Unvented Undersink Water Heaters. For D.I.Yers.

A plumber once told me that they are not permitted to be used where they are inaccessible. In other words, the situation is just like MF JBs.

I think the old rules were something like that - you couldn't use olive-type compression fittings underground, but you could use ‘manipulative’ fittings - i.e. ones that didn't have a loose olive but instead used a special tool to create an olive shaped bulge in the copper tube itself.

Probably the rules have changed these days to allow plastic compression fittings with MDPE underground pipework (where the seal is by O-ring but the restraint is often compression of some form). 

   - Andy.

OlympusMons: 
 

I have never found one that correctly installed has leaked. They are a fit and forget fitting. …

Z.

depends on the composition of the brass and its age. Dezincification can occur which leaches out the zinc leaving just copper. I have stripped out old copper pipes and the brass fittings are the first things to snap on bending. You often see it on the brass waste outlets in basins which turn to cheese, or perhaps that has more to do with unblocking chemicals used. Also, they do tend to weep slightly over time and need nipping up occasionally.

Perhaps the limescale on the insides of the pipework and joint will seal any minor imperfections, and/or protect the brass from corrosion.

Z.

Tin (or for non drinking water plumbing, arsenic) can be added to the copper/zinc mix of the brass to increase the resistance to selective leaching of the zinc. Fittings made of such alloys should be marked DZR if sold as dezincification resistant. 

If you live in a soft water area, such fittings should be pretty much de-rigeur, and copper olives.. Water that is slightly acidic (traces of sulpher dioxide and carbon dioxide will do this) are likely to be more problematic than those that are alkali or chalky. Pure zinc/copper can reduce to spongy orange copper at a rate of hundreds of microns per year in sightly chlorinated water. Cheap brass with impurities like traces of iron is far faster.

Mike.

As ever, Mike describes the science. So our rock-hard water is not a threat to brass, but acidic atmospheric moisture is. Every now and again, I have to descale my shower mixer. I must be careful not to overdo the application of acid. ?