DHW Cylinder - is this a common failure mode?

[Pentewan]

I have a 450 x 900 indirect cylinder that I put in about 10 years ago and I've noticed that there is now a very small (weeping) leak at the point where the cold water feed enters at the bottom. It's at the side so not easy to see (especially with the insulation around it) but it looks as if the leaks is where the tapped boss is fixed into the tank - the leak is definately not coming from any of the compression joints.

I would had thought a cylinder should have lasted longer than this. Is this a common failure mode?

Tomorrow I replace it with a new one so then I will be able to see what the real fault is. It will also be interesting to see how much scale buildup there is - we live in a very hard water area.

 

[croydoncorgi]

Could be just bad fitting in the first place - the joint might have started leaking along the threads where the compression joint for the feed is screwed into the boss on the cylinder. More likely, the flange joint itself will have failed. Again, most likely due to bad fitting, so the joint was strained. The wall of DHW cylinders start off thin.

Corrosion, especially the electrolytic type, can thin the copper even more, so that eventually the wall fails, usually around a flange. If this IS the cause of your leak, be very careful to drain the cylinder completely before touching the joint! Otherwise you might be paddling - quite a large area may be tissue-paper thin.

10 years is not long for a cylinder. But when you get it out, check whether there was originally a 'sacrificial anode' in the cylinder and whether there's anything left of it - should be visible at the bottom of the cylinder when you look through the immersion heater flange. Also, look for black (copper oxide?) discolouration on the outside of the cylinder and dezincification (pink colouration inside) of brass fittings: both indicative of electrolytic corrosion.

Having had a couple of similar leaks recently, I'd be interested to hear what caused your cylinder to fail.

 

[ChrisR]

I bet it's one of the "medium duty" ones, now gone, with the coming of Part L. It was mainly the bosses which made them not pass the Brit Standard . They had silly short coils in too.

 

[Pentewan]

Well, I finally got the new cylinder in!

I drained down the old one and removed it. It had been leaking from a hairline crack just to the side of the cold water inlet. There was no obvious reason why the crack was there as the surrounding metal was sound and not obviously corroded. There was some "creasing" in the copper around this area, possibly because the boss had been bent a little relative to the rest of the cylinder. But I'm still none the wiser as to why it failed.

I was surprised to find very little hard scale inside. Two cylinders I've replaced in other houses have had solid rock hanging onto the heating coil, but not this one. Possibly this is because it is normally used with a pumped primary rather than gravity primary circuit, and we keep the temperature down to 50 - 60 C.

Then the fun started!

The new cylinder didn't have a tapping for drain cock, so I had to add this to the inlet pipe. Then I found that the heating coil was longer with the upper and lower inlets further apart than on the old cylinder. So I had to modify the inlet pipes, involving soldering 28mm pipe fittings. Having done this I then got the new cylinder back in and coupled up, filled it and got the immersion working only to find that there was a leak on one of the new joints I'd made and I couldn't get to it without taking the cylinder out again.

So on day two I started again. I drained it down and got the cylinder out, resoldered the joint, decided I now had time to repaint the walls and skirting in the airing cupboard and got ready to put it back in. Then I realised that my new pipework in the feed side of the primary would form a potential air lock, so rather than get it all in again and then find it wouldn't work I bit the bullet and modified the pipework again. Finally it was ready - in it went again, this time testing the primary side first. It was OK when cold but once up to temperature one of my new joints leaked so out it came again. It was finally in place by 5.30 on day 2 and so far all is well.

There's no real moral to the story other than making sure you solder all joints well and thinking through the design carefully to avoid air locks.

It will be interesting to see whether the HW recovery time is faster now that the heating coil is longer.

 

[PEDANTICVINDICTIVEMAN]

Obviously you wern't waring the proper overalls, the brown ones with the front breast pocket with a packet of Woodbines sticking out, and a half smoked one behind your ear