Home: Kitchen: Sink
Hot water return line   (+12, -4)  [vote for, against]
Reduces water wastage

I think this is true across the baking world. You want hot water, to wash your hands perhaps. You turn on the tap and run it until it heats up. Several litres of water that was sitting in the pipes, cold, runs down the sink. You wash your hands and turn the tap off. The now-hot contents of the pipework begin to cool.

Lagging the pipes will reduce the rate of cooling. However, in these environmentally-conscious times, maybe we should think more about the water we're running down the sink?

I propose a diverter valve in the hot water line, adjacent to the tap, and a thermometer to indicate line temperature. This valve will switch the flow either to the tap or into a (clean) scavenge tank. When the thermometer indicates that the water is hot enough, the diverter is closed and the water comes out of the tap instead. The contents of the scavenge tank are pumped back up to refill the hot water tank instead of taking more cold water off the main.

Since this system is not open to the air to any relevant degree, it should be no more prone to bugs than an ordinary hot water system.
-- david_scothern, Feb 04 2008

Hot Water Return http://www.chilipepperapp.com/cwrcs.htm
Very baked [csea, Feb 04 2008]

Tankless Water Heater http://www.cpotankl..._water_heaters.html
On demand water heating. [Noexit, Feb 04 2008]

Excellent idea. I had this exact thought a few months ago.

The only potential problem is the cost of pumping the water and extra pipework, but without doing experiments I think it would be worthwhile.
-- Srimech, Feb 04 2008

If your pipework is 15mm diameter (which, in UK, it's most likely to be), each metre contains 0.177 litres of water. Each metre which is 22mm diameter (which some of it could well be) contains 0.380 litres. So if I had a tap at the front of my basement fed by 22mm pipe from the tank at the back of the loft, with the pipework running down three storeys and the full length of the house, I would waste less than two gallons. The energy required to return it to the tank is just under 2,000 Joules.

(All calculations subject to errors due to carelessness.)
-- angel, Feb 04 2008

This has been baked for a long time. [link], for example (scroll down for diagram.)
-- csea, Feb 04 2008

What you're looking for is a tankless water heater. Essentially they heat the water on demand so that there's no extra energy holding hot water til it's needed, or lag time waiting for the hot to get to you.
-- Noexit, Feb 04 2008

very helpful (+)
-- pyggy potamus, Feb 06 2008

[angel] 2 gallons doesn't sound like much, but multiply it by several times a day for each person and the numbers really add up. I'd be curious to find out how much embodied energy is involved in transporting and treating the water in the first place.

[csea] Although a hot water circ system is common, this isn't exactly what [ds] is talking about. HW circ systems waste heat, as your entire HW piping system is constantly acting as a radiator.

[Ne] Tankless water heaters are a good idea, but the type that are installed at every faucet have problems in that they're expensive, electric, and usually undersized. The one you linked to is a central unit, which wouldn't solve the problem of lag.

[ds] I like the concept. I do see a problem of using gravity for return, as you're introducing air into the system (which is bad). What you'd need for gravity to work would be vents at all of the high points, or your water wouldn't return - and this introduces (dirty) air.

To fix this, just add a pump, pressure sensor, expansion tank and check valve. At the faucet you press a button to cycle water until warm. This opens a valve, which increases the pressure in the return line that triggers the pump. Water is cycled until the temperature is right, and the valve closes. The pressure drops, which turns off the pump.
-- Worldgineer, Feb 07 2008

Have you ever wondered why most motels and hotels have instant hot water at the tap in the restroom? It's becasue of the hot water circulating system they use. At the end of the hot water circuit, there is a line that returns to the water inlet of the hot water system...the hot water line is pumped by a very small cirulating pump that keep the hot water circulating through insulated hot water lines through the building,. You can do exactly the same thing with yout house hot water very inexpensively. simply run a second smalle soft copper line ...you can do this through the attic...tee off from the hot water service line to the furtherst sink, then daisy loop from sink to sink and back to the hot water tank...but before you go back into the tank, you install a small circulating pump (some of the pumps are 12 volt DC types you might find in solar hot water systems or on RVs...you would, of course, use a small transformer to convert from AC wall service). Plug in the pump and presto, instant hot water. Be sure to insulate the smaller circulating line...you would run the pump from a timer, starting it a few minutes before you regularly get up in the morning and trun it off after you go to work...on again a bit before you bathe at night...or run it most of the day if you have lots of family at home during the day. This is a closed circuit system, always under line pressure...the pump operates by creating just a bit more pressure on the inlet side of the hot water tank on the hot water return line, thus circulating hot water continuously. This insulated line will not freeze, especially so long as you keep the pump circulating in times of really cold weather.
-- Blisterbob, Feb 07 2008

random, halfbakery