Avoiding heat loss in pipes
In general, the biggest source of heat loss in delivering hot water is due to amount of hot water left standing in the pipe when the faucet is turned off.  It is not unusual for there to be between one and two gallons of hot water left in the pipe (the longer it takes for hot water to reach the faucet, the more hot water is likely to be left in the pipes.  We measured two faucets in our current house with a bucket and a 1 liter container: both take about 1 minute to deliver hot water and leave 1.5 gallons of hot water in the pipe when we turn off the faucet.)  Of course insulating the pipes and locating them entirely in the heated space helps, but even still the water in the pipe cools down to luke warm pretty fast. During the heating season, some of this heat loss goes profitably into the house, but often the worst loss is due to pipes not fully in the heated space (note that a pipe located halfway in an insulated wall has an ambient temperature about halfway to outside temperature)  In addition to leaving hot water sitting in the pipe, the water that comes out of the faucet until hot water arrives is wasted.

One popular solution addresses the water waste by using a pump to pull hot water rapidly from the tank to the faucet while simultaneously pushing the water sitting in the pipe into the hot water intake.   This also saves some energy by feeding the room temperature water left in the pipe into the HW tank instead of the cold water coming in from outside.  While this is an improvement and worthy of consideration, it doesn't address the underlying problems and requires a pump, extra plumbing and extra expense.

The best solution is to consider these rules of thumb:

• Locate the HW tank centrally to all the points of use, and put it in heated space
• Keep the each point of use of HW as close to the tank as possible
• Keep all HW lines fully within heated space
• Keep the size of the pipe as small as possible

Getting a good room layout to allow for short runs between the faucets and the HW tank is probably the most difficult, especially as the number of bathrooms goes up.  Since we were building two separate living units (ie an extra kitchen, bathroom and laundry), the problem was especially difficult.  Keeping the pipes in heated space is best accomplished by not putting faucets on exterior walls, or at least giving consideration of how to reach the faucet largely via heated space.  Locating the HW tank within the heated space can also be difficult, and typically its in a basement because there is no room for it elsewhere.

What we did
Because we were building two living units in one house and our lot was relatively small (5000 sq ft), we had very little flexibility in building layout.  The main floor is crammed with the apartment on one side and our main living space on the other, leaving no spare room for a HW tank, and forcing some of the faucets farther away from each other than we would have liked.  Initially we considered having all the utilities in the heated attic space, but two things argued against this: (1) it didn't integrate well with the PEX manifold system (see below) (2) it was integrated into the heating system, which works best in the basement (see hvac).  Our solution was to build an insulated utility room in the central part of the basement, and although that works pretty well (it makes the tank centrally located to the faucets, allows the tank to integrate well with the PEX manifold and the heating system), it results in compromises in where the pipes are run.  Since the basement is unheated, the main floor is insulated (12" TGIs stuffed with cellulose), but since you can't just pop holes in TGIs anywhere, the pipes tended to run 4" or more away from the heated space.  Although we kept most fixtures on interior walls, kitchen sinks traditionally are placed below an exterior window, and in fact we did that.  In the main unit the kitchen sink is reach by a pipe under the floor that has only a couple of feet in the exterior wall, but the apartment kitchen sink is in a bump out over an unheated crawl space (in spite of all the warnings in the EEBA builders guide not to do this, but that's another story).  In this case there is about ten feet of pipe in the exterior wall.   Of course the walls are 9" thick, and the pipe is mostly tacked to the inner 2x4, so its mostly about 5-1/2" from outside (about 60% of the way in).

PEX pipe to the rescue
In the typical copper plumbing system, a 3/4" main HW line is run from the tank to 1/2" branch lines which then lead to 3/8" lines leading to most faucets.  This system is designed so that if multiple faucets are using hot water at once there is enough pressure to feed them all.  In real life this rarely happens, and the side effect is to significantly increase the amount of hot water left in the pipe when the faucet is turned off.  In the PEX system the only big line goes from the HW tank to a manifold, that acts like an electrical breaker box, providing a separate "home run" pipe to each faucet.  The key here is to keep the PEX manifold within a couple of feet of the tank, then each faucet is fed with its own 3/8" line, reducing the volume of water between the tank and the faucet by anywhere from thirty to fifty percent.  For more info on PEX, see www.vanguard.com.

PEX lines terminate on all the standard fixtures using a crimp on fitting (far left), in this case onto a laundry tray.  Hot and cold lines run in bundles through the walls and ceiling, just like electric wiring (center), originating at a manifold which has single cold and hot water inlets (far right).  The bottom manifold is for the ADU.

Low flow fixtures
Many building codes these days require "low flow" fixtures that ten years ago were considered state of the art.  In the case of faucets, the idea of low flow does not make sense to us, because low flow can be achieved by simply not turning the faucet fully on, and further 2.5 gallons per minute is actually a lot of water unless your trying to fill a bathtub.  In the case of showers, the issue is as much with the design of the shower head as with how much water is coming out.  Various places sell very low flow showerheads (as low as 1gpm, see www.realgoods.com for some examples), but our limited experience with them is that they produce a mist that many people don't like.  Our current shower uses a symmons valve with a volume control that we leave turned down to the lowest, resulting in a very acceptable shower at about 1.5gpm.  In the future we may experiment with alternative showerheads.

Everyone sells 1.6 gallons per flush toilets these days, but not all these toilets actually use 1.6 gallons per flush and their effectiveness varies greatly among models.  Recently a comparison of 35 common toilets was done by the NAHB in alliance with others including Seattle Public Utilities,  see www.savingwater.org for the list.  Each model is described by how much water they use out of the box, how much they use if they are repaired with a standard flapper and how well they flush a variety of materials (the real thing is simulated with wads of paper and sponges).  Going beyond these, some manufactures offer dual flush toilets (1 gpf for liquids, 1.6 gpf for solids), such as the Carmora which is made in Australia and sold locally in the northwest by the Environmental home center (www.environmentalhomecenter.com).   Other ultra low flush designs are either out there or in development, but at the moment we have no information on them.

After asking around about reliability, and looking at cost, we decided on the Toto drake, which was one of the top five performing models, has good reliability record, and was one of the top performers in the NAHB flush test.  They're a little pricey at about $230 each (that's with a good discount, list price is closer to $300).  We got recommendations for the Kohler Wellworth, which are closer to $100 each, but our experience with the 1994 model isn't so great (we have three of them), and they didn't score as well as the Toto, so we didn't want to take a chance.  It may be that the improved model is now quite good.  (The older model does not flush solids very well at 1.6 gallons, we usually hold the handle a little longer, but even then it seems to lack the swirling action that makes a toilet work well). 

Miscellaneous issues
If the HW tank is located in heated space and isn't electric, a power vented model should be considered for  indoor air quality reasons.

Building code requires a vent cross sectional area equal to the area of the drain and vents on every fixture as well as something called a "johnson tee" on every dishwasher, and as a result of this, there is about the same amount of vent lines as there are drain lines, creating additional voids in exterior walls and consuming a more plastic.  Like many other things, plumbing vents are sized by prescriptive path, which tends to result in everything being oversized.  Its not clear what criteria vent size is based on: convention wisdom was that vents just let out sewer gases, but since code requires a vent for every trap, this is clearly not the case.  The only hint I could find was that vents prevent the water in a trap from siphoning out (e.g. think toilets), but in most cases that would seem not likely, so maybe there is a better reason.

In places, there are vent and drain lines in every nearby bay, in this particular case we had originally planned to have the fan-coil heating unit in the leftmost bay and forgot to mark it as such (far left).  An example of a johnson tee (right), the thin copper line is the drain of the dishwasher.

Adaptability
Our system, like everyone else's, is installed by drilling lots big holes in the framing (not as big as HVAC ducts, but still big) installing everything permanently in place, and then hiding it all behind sheetrock.  Hopefully you don't have to repair you plumbing for a really long time, and typically you don't want to update or remodel it for a long time either, but eventually you end up doing one or the other, and in both cases you quickly find, that the standard system is difficult to repair and even more difficult to modify.  Like electric wires, plumbing lines, drains and vents, are strewn all over the house in whatever is the easiest path to do the job.  Part (and probably most) of the problem is that we don't plan where the plumbing lines and drains are going to go, we just assume that space can be made for them.  Designing the floor plan and framing with plumbing and HVAC in mind, would be a great benefit, and using framing systems that allow flexible placement (such a open-web trusses for the floor), while not necessary, can certainly make the job much easier. 

Sewer
The original house had only a crawlspace, and so the main sewer drain was located only about 3 feet below the floor.  We used a locator to determine how deep the drain was at the street and calculated that we would have barely have enough drop for a gravity drain (typically 1/4" per linear foot), but when we actually went to install it months later, the drop was about 2 inches too small, and so we had to break out a chunk of the street (at a cost of $1000 or so) to make it work.  The alternative is a sump pump, which although many people consider acceptable, we think is totally bogus.  Gravity works all the time, sump pumps only work when they have power and aren't broken.  Even though this principle of working with nature is a key part of green building, and so a key part of our project, we apparently didn't scream "gravity drain" loud enough.

In retrospect, we should probably should have designed the groundwork (the pipes under the slab) so that we could come out of the house as high as possible, although that might have meant pipes running along walls, instead of under the slab (its unclear which is better: having them out makes them easy to get at, but also in the way and subject to damage. In addition, drains are all ABS now, and it apparently goes up in flames very fast.  Other than cost, it all came out fine in the end.

After a long search for a side sewer contractor (its a separate license from plumbing), we ran across Earthwise excavation, who runs all their equipment on Bio-Diesel.  After a few more snafu's coordinating with the city, we had a sewer in a couple of days.