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Yearly boiler maintenance

Note:  This post is specifically for houses that have a hot water boiler and radiators.  If you have a furnace and forced air ducts you should still get a yearly checkup but it should be done by an HVAC professional.

If you have an older home with radiators, consider yourself lucky!  The consistent heat that comes off radiators is one of the most comfortable ways to heat a house.  It doesn't dry the air out as much and it doesn't blow dust around the house like forced air, plus radiant heat just feels great on a cold day!

Boilers, while expensive will last a long time with a little bit of maintenance.  Our house had a gas boiler installed in 1944 and it didn't need replacing until 2005!  Before I get into specific maintenance, I want to give a quick overview on how boilers and radiators work.

How boilers work

Radiators, a boiler, and the pipes that connect the boiler to the radiators are all filled with water.  Unlike sinks and tubs where water enters through pipes and then leaves the house through drains, the water in a hot water boiler system never needs to leave the system, or continually enter it.  It's called a "closed system" because of that.  Now, there are ways to temporarily add water or remove water, and I'll get into that, but for normal operation, the same water just keeps circulating.

The boiler heats up the water to around 180 degrees and a pump pushes the hot water through pipes and into the radiators.  The radiators then heat up because of the hot water and heat the room.  The water leaving the radiator has cooled so it's sent back to the boiler to be heated again and pumped back to the radiators.  It's basically a big circle.


There are a few different kinds of radiators you can use with a hot water system.  Older homes have cast iron radiators.

If you've ever cooked with a cast iron pan or dutch oven you know that cast iron take a long time to heat up on the stove but once it's hot it stays hot for a long time, even once it's off the stove.  Cast iron radiators work the same way, they might take 20 minutes to fully warm up but once they're hot they stay hot - sometimes for hours - even after the boiler has stopped heating the water.  You can buy used cast iron radiators but expect to pay roughly $30 per fin (the vertical tube) and for the radiator to wight about 20 pounds per fin.  We have a radiator in our living room that weights about 600 pounds!

Since cast iron radiators cost so much, newer radiators are made of steel or similar metals.  The work the same way but they're faster to heat up and faster to cool down.


Baseboard units can also be used with a boiler.  Instead of filling a large radiator with water, baseboard units use a pipe with dozens of thin metal fins attached to the pipe.  The fins heat up the air between the fins and the hot air rises out of the baseboard unit.  While radiators mostly "radiate" heat, baseboard units "convect" heat meaning radiators heat objects in the room while baseboard units heat the air.  It's a minor difference but the heat feels a little different so it's worth mentioning.


Finally, radiant, or in-floor heat, can be used in a hot water system.  The terminology gets confusing because cast iron radiators create "radiant heat" but radiant heat is now a term for in-floor heat.  Confusion aside (maybe??), radiant heat is created by running hot water through pipes that are embedded in concrete or a similar material.  This processes heats up the concrete and makes the entire floor a radiator.  While radiant heat can use a boiler, the set up is different enough that my maintenance tips won't always apply, it would be best to contact a HVAC professional.


Bleeding the radiators

If there's one thing you do each year for your boiler, it should be bleeding the radiators.  Don't worry, it's as easy as changing a light bulb.  Each radiator or baseboard unit should have a small valve that looks like this.

This valve allows one to remove air from the radiator.  I mentioned that boilers are a closed system but air can still escape from the water.  Since air is less dense than water it will rise to the highest point in the system - the tops of the radiators.  Radiators heat up much more efficiently if they're touching water instead of air so it's important to keep the radiators completely filled with water.  The water and air in the radiators are under pressure so if you open the valve, the air will be forced out and you're left with just water in the radiator.

How to bleed a radiator:

  1. You'll need a special bleed key that should have come with the house.  If you don't have one, you can usually use a flat head screwdriver or buy a new key at a hardware or antique store.
  2. Hold a cup underneath the opening below the bleed valve
  3. Turn the bleed key counter-clockwise.
  4. You'll hear the hiss of air leaving the radiator.
  5. Once the air stops coming and water starts going into your cup, close the valve by turning the bleed key clockwise.

The water in your cup will most likely be black and gross, that's normal.  I wouldn't drink it though...

Checking the boiler pressure

This task is a little more advanced so if you don't feel comfortable with it, have an HVAC pro do it.

Open the service panel to your boiler and check out the series of gauges.  One will be for the temperature of the water and another will be a pressure gauge.  A boiler should be around 12 psi when the water is cold and will rise to around 18 psi when the water is fully heated.  You'll want to bleed your radiators first since it will lower the pressure slightly, then, with your boiler fully cooled (best to do this before you turn it on in the fall), check the pressure.

If the pressure is below 12 psi you need to add water to the system to increase pressure.  Find the cold water pipe entering your boiler with a valve that's closed.

Slowly open the valve and watch the pressure gauge until you've reached 12 psi then fully close the the valve again.  Do not open the valve and leave it.  The pressure in your water pipes is too high for your boiler system.  Boilers have a pressure relief valve that opens if the pressure it too high (usually 25 psi).  If you leave the cold water pipe open your pressure relief valve will open and you'll spill gallons of water on your floor.  I know because I've done it!

If your pressure is too high, say above 15 psi when the boiler is cold, you'll need to remove water from the system.  Boilers have a drain for such a task.

Connect a hose from the drain to a floor drain or put a large bucket under the drain and open the valve until you've removed enough water to be back down to 12 psi.

Cleaning the combustion chamber

So far all the maintenance has been on the water side of the boiler.  There's also maintenance that needs to be done to the fire side.  If too much dust or grime gets into the combustion chamber then the boiler won't heat efficiently.  While there are ways to DIY this, any task that deals with gas/oil, fire, and explosion is best left to a pro.  Hire an HVAC pro to clean and tune up your boiler every few years.

Hopefully you know enough about your boiler and radiators to keep everything running smoothly for years!  Enjoy the heat!

If you want know even more about boilers, check out the post I wrote earlier this year about how to completely drain a boiler.

Comments (6) Trackbacks (0)
  1. Really good post. I’ve always stayed as far away as possible from radiators as possible, but mostly because I just don’t understand them. This post really removed a lot of the mystery.. Doesn’t sound as complicated as I once thought they might be.

  2. Are the rediators supposed to be bled when they’re cold, warm or hot?

    • Do it cold so you don’t have hot water coming out at you! If you’re asking because the pressure will be different, i always bleed cold, then adjust the water pr
      essure after.

      • I’ve never known what was correct. I’ve always done it hot, going by the assumption that hot water/air rises so I figured it’d be more likely to get the air out. All my bleed valves point down, so I’ve never really had a problem with getting burnt by the water.

  3. Thank you for that great boiler and radiator explanation. My radiators are hot and may house is warm.

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