depends on the boiler and the soundness of the wall. Is it a dry lined wall? (dab and board) if so then get the plaster'a to dab the entire area of where the boiler will sit and you will get goo fixings with 3 to 4" screws without worry. If its not dabbed well you may distort the wall area.
long coach bolts are a good idea also.
to be honest there are alot of different types of these blocks depnedant on the walls designed structual load, as the higher the load the more dense the block and the better the fixings will be.
thanks aw thats a good idea about dabbing the whole area, its an extention and luckily it's not been boarded out yet. i hate these type of blocks they're as soft as butter
got to say the rawlplugs and bolts that come with vailants seem to hold in most walls think there fischer fixings so probably exspensive to buy seperately
from what ive seen watching others a lot of the time bad fixings are caused by fitters who dont have the right drill for the fixing and try to get away with wiggling the drill which gives a crater not a hole and little chance of getting a good fixing
If I'm in doubt of the strength of fixing that I can get I normally fit 3/4 ply to fix the boiler to. It gives you scope to get a lot more fixings in than you can with the boiler bracket. I've even had to resort to chemical resin fixings on a couple of occasions.
Official Sponsors of Plumbers Talk
Reply to the thread, titled "fixing combi boiler to termalite wall" which is posted in Boiler Advice Forum on Plumbers Forums.