dishwasher install with standpipe

[brassmookie]

I am working on installing a new dishwasher. The location of the install is also the location of an old washing machine (and not directly adjacent to the sink) so I was hoping to use the old dishwasher standpipe for the drain (1.5" pvc pipe). The top of the standpipe is just a few inches above floor level and the p-trap (2") is about 35" below floor level in the basement (photo here). I know this is not necessarily standard installation for dishwasher drainage, but can it be done this way?

Will I need to add an AAV to the line? Possibly replace the current trap with something like this?

 

[MicEd69]

Is the existing P-trap vented? Like what does the piping look like at least 6 feet to the left of the P-trap?

And are you asking if this will work, or if this will be per code?

 

[brassmookie]

the existing p-trap is not vented. the closest vent is from the kitchen sink, which is about 10 feet away horizontally, and then 3 feet vertically up. only asking if it will work well, not per code.

 

[brassmookie]

You can see the rest of the drain piping here. The vent pipe is highlighted.

 

[GReynolds929]

Is that black iron pipe and fittings. I don't see any direction on the fittings. Those tees look like pressure type fittings, like for 1 1/2" - 2" gas pipe....

 

[MicEd69]

You have a very interesting mix of plumbing piping, PVC, copper, and carbon steel or iron. sing iron/steel pipe and fittings aren't particularly the best thing to use. And there are some huge issues with your existing drain piping. The screwed steel tees do not provide proper flow paths for wastes. The copper Sanitary tee where your PVC and steel meet with it should be a wye and 45 or a combo fitting to direct the flow towards the sewer. So, while I would strongly consider replumbing your complete plumbing draining system at some time, maybe very soon, I'll say you can PROBABLY get by using your old washing machine standpipe drain for your dishwasher, with some modifications.

A dishwasher should have an air gap just like your washing machine. However, they are usually a little fancier than a hose draped over the open top of a standpipe. So, I would suggest you purchase one of those and use it for your dishwasher.


Your dishwasher easily connects to the Air Gap, and with the appropriate hose adapters/PVC fittings, the existing 1 1/2" PVC washing machine standpipe can ve connected to the Air Gap.

The P-trap should be vented, but you don't have to change out the P-trap itself. Just install a Sanitary tee in the line between the P-trap and the cleanout and install an AAV as high as practical.





This isn't per code, but it should be workable. And remember, you do have what I would consider major issues with your existing drainage piping.

 

[user 32496]

I am working on installing a new dishwasher. The location of the install is also the location of an old washing machine (and not directly adjacent to the sink) so I was hoping to use the old dishwasher standpipe for the drain (1.5" pvc pipe). The top of the standpipe is just a few inches above floor level and the p-trap (2") is about 35" below floor level in the basement (photo here). I know this is not necessarily standard installation for dishwasher drainage, but can it be done this way?

Will I need to add an AAV to the line? Possibly replace the current trap with something like this?

I agree with @MicEd69. I don't see why you can't use this drain line for your dishwasher other than the problems MicEd69 points out. It's nice to run the dishwasher into a garbage disposal but it's not mandatory. You do need an air gap in the discharge line from the dishwasher.

Or at the very least, a High Loop in the line.

You have a very interesting mix of plumbing piping, PVC, copper, and carbon steel or iron. sing iron/steel pipe and fittings aren't particularly the best thing to use.

Here's what I found in our 50 year old house when we first bought it.



Steel pipe (galvanized in this case) doesn't last. The result of this leak was to replace several wall studs, the wall sole plate, and the sub-floor.

And there are some huge issues with your existing drain piping. The screwed steel tees do not provide proper flow paths for wastes. The copper Sanitary tee where your PVC and steel meet with it should be a wye and 45 or a combo fitting to direct the flow towards the sewer.

Sanitary tees shouldn't be laid on their backs for drainage (for dry vents it is ok). These problems should be addressed but you can still hook up your dishwasher before doing this other stuff.

So, while I would strongly consider replumbing your complete plumbing draining system at some time, maybe very soon, I'll say you can PROBABLY get by using your old washing machine standpipe drain for your dishwasher, with some modifications.

The P-trap should be vented, but you don't have to change out the P-trap itself. Just install a Sanitary tee in the line between the P-trap and the cleanout and install an AAV as high as practical.

I guess, the higher, the better for AAVs but they are only required to be 4" above the trap weir (per the manufacturer).

The IPC code allows AAVs
IPC SECTION 918

AIR ADMITTANCE VALVES

918.1 General. Vent systems utilizing air admittance valves
shall comply with this section. Stack-type air admittance
valves shall conform to ASSE 1050. Individual and branch type
air admittance valves shall conform to ASSE 1051.

918.2 Installation. The valves shall be installed in accordance
with the requirements of this section and the manufacturer’s
instructions. Air admittance valves shall be installed
after the DWV testing required by Section 312.2 or 312.3 has
been performed.


It's funny they sell things like this because it isn't allowable by code. Not because it is an AAV but because there is a minimum distance between the trap weir and the vent opening just like there is a maximum distance between the trap weir and the vent opening.




The minimum distance between the trap weir and the vent is there because there is a turbulence created by the trap that will push junk up the vent if the vent opening is too close to the trap weir. It is possible it could block the vent.

Of course, if the vent opening is too far from the trap weir, a siphon can form and suck the water (trap seal) out of the trap.

Our local Lowes sells "S" Traps which are against code too.

If you haven't already, you should Google your County and see what plumbing code they use (IPC vs UPC) and see if there are any local variances to the code. UPC is a little stricter.

Some plumbers (Okay, most) don't like AAVs. They relieve negative pressures in the drain which eliminate a siphon from being set up, which is the main purpose of a vent but AAVs don't relieve positive pressures. That is why, even if AAVs are allowed in your area, you still need a main stack vent that goes to the atmosphere to relieve positive pressure.

I have a friend who moved to Alaska and built a house. He was so dead set against putting a penetration in his roof that he used all AAVs for all his vents including the main stack vent. He put all the AAVs in the attic...which is okay because you have to have access to them (ie. don't put them in a wall). I didn't question him on it because he's a real smart guy but unless Alaska has an amendment to code somewhere, it probably is against code to use an AAV on all the vents.

 

[Twowaxhack]

What does the code say about the max vertical distance from the appliance or fixture to the trap ?

 

[brassmookie]

What does the code say about the max vertical distance from the appliance or fixture to the trap ?

Not sure. I'm not trying to follow code, just looking for what works.

 

[brassmookie]

Thanks to @MicEd69 and @RonVan for the replies. Extremely helpful information. I like the idea of installing the AAV off of a sanitary tee rather than replacing the p-trap. And I appreciate the insight into the other sections of the drain. I will definitely replace the other sanitary tee with a wye bend in the near future. I'm still not certain what I want to do about the air gap though.

You do need an air gap in the discharge line from the dishwasher.

Or at the very least, a High Loop in the line.

I was planning on putting in a high loop already. So with the high loop and the AAV, do you think I could get away with not adding the air gap kit? It seems like the install on that would complicate the setup quite a bit. I could put a hole in the countertop above the dishwasher and run the air gap kit through there, but I'd rather not if it wasn't entirely necessary. Or perhaps I could add the air gap kit later on the down the line if I started noticing issues? What kind of issues might I see without the air gap kit?

 

[Twowaxhack]

Why would you need a vent for the dishwasher if the washing machine in place before worked fine ?

 

[brassmookie]

Why would you need a vent for the dishwasher if the washing machine in place before worked fine ?

I don't know that the washing machine was actually working correctly. We bought this house about two years ago and there was an awful lot of water damage to the floor where the washing machine was, so I'm inclined to think that it wasn't set up correctly. As with many old houses, we inherited a lot of things that were not set up / installed properly.

Also, I don't know enough that I am willing to assume that a washing machine and dishwasher are equivalent appliances.

 

[Twowaxhack]

I don't know that the washing machine was actually working correctly. We bought this house about two years ago and there was an awful lot of water damage to the floor where the washing machine was, so I'm inclined to think that it wasn't set up correctly. As with many old houses, we inherited a lot of things that were not set up / installed properly.

Also, I don't know enough that I am willing to assume that a washing machine and dishwasher are equivalent appliances.

Have you ran water down the drain to see if it drains water ?

 

[MicEd69]

What does the code say about the max vertical distance from the appliance or fixture to the trap ?

Twowaxhack has a good point as the code says 24" between the fixture to the trap. And that is why I said the Air Gap is more than a suggestion but required in this installation. Otherwise, you could be siphoning out the water from the dishwasher. Again, not per code, but something that should be done here.

 

[MicEd69]

Also, I don't know enough that I am willing to assume that a washing machine and dishwasher are equivalent appliances.

They are not equivalent as the new washing machines discharge their water at a much higher rate than any dishwasher. And a 1 1/2" standpipe is often times not large enough for the washing machines. But it should be fine for a dishwasher. And yes, the P-trap should be vented to make sure drainage issues don't occur. And in this particular case, a Sanitary tee will work with no issues whatsoever.

And this minimum should be adhered to.

 

[brassmookie]

Twowaxhack has a good point as the code says 24" between the fixture to the trap. And that is why I said the Air Gap is more than a suggestion but required in this installation.

So if I'm understanding correctly, because the vertical distance between the fixture and the trap is greater than 24" (about 35" in this case), the air gap is required? That seems fair enough. So regarding the air gap install then, I am a little bit confused as to where to put the air gap. The kits that I am seeing for sale are all meant to come through the sink or countertop surface. Would it be appropriate to splice the air gap kit into the dishwasher drain line, where I was planning on installing the high loop? So basically the air gap would end up being the top of the high loop?

 

[user 32496]

That seems fair enough. So regarding the air gap install then, I am a little bit confused as to where to put the air gap. The kits that I am seeing for sale are all meant to come through the sink or countertop surface. Would it be appropriate to splice the air gap kit into the dishwasher drain line, where I was planning on installing the high loop? So basically the air gap would end up being the top of the high loop?

That's where the air gap is installed, however, you want the air gap on top of your counter and usually near a sink because they can spew water. Since you don't have a sink there, I'd go with the high loop. Make life easy...get-er-done.

If you are worried about the 35" to the trap...raise the trap up. You have room. Put the AAV in the trap arm before the drain line drops to the existing line.

 

[brassmookie]

That's where the air gap is installed, however, you want the air gap on top of your counter and usually near a sink because they can spew water. Since you don't have a sink there, I'd go with the high loop. Make life easy...get-er-done.

If you are worried about the 35" to the trap...raise the trap up. You have room. Put the AAV in the trap arm before the drain line drops to the existing line.

Thanks for this recommendation. I think this is the setup I will go with, so long as no one else chimes in to warn me against it!

 

[Twowaxhack]

Do you guys really think that a dishwasher will siphon a 2” p-trap ?

I’d like to see a pic where this old washing machine standpipe is a few inches above the floor.

 

[brassmookie]

I’d like to see a pic where this old washing machine standpipe is a few inches above the floor.

The standpipe was originally taller but I cut it when I removed the washing machine.