adding wash basin 1.25" waste to an existing 1.5" Drainage and wastage systems and plumbing help, advice and answers

[scouse52]

I am in the process of installing a wash basin (no basin exists at present). Can I simply connect the 1.25" waste for this basin, to the an exisiting 1.5" bath waste pipe or could this create problems. The new wash basin will have an anti-vac trap. I don't particularly want to drill a new waste hole to the outside wall or increase the size of the bath waste pipe. If connecting these wastes is not recommended then could I as an alternative boss the basin waste into the horizontal toilet waste pipe (bossed in from above)? Many thanks

[plumbbob]

It is not good practice, but you can combine the two wastes. Consider the length of run and falls of the two flows.

If the run is very long or the fall is slight, it may be the basin water will appear up in to the bath.

You can also connect to the toilet waste as you suggest. A right angled pan connector with a waste connection is available if useful.

[rosebery]

"I am in the process of installing a wash basin (no basin exists at present). Can I simply connect the 1.25" waste for this basin, to the an exisiting 1.5" bath waste pipe or could this create problems. The new wash basin will have an anti-vac trap. I don't particularly want to drill a new waste hole to the outside wall or increase the size of the bath waste pipe."

You could if thats the very best solution for you. Mind that you don't generate syphonage on the bath trap. You may find you need an antisyphon arrangement on the bath as well.

"If connecting these wastes is not recommended then could I as an alternative boss the basin waste into the horizontal toilet waste pipe (bossed in from above)?"

You could but I'd recommend the first alternative.

Cheers