"Hydronic Heating System - How to Drain the Expansion Tank"

 By Ron Stultz

5 November 2010

Background: One would think that the expansion tank on a hydronic heating system would need no maintenance and never need to be drained but this is not true. As part of annual maintenance, the expansion tank should be drained of all water. And one would think that how hard can it be to drain an expansion tank? Well there is only one right way.

How to drain a hydronic heating system expansion tank:

1. Hydronic heating system should be turned powered down either by switch on furnace or at circuit breaker panel.

2. System should be cool, not hot.

3. Hydronic heating system water pressure (when system cold) should be correct. If system cold water pressure is too high or too low correct system pressure before draining expansion tank.

4. Close the valve that allows water out of the furnace up and into the expansion tank. My system did not have such a valve when I moved into my current home. Without a shut-off valve, when you drain water from the expansion tank, the pressure in the hydronic heating system will simply push water up into the expansion tank and you will drain not only tank water but some water from your system.

5. Using a bucket, not a hose (hose may siphon water), place your bucket up under the drain faucet on the expansion tank and slowly open the faucet. Now as most expansion tanks are vent less, water will more or less gulp out of the tank.

Notes:

(A) I have a black cap on my expansion tank drain faucet. Should the faucet begin to leak, the cap should keep water from dripping out where I do not want it.

(B) Expansion tanks come in all sorts of shapes and sizes. Mine happens to be an older, large type.

(C) I like labels and use them everywhere. On my expansion tank are labels which say "drain annually" and "use bucket".

6. Fill bucket from expansion tank, close drain faucet, empty bucket. Repeat until all water is out of the expansion tank. And yes, water out of the expansion tank might have a very black or other color. This is normal.

7. Once tank is completely empty, tight drain faucet and replace faucet cap if you have one.

8. Slowly open the valve that allows water from the hydronic heating system into the expansion tank. Water from the water inlet pressure valve should now add water to your system back to pre-expansion-tank-drain-pressure. This filling of your expansion tank might take a half hour or more. To be safe, let your system stabilize system water pressure for at least 1 hour before you start up your system again.