Trickling sound in HW cylinder - possible ruptured coil?

[RigidRaider]

Some of you followed my post a few days ago about our new boiler and my concerns about the adequacy of the water heating.

Ever since the boiler was installed along with a new HW primary circuit on a Y plan, we have been concerned about its performance. In answer to those who say "Get the plumber back" - yes, he will come back at a moment's notice but this week he's away on holiday.

What concerns me is that I can hear a gentle trickling sound inside the cylinder. The first time the plumber came back he fitted a vent on the flow pipe beside the cylinder and got a lot of air out of the coil. Now why would I still be able to hear a trickling sound? If there was air in the coil and water was trickling past it, surely that air would have risen to the top of the coil and up to the vent by now?

There's another strange symptom: when you draw off hot water the cold supply pipe entering the bottom of the cylinder from the F&E tank becomes cold very quickly as you would expect. The boiler fires up as you would expect. As soon as you stop drawing off hot water both the cold supply pipe and the cylinder vent pipe, rising straight from the top of the cylinder, become very hot very quickly. Now I know water expands as it warms up but this seems remarkably sudden. The plumber did ask me to check that the pump isn't pumping over and this morning I went up and checked again while the boiler was firing and saw no evidence of hot water flowing over the top and back into the F&E tank.

Is it possible that while doing the work on the primary circuit the plumber may have indavertantly ruptured the coil? There's never been any smell of Fernox (a distinctive alcohol-y smell) in the hot water and there's absolutely no other evidence of the pump pressurising the cylinder or any contamination with primary water.

The HW cylinder is 20 years old - how common is a ruptured coil, in your experience? Surely if this had hapened due to a mechanical twisting force being exerted on the outside of a union, there would also be a leak outside the cylinder?

We will get the plumber back next week but as always I'm grateful for your thoughts.

 

[JohnD]

As soon as you stop drawing off hot water both the cold supply pipe and the cylinder vent pipe, rising straight from the top of the cylinder, become very hot very quickly.

the cold supply pipe goes into the bottom of the cylinder. is that the one you mean?

 

[RigidRaider]

Yes, it comes straight down from the F&E tank.

Been thinking about this since my first post - if the coil was ruptured there's no way you would hear trickling water, surely? The HW cylinder is full to the top so even if water was bursting out of the primary circuit you wouldn't hear anything.

The explanation has to be that air is trapped in the coil and what I can hear is heated water flowing into the top of the coil and dropping down onto the air bubble. If this is the case I still can't understand how the air can remain trapped in the coil, which slopes uphill.

Unless the coil was badly manufactured or had drooped inside the cylinder enough for half of each loop to be forming an air trap? The presence of air would certainly explain the poor heat transmission from primary to cylinder.

 

[JohnD]

the cold water supply pipe goes into the bottom of the cylinder. Since hot water rises, there is no way hot water can get into it unless it is being pumped, or sometimes if the cylinder is boiling. Go and look at the cold water tank (this is the big one, not the Feed and Expansion tank). If you see no sign of water coming out of the vent pipe, tie a rag round it so it will get wet if water comes out, and check it from time to time.

Have you got a shower pump? Go and turn it off.

If you can post a pic of the cylinder and the pipes around it, and indicate which one you mean, that will be helpful.
//www.diynot.com/forums/viewtopic.php?t=129539

**** STOP PRESS ******
have you got a shower mixer or a sink mixer that is fed with cold water from the mains and hot water from the tank? Especially a "joystick" one? Go and feel the hot pipe to this tap, see if it is surprisingly cold.

 

[RigidRaider]

Okay, I thought the big tank was called the feed & expansion tank. The big tank feeds the hot water system and cold to the showers and the cylinder expansion pipe curves over above it. This is not the same as the smaller heating F&E tank.

From the cold water tank a 22mm pipe drops straight down and into the bottom of the cylinder. When you draw hot water this becomes cold, as you would expect. However when you are not drawing water it becomes very hot, very quickly and the hot zone extends right up to the level of the water in the big cold water tank. I can't understand why this happens because there is also an expansion pipe running from the top of the cylinder to curve over the top of the cold tank. Also as you write above, the bottom of the cylinder has a layer of cold water, so how the heck is hot water being driven back up the supply pipe?

No shower pump and no mixer taps with cold mains feed.

 

[steved101]

do you have a mixer tap on your kitchen sink??


shower pump will not cause pumping over...... why did you suggest that??


what height roughly does the vent pipe over the large cold water tank rise to above the water level in the tank, BEFORE it bends over and into the tank??

btw when water in a hot water cylinder is heated it expands and puahses some water back up the cold feed pipe and into the cold water tank (large one). it will not be hot hot though.

 

[JohnD]

The big tank feeds the hot water system and cold to the showers

you do not mention the other cold taps.

Like steved101 I suspect your kitchen sink mixer, also your bidet mixer, bath mixer etc.

 

[charliechaplinspants]

You can always expect an amount of residual heat travelling up the expansion pipe ( top of cylinder) and the cold feed into bottom of cylinder when the cylinder itself is banging hot, have you got a cylinder stat? As for the trickling noise, does it matter? as long as your system is working why are you interested in listening to the noises in your cylinder, don`t you have a TV? I bet if you had listened as much before you`re new installation you may have heard stranger sounds. There doesn`t seem to be a problem. How many people stand next to their cylinder every morning turn on the hot taps and feel the pipes? It`s all in your `any chance of sueing the installer` mind.

 

[RigidRaider]

Thanks for the advice everyone. No, there's no suggestion of sueing the installer; we are on good terms with him, we know him well, he's done plenty of work for us in past years, we have always paid him promptly and he will come and rectify any problem at a moment's notice.

We are still concerned that the hot water is not as hot or plentiful with the new boiler as it was with the old. The trickling noise in the cylinder is new and I'm trying to determine if this could mean there's air trapped in the coil and therefore inefficient exchange of heat. Along the way I'm trying to understand why the expansion pipe AND the cold feed to the cylinder get very hot only a few minutes after drawing hot water, whether or not the circulator is running and the boiler firing.

The plumber is away his week but as as soon as he returns he will be back see if he can get the hot water performing better.

 

[muggles]

What height is your F+E tank installed in relation to your cold water storage cistern (CWSC)? Are they at the same level? Or at different heights? Which one is higher?

If you suspect that the coil is leaking into the cylinder, don't draw any water off for a while and wait for your CWSC to stop filling. Check that both tanks in the loft are not filling. Now go to a hot tap and open it (bath is best as this takes less time). Leaving it open, go up and watch the loft tanks. If they both start draining out and therefore refilling, you have a split coil. If they don't, the coil's OK.

Incidentally the question about different heights relates to a different way of spotting the same problem - if they're fitted at different heights then one tank will fill up from the other via the split coil, and if there's sufficient height difference it will overflow.

 

[RigidRaider]

Thanks, that's interesting. Both tanks (the large house cold tank and the heating F&E tank) are at the same height.

Is the HW primary circuit usually supplied from the heating F&E tank then? I hadn't worked out that bit.

I will check as you suggest.

 

[muggles]

Yes, all heating pipework is supplied from the F+E. Current installation guidance states that the F+E should be installed below the CWSC so that in the event of a coil split the pressure differential will cause water from the cylinder to flow back up into the F+E, and out of the overflow there, thereby preventing the mixing of central heating water and associated chemicals in the hot water cylinder, and also preventing those chemicals finding their way into the CWSC. Yours is obviously and older installation and so won't necessarily comply with this, although having the bases on the same level is good enough.