Dishwasher outflow - keep as 23mm if no standpipe? Manual says 32mm standpipe needed

[lardconcepts]

Long story cut short - very old building, second hand kitchen, new dishwasher. Without going into details, there are two good reasons for not being able to run the recommended 32mm standpipe behind the cupboards and through the 2'6" thick outer stone wall, in which a 23mm hole could easily be made, but a 32mm one would be "a nightmare" (plus I'd have to hack holes out of the back of the kitchen units.

One builder and one plumber can't see any way round it other than to just have a 23mm wastepipe.

As long as there was a good fall (there would be - 600mm over 6 feet - mixing my measurements here!) and the top of the pipe was the 800mm the manual specified, can anyone see anything wrong with it?

The entire pipe run will be sealed, so no trap needed inside.

I guess what I'm trying to get at is - it's not ideal, but in the case of their really being no other option, is there any reason it won't work? And I've already thought about build up, but as the wash is about 90 degress and full of evil chemicals anyway, I can't see it getting the kind of fatty deposit buildup associated with cooler sink water and normal "hand strength" detergent.

Just to pre-empt questions about building and water regs, they're not going to actually fit it themselves, it was left as "regulations, see, but if you thought about it, you could...", and the outflow WILL be teed into the sink waste, just outside instead.

[RobertL]

Would it help to use two 23mm pips in parallel to give the same flow area as a 32mm pipe?

Robert

[John]

"The entire pipe run will be sealed, so no trap needed inside."

Isn't this likely to cause siphoning?

[lardconcepts]

No, the pipe will be dra> Would it help to use two 23mm pips in parallel to give the same flow

Hmmm, subtle idea, but even drilling one hole through the wall is going to be enough aggro. If it has to come to it, I suppose it's an option, but drilling through stone-wall is going to be bad enough as it is.

[David Hansen]

On Thu, 9 Jul 2009 07:01:12 -0700 (PDT) someone who may be lardconcepts wrote this:-

Imagine the situation where the machine has just pumped out the last water. There is now a pipe full of water propelled by the pump down a "good fall".

Depending on the arrangements inside the machine one of two things is likely to happen. If you are lucky the water will draw air into the pipe behind it, drawing the air through the pump. This will empty any trap in the machine.

If you are unlucky the water will not be able to draw air behind it and will not be able to draw air along the top of the pipe (as it would be able to do with a larger pipe). In this case your pipe will be flattened by atmospheric pressure on the outside of the pipe.

Either find out what is inside the dishwasher, or arrange an air admittance valve where the end of the hose and the start of the fixed pipe starts. Do make the long run in proper pipe, suitably supported and with easy bends if there are any (to minimise back pressure), I would use copper pipe.

Done properly there should be no problem even with decades of use.

[neverwas]

Can you start with a vertical run of 32mm and then join that to the

23mm? I ask as I seem to recall that you get a much greater flow rate through a given run of downpipe after the initial section in which the flow may be especially turbulent. That applies in spades with rainwater downpipes (which have a greater capacity if they collect water from a wider inlet than from - say - juts a running outlet.) Might it also mean you can avoid the issues of siphoning by using a conventional pipe hooked into the 32mm pipe?

[Ian Middleton]

Been there seen that with a dishwasher outlet plumbed in 22mm copper as being the only way to get the waste behind a kitchen unit and through a limestone thick wall.

Apparently worked fine for couple of years (2-3 ?) then dishwaser had issues emptying. Fault finding guide said "blocked drain/outlet pipe" which it was. Removing the fitted dishwasher and using drain unblocker got it going again but blocked again in a month or two. Also had issues connecting the drain hose to 22mm pipe in a leak free manor. It always had a slight leak.

The 22mm pipe was changed to a longer washing machine type hose (5 meters ?) and that was better but eventually leaked at a tight bend after about 5 years.

The whole lot was then redone correctly in 32mm plastic pipe, which involved removing kitchen units from under the work top, chiseling the wall to get a

32mm trap in and drilling a 32mm hole.to outside.Worked fine for years no issue until dishwasher died !!!.

I have personally changed a dishwasher hose from the supplied 1.5m hose to a longer hose 3m (involves getting inside the dishwasher to attach the hose) so the outlet hose could pass behind a kicthen unit to the drain spigot on the kitchen sink U bend. Still working as far as I know.

[David Hansen]

On Thu, 09 Jul 2009 14:52:49 GMT someone who may be "neverwas" wrote this:-

Larger pipes going into smaller ones are not appreciated. I would want to do some calculations before installing one.

[Andrew Gabriel]

I have a dishwasher installed some distance from the sink/drain. I used 21mm solvent weld overflow pipework around under the units, probably a 5m run, before it rises up into the cupboard under the sink and enters the sink waste before the U-trap and that forms an air break. It's worked fine for 9 years. The dishwasher is specified to allow the outlet to be pumped quite high (IIRC, high enough that you could have it in a basement, pumping out to a waste on the floor above, although I'm not using that capability). However, this means the dishwasher does have a non-return valve in the waste outlet, but that's not really relevant in the situation I have installed it.

IME, dishwasher detergent is excellent at cleaning the pipes it runs through. For a brief period, I had it pumping out into the sink, and I never had to clean the sink during that period -- it was always bright and shiney!

[lardconcepts]

To follow this up, so you can all say "told you so!", after a couple of quotes around =A3200, I decided to try it myself, and rented =A330 of drill and diamond core.

Started at 8pm, should've taken about an hour to go through the wall.

Except..rubble and stone wall. Once I'd put the extension bar on, it was still going OK until it snagged on something. I'd been clearing it regularly, taking it slow, letting it take it at it's own speed. I tried to pull it back out, but what had happened was some rubble had fallen onto the extension bar, trapping the drill. I'd measured it, I was ONE INCH from the other side of a 20" wall. Totall stuck, wouldn't move or rotate at all.

After about an hour of trying to free it, I got a grip wrench (the motor / clutch was just stalling) and after much to-ing and fro-ing managed to free it by using all my might, putting all 14 stone of me on the wrench having propped it below so the force would be rotational, not downwards.

It was about 11pm when it went "ping" and would turn again. However, it just wasn't going any deeper now. In hope and desperation, I just kept pushing for about half an hour, but nothing. Probably one of the best ideas I've ever had struck me - I went outside, felt for the "warm bit" on the wall (it was a cool night), got a normal drill, drilled loads of little holes, and bust in with a crowbar.

By 1am, I'd got the core drill out from the other side, fed the pipe through,and inhaled enough dust to setup a small cement factory in my lungs. To my horror, I also found that in freeing the drill, I'd bust off one of the diamon teeth. And this was a very expensive drill - even with the hire place taking pity on me and only charging me cost price (he showed me his trade list when I still baulked at the price) AND taking into account discount of existing wear, the whole episode ended up costing me =A390 inc.

Bottom line is, I still saved over =A3110, however it's one learning curve I don't ever want to repeat in a hurry!