Why Are My Pipes Knocking When the Toilet is Flushed?

Why Are My Pipes Knocking When the Toilet is Flushed

The sound of knocking pipes in the wall when you flush the toilet sounds like a major plumbing repair emergency call is imminent. But as disastrous as the knocking pipes may seem, they have a simple cause and, in most cases, can be resolved quickly. 

However, you do not want to wait to address this problem. If knocking pipes are not corrected, the pressure and sideways force brought to bear on the pipes can eventually break poorly soldered joints and fittings. It can also loosen the straps that secure your water pipes to the beams inside your walls. If this occurs, the knocking sound will progress from occurring only when you flush the toilet to any time you use water that passes through wherever the pipes have come loose. The knocking sound will be louder, and the risk of joint breakage increased.

Your cold water pipes involve water, water pressure, water flow rate, pipe diameter, and strategically placed valves to stop and start the flow of water. These valves are the manual ones you use to turn on sink faucets and also the automatic shut off valves used in toilets and washing machines and dishwashers. When you flush a toilet, the valve opens. When the toilet tank refills, the valve shuts off.

Your Toilet Water Fill Valve is Where It Begins

The water that operates your toilet is managed by you when you flush the bowl and by the water fill valve that opens to allow water into your toilet tank and bowl to replace the water.

Your water feed is under pressure at all times. That’s why it can go uphill and travel from your basement to your second-floor bathrooms. When your toilet water fill valve is closed, the water in the pipe on the other side of the valve is still. It is under pressure, of course, but it isn’t moving, so it has less energy.

When the valve is opened, the entire pipeline of water – back to your water main – rushes forward toward the toilet tank with force. When the toilet’s automatic water valve shuts abruptly, the rushing water has nowhere to go and “crashes” into itself. The dispersal of energy pushes against the pipes and that’s why you hear knocking. It’s crashing water with nowhere to go. We call this a “water hammer.” It is merely a water pressure spike.

The Built-in Cure for Water Hammer

The way to apply “brakes” to this crashing water issue is with air gaps. Water flow that is suddenly stopped needs to release its energy. Sending it up into an air pocket acts as a shock absorber. Plumbing systems built before the 1960s usually included T-sections of pipe containing only air. They are vertical sections of pipe joined to the main water line with a T-coupling that creates an air cushion. 

Since the 1960s, residential plumbing systems upgraded the air chamber with water hammer arrestors. These devices are vertical pipes set just behind a quick closing valve. These arrestors contain a permanent, pressurized air cushion and a piston that compresses it when water pressure spikes and drives the water up against the piston. When strategically placed behind quick closing valves, they are effective management of sudden spikes in the water line pressure.

How to Eliminate Pipe Knocking Brought on by a Toilet Flush

Because the modern water hammer arrestors rarely fail, if you are experience pipe knocking, you may have either the earlier air chamber or of no concern. Occasionally arrestors have been omitted from the line, especially if the previous work was a DIY project. 

Air cushions and, much less often, arrestors can fail when they gradually replace the air with water and lose their ability to buffer the line. Fortunately, it’s usually a simple fix for people who are familiar with plumbing repairs. If not, call Benjamin Franklin Plumbing, and we will solve your problem fast. All water must be drained out of your plumbing system. If this sounds overwhelming, call us. Otherwise, use this method:

  • Locate the water main shut-off lever to your house. It is in the front and runs under your lawn to the house from the road. It may be just to the outside of your foundation on the wall, or under a metal plate and concrete box next to the house. Some houses have the water main shut-off on the inside wall. If you can’t find it, give Benjamin Franklin Plumbing a call. We may be able to walk you through finding the shut off over the phone.
  • Turn off the water to your home.
  • Open the highest faucet in your house and drain water from the lowest faucet (usually found in the basement or on the first floor). 
  • Flush all the toilets in your home. 
  • Run the dishwasher and washing machine for a few minutes to drain them.
  • Once your plumbing system is drained of water, water clogged air chambers will have refilled with air. If water got into an arrestor, it will also be drained. 
  • Close the lowest faucet and turn on the water supply again. The top faucet will sputter briefly. When it turns to a constant stream, turn it off.
  • Allow the toilets to refill with water if they have not done so yet.
  • Test the line by flushing the toilet that created the noise to see if the issue is resolved.

If it Doesn’t Work

If the above procedure doesn’t work or only works temporarily, we strongly recommend you give Benjamin Franklin Plumbing a call. Several things could be the cause of continued water hammering, and while they are usually fast to repair, they require a professional. 

  • Excessive water pressure in your pipes can defeat air chambers and arrestors. Your pressure should be set below 50 PSI. This action can be achieved by dedicated do-it-yourselfers, but we recommend a professional inspection.
  • The pipes may have rattled loose from their straps and shake and bang when the water pressure increases. Upon inspection, these can be secured or padded.
  • If your water hammering has gone on for a long time, an inspection of your water line should be performed to ensure no leaking joints or pipe damage was sustained.