Hot water pipes knocking

The noise you hear is the expansion and contraction of pipes rubbing against wood studding or floor beams. Securing pipes tight to beams will help to eliminate the noise.

U can use pipe stand offs which has a plastic ring for pipe to go through.

If there is space between clamp and beam, this slack should be removed.

Chuck

Knocking, popping, creaking and crackling pipes are a common complaint, especially in colder weather. I can tell you what's happening and take the mystery out of it but you're not going to like the repair. When a draw is made on a hot water line the line expands against a pipe strap next to a stud and when you drain hot water from your tub or shower it goes into a chase that is a little cooler then room temperature. The heat expands the pipe causing it to rub against the stud to which it is pipe strapped. This is the sound you hear. As it cools it contracts and the noise is heard again. To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight. Most people when they learn what causes it just elect to live with it. Good luck and thank you for rating my answer. Tom

Thank you very much to both of you for your replies and I do not doubt your expertise but I am not convinced that expansion of pipes is the answer and I apologize if I did not give you enough information to diagnose the problem. I will add some info here and if you still think that is the problem then I will accept your expert opinion.

I live in south Florida where cold is a rare occurrence. The pulsating actually comes right out of the hot water heater and is then transmitted to every pipe in the house. If you grab a hot water line under any sink you can feel the rhythmic pulsating like a heartbeat. You can set a watch to it because it is so regular and even with the water running for 30 minutes or more at a time it remains at a steady rhythm which makes me believe it is not just temperature related expansion.

In addition, with the hot water circulator system running (the system we have so that the hot water pipes are always filled with hot water so you don't have to wait for the water to heat up when you are on the other side of the house from the water heater) I would imagine that the pipes stay a relatively narrow range or near constant temperature during the time it is running yet the noise still exists. This circulator works on a timer pump and will circulate continuously if I set it to do so, all the while the beating is occurring. There is no water going down any drain pipes when just the circulator is running and no faucets are on.

I thought maybe this circulator pump was the problem because the source of the beating seems to be very near or inside the hot water tank and the pump is attached to the outside of the tank. However when I turn off the pump the noise indeed stops but only until someone turns on a hot water faucet and then there it is again at the same frequency beating (even with the pump now off) until the water is shut back off in which case the sound immediately stops. I would again imagine that if temperature changes were the key, then the sound would continue for a while as the water pipes then cooled down when circulation stopped rather than instantly.

Do you still think that the problem is pipe expansion/contraction from temperature change?

OK! Now I don't think it's contraction and expansion.
Is your water heater gas or electric? My bet's on gas. Am I correct? Tom

It is electric 120 gallons and I have solar hot water panels on the roof.