[Greenbuilding] Greenbuilding Digest, Vol 68, Issue 6

[barbara deane-gillett]

most tanks r not .95
tests r in labs not basements
a lot of loss is around the insulation thru steel support legs pipes wires etc

still about a kwh per day is typical

gas r even worse cause of hole up middle vent

basic rule triple the insulation and leave off when not in use
also set lower tstat as low as possible or off

Sent from my iPhone

On Apr 9, 2016, at 12:01 PM, greenbuilding-request at lists.bioenergylists.org wrote:

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>   1. Re: Water heater timer energy savings? (Nick Pine)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Sat, 09 Apr 2016 12:15:56 -0400
> From: "Nick Pine" <nick_pine at verizon.net>
> To: <greenbuilding at lists.bioenergylists.org>
> Subject: Re: [Greenbuilding] Water heater timer energy savings?
> Message-ID: <5F3835D6E10F479A81F3256A1B225090 at Fred>
> Content-Type: text/plain; charset="iso-8859-1"
> 
>> This water heater http://m.lowes.com/pd/Whirlpool-50-Gallon-240-Volt-6-Year-Regular-Electric-Water-Heater/50397576 has an EF = 0.95 energy factor, ie it uses Qdm Btu/day, about 5% more than 
> the average daily useful water heating energy https://en.m.wikipedia.org/wiki/Energy_factor  as tested in a 67.5 F room with 6 64.3 gallon draws of 135 F water heated from 58 F in the first 6 hours of the day, with an 18 hour rest period after that.
> 
> Oops. https://en.m.wikipedia.org/wiki/Energy_factor says:
> 
> "During the test 64.3?1.0 gallons of water are drawn from the water heater in six equally spaced draws that begin one hour apart."
> 
> After I calculated the 88 F water temp after 6 hours and realized that 6x64 gallons is a lot of hot water for a family and reflected on the R1.82 wall insulation calc, I realized that the total draw is 64.3 gallons, which makes the useful water heating energy Qu = 64.3galx8.33lb/gal(135-58) = 41243 Btu, approximately. And EF = 0.95 = Qu/Qdm by definition, so Qdm = Qu/0.95 = 43413 Btu, and the standby loss with no water draws is 43413-41243 = 2170 Btu/day, approximately, ie 0.64 kWh/day, or 1628 Btu during the 18-hour rest period, when a timer might help save energy.
> 
> With no timer, 1628 Btu = 18h(135-67.5)G makes the water heater's thermal conductance G = 1.34 Btu/h-F. With 50 gallons of water, thermal capacitance C = 50x8.33 = 416.5 Btu/F, approximately, so the cooling time constant RC = C/G = 416.5Btu/F/(1.34Btu/h-F) = 311 hours, so with a timer, the water would cool from 135 F to 67.5 + (135-67.5)e^(-18h/311h) = 131.2 F in 18 hours. Reheating it to 135 before the next draw would require 416.5(135-131.2) = 1583 Btu. So the timer saves 1628-1583 = 45.3 Btu/day, or 1.9 Btu/h or 1.9/3.412 = 0.55 watts, on a continuous basis.
> 
> How long would it take this $21.57 R10 water heater blanket to pay for itself at 15 cents/kWh? http://www.lowes.com/pd_24399-1410-SP57/11C___?Ntt=water+heater+blanket&UserSearch=water+heater+blanket&productId=3133229
> 
> Nick
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> End of Greenbuilding Digest, Vol 68, Issue 6
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