[120336]

Whenever I turn on any hot water tap in the house to more than a trickle I get a terrible knocking. Is this dangerous and how do I stop it?

[speedball1]

This tells you that something's loose and vibrating. In situations like this you have to be there to fell where the vibration's the strongest or the loudest. Start the pipes knocking, start looking at the water heater and began to track back to the strongest spot. Let me know what you find. Good luck, Tom

[Credendovidis]

This indicates that there is air somewhere in the hot water pipes, causing these bangs (water hammer).
Find the hotwater pipe release valve for air, to let the air escape. Normally near the boiler or heater. It may take time and several release actions to end the problem.

Central heating systems always have air in the pipes, so they are supplied and installed with an expansion vessel, also with a gas-release valve to allow too much air to escape.

[speedball1]

I disagree with Credendovidis! Both with his analysis of the situation and his explanation of water hammer.

This indicates that there is air somewhere in the hot water pipes, causing these bangs (water hammer).

Water hamer's caused by water, not air. Water hammer is usually recognized by a banging or thumping in water lines. The noise occurs when the flow of moving water is instantaneously stopped by a closing valve. This sudden stop results in a pressure spike behind the valve which acts like a tiny explosion inside the pipe. This pressure spike will reverberate throughout the plumbing system, rattling and shaking pipes, until it is absorbed.
That's water hammer. Air in the lines may cause a faucet to spit at you but opening a faucet at the top of the system will clear any air out of the line.
It justs keeps getting better and better.

Central heating systems always have air in the pipes,

There was never any mention of a centrap heating system in the askers post. If this were water hammer he would be getting a big bang whenever he shut the water OFF not turning it on. What he has is called "chatter"not "hammer"and hasn't a thing to do with his central heating system. Tom

[Credendovidis]

It justs keeps getting better and better.

What a silly man you are. I may have been wrong, but I provided my input in trying to help here. Not to be pissed on to by you. Who do you think you are ?

:(

[hkstroud]

What a silly man you are. I may have been wrong, but I provided my input in trying to help here. Not to be pissed on to by you. Who do you think you are ?
:(

You are wrong. Speedball1 did not piss on you post. You did not read the post and were incorrect in you response. Speedball1 corrected you factually and politely. I don't know who Tom thinks he is but he IS most knowledgeable and experienced plumber on this site. You are wrong, you were rude and owe Tom an apology.

[speedball1]

Credendovidis disagrees: What a silly man you are. Why do you piss on my post? Who do you think you are?

Who am I? Why I'm the guy that corrected you when you gave a faulty and misleading answer. That's who I am.

letmetellu agrees: Don't feel bad it has happened to a lot of us in the past, best to just ignore it.

Letmetellyou, If you have a problem with me then "spit it out in Daddys hand" and we'll talk it over.
In the meantime , Credendovidis , You cool your jets! You don't attack another expert on this page, whether it's me or someone else. If you give faulty information expect to be corrected. If that hurts your feelings then perhaps you're too sensitive to reply to plumbing questions. This is your first warning. Tom

Whenever I turn on any hot water tap in the house to more than a trickle I get a terrible knocking. Is this dangerous and how do I stop it?

To get back to your question do you have heat trap nipples,(see image) connected to the heater supply and outlet? Can byou feel where the vibration's the strongest? Sorry we got sidetracked But one of my jobs is policing the site and correcting wrong answers or deleting dangerous ones.
Let's hear back from you. Regaqrds, Tom

[bwechner]

I have the same problem out of the blue. Last night my kitchen hot water tap just started spitting in bursts, which actually scalded someone filling a kettle.

It looked indeed a lot like air pockets being spat out with the water stream and it would come in waves.

I have a vent at the top of the hot water cylinder exit pipe, which I vented last night, and that seems to have helped. I shall repeat if it returns.

My curious question, for speedball1 who seems very knowledgeable, is how and why would this suddenly happen after years and years of trouble free use?

There has been a sudden and almost violent drop in temperature in the last week here, winter came with an unseasonal and sudden force it seems. I wonder if that's correlated. It's not frozen though we don't freeze here or get much frost at all, but it is decidedly chilly compared to the week before (perhaps 10 degree drop in average temperatures)

My reason for concern is it kind of implies a leak in the system somewhere. Perhaps only a small one, a very slight drip at a brazed joint in the pipe say such that water trickles out, possible not even dripping, just evaporating, but allowing the ingress of air into the pip which then works it way to the high point in the pipe (where my vent is). If that's the case, I'd rather find it before it gets worse and starts leaking proper and flooding my house or wall cavity.

I had such an experience in the cold water feeder to the house last year, which was evidenced only by flooding in the cellar. I traced it to a small and steady leak in an elbow where the underground feeder comes up from the under ground beside the house (and then enters the house). Alas the flooding did some considerable damage to the ground in my basement (clay, which swelled in the wet, and then shrunk leaving deep deep cracks all over the floor. I'd rather not experience something equally bothersome...

[speedball1]

Do you have a gas or electric water heater?

Last night my kitchen hot water tap just started spitting in bursts, which actually scalded someone filling a kettle.

Makes me wonder if what you're seeing isn't steam. What's the temperature setting on the thermostats? Hot water,yes. Scalding water, too hot. Regards, Tom

[bwechner]

Solar in fact with electric boost.

It wasn't too scalding. The kettle had just been used in fact and emptied, so was being refilled while dry. He lifted the lid and ran the tap. It spat back at him and didn't scald badly but enough for us to be worried and run his face under cold water a while. Settled down fine. I'm not sure where the thermostat is but I would guess 65 degrees C. It was at 70 before we replaced the system some years ago, but the old system was a low pressure tank and fed from a header tank only 3 or so metres above ground, so to get a shower out of it the water was presumably set so very high.

I seem to recall a fashion here for 70 degree C hot water too in response to Legionaires concerns and it being described as the upper limit in terms of safety, and not generally a good idea especially with kids in the house.

It may be that the kettle was filled with cold water and this was in fact the water hitting the hot element and vaporising suddenly causing spit back. That was our assumption.

Later in the evening though, oddly enough, filling my hot water bottle for bed (I told you it got cold ;-), I noticed this hot water spitting from the tap.

I have guessed since, that this was in fact what happened earlier and that the man concerned was filling from the hot water tap (not something I do as a rule, as we've also been discourage in our upbringing from drinking hot water, which has been sitting idle and warm in a tank for so long, we boil cold water instead, but I know other people have other habits)

Anyhow it's solar with a collector on the roof and a small circulation pump that cycles water via the collector and an electric element as a backup which in this cold weather I enable after sundown to ensure we have hot water for morning showers. The hot water is drawn from the top of a tank where there's a pressure release valve, which I was in fact advices to activate twice a year or so, so I cut a small door into the wall cladding behind which the tank rests, to access that valve.

I vented it last night.