They are very efficient compared with storage type units (solar excepted). We're a family of 4, in summer our gas bill for cooking and hot water is about AU$40 (US$30) for 2 months.

   Most of the units I've had were mounted on an outside wall. Winters here aren't cold enough to burst pipes. For outside installation the local standard specifies a distance of at least 1.2 metres (4 feet) from any window or corner, presumably so you don't suffocate on the exhaust.

   I lived in a house many years ago with an indoor unit that was not properly flued. You could smell the exhaust building up in the bathroom. This made the unit even more efficient, as we were all too scared to spend more than a couple of minutes in the shower.

   You're right to be wary of propane, it's dangerous stuff. A lot of cars in Australia are converted to LPG, a propane/ butane mix, and every now and then a dodgy backyard conversion causes a tragedy. It's pretty uncommon these days, licensing and standards are well monitored, but it still happens.

   Propane is heavier than air, and collects in low-lying areas if it leaks.

Did you ever think of using solar panels for your hot water, I know solar panels are expensive but you can make your own for cheap, I just became a distributor for a company who manufactures solar panels. You can buy the absorber (alumminium/copper) 4x7 or 4x8 to make your solar panel, you just have to make a insulated box and add glass on top.

I am bulding 4 solar panels to start with, I will use them for my hot water (family of 4) and if I can I would like to make 12 in total for my hot water and for radiant heat. Each solar panel will cost me $300 to make compared to $800 in the store. I am just missing a tank for storage.

The solar fins retail for $280cnd 4x7 with 3/4" header,  not too bad but you can buy the fins and make you own header.

I will keep you posted on the progress of my project.

P.S. I am in process of making a web site for the fins it will be cheaper than the manufacturer's retail price.

A simple/cheap and effective way that I use, is, 150' of 3/4" black pvc water pipe layed out flat in a round coil, placed on the sunny side of your roof or on the ground where you get the most exposure to the sun. It will supply you with all the hot showers you want.
This of course will not work in the winter in the Burksfall's area.
Just for a test try leaving your garden hose laying on the ground, I guarantee it will be hot enough to burn you on a good sunny day.

I'm new here and maybe this will help.  

Few years back I used a heat exchanger made of copper pipe mounted on an old galvanized highway billboard sign.  I had the system charged with antifreeze and covered the panel with a brand name Solarex fiberglass cover.  The panel was very crude just six; sixty feet reels of ½ inch copper pipe soldiered together on the panel and two reels in the bottom of an insulated non-pressurized plastic 400-gallon tank.  I used a circulating pump hooked up to a lamp timer to turn it on and off.  

At the top of the tank I ran four 60 feet reels of ½ inch pipe to collect the heat from my service water leading to the hot water heater.

I'm doing test on cooking oil as a replacement for antifreeze now to see if it hold up to freezing and thawing.  If it does then in a closed loop used cooking oil would do the trick for a lot less cost.

Conducting the controlled testing cooking corn oil as a replacement of antifreeze I followed these control measures.

Filled 16-ounce plastic bottle with "new" oil to the exact rim line below the cap and allowed air bubbles to rise and escape before capping

Did the above with tap water.

Placed both bottles in freezer.

Checked two hours later with following results.  
Water: freezing and bottle expanding.
Oil: contracting in bottle, with ½ inch of air above oil; oil becoming consistency of chocolate syrup.

Checked 12 hours later with following results.
Water: container split open, water frozen solid.
Oil: Container further contracted (bottle somewhat flattened) with nearly ¾ inch air space at top of bottle.  Oil slightly cloudy and mushy with consistency of molasses.

Removed oil from freezer and without disturbing left sitting in shaded area to come to room temperature.
Results:  The bottle had regained roundness and oil returned to the original fill line.  

 Conclusion:  New corn oil could be used as a replacement for antifreeze in a closed loop system if an in-line reservoir or pressure tank were added to compensate contraction.

NEXT STEP:  Conduct above test on used cooking oil being discarded from a local restaurant after screening out impurities with wire mesh and coffee filters.

We've used the rolls of black water tubing.  Note, that's not PVC, it's polyethylene.  We use 1".

It ain't an optimum system, but it works whenever the air-temp is above 50 or so.  Presumably, if you took a couple of flat coils of it 4' in diameter and made the usual 2x4/plywood/patio-door box, it'd give you a hot shower down to 30 degree outdoor temps.  Well, at least a -warm- shower :grin: