old expansion tank, what the heck is it doing?

steamnoob

So I think I have a very outdated and weird setup. Two zones (one cast iron rads, one baseboards), each zone with a separate pump and t-stat. I use the rads far more than the baseboards. I just purged a lot of air out of the radiators, adding quite a bit of water to the system to do so. Pressure in the system is steady at 18 psi (jumped up to 25 when filling, but went back down after purging the rads) and all are warming up now.

I have this old style expansion tank above the boiler, and I don't really know what it does, if anything, for the radiators, or what kind of maintenance I'm supposed to do with it. It appears to only be hooked up to the baseboard zone. The valves from the top of the air purger are all open, but the valves back to the cold water return (before the baseboard zone pump) are closed. I unhooked the tank return hose and cracked the old valve (which seems to have been closed for a very long time), and out came a bit of rusty water, followed by relatively clear water. I can't really tell how full the tank is. If I ever needed to drain and refill, how does water get into the tank? And why is the expansion tank drain hose be routed into the water return?

Jamie Hall

That expansion tank works -- for your whole system, since it's connected through the boiler to everything -- to allow for expansion (doh… sorry about that). What happens -- or is supposed to happen -- is that air which gets into the system from whatever source should be separated at the air purger and goes into the tank to provide an air cushion. Ideally the tank would be about half full of air when the system is cold. Systems like yours don't usually have fancy water level controls, nor any easy way of checking the water level. However, the best way to get the proper air charge is to find a valve somewhere where you can isolate the tank from the system and empty it, and then reconnect it to the system and let the system pressure compress the air. That will usually be about right.

What you don't want to do is to remove the air from the system elsewhere, if you can help it (you would for an initial purge, of course), as you need that air to maintain the air cushion.

steamnoob

air would only be removed when the baseboard zone is on though, right?

and if i did drain it... how would i refill it? there is no supply line in... just the air line from the purger, and the drain line. hook the drain up to a hose?

Jamie Hall

You don't have to refill it. You open a drain and empty it while it is isolated from the system. Then you close the drain and reconnect it to the system. Water will come in from the system (which will happen automatically if you have autofill, or you will have to add water manually) until the system pressure -- and therefore the air in the tank -- is at your cold static pressure.

Originally, when the flow control valves weren't there, it was easy enough for air from either loop to accumulate in the boiler, whence it would make it's way to the tank. As it is now, you are correct -- only air in the baseboard loop will get there. Air in the radiator loop will go to the boiler and eventually most if not all of it will get to the baseboard loop and be taken care of. Which means you may have to bleed radiators now and then, if you find they aren't heating completely.

steamnoob

ok, so that's why the tank "drain" hose is hooked up to the return line. Once it's drained (closing off valve from air purger), I could hook back up to return line, open those valves, and it will repressurize with water, assuming I can find some sort of air bleeder on the tank.

I do have an autofiller, but I don't think it's working correctly. When I opened the water line it ran and ran, enough to put the pressure in the furnace close to 25 psi. So either the furnace pressure gauge is off, or the auto fill valve isn't working. Next project.

neilc

could you post a picture of that tank, and its 2 connections?

EBEBRATT-Ed

You do not want to bleed air from the tank. The tank needs to have air in it...usually half air and half water.

Water expands as heated. The air in the expansion tank acts as a cushion to the heated water as the water is heated it expands and puts pressure on the air in the expansion tank.

Typically the pressure when filled with cold water the system should be at 12-15 psi. As the water is heated the pressure will rise to 20-25 psi. If you have no expansion tank (or a waterlogged tank) the heated water would have no place to go, pressure would rise and blow the pressure relief valve. Water and other fluids cannot be compressed

Jamie Hall

You keep trying to find ways to get air out of the tank. Stop it. You need to have air in the tank. That's what it's for.

steamnoob

steamnoob

first pic is purger, copper line out, through two valves (both open) to expansion tank (3rd picture). Hard to see, but the red valve in upper right corner is the intake.

2nd pic is the "drain". Valve at tank, and another at the return line for baseboard loop. Both valves have been closed.

steamnoob

Sorry, that 3rd pic sucks, but the purger is right behind those service tags. There is one valve above the yellow tag, and another valve (red) right by the tank intake. the tank is directly above me in that photo

neilc

how about one more picture showing all 3 of those, in one shot.

steamnoob

hard to do in tiny area without standing on my head. i circled the purger in red. the red line goes to the exp tank, circled in blue. then there is a hose (green) that comes down of the back of the tank, and hooks up to the water return line (blue).

those are the only things going into or out of exp tank... one line from purger, and one to return line. in my two years, the valves to the return line have been closed, the valves to purger have been open. should those valves be open?

mattmia2

I don't know what is with that washing machine hose, looks like someone just connected it to 2 random boiler drains on the system.

Jamie Hall

The valve from the air purger to the tank should be open. The other one doesn't appear to do anything useful one way or the other...

steamnoob

So im guessing its just the drain line hooked up to another drain line... i guess it would help if either spigot was leaking.

neilc

I'm wondering if someone tried that hose as a connection to "Pump Away".
If that hose is on a drain valve on the tank, and connected to a drain on boiler return,
then I would say make sure both drain valves are shut, and remove the hose, it doesn't belong there, unless you're using it to drain the tank, to daylight.
Or did you say both drains were leaking, and the hose was a temp fix to stop both leaks?
Both drains are shut, correct?
Are there any valves between the air device and the tank ?
I didn't see any, if there are they should be open.
How is the boiler pressure behaving?

steamnoob

Yes, both drains are shut on that side. I don't seen a leak, but the valve on the expansion tank looks pretty crusty. There are two valves between the air device and the tank... both open. Pressure seems okay, reading about 18 psi (2 stories with basement) at about 145 degrees currently. When running before, I recall it was getting up over 20psi, but that was before I added water and purged air from rads. I'll keep an eye on it, but I'm not sure if I trust the gauge since it should be holding at the LO temp of 110 degrees (until I put a new aquastat in). And when it is firing, it doesn't seem to get all the way up to 180 HI setting before snapping off. I guess I should be budgeting a replacement. That giant oak hanging over the family room will have to stay put another year.

steamnoob

If anyone is still reading this thread, I'm still mightily confused by the air purger setup, which sort of is the root of the original question. I've read on another site that this is how these were hooked up "once upon a time", but the physics ain't jiving in my head. The line from the top of the air purger is supposed to contain "air" right? And that is what is routed into the old expansion tank. The bottom "extol" doesn't have anything connected. Looking a cutaway of air purgers, I guess I can see how pressurized water could go up and through to the expansion tank, but if so... how does it actually function to remove air bubbles from the system.

Jamie Hall

Basically the idea is that as an air bubble wanders by it moves up the pipe to the tank. They work best if they are piped directly to the top of the boiler... but they can work moderately well as yours is set up.

JUGHNE

The air purger is built such that it tends to pass the water thru a scoop that is designed to provide a little pressure drop which is when the air wants to release from the water. The air being lighter gathers at the top and passes thru the pipe to the tank.
The pipe is full of water and the tank should be about 1/2 full of water. The pipe going to the tank should be running uphill to encourage the air bubbles to migrate up hill and into the top of the tank.
This air migration is a fairly slow process and eventually with the pump running the air will be scooped out of the water.
You do have to first manually purge the excess air out of the system. After that all addition air that comes out of the water must go to tank. If you had an auto air bleeder on the system then that is where all the air would go. The tank would eventually get water logged and have no air to compress as the water heats up and expands. Eventually opening the 30 PSI relief valve.
If the tank eventually gets waterlogged, then you have to drain the water out. With the system power off, you isolate the tank with the valve between tank and purger. Then drain the tank completely dry and allow the air to naturally go into it.
I would drain it slowly burping air into the tank.
There is a very slim chance that draining a tank without a second opening to break the vacuum could cause the tank to collapse......very slim chance but apparently it can happen.

Drain the tank dry, it will naturally fill with air, close the drain valve. Then open the valve to the purger. Your auto fill valve should add water up to the 12-15 PSI point. This added water will go up into the tank compressing the air to stay at about 1/2-1/3 of the top portion of the tank.
That compressed air is then your expansion cushion for applying pressure to the entire water system.

Keep the hose off of either of the drain valves, it will just cause confusion.

Maybe the book available here....... "Pumping Away" will help you more.

steamnoob

so ideally the line from purger to the expansion tank should be sloped upwards, right? That was my initial confusion, I thought the air purger was like an air valve on a steam heat system. But I guess water just flow through it too.

JUGHNE

Gravity will allow the air to pass uphill, the slope of the pipe helps that happen. That is why the tank has to be above the purger.

Ideally the purger would be on the single pipe before the tee splits the circuits.

But eventually all the water getting mixed in the boiler and will pass thru the purger.