Old style pressure tank

My pressure relief valve (30#) was blowing off so I replaced it. But it still blows off about 2 gallons when heating up my water -radiator system. I do not have a bladder pressure tank but an old style set on the floor (4' by about 10" and it appears to be empty and maybe not working. How do i re pressurize such a tank or test it ? Is it likely my problem is that it will not hold air so no pressure is created when the heated water flows to it?

 

Can you show us a picture of that tank Wayne?

Are there any valves on the tank itself? Is there a valve on the pipe leading into the tank from the boiler?

 

Wayne,
Do you have a shutoff valve between the tank and the boiler to isolate the tank for draining.

 

From the original post:

I have never seen a floor-mounted expansion tank that did not contain a bladder. A "conventional" compression tank MUST be located above the air removal device to allow the air to return to the tank.

It appears that either Wayne was scared off or he is another "one-post wonder." Hopefully I am wrong about that.

 

I suspect that some folks who post expect that they will get an email alerting them of replies to topics they start... and don't come back because that doesn't happen automatically... but I do suspect that you're not wrong Furd!

Hey, I was just reminded that I owe you a PM!

 

This system has worked at least since 1974 when the boiler and tank were installed..

I do have a shut off valve.

I now recalled this happening about 5 years ago. So yesterday, I drained the tank which is hard to do unless I let some air into the tank which I figured out how to do. There is a plug half way up the tank. So i succeeded in fully draining it with the valve from the boiler shut off. Then I 'pressurized" it with lung breath closing plug and valve at same time. I reopened the shot off valve and the system repressurized the tank further -I am guessing maybe 1/3 full with water. ( I am guessing the tank is 20 gallons) so I basically have fixed it .
My error- which was why I wrote for help- was that I could not confirm the tank was water logged because I cold not really drain it until Ii let some air in via the plug.

Thanks for the responses.

 

I had mistakenly thought the tank was empty of water when in fact it was waterlogged. I realized I had to relieve the tank plug midway up on the side of the tank in order to effectively drain it at the floor because of course it needed air intake. (When I had previously tried to drain the tank, it seemed empty because very little water would come out)

I shut off the valve to the boiler, drained it completely, filled the tank with lung breath (and closed the drain and air plug). The tank pressurized further after I opened the valve from the boiler -filling the 20 or so gallon tank with about 1/3 of that amount of water. It is now working again.

Thanks for the responses. This floor tank system must work some how because it has since 1974 when it was installed.

 

They do in fact make floor model tanks without bladders but I've only seen them on commercial jobs in high rise buildings. I used to maintain them.

 

I've seen several floor-mount tanks with bladders. Most of them had a half-coupling welded to the shell with a plug screwed in hand tight. Removing the plug revealed the tire valve for pumping up the bladder.

Again, if the tank is lower than the discharge from the air separator there will be no way for the air to find its way back into the tank. With the tank being mounted next to the boiler it cannot possibly be located above the air separator.

I spent more than thirty years in the field of hydronic and steam heating, commercial and industrial sized equipment.

 

The only way I can see a tank like that working is by a constant routine of having to add air to the tank and bleeding the rads... but maybe that's what Spott meant by 'maintaining them' ?

 

Or maybe he meant the tank was in the penthouse and the boiler on a lower floor. I have seen such on chilled water systems. Putting a standard compression tank on the top floor and running an air-only pipe down to the air elimination device on a lower floor would definitely work.

 

I did the heating for Worcester Housing Authority for 15 yrs. and they do exist without bladders. The were installed in all the high rise buildings that had forced hot water.
I did on occasion have to drain them.

Bear in mind these were on boilers probably 10 ft. tall and ladders were needed when working on them except for the burner. One was an HB Smith dual fuel that when on oil fired 60 gal. An hour. 20,000 gal. in ground tanks.
The expansion tanks themselves were about just as tall.

I had no idea that it wasn't commonplace.
Anyway I just wanted to let you know they do or did exist.

Furd,
I didn't see your post until I got done but no, they weren't in the penthouse although we did have one 5 story with that setup.
These were in the boiler room, next to the boiler.

I can't get in a discussion about this because at the time I never gave it thought. They had been in about 20 yrs. when I got there and they worked fine.

 

Oh, baby boilers. My last job we routinely burnt 6,000 gallons or more per shift.

 

I am the original questioner. In all this discussion, I am confused about the term "location of the air removal device."
What is the air removal device and where is it usually located relative to the boiler?

My boiler is about two feet tall and the pressure relief valve come out of the top about 5 inches above and off of it is the lateral pipe with valve inline that goes to the mid height point of the pressure tank . The tank extends from the floor to about 2 feet above the boiler top.
By the way, I bleed a small amout of air from the highest radiator at the beginning of each heating season.

And indeed, like someone speculated and like many threads I follow, i expected some sort of notification when the thread had been added to and thus had not checked back promptly on my query..

 

WayneLee, I would really like to see some photographs of the system if you wouldn't mind posting them.

Air scoop would most likely look like this , but I don't think you have one.


image courtesy taco-hvac.com

At the top of the thread there is a 'drop down' that says "Thread Tools". Drop that box down and there are a few choices, one of them being "Subscribe to this thread" which if you select that you will receive an email of thread activity. You don't get automatically subscribed to threads which you start for some reason... BUT, there is I believe a choice in your 'user profile' settings that WILL auto subscribe you to your own threads... I think.

 

AHA! I was envisioning a tank that was not as high as the boiler. That pipe with the valve from the boiler safety valve connection to the tank IS the air return and the air separation device is the boiler itself. The output to the heating system either comes out the side of the boiler or if it comes out the top it is connected to a submerged nozzle inside the boiler.


And that is where you are losing the air. With a conventional (compression) tank the air is continually being absorbed by the water and later released at the air separation device to return to the tank. Ideally the piping between the tank and the boiler should be at minimum 3/4 inch nominal size AND tilted upward towards the tank connection but eventually the air should return to the tank if you are patient.

Like Trooper I still want to see some pictures.

 

First time I have attempted to upload pics to this forum. Hope it works

 

I see the problem. That pipe turns down to enter the tank. The air is trapped at that point. The best thing you could do is to raise that tank so that the pipe is always on an upward slant into the tank.

Also, that pipe from the boiler is 3/4 inch nominal but then decreases to a 1/2 inch or maybe even a 3/8 inch. It needs to be the full size the entire route.

Probably a job best for summer as it may require moving the safety valve a bit to get everything to line up properly.

 

I need further education on these issues.

This is my understanding of what is happening and why my system is as it is:

My tank is sealed containing water and air. When i sealed it and opened the valve from the boiler the water flowing in thru my water infill system flowed into the tank until the air pressure inside equaled the air pressure in the boiler and whole heating system. When cold, this pressure is set by the intake valve at about 20 lbs ( much less of course than the city water pressure)
When the boiler heats the water, the water expands and excess flows into the tank increasing the pressure in the tank and in the system. If the water can go no where , it would blow off the relief valve at 30 lbs (which is what it did when my tank was waterlogged).

When the system cools back down again, the water shrinks and due to the pressure in the tank, some water flows back into the system -all equalizing at the the cold set pressure.

There is no air exchange in or out of the total system throughout this process.

What am I misunderstanding please?
thanks
Wayne

 

Your understanding is good WayneLee...

I might just add the tidbit that water can not be compressed at all, but of course air CAN. That's the actual physics that allows the expansion/compression tanks to work and control system pressure at all.

The fact is that there IS air exchange.

AIR is SOLUBLE in WATER!

So at the air/water boundary layer inside the tank air is going into solution in the water in the tank as the water cools and it's capacity to dissolve air increases. When that air laden water goes back into the system, and the water is HEATED, the capacity for the water to HOLD that dissolved air is greatly reduced.

The result is that not only is there an exchange of WATER, there is an exchange of dissolved air that comes back out of the water and travels through the system.

The reason that the piping to the tank should be LARGE ENOUGH, and SLOPING UPWARD to the tank is so that the bubbles that are caught at the top of the boiler can make their way BACK to the tank where they belong.

Furd's suggestion of raising the tank a few inches or so and doing a bit of re-piping with larger pipe (NOT reducing the size as it is now) so that the air can float back to the tank would likely improve it's performance and greatly reduce having to bleed radiators and drain the tank.

Ask yourself these questions: "Where is the air in that tank going that I have to periodically drain the tank and introduce new air? Why do I get air at the top of the system that I have to periodically bleed out... where is it coming from?"

I think that after reading the above, you will have the answers to those questions!

 

Thank you very much NJ. That increases my understanding and at some point I will adjust the tank connection.
So, I also conclude that my tank if it had with an upward sloping connection pipe is actually the equivalent of an an "air scoop" right?.

A couple other things. Previously, I just thought I got a waterlogged tank over the course of 5 years or so because slowly air escaped due to leakage at threads etc or maybe it was the cumulation of the small amount of air I bleed each season .
The air bled from the highest radiator once a year is not much. Maybe I guess the equivalent of one gallon of water because it bleeds out quickly and the the full radiator shoots water from the bleeder valve say after 40 seconds or so.

Also, I'd like to say that bleeding once a year and draining the tank once every five years will not be a burden.

One additional question. Does anyone know the formula so that I could guess at how much water is in my tank at the beginning of my refresh refill. Basically it is a question of (say my infill pressure is set at 20lb) how much water is needed in that size tank to pressurize a tank full of air to the 20lbs.

Thanks again for the serious attention to my problem.

 

Here is an old one piped in an attic... For reference....

 

Probably on an old GRAVITY SYSTEM.

That pipe out the top probably goes to a drain under the eaves of the house...

By the way, for 'historical' reference, this is the reason that boiler pressure gauges have an 'altitude' gauge on them, scaled in FEET. The operator knew how much water he needed in the system in feet...

====================

Well... no, not exactly. In your case being piped off the top of the boiler, the boiler itself is acting as the 'air scoop'. Bubbles rise to the top of the boiler and are collected there and make their way back to the tank.

Yes... this is more likely. The air that collects in the radiators ultimately has come from the tank via the air being dissolved into the water.

Then by all means, leave as is! Why make work for yourself if there's nothing to 'fix' ?

If you had the formula you wouldn't have to guess!

If you calculate the volume of a cylinder, you can determine how much water it would hold if there were no air at all in it.

Let me google "what is the volume of a cylinder?" for you

and the volume of a gallon of water

Let me google "how many cubic inches in a gallon of water?" for you

and then simple division.

The tank full of air is a little different...

"Gas laws" of physics will tell you that pressurizing a tank of air with water to 20 PSI will be APPROXIMATELY a little less than half air / half water.

If you google it, remember that when you see PSIA it means PSI ABSOLUTE, and you nned to ADD 14.7 PSI to that number. 20 PSIG (Gauge) is 34.7 PSIA (Absloute).

If you want to know exactly, I'll just tell you that if you start with a tank full of air, and add water until you have 20 PSI in the tank, the tank will have 42% air and 58% water.