View Full Version : Changing an Expansion Tank & T&P Valve

09-18-2009, 10:52 AM

Can one of you provide some detailed instructions on how to safely R&R an expansion tank mounted above my boiler and also how to R&R the T&P valave mounted ont he boiler?

Also, do any of you have an idea of what the pressure reading on my boiler should be? This morning about 5:00 am when everyone was asleep and the hot water had not been used for over 6 hours the pressure was about 10 psi, dropped to 5 & then almost zero psi while I took the photos. The boiler came on and pressure went up to about 25 psi.

I have attached as many pictures as I thought would be needed to help.
1) What valves do I turn off?
2) Do I need to drain the 81 gallon pressure tank?
3) Do I need to turn the gas to the boiler off?
4) How do I release pressure in the expansion tank when water comes out of the schrader valve?


09-18-2009, 12:21 PM
If your pressure is dropping to zero, you may need more than the T&P and expansion tank...the T&P may not be resealing, or you have another leak since it normally is set to relieve pressure at around 40# or so. Your boiler should say what its normal operating pressure is...typical is around one atmosphere 14-15#, and you'd set the pressure of the expansion tank before installation to the normal operating pressure.

Shut the boiler off. Relieve pressure by opening a drain valve. If it has an autofill valve, shut that off. You don't need to drain the whole thing. If the water is hot, you may want to wait awhile, since you'll probably spill some, and you don't want to get burned. If you have any valves around the expansion tank, you could close them to limit how much water drains when you take it off.

The expansion tank is probably full of water, so when you unscrew it, support it well, it will be heavy. The new one is mostly air, and will be light. Make sure to use either pipe dope and/or teflon tape on the threads when installing the new one. You should have enough strength with that tank to hand tighten, no need for any tool to install or remove.

The T&P valve likely has a drain line attached. To unscrew it, you'll need to first remove that. If you don't have any soldering skill, it is time for a pro. Same thing on it, the threads need sealant - pipe dope and/or teflon tape. You may need a pair of pliers or a wrench to ensure it is tight enough not to leak since it is nowhere near as big around. you'll likely need them to remove it as well.

09-18-2009, 02:46 PM
Thanks for the reply..again! :) You are really helpful.

I definately know there is water in the expansion tank so that needs replacing.

I already replaced the T&P valve on the indirect water heater about 19 months ago when it was a little less than 6 yrs old. The boiler T&P valve has never been replaced and it looks like it has been leaking, as there is white stuff on the pipe the T&P valve is screwed on to.

Replacing those two things will be a start. I will have our resident neighborhood plumbing expert on-hand to help in case anything goes wrong.

I will write back with the results later this weekend.

09-18-2009, 05:54 PM
Jadnashua, I'm glad that you state you are not a pro because you have written things that are not true.

Firstly, boilers do NOT have T&P valves. The safety valves on boilers are actuated only by pressure, not temperature.

Secondly, there are very few residential boilers that have an operating pressure of 40 psi. Most are limited to 30 psi.

Thirdly, you are off slightly on the cold pressure. Most are between 12 and 15 psi with the majority being on the lower end. Homes with more than 3 levels may be a bit higher than 15 psi cold.

I can't unlock the pictures so I can't comment specifically on Amkazen's system but tightening the expansion tank by merely twisting the tank itself is WRONG! There is a hexagon on the attaching fitting for a reason and that is to use a wrench. Tightening by twisting the tank puts undue torque on the fitting-to-tank weld.

The safety valve on residential boilers should be replaced every five years. Use a wrench to tighten the new valve in place. You can use a shark bite fitting to replace the drain piping or you can install threaded pipe using 90 degree elbow and unions. The stem on the safety valve should be vertical in a proper mounting arrangement. If the safety valve tapping is in the side of the boiler use a short nipple and an elbow to change it so the safety valve stem is vertical.

09-19-2009, 11:30 AM
Boiler pressure relief valves are available from around 30-150#, so use whatever your boiler specifies. The two boilers I've had, had low-pressure switches that cut it off if the pressure wasn't above 12, the systems were set to 14 on the autofill valves...not sure if they came set to that value, or were adjusted to it. Most boiler's high temperature limit safety controls would shut the thing off before you'd boil the water, but if not, the pressure increase would cause the valve to open as the temperature rose with the steam. I was under the (mistaken) assumption that the relief valve could also be triggered by temperature, but the idea is the same...on a run-away situation, if the overtemp safety didn't shut things down, the relief valve would open so you didn't get a steam explosion. If it continued to run and it ran out of water, it would melt down, but numerous safety circuits would need to break first.

09-20-2009, 05:41 AM
Furd, thanks for clarifying what I thought was a T&P valve on the bolier is actually a safety valve. And, also on how to remove the expansion tank. I am a little confused about the people not being able to open the pictures. I juts went to my post, clicked on one of the picture links, a small window popped up and gave me the option of opening, saving, or cancelling. I selected open and a WinZip window opened up, listing all the pictures. At this point the pictures can be extracted or just clicked on & viewed. WinZip is a common application that has been around for as long as I can remember and I believe Windows XP can unzip Zip files without installing any additional utility.

jadnashua, thanks for your help also. You have given me some good directions and thoughts on how to proceed.