Air admittance needed for dishwasher connection?

[endecotp]

Dear Experts,

I am attempting to connect the waste from a dishwasher. It is not next to the sink and the plumbing is awkward at the back of a corner unit. For extra fun, the drain goes into the wall relatively high up.

I am considering running a length of waste pipe from near the sink with a running trap at an accessible location half way along. Then near the dishwasher, connect the appliance hose directly onto the end of the waste pipe.

I’m aware that normally an appliance connection has some sort of air admittance, e.g. if it connects just below a sink waste or if a standpipe is used. With my proposal there wouldn’t be any air admittance. Is this a problem? If so, what it the solution?

Many thanks, Phil.

 

[CBW]

If it’s on the same run as the kitchen sink, then air admittance will not be required, as long as it’s trapped, which you’re doing with your running trap.

 

[endecotp]

Thanks CBW.

Pipe disappears into the wall in the middle. There is a tee. To the right is the sink, with a conventional trap and washing machine connection. To the left is the dishwasher.

Thinking a bit more… (dangerous I know!) … air admittance is normally downstream of a trap, to avoid suction from downstream emptying the trap, right? What I’m worried about is upstream of the trap, i.e. where the dishwasher connects to the trap. As the dishwasher starts to pump out, where does the air in its outlet hose go? Does it bubble through the trap? It would escape if there were a standpipe or sink waste connection.

 

[endecotp]

Maybe I should be using a HepVO ?

 

[Madrab]

How long is the run? You could always us an anti vac running trap, will avoid any gurgling in the waste and the reduce the possibility of the trap being self syphoned

With a HepVO ideally it needs to be somewhere serviceable, they do need periodic maintenance/checking, especially on an appliance run as they dry up between uses and can get crusty.

 

[endecotp]

Thanks Madrab. That’s air admittance on the downstream side of the trap, right? Any thoughts on my concern about the upstream side? Googling finds lots of American references where they need to have an air gap upstream of the trap for regs compliance. I know we din’t need it for that reason, yet still we do nearly always have some sort of air gap e.g. a standpipe or a connection below a sink waste. If I just connect a dishwasher hose to the input of a trap like that one, are there any problems?

I’ve found this part:
https://www.screwfix.com/p/mcalpine-twin-hose-connector-40mm/206hr
McAlpine twin hose connector
For plumbing 2 domestic appliance discharge hoses into a standpipe trap
Incorporated Air Break

Is this the sort of thing that I need?

 

[endecotp]

Hmm, actually that “twin hose connector” doesn’t seem to have any sort of valve in its air vent; it is literally just like sticking two hoses into one standpipe. But McAlpine do have this, which does have a valve. But it is rather long; can I cut that down to, well, almost nothing and still work?

 

[CBW]

Why do you need an air gap/break?

 

[JohnD]

to prevent syphonage

 

[endecotp]

Why do you need an air gap/break?

I don’t know - do I? Please tell me.

My feeling is that in most installations there is an air gap (i.e. in standpipes or under-sink-waste connections) so perhaps I always need one.

 

[Madrab]

Always good to have an air gap, stop the risk of self sypohing especially if the waste starts to get restricted further downstream of any trap seal.

Hmm, actually that “twin hose connector” doesn’t seem to have any sort of valve in its air vent

The McAlpine twin spigot has an air hole in that slot in the top.

The Air Admittance side of the trap could be either up or downstream I guess, I'd probably orient it downstream though given it's to break any vacuum that a full pipe downstream of the seal may create.

 

[CBW]

I would have thought the inline running trap would be sufficient. However, the mcAlpine double spigot or the trap Madrab suggested would be more than enough.

 

[Madrab]

Hmm, actually that “twin hose connector” doesn’t seem to have any sort of valve in its air vent

The McAlpine twin spigot has an air hole in that slot in the top.

Sorry, just re-read that, you're right' it's not valved, that'll be because the standpipe is trapped so doesn't need a one way.

 

[endecotp]

The sort of valve in the “TV1” i.e. a float so that air can go in or out but not water is, I think, the ideal solution. But i bet no-one ever uses them!