UPDATE... First off, thank you all for your comments and suggestions. I thought you'd like to know the final outcome of our plumbing & venting issues.... We have moved the shower drain pipe. After getting a totally unacceptable estimate of $4,800 to move our shower drain pipe & vent from our kitchen contractor that included punching a hole in the roof to vent the drain, we solicited comments and estimates from 3 outside plumbers. Of those 3, 2 proposed the same solution and a third proposed a different solution...none of these solutions involved punching a hole in the roof. Why not? Well, it turns out we have a whole-house vent stack in the attic that we can tap into. All 3 outside plumbers checked our attic to see if we had one and then proposed solutions using that vent stack. Our kitchen contractor's plumber never checked the attic for that stack so assumed he would have to go through the roof. (You know ASSUME does...) Our first outside plumber proposed tapping into the toilet waste pipe. But, all of the other plumbers were uncertain if that would pass code...this estimate was for $550 + $150 for an express permit The other two plumbers proposed rerouting the shower drain back to the main 2nd floor waste pipe...one was for $925 and the other for $1,000 both + cost of drywall repair. We went with the $925 estimate for a couple of reasons...(1) they were local and have an "A" rating across the board on Angie's List (the first time it's been useful!) and (2) the other plumber was not a master plumber (journeyman), he was not insured, and wasn't even sure if his license was up-to-date in our county...all red flags to us. Anyway, so our plumber came out and moved the drain pipe. He then told us he would try to run the vent to the attic without cutting into the wall (saving us the $500 for drywall repair). It worked! He was able to push the vent pipe up the wall and into the attic without any problems. While he was working on this we asked him about the Polybutelyne water pipes...he, as well as all the other plumbers, had recommended changing them out. We're on well water so there's no chlorine involved, but they did tell us that it would eventually become brittle and split. So, we got estimates for that and, since our kitchen project has been stalled for over 8 weeks, went ahead and had it done ($3,200 for replacing water pipes + $1,900 to repair drywall in 3 bedrooms + LR textured ceiling = $5,100. We already have the ceiling down in the kitchen and the main wall opened up so no charge for repairing those) This was all done fairly quickly and we have had both our final inspection done and the drywall repairs completed...in time for our kitchen to restart Wednesday. The only drawback...previously, since each fixture had it's own water pipe, flushing the toilet had no affect on the shower. Now, however....a very different story! It's taking some getting used to! BTW...it turns out the first plumber's solution would have met code and passed inspection since he had planned to use a sanitary T. I was curious so I asked the inspector about it when he was out Friday. If you want pictures, let me know and I'll post them. Thanks again everyone!

That existing 4' high loop is really delving into a rather gray area of the code. Normally the trap is considered the official point of transition between an appliance drain line and the house DWV system and in that case the manufacturer of the appliance may determine the required diameter of the appliance drain line which is determined by the volume and velocity of the discharge from the appliance. Generally the manufacturer will print a table in the installation guide that defines the size, length and permissible vertical lift of the drain line with recommendations to increase the line size if you need to exceed the length or vertical lift. That table will also define a maximum length or lift which is determined by the output pressure of the pump in relation to pipe friction loss or maximum vertical static head. In most cases the code will require that the appliance drain must be in an accessible location to permit servicing in the event that line should become obstructed. Whenever a drain line is permanently installed in the wall it is then classified as a portion of the structure DWV system and it would require that there must be a trap at the beginning of the concealed section of the line and the minimum permissible diamter of the line would then be determined by the DFU (drainage fixture unit) load value of the appliance or fixture being served by that drain. In the case of a dishwasher the DFU load is 2DFU's which would require a minimum of an 1-1/2" drain line. The plumbing codes also require that a dishwasher, washing machine, HVAC unit or a home water treatment discharge line must also be connected by means of an "indirect waste receptor". Under some local codes you could achieve the indirect waste receptor by installing an "Air Gap" or they may require you to install a standpipe on the trap and connect the dishwasher drain to a standpipe in the same manner as a washing machine line is commonly connected to a standpipe. (Some codes require all dishwashers to have an air gap, regardless of how the drain is connected. Check your local code.) Under the UPC whenever we have two common vented gravity flow drains connecting to the same vertical riser we are required to use a "Back to Back Tee" (4-way) however code prohibits connecting two commonly vented drains by means of a Back-to-back if one of those drains is receiving the discharge from a powered drain I.E. a dishwasher or washing machine. The discharge from a powered drain is required to be connected by means of a directional fitting (baffled Tee) and generally the baffled tee should be placed lower than the gravity drain. In your case the simple solution would be to connect the dishwasher to the disposal input port, providing that is permitted by your local code as well. (For reasons known only to the AHJ some codes prohibit connecting a dishwasher to a disposal input port..One code that immediately comes to mind here is the Illinois State Code.) Your screen name is Tom-in-Seattle so I am assuming that you are actually in Seattle. I am not certain but i believe your code is modelled after the UPC (Uniform Plumbing Code). If so the defined trap and waste arm size for a kitchen sink is 1-1/2" but under the UPC all vertical risers and drain lines serving a kitchen drain must be a minimum of 2" pipe. You may use an 1-1/2" pipe for the vent. There is some question about whether the length of the horizontal offset on the vent in your attic would pass, but I doubt that it would even come into play unless you happen upon a hardnose inspector that is really intent on finding a problem.

Your second plumber is correct, you should not use an "AAV" (Air Admittance Valve) in this situation. Sewer gasses are primarily composed of Methane, Hyrdogen-sulfide, Carbon-Dioxide, and Sulphur-Dioxide. All of these gasses are toxic and some are extremely flamable or in even low concentrations and can be explosive in higher concentrations. (Methane is the base component of Natural Gas). In addition, there are a number of airborne pathogens that are known to commonly be present in sewer gasses. Some of the gasses present in sewer gas are lighter than air while others are heavier than air and the concentration of these gasses will continually change day to day or even hour to hour. When the mix is predominately heavier than air the gasses will settle downwards and ultimately go out your sewer line to the septic tank or municipal sewer. On the other hand, when the predominance of the gasses is lighter than air the direction of flow is upwards and out the roof vent thus all vents should allow a two way flow of air. As water flows through a drain pipe it pushes the air in the pipe ahead of it. This can then cause a negative air pressure behind the water flow and that negative air pressure will result in sucking the water out of the fixture trap. This then leaves the trap dry and allows sewer gasses to enter the structure. For this reason the codes are very explicit in regards to the length of pipe in a fixture arm measured from the trap to the vent. An AAV has a spring loaded diaphram that will allow air to enter the line to prevent sucking the water out of the traps, however it will not aleviate the problem of the lighter than air sewer gasses collecting in the line. In addition, because the AAV has a spring loaded movable diaphram it is subject to malfunction in which case the sewer gasses would again enter your home. The Uniform Plumbing Code limits the use of AAV's to a maximum of one per structure and only with the expressed written approval of the local inspecting authority. (Generally they will only approve an AAV for use as a kitchen Island vent and even then most inspectors would prefer to see an Island Loop Vent rather than an AAV). The International Residential Code requires all structures to have one "Main Vent" which must run undiminished in size from the house main drain up and out through the roof. Once the main vent is created they will permit terminating a vent; 1. in the attic space (very bad idea because it causes excess moisture to build up in the attic insulation and sewer gasses venting into the attic can infiltrate back into the living space.) 2. Through the wall (subject to a number of limitations). 3. May use an unlimited number of AAV's however AAV's should not be used where a common atmospheric vent can be achieved) In your case the atmospheric vent already exists so installing cabinets or changing a room decor would not be a justification for discontinuing the vent and installing an AAV. You could very easily drill a hole in both the top and bottom of the cabinet and extend the vent line through it.

As long as there is power, the unit itself is protected. It's wrapped in a small resistance heater that draws about 100 watts if it needs to kick in. All bets are off if you lose power in a serious cold snap. I'm no fan of outdoor plumbing ever. But I live where Winter temps can get -30 on a bad day so we know how to protect plumbing. Everything needs to be re-thought with an eye towards "unusual weather events" being much more the new normal. A "redesign" of this would be to put an indoor unit on the inside and keep all the plumbing inside of an insulated wall. Then do a sidewall vent to the outside. Doing that would require a new tankless unit. You could improve on the current setup with a recessed cabinet and cover, such as this one - and still use the existing heater. It's not for extreme cold weather with no power - but it's better than having things completely outside. Mine is in my basement and 1" of foam board insulation from the outside wall. It wouldn't freeze even if I lost power for a while since the basement is below the frost line.