[RE-wrenches] Electric Water kWh/ person

Drake drake.chamberlin at redwoodalliance.org
Tue Mar 20 11:50:38 EDT 2018 

Daniel,

That sounds like an intriguing technology, much 
along the lines of an air to air heat exchanger. 
Thanks for the recommendation.  These systems 
could keep a lot of CO2 out of the atmosphere if widely used.

Drake


At 10:09 AM 3/19/2018, you wrote:
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>Drake,
>
>Have you or the client looked into the 
>drainwater heat recovery units? They make both a 
>vertical and horizontal mounted version. They 
>just help recover the waste heat from shower water going down the drain.
>
>My understanding is that economically they are 
>barely past the breakeven point when considering 
>offsetting grid power. If using PV costs as the 
>benchmark, the economics would be better. If 
>somehow this is off grid, then I’d say it’s a 
>home run. (Hopefully though this is not an all-electric off-grid building.)
>
>The main benefit, as far as I understand it, is 
>that you can get by with a smaller unit, with a 
>smaller heating element to perform the same 
>task, OR you can get much longer showers from 
>the same sized unit. Depending on the clients 
>goals, this might be a useful tool to bring into the situation.
>
>Here are two brand names that I know of.
>Vertical mount: renewability
>Horizontal mount: ecodrain
>
>I have not used these products, so I cannot 
>endorse any of them, just though it may be worth consideration.
>
>With Regards,
>
>Daniel Young,
>NABCEP Certified PV Installation ProfessionalTM: Cert #031508-90
>
>From: RE-wrenches 
><re-wrenches-bounces at lists.re-wrenches.org> On Behalf Of Drake
>Sent: Thursday, March 15, 2018 1:00 PM
>To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
>Subject: Re: [RE-wrenches] Electric Water kWh/ person
>
>This discussion has been a great help. The house 
>is built and the plumbing is in, but could be 
>modified. Wood stoves are not a likely solution. 
>I think we need a different water heating strategy.
>
>Thank you all!
>
>Drake
>
>
>
>At 09:21 AM 3/15/2018, you wrote:
>
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>This is what I came up with:
>
>The calculation is Q = mcp delta T
>
>Water Temp out =Â  120
>
>Water Temp in = ground source Temperature @ 50F?
>
>delta T = 70F
>
>m =  mass flow rate.  ASHRAE is the source for 
>this. 20gals per person per day = 20 gallons x 
>8.34 lbs/gallon =Â  167lbs/day/person
>
>cp =Â  1 BTU/lb-degree F
>
>Q = 167 x 1 x 70 = 11,676 BTU/day/person
>
>Divide this by the efficiency of the water 
>heating system (I'll assume = .86)Â  =11,676/.86 = 13, 577 BTU/person/day
>
>convert to kWh if necessarym 13,577/3412 = ~4 kWh/person/day, 120 kWh/month
>
>I checked this against one month and it seems reasonable.
>
>Website assuming ~3.5 family size comes up with 
>@ 400kWh per month. They approached it another way with assuming run times.
>
><https://www.google.com/search?safe=active&source=hp&ei=Z3KqWvXiNsGb5wKJ2Kf4DA&q=electric+water+heater+use+annual+consumption&oq=electric+water+heater+use+annually&gs_l=psy-ab.1.2.33i22i29i30k1l3.279.9222.0.12475.35.19.0.7.7.0.451.2360.0j6j4j0j1.11.0....0...1c.1.64.psy-ab..18.16.1952.0..0j0i67k1j0i131k1j0i22i30k1.0.CiU8wYZigpI>https://www.google.com/search?safe=active&source=hp&ei=Z3KqWvXiNsGb5wKJ2Kf4DA&q=electric+water+heater+use+annual+consumption&oq=electric+water+heater+use+annually&gs_l=psy-ab.1.2.33i22i29i30k1l3.279.9222.0.12475.35.19.0.7.7.0.451.2360.0j6j4j0j1.11.0....0...1c.1.64.psy-ab..18.16.1952.0..0j0i67k1j0i131k1j0i22i30k1.0.CiU8wYZigpI 
>
>
>
>Chris
>
>
>
>
>On 3/15/2018 8:09 AM, Mike Kocsmiersky wrote:
>
>Barton makes a good point with regards to the 
>incoming ground water temperature and the amount 
>of heat required to bring it up to shower temp. 
>Â My recollection is that ASHRAE states 
>20gal/person/day for the first 2 people, then 
>15gpd or 12gpd thereafter.  However those rates 
>vary wildly with consumers, from those that 
>never bathe to the high schooler that needs 3 
>showers a day.  In the northeast water heating 
>can be up to 19% of the total household energy 
>load for the year.  Also, I would recommend the 
>air source heat pump water heaters if you have 
>significant humidity or an on demand 
>unit.  Water tanks are typically under 
>insulated and can lose 2kWh/day in standby losses.
>Â
>Mike Kocsmiersky
>Principal
>Spirit Solar Inc.
>(413) 734-1456
>Â
>Â
>Â
>From: Barton Churchill 
>[<mailto:barton at solarips.com>mailto:barton at solarips.com]
>Sent: Wednesday, March 14, 2018 7:31 PM
>To: RE-wrenches
>Subject: Re: [RE-wrenches] Electric Water kWh/ person
>Â
>You first need to know your ground temperature. 
>Then figure a delta to 100 or so degrees. Most 
>but not all showerheads flow 2.5gpm and showers 
>are typically 10 minutes. You can then find a 
>calculator online to calculate how many Btus are 
>required to raise water temp for your delta per 
>gallon. Convert to kWh, add some contingency and you should be good.Â
>
>On Wednesday, March 14, 2018, 
><<mailto:drake.chamberlin at redwoodalliance.org> 
>drake.chamberlin at redwoodalliance.org> wrote:
>Hello Wrenches,
>
>Is there a good way to estimate the power that 
>will be consumed for electric water per person 
>per day? We are estimating the amount of power 
>that will be used by an all electric duplex, and 
>the water heater amount seems high. The current 
>model we have claims 3493 kWh/ year per 26 
>gallon tank will be consumed. With the two 
>units, that would add up to 6986 kWh / year for 
>hot water, pushing the system size to humongous for a 1300 ft sq building.
>
>Does that seem high? There will be a maximum of 2 people per unit.
>
>The owner is considering going to 115 V units 
>with a 15 gallon tanks. I think the smaller tank 
>would help, but it would seem that the amount of 
>hot water consumed would be the main issue.
>
>How can we get a good estimate for a per person 
>energy usage for electric hot water? Any other suggestions are welcome.
>
>Thank you,
>
>Drake
>
>
>
>--
>--Â
>Barton Churchill
>406.587.5295 Â
>2430 North 7th Bozeman, MT 59715
><http://www.solarips.com/>www.solarips.com
>Image removed by sender.
>
>Â
>
>
>
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>Â Â Â Â Â Â Â Â  Christopher Warfel, PE
>Â Â Â Â Â Â  ENTECH Engineering, Inc.
>PO Box 871, Block Island, RI 02807
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