Kitchen waste - compression vs push fit for outside? Which pipe? And do I need an AAV?

[DIYNotIan]

Hi all

Sorry for the long title but thought best to bung all these questions together.

I'll be fitting a new kitchen sink soon and am unsure on the following...

1. Compression vs Push Fit for outside? I know solvent weld is the preferred and professional choice, but I'd really like to be able to dismantle every so often and clean out the pipes...

2. Which pipe for outside? If I'm using push fit then I guess the answer is obvious! but if using compression I've read I can use either type of pipe. Which stands up best to the elements? And do both types need painting (have read various answers on that question)?

3. Do I need an AAV valve? I'm totally confused as to when they are/aren't needed. For context, the kitchen sink is not connected to any other wastes. And it is open ended , i.e. the open end of the pipe sits just inside the drain cover grate. EDIT - I should add. There is no AAV in the existing set up. However, in case it matters, I will be going from a sink and a half to a single sink, and I'll be adding a dishwasher where does was none before.

Any help would be much appreciated.

Thanks for reading

 

[Hugh Jaleak]

Solvent weld every time. If you've a slightly more complicated run, fit an cleaning eye where needed to allow access to the pipework. Use swept bends as opposed to elbows and provided you don't chuck anything down the sink you shouldn't be doing, it's highly unlikely anything will block. My sink has been in situ for nearly 25 years, never had to clean the waste pipe out.

Push fit, the seals will go brittle over time, so chances are if you take it apart to clean it out, it'll leak like a sieve afterwards. It also suffers more from the weather, although it is recommended to give all external plastic pipework or whatever type a couple of coats of decent gloss paint to provide some UV protection.

You wont need an AAV on a single sink, provided the run isn't ridiculously long, and there's no other appliances connected to it.

 

[DIYNotIan]

Great - thank you very much for the super quick reply.

Thanks for explaining the down sides of push fit. Is there a reason compression is never used outside? Same thing about the seals perishing or another reason?

no other appliances connected to it.

Funnily enough, I expect as you were typing that out I was editing my original post! I added: "There is no AAV in the existing set up. However, in case it matters, I will be going from a sink and a half to a single sink, and I'll be adding a dishwasher where does was none before."

So does that mean an AAV might be needed?

Cheers

 

[Hugh Jaleak]

Thanks for explaining the down sides of push fit. Is there a reason compression is never used outside? Same thing about the seals perishing or another reason?

Very few Plumbers will use anything other than Solvent Weld, its better, neater, and looks professional, so there's no need to use anything else. I would wager the only time compression waste is used anywhere, (other than joining different types of pipe together), is by DIY'ers.

"There is no AAV in the existing set up. However, in case it matters, I will be going from a sink and a half to a single sink, and I'll be adding a dishwasher where does was none before."

So does that mean an AAV might be needed?

No.

 

[DIYNotIan]

OK, great, thanks for that. Solvent weld it is then, and pleased no AAV is needed. One less thing to have to think about!

On a different but related matter, I've been advised to avoid using a sink trap with stubby connector for dishwasher (before the U bend trap), as with these you hear all the water discharges via the sink plug hole. Rather, it is better to use a standpipe for the dishwasher and then connect the sink waste and standpipe before exiting the building.

That all makes sense to me, but wondered what anyone on this forum thinks?

Cheers

 

[Scally-Ho]

Correctly trapped stand pipes are not exactly noise free and really it's down to personal choice. My choice is to run separately, but only to avoid the possibility of the hose breaking free from the under-sink connection option.

 

[ree]

OP,
Yesterday you were given some very bad advice - here is a correct, professional way of using AAV's under a sink - & why AAV's would be used under a sink.

1. Its not solvent weld fittings or pipe. If possible, you dont mix your materials.
2. Only compression fittings are used until the waste pipe typically exits outside the wall.
3. AAV's are mostly also compression.
4. AAV's are necessary under Island sinks - and where drainage is slow, & trap seal keeps getting lost.
They prevent backflow, & negative pressure siphoning while maintaining the trap seal.
Where appliance waste hoses are being pumped into the trap then AAV's come into their own.
5. McAlpine & others do P-traps & Bottle traps with built-in anti-siphons.
They also do a large selection of various AAV's for under sink & similar drainage situations.

6. Stand pipes are nowadays used when the appliance has a long waste run to the sink trap.
7. Appliance waste hoses can be extended but I would prefer a stand pipe rather than a v. long hose run.
8. Sink traps can have nozzle connectors for appliance waste hoses. DW & WM can both be connected to the trap nozzles, & pump directly into the trap. No stand pipe needed.

FWIW:
1. You dont go painting plastic pipe - dont ever paint plastic pipes.
2. Push fit seals often last for decades - I've dug up clay, push fit sewer pipes that had been in the ground for 50 years & the seals were still supple and sound.
No matter the condition of any seal - as a matter of course, we always replace seals as we come upon them.
Most professionals do the same.

 

[Hugh Jaleak]

Please ignore advice from people who don't understand UK Plumbing systems. I advise as a City and Guilds holder, having served an Apprenticeship with a time served Plumber. I have never heard such drivel as some people are talking!

AAV's should not be needed on a correctly designed sink waste. No professional Plumber will use compression fittings after the pipework leaves the trap, internally or externally. If an appliance is pumping into a Trap, it'll be discharging above the water level in the trap so an AAV is pointless, its open to atmosphere above the discharge point!

You will get some noise when a Washing Machine or Dishwasher discharges, as they need an air break between the outlet and the water seal in the trap, so the water has to fall some distance as it pumps out. My Dishwasher is connected to the Sink Trap, yes there is a slight gurgle when it discharges but it's not loud enough to cause any real disturbance.

It is also advised in the UK to paint external plastic pipework with a suitable Gloss Paint to provide protection from the UV in Sunlight.

Would love to know how to replace seals in pipework, the fittings are manufactured with the seals in place, and not designed to come apart!

 

[ree]

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