but on most, I put a tee in and a short nipple and ell to put the valve off to the side pointing up. I put a reducing coupling on the straight run with a float vent to capture steam bubbles that seemed to form. The air/steam would be re-absorbed in the chamber. It kept the steam/air vapors off the valve seat which caused them to foul and fail. It cut down on replacing relief valves on boilers.

I hope this helped. Its my opinions. Anyone is free to disagree with me. Its just my experience.


  • Henry
    Henry Member Posts: 996
    P & T relief valves

    Here is a good video on why you need P & T valves on hot water tanks. I have seen one that went through the floor into the ceiling above!

  • icesailor
    icesailor Member Posts: 7,265

    If Mythbusters has available videos for viewing, they did one recently that is better than that one. They took an electric water heater and worked their behinds off to get it to explode. They could have made it worse, but it was just as spectacular.

    The Watts film is using a Brown Bros. Copper Tank. The two ends are soldered together with a copper band with 50.50 solder. Once the water gets to 360 degrees, the tank comes apart and the water instantly turns to steam. They powered ships and locomotives for railroad trains with that.