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Adjusting boiler expansion tank?
QUESTION:My boiler expansion tank (15 gallons) was found to be nearly full, so I
drained it. I added a bit of water back in to fill about 1/3. I'm having a
problem getting higher pressure, however. When I add water to the system,
hopefully to increase the pressure, it seems to go straight to the expansion
tank and fill it up. So then I get another full tank and still low pressure.
What am I doing wrong? I just had the boiler serviced, and everything was
found to be fine. I've bled the radiators. There are no leaks.
ANSWER: The key to making a standard "compression tank" work is to have air in
the tank before you add water.
Here is how it works:
When the tank is empty of water and full of air, the absolute pressure
in the tank will be about 15 psi (0 psig). If you then fill the tank
to about half full of water allowing no air to escape, the pressure
will double assuming a constant temperature, which for all practical
purposes it will be. The pressure in the tank will then be about 30
psia (15 psig)
For most home systems this should be enough.
Now, how to do it?
It depends on how the tank is plumbed. I prefer to have all valves
separate and all functions manually controlled. However, this in not
normally the case. Most systems have a 3-way valve that you move to
one position or another in one position, your system is disconnected
and the tank is running to a drain. In the other the system and the
tank are connected.
Here is what you need to do:
1. Shut off the boiler. Leave the pilot light on if it is a pain to
relight.
2. Shut the circulating pump off.
3. Fill the system with water making sure that all vents are open and
venting. It is good to have vent isolation valves that you can close
allowing you to remove the automative vents if needed without draining
the system.
4. Shut off the make up water.
5. Isolate the tank from the system. Hopefully you have a dedicated
valve to do this with.
6. Drain the tank completely allowing the tank to fill with air.
Ideally, there is an air vent valve on the tank that you can open.
7. If you have a sight glass, check the packing nuts on it. The upper
nut usually dries out and allows air to leak. If you have valves for
the sight glass, the packing gland on the upper one will leak air too.
8. After the tank is full of air, and you have ensured that all leaks
are stopped on the "air side" of the tank (including closing the vent
valve), reconnect the tank to the system.
9. Open up the make up valve and allow the tank to fill a little below
half full. If done correctly, Boyle's Law will work and the system
pressure will come up.
10. Restart the circulating pump.
11. Restart the boiler. As the system heats up, the water will expand
and the tank will fill a little. Once the system reaches operating
temperature, you should be at or near the correct pressure.
Be sure that the make up regulator and relief valve are in good
working order.
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