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Old 10-11-2009, 01:33 PM   #1
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Boiler: Changing T&P Guage


I have Weil-Mclain CG-4 88K BTU Boiler installed in 1994.

The T&P Guage has water dropplets forming on the inside of the glass - enough so it is hard to read the guages..this is actual water not fog.

I assume this guage is going bad so I need to change it - does anybody disagree?

MY boiler has an inlet shut-off valve but no return shut-off. IT also has an old style expansion tank.

I assume I have to drain the system to replace the Guage.. since I can't isolate the boiler from the rest of the system. I can isolate the expansion tank from the boiler. IS this Correct?


Tx,

Tom


Last edited by roztom; 10-11-2009 at 01:53 PM.
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Old 10-11-2009, 01:43 PM   #2
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Boiler: Changing T&P Guage


Drain it all. The expansion tank will recharge itself with air when refilling.

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Old 10-11-2009, 01:50 PM   #3
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Boiler: Changing T&P Guage


Thanks.. SO drain the whole thing down..

DO I just put some plumbers pipe thread paste or thread tape on the Guage threads?

Also if I use paste, does it have to set before I put water back in the system?

WHen I refill the system do I have to do anything special with the expansion tank ?

I assume I just put some plumbers dope on the threads of the new guage and refill and bleed out the boiler/system, anything else I need to know?

BTW: IT seems that Honeywell makes a T&P Guage that will fit the Weill-Mclain, are they pretty much interchangable? Costs about $30.

Tx

tom

Last edited by roztom; 10-11-2009 at 01:54 PM.
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Old 10-11-2009, 02:01 PM   #4
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Boiler: Changing T&P Guage


Tape should be fine. would not even touch the tank drain it replace the gauge refill, bleed it and you should be good to go. I always try to use OEM parts but if a generic will fit it should be ok
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Old 10-11-2009, 02:06 PM   #5
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Boiler: Changing T&P Guage


THanks for the help!!

I assume the water droplets on the inside the guage glass are reason enough to change this out, do you agree?

Tom
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Old 10-11-2009, 02:11 PM   #6
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Boiler: Changing T&P Guage


Before you go and drain the system down check the return and supply for shut off valve. Some installers will install valve so that you can isolate the boiler without draining the system and fighting air pockets later because and auto bleed doesn't work
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Old 10-11-2009, 02:17 PM   #7
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Boiler: Changing T&P Guage


Thanks. I'm one of those lucky guys that doesn't have a return shut-off valve....
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Old 10-11-2009, 03:14 PM   #8
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Boiler: Changing T&P Guage


I noticed that there is a nut right behind the guage that looks like it goes into the stem and then the step has a nut/fitting that goes into the boiler. Are these one in the same or can/should these pieces be seperated or replace the whole deal??

Also I am looking for an online source is there one that is well known?

Tx,

tb
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Old 10-11-2009, 03:54 PM   #9
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Boiler: Changing T&P Guage


Never drain a water boiler unless you absolutely have to.
Fresh water is an enemy of hot water heating systems. It can cause corrosion.
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Old 10-11-2009, 04:07 PM   #10
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Boiler: Changing T&P Guage


IS there any way to replace this Guage without draining the boiler? I do not have a return shutoff valve.

tb
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Old 10-11-2009, 04:22 PM   #11
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Boiler: Changing T&P Guage


Yep.

But hard to explain over the internet.

Need to put system into a vacuum(the part that can be hard to explain when not there). And have the new guage sitting there ready to be changed instantly.
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Old 10-11-2009, 04:33 PM   #12
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Boiler: Changing T&P Guage


All things considered I only have the inlet shut off and/or the auto fill valve... so - what else is there?

If I close everything off, as soon as I pull the Guage or even lossen it, air will get in and water will come out... or will it take a minite or so before it starts coming out? What else is there?

tb

tx
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Old 10-11-2009, 05:14 PM   #13
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Boiler: Changing T&P Guage


Shut off the auto feed.
Close the valve between boiler and expansion tank.
Connect garden hose to boiler drain cock, and run the hose to a drain that is lower the the temp guage.

Have the new temp guage ready to install(threads already taped or thread dope on them).

Open boiler drain cock. Wait till water stops flowing(I usually loosen old guage slightly before opening boiler drain) boiler .
Quickly turn out temp guage and put new one in very quickly(I place new temp guage right infront of me while I do this).
As soon as you have the new guage turned in a thread or 2, close boiler drain.

If you do everything right. Less then 1 cubic foot of air will enter the boiler.

Then open all other valves that you closed.
If you have rads, run heat for a couple minutes, and then bleed rads.

PS: I would also drain the expansion tank after changing the temp guage. Hard to tell how much water it has in it, so its easier to drain it while your there. then clean up water in an hour or a day or 2.
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Old 10-11-2009, 05:55 PM   #14
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Boiler: Changing T&P Guage


That sounds good.

Will water also be squirting out of the Guage connection? How long will it hold before it does? I do get it..

The water draining from below the guage will pull a vacumm - I guess it has to do with the drain flow from a lower point being greater than the drainage from the guage inlet. Is that kind of it?

Tx,

Tom
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Old 10-11-2009, 06:03 PM   #15
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Boiler: Changing T&P Guage


Thats pretty much it.

You don't have much time. The longer the guage is out. The more air drawn into the boiler. And after so much, water can and will come out the guage port.

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