Both the ASTM (American Society of Testing & Materials) standards and the plumbing codes require that the sensing probe must be in the upper 6" of the water heater pressure vessel.

Typically they are side mounted on gas water heaters while they are commonly top mounted on electric water heaters. Either position is fine providing the T&P valve sensing probe is extending into the upper 6" of the internal pressure vessel.

Years ago water heaters did not have Temperature & Pressure Relief Valves. Instead they had "Pressure Blowout Plugs" which look like the freeze plugs in an automobile engine block.

When T&P valves first became available water heaters only had two threaded ports, hot & Cold, so they would install a close nipple and Tee on the Hot water discharge line and place the T&P on the top of the TEE with a long probe that extended through the TEE and nipple and into the upper part of the tank.

If your old water heater has a third threaded tap on the pressure vessl where the probe is extended directly into the water in the tank you may use a standard T&P valve. If the T&P is on top of a tee you must use an extended probe type valve.

If you need an extended reach type look for a Watts 100XL. The XL on the model number identifies it as an extended length probe.

Thank you much for the info. This valve is located directly on top of the tank in a threaded tap(not on a tee)
But the probe in the new valve only extended out maybe 3 inches, so I know it isn't 6 inches into the tank. Can you tell me where to get the Watts 100XL? I think I am going to replace this....just to be safe.
Thanks again,
R

Addendum---So sorry, but after re-reading your posting and my response it looks like I didn't understand what you were saying. I will try and re-word the response as well as I can, and do believe I know what you're saying.
----The valve is in a third threaded tap (on top) and isn't in a tee, do it goes directly into the tank. I understand what you said that it must be located 6 inches from the top of the tank (so disregard the previous msg about it not extending down six inches) I guess my question is concerning the water level itself. Does the water level actually extend to the top of the tank? If that's the case then I believe this shorter probe will in fact reach the water. If there is a space between the tank top and the water level then I think I need the longer probe.
Thank again for the great info !!

The top end of the inner pressure vessel is dome shaped with the ports at the high part of the dome. This insures that the tank will remain full of water and no air will be trapped in the water heater.

If your T&P valve is mounted into a threaded port fitting on the inner vessel than a standard length T&P valve is fine.

Remzy,

I was in a similar situation a few years ago. My T&P valve started "burping" about a cup of water once or twice a day. Did some research and decided it was a defective T&P valve. Put the old T&P valve in my pocket and went to the local hardware store (yes, still have one of those) and they looked at my old valve and said "they are all the same" and offered me one that was not the same length and more important, the temp was different. I thanked him and off I went...

... To the Home Depot where they only had one length and one temp (the wrong temp) and maintained thet "they are all the same". Off I went.

Third time was the charm, another small hardware store and was asked "did you bring the old one?" aha, the right question! I offered it up and the gentlman showed me where the part # is cast in and the temp also. I knew that but was waiting for someone to ask or show me before I bought a new valve. We ambled over to the correct isle and he produced T&P valves with both lengths and both temps. He offered me the correct replacement for $.18 less than the Home Depot and away I went.

Turned out, the T&P valve was not bad and was doing it's job. I had a thermal expansion problem and installed a thermal expansion tank. Was a good idea to replace the T&P as they are safety devices and not designed for repeated operation.

I probably could have gotten the correct valve at a plumbing supply but, at least around here, they are abusive and not at all interested in selling to civilians.

The moral to this story is that, often the answers to our questions are right there on the old part. When told "they're all the same" my reply is "then why do they make different versions. Before you replace anything with a part that is not the same as the one you took out, especially if it is a SAFETY device, MAKE SURE you didn't buy it from the first two people I ran into.

The guys at the hardware store were correct. all T&P valves are made to the same temperature rating.

The ASTM specification says a T&P valve must be rated at 150psi & 210degF.

Do not confuse the BTU rating on the certification tag as a temperature rating. The BTU rating defines the diameter of the internal orifice and the diameter of the output discharge port. The minimum BTU rating of a T&P valve must be equal too or greater than the maximum BTU input rating of the burner or heating elements.

It is also vitally important that the discharge pipe must be equal to or greater than the diameter of the T&P discharge port. Down sizing a discharge port line one nominal trade size will reduce the relief capacity of the T&P valve by 50%. In fact, if the length of the discharge line is greater than 40' we are required to INCREASE the size of the line by one nominal trade size.

Lazypup, you are da man but with respect...

Watts, the manufacturer of many or most T&P valves seems to offer than one T&P valve http://www.watts.com/pro/_products_sub.asp?catId=64&parCat=294

I was in error in my previous post, I saw different length and presure T&P valves not different length and temp valves.

Agreed, but when I was shopping for my replacement the first two places wanted to sell me the "they are all the same" with a much shorter probe. That's didn't seem to make sense to me.

The third place had the standard(?) length probe and the "x(tra)l(ong)" which was what I needed.

I can see where the "xl" might be an approved replacement for the shorter probe but is the shorter probe an approved replacement for the "xl" probe?

Just seems to me that a logical approach is to take the old one with and replace it with what you took out unless there is documentation saying the substitute is acceptable.

There are far too many badly or untrained people selling stuff that live by the creed "they don't make that", they used to make that and this will work", and the famous "they never made one like that" and then you open your hand to show them one.

What may be the three most valued words in the english language are rarely used in retail anymore... and they are "I don't know".

In regards to you observations about retail store employees I could not agree more.

In fact, while I don't recall ever seeing the law enforced, in many regions it is illegal for a store clerk to offer recommendations of the suitability of a plumbing or electrical product unless that clerk is also a certified journeyman in the respective trade.

Those of you who have had the experience of shopping in a trade specialty plumbing or electrical store know only too well that the clerks will go to great length to find the item you specify or perhaps an equally interchangable item from another manufacturer but they will not determine the type, size or quality of the item you need. If you don't have your specifications they will simply tell you that you will need to provide more information.

Hey guys, Thanks so much for the info. I really appreciate the help. I'm in the middle of 'Wal*Mart' country....and the last REAL hardware store succumbed to them a few years back. In fact now there isn't a hardware store for 40 miles or so. Got the same dumbfounded look and response as justalurker at Home Depot and Lowe's. Both stores only offered ONE valve, but I knew the one I removed was different. Which is why I turned to this site for help. Did we really once live without the internet?.....but I digress. Anyway, thanks again guys !
Remzy