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How to Drain a Boiler

Many older homes (like ours!) are heated by radiators filled with hot water instead of by a furnace that blows hot air through vents.  The water for the radiators runs through a boiler that heats up the water and pumps it through the pipes and radiators.  I love boilers and radiators - not only for their warming consistent heat - but for how easy they are to maintain and how long they last.  Our house was originally heated with a coal burning boiler but was replaced with a gas burning boiler in 1944.  That boiler lasted until 2005, that's 61 years!  Hopefully this one lasts until 2066.

Boilers can keep their longevity with an hour of annual maintenance that's so easy any homeowner can do it.  That maintenance is done in the fall and I think I'll write more about it this fall.  The only other thing you may need to do to a boiler is drain all the water from the system and fill it back up.  A boiler could go it's entire life without being drained and filled but you may need to drain it to work on the plumbing or radiators themselves.

When we first moved in we refinished all the hardwood floors and it's almost impossible to sand underneath the radiators.  We had to disconnect all the radiators and move them around the house in order to sand.  Did I mention that our radiators are made of cast iron and some of them weigh over 300 pounds??  Before we could disconnect the radiators we had to drain the boiler or the radiators would have spilled a few gallons of water on our floors when we unhooked them.  You can read about that mishap of a journey here.

Radiator unhooked

Now, a year later we're draining the boiler again to move some of the asbestos-covered pipes that connect the boiler to the radiators so they're out of the way when we finish the basement.  While it's drained, we also want to paint the kitchen radiator so it looks less conspicuous underneath our revamped kitchen cart.

If you ever need to work on the pipes or unhook a radiator, you'll need to drain the boiler too.  Here are the steps:

  1. Turn off your thermostat and power to the boiler.  You definitely don't want it to turn on while there's no water in the system.
  2. Find the boiler's drain, it will looks like a spigot for hooking up a hose.
    Boiler Drain Valve
  3. Hook up a hose to the boiler drain and set the other hose into a drain or utility sink.
    Draining To Sink
  4. Open the valve on the the boiler drain.  Be prepared, the water is going to come out about as strong as a hose hooked up to a spigot outside.  Be prepared for the amount of water and pressure that will shoot out!
  5. Once the water stops coming out (about 5 minutes) you're half way there.  So far the water has been forced out by the pressure in the pipes.  Radiators have a pressure value and if you were watching you'd see it go from about 15-20 psi to 0.  There's still more water in the system though.  Because radiant heat is a closed system there's a lot of water being held in by suction, it's like filling a straw with water and putting your finger over the top to hold it all in.  We need to lift the finger off the top.
  6. You'll need a radiator bleed key.  Hopefully one came with the house, but if not I've seen them at antique stores (in different sizes).
  7. Find the tallest radiator on your highest floor, insert the bleed key into the radiator's bleed valve, and turn it counter-clock-wise.  You'll hear a rush of air, that's actually air being sucked into the radiator to replace the water in the radiator.  When you stop hearing air close the bleed valve.
    Bleeding A Radiator
  8. Repeat step 7 for each radiator, working from the highest to lowest radiator.
  9. Finally, close the boiler drain valve and unhook your hose.  Congratulations, there's should be no more water in your heating system!

We were very excited to finally be draining the boiler last night for two reasons.  First, eliminating all that water means we don't need heat anymore for awhile!  This spring has been a huge tease, but we're confident that we won't need the heat again until the fall.  Secondly, this all means that we're making more progress on the basement.  The asbestos pipe remover guys are making LOTS of noise as I type and we can't wait to get back into the basement to see for ourselves and document for all of you how much more head room we'll have with those bulky pipes out of the way!  We still don't have much to show, but we're getting closer to having our basement finished!

Comments (3) Trackbacks (1)
  1. This is a great informative post guys! I am sure we will need these tips someday! How did the asbestos removal go? I just did a post today about lead and asbestos testing in our house before they can start repairs. It was pretty interesting. We also have asbestos covered pipes in our basement… someday those will have to go too.

    • The asbestos is out! We’ll be posting about it today or tomorrow :) I just read your post! I guess dangerous materials come with old homes. Don’t go eating those paint chips!

  2. How do I fill the boiler after draining.