Why is my steam boiler knocking and making loud noises? The basics about water hammer.

A water made of hammer on a red background.

When we meet new clients with steam boilers, we will ask, "Do you hear any loud or unusual noises when your boiler is running?" The answer we often hear is, "Oh you know, just the normal noises."

Of course there will be the sound of any mechanical system turning on and running, but generally speaking your steam boiler shouldn't be making any of those "normal" noises, which includes loud noises, knocking, hammering or banging. If you do hear these noises, that is a sign of water hammer, a detrimental condition that harms the longevity of a steam heating system.

What is water hammer?

Water hammer occurs when steam comes in contact with water/condensate that is not properly draining from the piping. Steam can travel through piping at speeds upwards to 100 MPH. That's fast! While steam is brilliant at flying through the twists and turns of piping to your radiators--water, not so much.

As the steam travels through the piping, the steam will pick up this water that hasn't drained properly and slam it against the first turn in the piping system that it encounters. It results in that knocking, hammering or banging sound. Thus the name water hammer.

Water hammer, specifically because of the incredible speed that the water is traveling at, can become very violent and damage system components due to the shock that accompanies the water slamming around the piping system.

If you think about the power of water, even a slow steady drip can carve rocks. Imagine the increased power of water when it's given speed and force like a power washer. That's why when these sounds are heard, it's important to address them to save the system from damage.

What causes water/condensate to improperly drain to cause water hammer?

Water hammer is caused by areas within a steam heating system that have improper drainage, mainly due to improper pitch. The proper pitch of piping and radiators allow the condensed steam (also called condensate) to drain back to the boiler as it should. 

There are many different areas of a steam heating system and reasons why a system might experience water hammer. The following are just a few:

• Many steam heating systems and the homes they are in are more than 100 years old. It's natural for the home to continue to settle, and for the piping to begin pitching the wrong way.

• Clamps and hangers that are meant to support the piping in the basement often fail. These clamps and hangers are made of steel, wood and nails. Eventually the material degrades and wears out from supporting the heavy steel piping. This causes the piping to sag. Again, improper drainage due to improper pitch. 

• One-pipe steam systems are a type of system that has only one pipe coming through the floor to connect to each radiator. Each radiator needs to be properly pitched downward towards that one pipe to allow proper drainage. The radiator supported by that pipe can often cause the floor to sink on the unsupported side, which changes the required pitch.  

• Piping repairs and/or changes made to a steam heating system are unfortunately not always done properly using the proper and required pitch. What is additionally unfortunate is that sometimes the problem doesn't immediately appear, and the knocking begins well after the repair is completed and the contractor is gone.

What are some solutions to stop water hammer?

1. In one-pipe steam heating systems the radiators should be checked with a level to determine whether the radiator has the proper pitch toward the supply pipe. 

2. Two-pipe steam systems are not as prone to getting out of level, but it's always best to double-check them. In the case of a two-pipe system, the pitch goes down toward the end of the radiator opposite of the hand valve for proper drainage. (If you want to know more about re-pitching radiators we'll create a follow-up post to go over it in more detail.)

   

   drawing of one-pipe and two-pipe radiators

3. Two-pipe systems can suffer another ailment that involves the radiator trap on the opposite end of the radiator (meaning the opposite side of the the hand valve). These traps are meant to drain the water from the radiator once the steam has changed to condensate. Sometimes these traps can get plugged or fail to close. The condensate builds up in the radiator where it meets live steam.

4. Piping in the basement needs to be pitched much more to allow for proper drainage and here is where you’ll find most water hammer in steam systems. Like we said above, improperly draining piping is often caused by years of settling building materials and the sagging of the hangers that suspend and support the piping. (If you’d like to attempt this repair yourself we'll also do a follow-up post to go over this in more detail.)

5. Sometimes when new boilers are installed the installation can expose piping that’s supposed to be flooded (thus keeping live steam). This is due to the fact that new boilers are usually much shorter than the old boilers that they are replacing. When that happens, once flooded returns (typically called wet returns) will be drained because the boiler water level is now much lower. You can usually tell if a return pipe was once a wet return by they're placement. Typically they are mounted to the basement walls usually no higher than about 36”. They run around the basement and are meant to carry the steam condensate (water) back to the boiler.  If live steam gets into these wet returns it’s likely to meet up with condensate and cause water hammer. The fix for this is to install additional piping at the boiler which creates a higher, false water line that again fills the wet return with water. This is a fairly extensive repair and we would recommend a reputable contractor that knows steam.

   
   

   While there are other causes of water hammer, these are the most common problems. Some of these problems can be taken care of by resourceful and handy home-owners, while others need to be taken care of by an experienced contractor. Servicing steam boilers is not incredibly difficult because they’re fairly straight forward units. Though, that being said, try your best to ensure when someone works on your steam heating system, especially the piping system or a boiler replacement, that you’re hiring experienced contractors that won't leave your system in disarray.