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.