Boiler leak.. then another... then another

[Andrew Baker]

OK, so am I missing something here? I fix a leak and another appears..

We've two system boilers that run in parallel for some reason. Vaillant thermocompacts - both of them - about 10 years old now.

One stopped working yesterday. Took cover off to check the flue sensor and noticed a the hot water output from the H-ex was weeping on both boilers. One obviously had been doing it for a while (rusty bottom!)

Bought the correct, official "packrings" (o-rings by any other name) and replaced them.

Put water back in.. the input side leaks on one of them... Took it all apart, replaced both of those.

Put water back in. The PUMP connection starts leaking.. ON BOTH BOILERS.. Replaced the packrings (same one - lucky I had to buy a pack of 10!)

Put water back in. The pip next to the pump is now leaking ON BOTH BOILERS.

Is this just coincidence (combined with 10-year-old boilers)? Are these packrings all failing, in the same order, at the same time? Or is it, say, because the boiler has been cold and empty of water, which is causing pretend-leaks that will go away if I fill it (with bucket handy) and run it to temperature?

It's too cold for this!

[Jethro_uk]

You'll find as you strengthen one part of a system, the next weakest part will fail - especially on older systems. Which is why you're better off overhauling everything at once.

[harry]

As pipes heat and cool, expansion puts a lot of stress on the pipes depending on the layout. Could be these effects causing your leaks. They may have been piped up in a poor way, ie without regard to expansion stresses. Especially in larger systems.

Sludge accumulations can cause local overheating/stresses too in the heat exchangers.

Two boilers is a good thing for fuel efficiency if they are piped up correctly.