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Tmb9862 01-06-2007 01:01 PM

Air in boiler / Over pressurizing
This is a hot water boiler, the pressure reducing valve seems to be set at around 20lbs. It stays at 20psi usually but sometimes the blowoff valve goes, and other times it won't go off for a few days.

This started when the relief valve began leaking. It was dripping last winter, then it sat for the summer and the relief valve didn't leak as the boiler wasn't being used. So I'm not sure how long exactly this problem has been there as it wouldn't have over pressureised since is was constantly de-pressureizing.

Now a new 30psi valve is installed I bleed the radiators, air comes out, then a week later their seems to be air in the system again. I don't know if I just have a lot of air that is being forced out or if I'm getting it in there somehow?

Additionally the expansion tank is full of water. I know years ago a plumber increased the pressure of the boiler, it may be possible he didn't increase the pressure of the expansion tank as well right? The tank is pretty old, probably 30+ years, no tags or markings, and no valve that I can find to add/remove air. Were the old tanks just unajustable? I tried shutting off the supply to the expansion tank and draining it. I got about seven gallons of water (tank has to be around 20) and it stopped comming out or rather slowed down to almost nothing. At this point the tank was still full of water. I turned the water to the tank back on and it took those seven gallons of water right back.

This is the expansion tank I have.

#CARRIERMAN 01-06-2007 03:26 PM

Hi tmb9862

I am not quite sure why the reducing valve is set for this high of pressure. Typically speaking all that is required for a hydronic system is 12 to 15 lbs of pressure. The high pressure is more than likely what caused the pressure relief failure. My reccomendation is to get the pressure down on the reducing valve, if you cannot you may need to replace or rebuild the existing pressure reducing valve.

Good luck

trollmastergeneral 01-06-2007 04:25 PM

IF you have a full expansion tank this will cause higher presure,sometimes the gauge may be may have to hook up another guage to see if the other guage is acurate you can attach another guage to the boiler drain with a few adapters from your local hardware store.This way you will be able to know if the safety valves is going off at 30 psi or if it is defective and needs to be replaced

#CARRIERMAN 01-06-2007 04:44 PM


Trollmaster is right, a bad expansion tank can cause this to happen also. Good call trollmaster.


Tmb9862 01-10-2007 06:33 PM


Originally Posted by #CARRIERMAN (Post 29155)

Trollmaster is right, a bad expansion tank can cause this to happen also. Good call trollmaster.


I shut off the boiler, turned the pressure down to 10psi, drained the expansion tank the best I could (I get about 5 gallons out of it but it must be a 20+ gallon tank and the thing weighs a ton so its full of somthing), and opend the valve to the expansion tank again. The water rushed right into the tank.

Afterwards I set the pressure back at 15 and turned the boiler back on. It's no longer blowing water out but the pressure goes up to about 25 when the water gets hot then seems to go back down when its cold.

Sounds like an expansion tank right?

If I have to change the tank can I get one of those nice little 4 gallon or so ones or do I need a monster like I have?
If I have to change it when I take it down I can just drill a hole in it to drain it right?

#CARRIERMAN 01-10-2007 09:25 PM

Hi Tmb9862

If your expansion tank is completely full of water, there is no doubt that it has a leak in it. You should be able to install a small self contained expansion tank on this system. However keep in mind, it has to be for hot water to work. A standard expansion tank will probably not even last a season. I can't remember the manufacturer of the one I use off the top of my head. But it's part # is ET15. Hope this helps.

Good luck

Tmb9862 01-11-2007 08:24 AM

How do I find out what tank is large enough? Their is no BTU rating on my boiler that I can find. I do know the system must have a fairly large capacity though. About 6000sq of house with cast iron radiators and it's a converted steam system (large pipes).

Will somthing like this handle it?
Should this be adaquate?
The next biggest one I see is this:
Anything bigger than that has a 3/4 or 1in inlet and I currently have 1/2in pipe. I'm pretty sure that's against some code and I don't really want to re-pipe it.

#CARRIERMAN 01-11-2007 11:40 AM

Hi Tmb9862

The 4UN87 that you have posted here is the most common on a low pressure hydronic system. The common misconception about a expansion tank is that it takes a huge tank to do the job. In most residential applications thats not so. All an expansion tank does is act like the shock absorbers on your car. As the pressure builds on the system it allows for a steady pressure. As far as repiping goes, you will want this expansion tank as close to the air scoop if thats what the original tank is hooked to. Basically all you will need to do, is drain the old expansion tank. Where the 1/2" copper going to it is hooked up on the boiler. Remove this at the boiler connection and screw your new expansion tank on at this point. Make sure to use a good grade pipe thread sealant, you will some day need to replace this one, you will want to be able to get it back off. Any further questions, stop back in and see us.

Good luck

redline 01-11-2007 12:10 PM

Is there a shut-off valve between the expansion tank and the boiler?

Tmb9862 01-11-2007 12:14 PM

I don't believe I have an air scoop. This boiler is between 30 and 40 years old, did they use them then? Should I add one?
I have the pressure regulator, immidiatly after that it T's off, one part goes into the outgoing water supply for the radiators, the other part goes to the expansion tank.

Can I stick my new tank right where my old one was or does it have to be moved? If it has to get closer to the boiler I can run that T a little towrds the old expansion tank and have it right over the boiler. I would then have to support it with straps hanging from the celing I suppose.

It's a little more than just a screw/unscrew operation. If I want to put it in the ceiling I'd have to add some pipe as the inlet is currently on the bottom of the horizontal tank. If I put it right above the boiler I'd have to add some pipe and put a threaded fitting on.


Originally Posted by redline (Post 29681)
Is there a shut-off valve between the expansion tank and the boiler?

Yes. It is visible in the picture. Blue handle at the top of the picture.

#CARRIERMAN 01-11-2007 12:47 PM

Hi Tmb9862

The pipe just above the tee marked expansion tank. Shut the boiler down and drain it down enough to cut the pipe. Cut the pipe about 2" above the tee, install a 1/2" copper female thread adaptor. The rest will be history. You will need to make sure to bleed the air back out of the system when you are done. You may be able to find the piece I said in a compression half union style, this will save you from having to do any soldering.

Good luck

Tmb9862 01-11-2007 01:08 PM

I'd rather soder it, I've seem compression fittings go bad much more than soldered joints.
What you say can't happen because of clearence. What I propose is in the picture, that should be fine, right?

Also is their a reason the new one has to be closer to the boiler?

#CARRIERMAN 01-11-2007 01:22 PM

Hi Tmb9862

The answer to the first question, that is fine. To the second question, the smaller expansion tanks react about twice as fast as the old ones due to the bladder. The further away you put them the more exaggerated the action. This will sometimes but not all of the times lead to nuisance relief valve trips. The old expansion tank you have used a air cavity above the water to do the same thing. Hope this helps, I don't mean to confuse you.


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