Our combi boiler (a wall-mount unit that is both a central heating boiler and tankless water heater) is as old as the whole bathroom, late 1977 vintage. The manufacturer suggested a likely life span of 15 years for that model. For the last three years we have been losing water from the system and had to refill every couple of weeks sometimes. Our regular plumbing/heating guy who checks the unit every year told us to live with it or get a new boiler. There is one company that bought all spare parts when Junkers dropped support for those dinosaurs. We called them and he confirmed my suspicions - broken expansion vessel - and claimed the boiler wasn't worth the repair.
We are talking about a closed, pump fed pressurised system here. When heated, water expands (like pretty much every other substance) and the presure in the system increases. Since water cannot be compressed (like air in a tire) a rise in water pressure eventually leads to exploding pipes, which is why a pressure relief valve is fitted to every system as an emergency relief feature. In normal operation, the pressure needs to be relieved differently or you ned to refill periodically. This is what the diaphragm expansion vessel is made for. A steel calinder with a rubber membrane, one part filled with water, the other with nitrogen or air. The gas compresses as the water gains pressure and balances the whole system. If the membrane tears from age, the vessel is shot.
A freind suggested simply installing an external expansion vessel. After some research I decided he was right. €40 in materials later and a day of help from my uncle the plumber, the system was up and running again. While he refileld the system he also found a loose connection that dripped. After he retightened that slip nut, the system held pressure perfectly. Now why were two service technicians incapable of finding and stopping that leak?!?
The red vessel next to our boiler (above the bath tub) looks borderline ugly, but it works again! I certainly value heat over looks and even my mom, the main style watchdog in the family, approved!
However, our sink that had already been a little loose for a while must have taken a beating during the process and now threatened to fall off the wall. At the time of one of the many plumbing leak repairs (we some lead cold water pipes until 1998) the plumber had to move the sink (which is bolted to the brick wall) up by 10cm to avoid drilling into his new pipe. He drilled several holes and finally claimed "now it should work".
When I took down the sink (close both shutoff valves, disconnect braided hoses from valves to sink, get someone to hold the sink while you're crouching underneath and undo the nuts, pull the sink off the walll including the trap which is only pushed into the drain pipe with a rubber o-ring, thanks heaven not soldered into a lead drain any more) it became soon clear why it hadn't really worked.
A) When my dad redid that bathroom he wanted it done fast and cheap. Thus the plumber friend of his, named Gustl (famous for his dodgy plumbing by now, like solder joints that held merely by chance and fell apart as soon as the pipe was touched or reversing the hot and cold at the boiler, with the result that the radiators took ages to get warm) tiled over the old white tiles. Where they were removed for plumbing work the void was simply filled up with plaster to the surface of the old tiles, eventually resulting in an almost 5cm (2") thick layer of plaster.
The bolts that hold the sink were 150mm (ca. 6") long, half machine thread and half wood screw. That means 5 out of the 7.5 cm supposed to go into the wall were in plaster rather than brick. Very very soft plaster, modern lime-cement mix with a very high perlite content, which is even softer than the cheapest old plaster. You can crumble it away with your fingers.
Furthermore, drilling 14mm diameter holes into the brick and inserting large screws caused both bricks to crack, weakening the hold of the bolts even more. Normally this is prevented by the bond of the bricks and the mortar joints that evenly distribute the pressure of the screw across the wall but in this case there was a deep hole below from some old plumbing, putting the whole stress on one brick.
This picture shows you the whole sorry mess with most of the plaster removed but the old bolts still in place. Lots of unused old plumbing as well as some new. The white and red stuff is plastic and foam pipe insulation, serving two purposes. First of all it makes sense to keep your hot water lines from heating your walls, then around cold water lines moisture can condensate in summer and cause trouble, and lastly copper needs to be protected from caustic mortar and plaster. The red section is from when we moved the hot water supply to the kitchen (originally exposed on the walls below the ceiling) under the hallway floor. That was in 2004 and we never replaced the plaster and tile to the right of the sink (well, that's behind the washing machine, so who cares...). The two yellow objects at the bottom are the taped shutoff valves, the thing with an old sock is the drain (it stank badly without the trap).My first attempt was to reseat the bolts by using new plastic wall plugs and repairing the cracked brick, supporting it with plenty of mortar in the hole below. The I squeezed plenty of spackle into the holes, inserted the plugs, plastered the whole section (not bothering with tiles since this is again a temporary job and hidden below the sink/behind the washing machine) waited overnight and re-installed the bolts. Since most of the bolts ended up embedded in plaster I left holes around them and only closed up everything when the bolts were fully screwed into the wall.
First coat of plaster:
I used lime plaster because we had a bucket of lime putty and a bag of sand left over from the WC project. When I ran out of lime plaster on the right, I used up a leftover bag of perlite plaster. Finish coat with one hole already filled in: 
Once this had cured enough I tried to install the sink, only to find that the bolts were too deep into the wall or too short, depending on perspective. Clearly, a new plan was necessary. First I asked around for 200mm long bolts. "Yeah, they make them, but we don't have any in stock!" was the reply of one of our local plumbers. OTOH he gave me two shiny new 150mm ones for free. Off to the big box store. "Wall bolts M14? We have 80mm and 100mm!" Yeouch! 150mm are too short and they try to sell me 80mm ones? Off to the painting supplies, grab a box of quick-setting concrete (useable for 3 minutes once mixed with water according to the instructions). Pull out old plugs using pliers and an old screwdriver, clean out holes and insert new plugs, flush with the wall surface. Fill holes with concrete rather than plaster. Wait an hour just to make sure. Insert bolts and hang sink. Done!
Just need to caulk where it meets the wall, with the old tiles being laid so bumpy there is a sizeable gap there.
After that experience I don't want to touch any plumbing again soon, at least not without wearing wellingtons (rubber boots)! The serious advantage of electricity (my learned trade) over water and gas is that it absolutely won't come shooting out of the wire and puddle on the floor!
