Wasting@Home - Confessionals

[bennelson]

That ChilliPepper sounds like a good idea.

Note that it is different from some other automated systems because you have to initiate it.

There are some systems out there that constantly cycle hot water through your pipes so that you always have hot water. But they WASTE a lot of heat because they do that.

On-demand water heaters that are located where you use them (like right under your sink) are even better.

I have always thought that solar pre-heat with a point of use on demand heater would be a fantastic tag-team for eco-hot water.

[boxchain]

Originally Posted by jjackstone

Here is a product I've run across to help alleviate wasted water. I haven't purchased one myself but the idea seems reasonable.

http://www.chilipepperapp.com/?darelldd

JJ

I've thought about something like this, glad they're actually making it. Problem is, you have to run a return line to your HWH, which costs $$.

[bennelson]

Another great idea is to use a heat exchanger on your shower drain.

Basically run your cold water from whereever it comes in your house past your shower. Have the cold waterpipe wrap around your shower drain and then go to the waterheater input.

As you use water from the heater, it needs to pull in cold water to replace the used hot water. The water that it pulls in is already partially heated from the wasted heat you put down your shower drain, so it uses less energy to heat it.

There are commercially available heat exchangers you can buy to do this.

[mopo3]

Okay so I did the heat exchanger calculation on your shower drain. I'll set it up so you can change the numbers to fit what your set up is.

Cp for water: (75.4x10^-3)(KJ/mol C)
Gallons of water per shower: 40 Gal
# of showers per day: 1
Temperature of in coming water: 65 F
Temp of shower drain water: 95 F
Cost of electricity: $0.147/Kwh
Cost of Heat Exchanger: $300 - $500
Cost of instillation: DIY


First lets convert the water temp from F to C:

(95F-32)/1.8 = 35 C
(65F-32)/1.8 = 16.3 C
The difference of our temp is 16.7 C

Now lets convert Gallons of water to mols

the density of water is:1000Kg/m^3
(1000Kg/m^3)(1000g/Kg)(1mol/18g)(1m^3/264.17Gal) = 210.3mol/Gal

40Gal(210.3mol/Gal) = 8412 mols of water per shower


The energy balance:

Delta H = Cp(Delta Temp C)(mols of water) = Q

Q is the amount of energy transfered from the hot water to the cold water (assuming the amount of water in = out and that the amount of cold/hot water is the same, with the hot water coming out at the temp of the cold water and the cold water coming out at the temp of the hot water, 100% heat transfer)

Q = (75.4x10^-3)(KJ/mol C)(16.7 C)(8412 mol) = 10592.22 KJ

Let convert the KJ to Kw*h

10592.22KJ(1000J/1KJ)(2.778x10^-7/1J) = 2.94 Kwh saved

Money saved per day

(2.94Kwh)($0.147/Kwh) = $0.43 per day

(365days/year)($0.43/day) = $156.95 saved per year

Remember I only take one shower a day, you might have more people in the house or take more than one shower per day.

How long does this take to pay for it self?
(Cost of heat exchanger + instillation)/($0.43/day)=(# of days to pay off)

$300/(0.43) = 697 days = 1.9 years

$500/(0.43) = 1162 days = 3.2 years

I found this website where I got the heat exchange numbers from:
http://www.toolbase.org/Technology-I...-heat-recovery

I personally think you could make your own system cheaper, but even if you had someone else do this for you it is generally good idea to do anything with a less than 3 year pay back period.

if you use gas here is the Kwh to Btu conversion:

2.778x10^-7 Kwh = 9.486x10^-4 Btu

[Who]

I think the heat transfer ratings are typically stated to be around 50 to 60% for 6' units and around 30% for 3' units. In terms of costs for DIY there are 3" copper transitions at each end to whatever the drains are now, plus piping the water to and from the drain. The payback is probably much longer, but the simple mechanical beauty and the expected long life of it adds a value beyond dollars. I have space for a 3'er and want to use one when I redo the drains for the bathroom.

[WTFM8]

Originally Posted by mopo3

Okay so I did the heat exchanger calculation on your shower drain. I'll set it up so you can change the numbers to fit what your set up is.

Cp for water: (75.4x10^-3)(KJ/mol C)
Gallons of water per shower: 40 Gal
# of showers per day: 1
Temperature of in coming water: 65 F
Temp of shower drain water: 95 F
Cost of electricity: $0.147/Kwh
Cost of Heat Exchanger: $300 - $500
Cost of instillation: DIY


First lets convert the water temp from F to C:

(95F-32)/1.8 = 35 C
(65F-32)/1.8 = 16.3 C
The difference of our temp is 16.7 C

Now lets convert Gallons of water to mols

the density of water is:1000Kg/m^3
(1000Kg/m^3)(1000g/Kg)(1mol/18g)(1m^3/264.17Gal) = 210.3mol/Gal

40Gal(210.3mol/Gal) = 8412 mols of water per shower


The energy balance:

Delta H = Cp(Delta Temp C)(mols of water) = Q

Q is the amount of energy transfered from the hot water to the cold water (assuming the amount of water in = out and that the amount of cold/hot water is the same, with the hot water coming out at the temp of the cold water and the cold water coming out at the temp of the hot water, 100% heat transfer)

Q = (75.4x10^-3)(KJ/mol C)(16.7 C)(8412 mol) = 10592.22 KJ

Let convert the KJ to Kw*h

10592.22KJ(1000J/1KJ)(2.778x10^-7/1J) = 2.94 Kwh saved

Money saved per day

(2.94Kwh)($0.147/Kwh) = $0.43 per day

(365days/year)($0.43/day) = $156.95 saved per year

Remember I only take one shower a day, you might have more people in the house or take more than one shower per day.

How long does this take to pay for it self?
(Cost of heat exchanger + instillation)/($0.43/day)=(# of days to pay off)

$300/(0.43) = 697 days = 1.9 years

$500/(0.43) = 1162 days = 3.2 years

I found this website where I got the heat exchange numbers from:
http://www.toolbase.org/Technology-I...-heat-recovery

I personally think you could make your own system cheaper, but even if you had someone else do this for you it is generally good idea to do anything with a less than 3 year pay back period.

if you use gas here is the Kwh to Btu conversion:

2.778x10^-7 Kwh = 9.486x10^-4 Btu

Show your work next time...

[roflwaffle]

Scooty Puff Jr. sucks!

[mopo3]

I made up an excel sheet so that other people could do the calculations more easily, but it is 20 kb too big. So I put it up on my web page.

http://matthewmoroz.googlepages.com/...tExchanger.xls

[WTFM8]

Originally Posted by roflwaffle

Scooty Puff Jr. sucks!

Yeah, sucks very little fuel! Viva la 1980s Hondas!

[Who]

The math is pretty simple using BTUs and ΔTs.

Shower is 110°, groundwater is 50° so ΔT is 60°.

8.3 pounds of water per gallon x 2½ GPM max x 60° x 60 minutes = 75 MBH

3413 watts / MBH if you want to calc the cost of an electric shower.

Anyway, the drain would probably only be about 80° on average over the course of a shower depending on how much mass is there like granite tiling that will absorb BTUs before it goes down the drain. Plastic showers would be best here. Anyway, with the incoming water temperature of 50° that gives a ΔT of 30° x 8.3 x 2.5 x 60 min which is 37½ MBH. Half the ΔT, same flow half the BTUs required.