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Hot Water

Page history last edited by Jonah 13 years, 6 months ago

Hot Water


Hot water for personal washing and dishwashers (but not necessarily hand-washing of dishes or for clothes washing) is a modern convenience. Yet many of the means we use to heat and store hot water are decidedly old-fashioned.


Without discussing the energy transfer into the hot water, Drain-Water Heat Recovery spells out four things which determine how much energy is in a particular hot water application from fixture to drain:

time = time of use in minutes

rate = rate in gallons per minute

highTemp = water temperature in degrees F

baseTemp = your tap's "cold" temp, in degrees F


Take those values, and use them thusly:

time x rate x (highTemp - baseTemp) x  0.00244 = energy in kWh


This conveniently suggests five ways to improve the energy performance of your hot water use

  • reduce time: spend less time running it
    • shorter showers, duh
    • turn off the water in the sink while scrubbing
    • get a cut-off valve and reduce or eliminate flow while scrubbing in the shower
    • perhaps get automated (motion-sensing?) on/off switches  
  • reduce rate: have the water flow slower
    • get a high-quality, low-flow (1.6 gpm) shower head
    • install low-flow (1.0-1.5 gpm) aerators on sinks
    • consider a spray nozzle to accelerate rinsing and related actions
    • make sure your outdoor hoses have good nozzles
  • reduce highTemp: at the heater
    • turn down your hot water tank's temperature (less effective with tankless)
    • take colder showers
    • have a timer that makes it hot only just before (but not just after) you need it
      • if there's a pilot light, this lets that energy be slowly and efficiently(?) captured

I'll add "maximize deliveryTemp"

  • reduce heat loss during transfer to delivery point
    • direct "star topology" piping
    • thinner, lower-heat-capacity pipes (PVC < copper < lead)
    • insulate pipes

increasing baseTemp gets into alternate water heating technologies

  • make the incoming water warmer
    • Drain-Water Heat Recovery  
    • solar pre-heat
    • ambient pre-heat 
    • burner cut-off to let pilot or ambient slowly heat the water
    • ground-source passive
    • ground-source heat pump (typically via a superdeheater)
    • other available waste heat 



One problem people solve in various ways is the one of having to wait for hot shower water and the water wasted during that wait.



They install pumps that circulate hot water constantly (or, more intelligently, on a timer), not only wasting electricity, but also making the hot water heater do a lot more work as the hot pipes heat the walls of the house. Better products (see below) only bring the water to you when you've asked for it.


A friend wrote to me about a similar product that was passive; I wrote back:




Unless you have *extremely* well-insulated pipes (impossible to beat

good insulation on the tank and even harder to beat eliminating stady

losses with an instant heater), this thing effectively causes your hot

water tank to keep your hot water pipes full of hot water all the time.

That means they are radiating heat all the time in your walls and the

floor. This has the opposite energy effect of an on-demand heater which

doesn't keep a tank hot all the time.


They admit as much:

> MORE HOT WATER! Adds as much as 10% more capacity to your current hot

> water system (by keeping the water in your hot water pipes hot!).

bigger tank = more energy, especially when 10% of it is uninsulated!


I'd guess that half the energy used by your hot water heater is lost

out the sides of your tank. Adding this device would cause you to be

losing heat all through the house (and making it less comfortable on

warm days).


This product is terrible. The only way it is "better" is if you compare

it to an always-running pump which wastes electricity as well.


The thing that you *do* want pumps cold water from the hot pipe back

down the cold pipe *only when you turn the hot water on*. To save

water, you *don't* need hot water in the pipe all the time. The

on-demand pump pumps water back until it's hot and then lets it flow.

The \"chilipepper\" is one such product.


That's assuming, of course you are more concerned about wasting water

than waiting. Waiting two minutes is a pain, but if you want to solve

that problem, don't heat your walls with your hot water heater; instead

get a tankless heater closer to the bathroom.





ps, at our house, we have had our hot water heater on "Pilot" most of the

summer. This cuts our gas bill (at least) in half. We only pay for the

pilot light ... which produces enough heat for two reasonably-long hot

showers per day. Sometimes a cool week or night will cause a shower

to need a bit more heat, but some extra insulation should fix that ...

perhaps even allowing us to stop using the "on" burner all winter. :)


pps, we've also been putting the cold "wait-water" into a bucket and

then pouring that in the toilet when the time comes. This manual method

of doing "greywater" is pretty annoying, but is less annoying to us than

watching the water go down the drain. Next up is some drainage or second

"window sink" for the kitchen so fruit and veggie washing water (if not

hand-washing water) can go into a bigger bucket in the back yard.




> how come the hot water lobster does the recirculating part without a pump?


To save money for the company. Not to save money for you. I suppose if you had the lobster plus a timer (which would require at least batteries for a clock) that enabled and disabled it, that might do what you wanted.


> I found the non-electrical aspect of that device appealing. If that had a

> temperature -sensitive shutoff then it woudl be ideal...


Except that it takes a long time for hot water to move up through the pipes on its own due to density. We had radiators in Evanston that were set up to use a pump or not use a pump. If I didn't turn on the pump, we had to wait at least 30-60 minutes before heat made it way up to the radiators ... and that was only from basement to first floor across a short distance (dunno if your water heater is in the basement).


So for 30-60 minutes, you'd be heating your walls while waiting for the water to slowly come upstairs. Better just to pump it for 30-60 *seconds*, take a shower for ten minutes, and then be done with it.


I think the electric pump makes sense: it detects when you want hot water, it moves the cold water out of the way, and then gives you the hot water, on-demand.


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