Expansion Tank Draining - Boiler Pressure Not Holding

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Old 11-20-07, 05:20 PM
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Expansion Tank Draining - Boiler Pressure Not Holding

Hello,

We have a two story house with a hot water system. The boiler is located in the basement. Few years back, I have installed a new boiler (HydroTherm), but the expansion tank was left in place (steel cylindrical type, mounted in the basement ceiling, approx. 12" dia by 36" long). Until now we had no problems. Last Saturday, for some reason I decided to drain the expansion tank and since then the pressure on the boiler keeps going up to 30 psi tripping the pressure relief valve.

What did I do wrong? Is there a special way to drain the expansion tank? What are the steps to properly drain and refill the expansion tank? Any help and feedback is greatly appreciated.

Thanks in advance
 
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Old 11-20-07, 05:33 PM
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You could possibly have unintentionally waterlogged the tank ...

There should be a shutoff valve in the line from the boiler to the tank. You need to close that. Connect your drain hose to the drain and get ALL the water out. Close the drain valve and re-open the valve in the line leading to the tank.

SOME water will go back into the tank, this is normal, but there will be an 'air cushion' above that water.

When I say get ALL the water out, you should be aware that the tank may 'vacuum lock' when partially drained, and no more water will come out. It's the same principle as when you put a straw into a glass of pop, put your finger over the end of the straw and lift it out. The pop stays in the straw.

A larger diameter drain hose works best.

There are a few tricks... you can blow into the end of the hose if you have healthy strong lungs and don't mind possibly getting yucky boiler water shooting out in your face.

You can use an air compressor to blow air into the hose.

You can close the drain, remove the hose, drain the hose, re-connect it, and open the drain again. Repeat as necessary.

You can use a bucket instead of a hose because the water will flow easily and air will be able to get into the tank. Be prepared for many buckets full.

Bottom line is all the water has to go, and you must have a valve in the line to the tank which you can close to isolate the tank.
 
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Old 11-20-07, 06:01 PM
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I think I did this (waterlogged the tank) first time when I drained it. I did not close the valve that is between the boiler and the expansion tank. That was Saturday, on Monday I drained the tank the second time with the valve closed. I also closed the boiler feed valve. I used the bucket instead of the hose. I seemed like all the water came out, toward the end it was gurgling and the water was coming out. Then, I closed the expansion tank valve, opened the water feed valve and the tank started to fill up. The boiler ran for a while at approx. 20 psi and then the pressure gradually started building up until it was almost 30 psi and the pressure relief valve started dripping water out. Is it something that I'm doing wrong when I start opening the valves? Should the boiler be running before I open the expansion tank valve? Do I even have to close the boiler feed valve before draining the tank?
 
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Old 11-20-07, 06:12 PM
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If you can do any piping, you could hang a bladder type from where you expansion tank is now and then just find a strategic spot for an autovent and have a good solution for under $100.
 
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Old 11-20-07, 07:26 PM
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Originally Posted by sborgula1 View Post
...seemed like all the water came out, toward the end it was gurgling and the water was coming out...

...The boiler ran for a while at approx. 20 psi and then the pressure gradually started building up until it was almost 30 psi and the pressure relief valve started dripping water out.
If it was gurgling, you may have gotten all the water, but it's possible that there were still a few gallons in there. If you heard gurgling all the way to the end, then there obviously had to be water still in the tank... one way to be sure it's empty is when you THINK it's empty, hook up a short hose and blow into it. Be prepared with a bucket to stick the hose into quickly, and to get your face out of the way when you stop blowing, JUST IN CASE!!!

If you are starting out with a cold boiler at 20 PSI, that's probably a bit on the high side. Unless you have a tall building (i.e. 3 stories) there's no reason for the cold pressure to be above 15 PSI. Also, keep in mind that the gauge could be inaccurate! You can get a gauge at the Depot or Lowes that screws onto a standard 3/4" hose bib for under $10. Put it on a boiler drain and check the boiler gauge against this other one.

Do you know where/what the pressure reducing valve is ? and how to adjust it ? I would lower the cold boiler pressure to 15 PSI. This will give you a bit more 'headroom' on the relief valve.

The only valve you NEED to close is the one between the boiler and the tank. It won't HURT anything to close the feed valve, but it isn't really necessary.

Generally it's probably SAFER to drain the tank with the boiler cool, just from the standpoint of no chance of accidentally being scalded in the event of HOT boiler water coming at you. But either way, the tank is being drained. You _may_ have slightly more capacity for expansion if the tank is cool when you close the drain, because cool air is denser than hot air, meaning there will be more air in the tank when you close the drain if it's cool. (probably not enough more to make any functional difference though)
 
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Old 11-20-07, 08:38 PM
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NJ Trooper,

Thanks for all the info.
It was gurgling until the water stopped running. Is it possible that there was some more water left in the tank?

I will try completely draining the tank again tomorrow just as you explained when the system is cold.

Like I mentioned before, the boiler is around 20 psi when cold. The elevation is about 18 ft from the basement floor level where the boiler is installed to the top of the highest radiator on the second floor. I'm not sure why the gauge is reading 20 psi. Shouldn't it be around 8 psi (18 ft * 1 psi/2.3ft), or am I missing something here? Or, should it be 12 psi just like the pressure of the feed water? As you mentioned the gauge might be incorrect, but when the pressure relief valve starts to drip the gauge reads about 30 psi which seems to be right. Any advice?

The pressure reducing valve on the feed water valve seems to be set to 12 psi (that's what it reads on the plate).
 
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Old 11-20-07, 08:47 PM
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You probably did drain completely, try lowering the system pressure first, before you go to the trouble of draining again and see what that does for ya.

Generally, you want about 4 PSI on top of the number that you figured. so, 18 * 1/2.31 PLUS 4 puts you at 12. You can simplify by just saying about 1/2 PSI per foot, plus 4, and that's close enough.

Just cuz the plate says 12 doesn't mean that's what it's set to ! They come from the factory that way, but how many years has it been installed ? How many techs have wandered through with a screwdriver in their hands ?

Yeah, if it reads 30 when the relief goes, it's probly OK , good point!

Do you need instructions on re-setting the pressure ?
 
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Old 11-20-07, 09:54 PM
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I would appreciate if you could post the instructions to reset the pressure on the system.

Thanks again,
 
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Old 11-21-07, 04:14 AM
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If it wasn't why did you try fixing it? A properly installed ceiling tank never needs draining. We drain due to losing the air charge. The air charge is lost through automatic air vents. These tanks are designed as no-service-required unlike bladder type tanks.
 
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Old 11-21-07, 06:30 AM
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rbeck,

I thought that the pressure on the boiler (approx. 20 psi) was too high and that draining of the expansion tank would help to lower the pressure.
 
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Old 11-21-07, 03:09 PM
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Originally Posted by sborgula1 View Post
I would appreciate if you could post the instructions to reset the pressure on the system.
What is the make/model of your pressure reducing valve ?

There may be some specific differences based on what you have (as and example, some Watts brand units have a 'fast fill' handle on top that must be removed to get at the adjustment shaft), but generally there is a 'lock nut' on a threaded shaft on top of the valve. You need to loosen the lock nut, and turn the threaded shaft counter-clockwise to decrease pressure.

Start by turning 1 turn counter clockwise.

Connect a drain hose (or use a bucket) under a boiler drain. Watch the pressure gauge and drain a little water until the pressure goes down to say 10-15 PSI. Close the drain.

Wait several minutes for the pressure to 'line out' at the new setting.

If still too high (but decreasing OK) turn a bit more CCW and repeat. Do this until the cold pressure on the boiler is between 12-15 PSI.

It does take a few minutes for the reducing valves to 'settle' at the new pressure, so don't rush...

When done, SNUG the locknut to prevent the adjustment from turning, but don't MUSCLE it tight... just snug enough to hold the shaft.

After adjusting, keep an eye on the gauge for a few days to be sure it stays where you want it. It will still normally increase as the boiler gets hot, but dropping this initial pressure will give you a bit more 'headroom' before the relief valves becomes disgruntled and opens up on ya.
 
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Old 11-21-07, 07:48 PM
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"conventional" expansion tanks

Originally Posted by rbeck View Post
A properly installed ceiling tank never needs draining. We drain due to losing the air charge. The air charge is lost through automatic air vents. These tanks are designed as no-service-required unlike bladder type tanks.

Why do some systems without automatic air vents get waterlogged?
 
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Old 11-21-07, 08:43 PM
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Because that sneaky air likes to go into solution so that it can escape the not so captive tank and then hang out upstairs where it can choke the flow of heated water in the bedrooms...

And bleeding those bedroom branches just admits more water into the tank.

Ahhh those fond memories.
 
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Old 11-22-07, 10:39 AM
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Thank you all for all the helpful advise!

After properly draining the expansion tank the pressure on the boiler is around 12 psi when cold and it goes up to around 18 psi when hot.

I drained the tank using a hose and taking off a plug that is on a pipe feeding the tank. The pipe that is feeding the tank has an 90 degree elbow and in that elbow there is a plug. All the water came out and the tank got filled with air as it was supposed to.

Another question that I have is the temperature of the boiler. The boiler's temp. was up to 180 F. Is it ok? or is it too hot?

Thank You all again and Happy Thanksgiving!
 
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Old 11-22-07, 11:01 AM
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Most residential systems are designed to operate at about 180 degrees. Under some situations you can operate at a lower temperature but there are a few other factors that come into play when operating at lower temperatures.
 
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