Hot-h2o-ked

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Estimating the energy cost and hot water usage for Miller Farm turned out to be an involved process. In attempting to produce a reasonable estimate, we used multiple online hot water cost and energy usage resources, online manufacturer provided equipment model information, experimental data collection, and our best guesses.

We began with a list of hot water considerations:

The next logical step is data collection. This proceeded in a piecemeal fasion as we worked. Special thanks to Kate for finding the online model information for the hot water heater and to Dan for the online Washing Machine model info.

Miller Farm Water Fixtures (w/ hot water):

Appliance Information:

Armed with the hard data on the water heater and the washer, we proceeded to address the question of how much hot water is used at Miller Farm. Unfortunatly, without installing a water meter on the hot water line (might not be a bad idea), the hot water usage data will be approximate at best. We began with online hot water heater sizing tables to get an idea of the average hot water use of everyday activities.

Average Hot Water Usage Sources:

All three sources have different hot water use estimates for given activities (even the two tables from the same DOE department). The averages of the three tables were calculated and are as follows

Hot Water Usage Estimates from Online Sources:

Given the limited time that our group had to work together, we decided to focus our efforts on finding our own shower hot water usage data. Some questions that came to mind:

We answered the first two questions as best we could by conducting the following experiments:

Tools

Finding Average Shower Temperature: How do know what temperature the average Miller Farm dweller likes for a shower? Well, we used ourselves as a typical sample of EC students. We each set the shower to the desired temp., filled the bucket and then made the measurement.

Results:

Average temp.: 106.3 degrees F.

Finding Average Gal./Min. Hot Water Use: When filling the bucket for the above experiment, we also recorded the amount of time it took to fill the 6 quart bucket. Average: 40 seconds. That's 1.5 gal./40 seconds or 2.25 gal./min. total water use. But, how much of that water came from the water heater? The answer to this question would have been easy to find if only the shower fixtures had separate hot and cold handles, but no dice. So, we played with the sink water temperature until it was close to 106 degrees. We measured the flow of this mixture over a 40 second period, then shut off the hot water and measured the amount of cold water over a 40 second period. The ratio of hot to cold turned out to be about 4:1. That means that on average 1.8 gallons of hot water are used for every minute in the shower.

How long is the typical Miller Farm shower? Who knows... but if we assume the average shower to be ten minutes (as the GRU website does) then a typical Miller Farm Shower would use 18 gallons of hot water, which is only a 10% difference from the online average shower use estimation. Not bad.

We can check to see if the 4:1 ratio makes sense by finding the temperature of four gallons of 125 degree water and 1 gallon of 46 degree water: (125x4+46)/5 = 109 degrees. Since the 4:1 ratio was approximate this result seems reasonable.

Inlet Temperature: Additionally in order to calculate the cost of heating our hot water we needed to know the average inlet water temperature. The below study on residential hot water in the US shows regional averages for inlet temperature, average daily draw and more!

Estimating Household Usage:

The RERC says that five occupants will use 75+ gallons of hot water in a day. Our average occupancy is 7 adults, so we expect our daily usage to be a good bit higher.


The Estimate (figured on a weekly basis and divided by 7):

The average daily usage is therefore: 192 gallons of hot water


Questions that still need to be resolved

http://home.inu.net/davidstua/wh_cal.htm and http://www.hydroquebec.com/residential/energywise/calcul_consom.html

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