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I'd be interested in having the (cooled down) hot water lines in my house purged when I walk into the bathroom in the morning

That can be done by a plumber... or yourself if you are ambitious. Run a return line from your bathroom branch of your plumbing to a lower point on your water heater (there's a drain valve at the bottom... you can 'T' off before that) and natural convection of water will ensure that your pipes are always "charged" with hot water from your water heater. It's called a hot water loop or a recirculating loop.

Link
Page 5 (You can usually forgo the pump if your water heater is in the basement)

Brother, you need to check your facts. Most dimmer switches have been Rheostats for the past 100 years or so. Maybe the new ones are 'choppers' as you say, but that dimmer switch on the wall in Grandma's parlor is still a rheostat.

Reference: http://www.askthebuilder.com/414_Dimmer_Switch.sht ml

If a tankless water heater saves 30% over a tank one, then for it to make sense the solar system needs to last as long, be no more expensive than the tankless premium and add a 30% efficiency gain (or some combination that adds up to the same thing). I'm dubious that it can achieve this.

I can buy a solar retrofit kit right now for $1kau (and I've heard that similar things exist in the US for $600) that has a 10 year guarantee. It would cost less than 20% of the gas of a tankless system, maybe even none of the cost. If the solar part is good enough you don't need to spend much on the water heater - a 20% efficient water heater in conjunction with a 90% available solar system uses half the gas of the best fully gas systems. With modern evacuated tube collectors 90% seems like aiming low.

Evacuated tube collectors are such good collectors that the heat output end gets warm to the touch when they are sitting in my lounge room under only incandescent light. And they are cheap. I predict that within 10 years most people in melbourne will have some kind of tube collector system.

An interesting (though maybe old) article:
http://www.askthebuilder.com/451_Tankless_Water_He aters_-_Some_Surprising_Facts.shtml

I'm dubious about the method used to measure though. His claim that tanks last a lot longer than people expect runs true. My dad has had his storage hot water unit for at least 15 years now, having had two anodes installed.

The less you use, the better deal the tankless units are.

The less you use the easier it is to provide all the energy with solar.

But in the limit, the water doesn't get to the target temperature with a tank unit. A tank unit has all day long to get the tank back up to temperature after my shower is over. That's why a tank water heater has 34 kBTU input and a tankless one has 175 kBTU.

They're all fast recovery types these days, which means that they can keep up with most demand rates at higher efficiency. The main advantage to a large tank is the thermal mass, which means that you can use a higher efficiency burn, and the large size, which gives you more room to do your heat exchange.

Btw, BTU is a measure of energy, not power.

The Bosch 250SX tankless heater has an energy factor of 0.82, which I believe means that 82% of the energy is used to heat the water, and 18% goes up the vent.

Sure, and I can buy an aquamax storage gas heater than costs half the price and gives me an expected efficiency of 94%, including tank losses - I just rung the company, who also sell a rebadged bosch (rebodged basch?).

What sort of house heating are you using?

I used to make duct tape wallets in high school but had a very bad experience with them. Most duct tape has a shiny backside that isn't very cohesive with the sticky substance on the underside. What happens is when these overlap or your design depends on these two surfaces to stick together, you get a mess.

Now, there may be some kind of duct tape out there that avoids this issue, I'm not sure. Maybe these guys would know which brand is best but my wallet nearly destroyed my license at the time. Essentially, you need to look for duct tape that will adequately hold against itself when you need it to. After years of sliding it in and out of my back pocket and sitting on it, the tape started to smear against the shiny part and separate. As a result, the goo (bottom ply) and meshing (middle ply) were slipping out from underneath the shiny part (top ply) and getting goo all over my cards and ID.

Essentially, it comes down to how many mils of adhesive you are working with. A lot of times, the more expensive stuff will have around 12 mils of glue which means that it is ideal for construction. However, this may result in more sliding and more goo seepage in your wallet. I would recommend something in between the range of 7.5 and 12 mils. You're not going to be concerned about tensile strength, just thickness and what the coating is on the back. If the coating is non bonding to the adhesive used, you don't want that tape!

The actual design can be very much up to you although the article does provide a link to a nice standard model. I would suggest to try to use as many whole pieces as possible and when you're thinking about the design, rely on adhesive/adhesive bonds instead of adhesive/polyethylene since the cohesion is vastly different.

So if you venture on this, for the love of god, RESEARCH THE TAPE! Honestly, I estimate the life of these things to be a month unless you want to also pay for a new license often. I think back to my dad's wallet which was this old beat up piece of leather with its seams showing. But he's had it as long as I can remember and that thing has taken a beating. There's a reason why wallets are made from leather.

Remember, if you can't fix something with duct tape and WD-40, you're just not trying hard enough :-).

If it's not stuck and it's supposed to be, duct tape it. If it's stuck and it's not supposed to be, WD-40 it. If it's not broken--keep me away from it!

>Windows have the distinct disadvantage of being very poor insulation,

double paned or triple paned windows, pretty common actually

>being nearly useless in large (office) buildings, and letting UV and infrared light through.

Special coatings are available to block out UV light. I've got them on my glasses. Also check out low e glass.

Here.

Other reasons:
Solar passive heat. In the right climates, the tank actually works to your advantage. I'm thinking Nevada, California, New Mexico, Colorado, Utah. And even in other places, it still may work to your benefit to have a tank and a solar heater. Essentially you paint a secondary tank black, place it in an insulated niche, and let it absorb sunlight all day. If you place it in the attic it will also absorb waste heat from the house and additional waste heat from the sun hitting the roof. Of course you can still couple this to a tankless, but as far as I can tell tanks aren't totally without use in a properly designed house.