Incomplete Installation

2/18/15

We ran out of hot water this morning.  After some googling, I found out that the tank we have will only provide 40 gallons of hot water when it is running solely on backup because it only has one heating element.  Regular hot water tanks have 2 heating elements. I sure hope once the solar system is up and running that this doesn’t happen again.

This experience led me to do some research on heat loss.  If we get cloudy days, how long will the water in the tank stay warm?

According to one site, “due to advanced insulation technology, water heated by the sun can remain hot for up to 80 hours after being exposed to sunlight.”

In the owner’s manual for the system we are getting, it states: “During normal operation, without any use of hot water, your solar storage unit should not lose more than 2 degrees per hour.”  That sounds overly conservative to me based on a formula for heat loss at Build it Solar, which is one of the most useful web sites on residential solar energy.  I think I am doing these calculations correctly, but I was an English major so there are no guarantees.

Heat Loss = (Area)(Tin - Tout)/(Rvalue), where Area is the area of the inside of the tank, Tin is the temperature in the tank, and Tout is the ambient room temperature where the tank is kept.

The Rheem tank we are getting has an R factor of 17.3. The area inside the tank is 38 sq ft.  Assuming a temperature in the tank of 120 degrees and room temperature in the garage of 60, you get the following:

Loss = 38(120-60)/17.3

Loss=131 BTU/hr

A BTU is the amount of energy needed to raise (or lower) the temperature of one pound of water by 1 degree.

1 gal of water = 8.3 lbs

80 gals of water = 664 lbs

664 BTUs are needed to raise temp in tank 1 degree.

664/131= 5.06  hrs to lose 1 degree based on the above calculation.

If you assume other factors for heat loss and take a more pessimistic scenario, we could postulate a loss of 180 btu/hr.   Then

664/180 = 3.7 hrs to lose 1 degree

Based on these calculations, it would take 3.7 to 5 hrs to lose 1 degree if temp in garage is 60.  In 15 hrs overnight, the loss should only be 3 to 4 degrees.  Of course these calculations don’t account for cold water entering the system so they would be best case.

If you use the 2 degrees per hour loss mentioned in the owner’s manual, that would be a loss of over 1300 BTU/hr, which seems high.  According to the formula, the R value of the tank would only be 1.7, which is basically uninsulated.  Like ROI, my heat loss numbers have a wide range. Experience with the system will be the true test. However, since we don't have any experience with the system, I spend a lot of time doing theoretical work that may, in the end, be totally off.

Here is some more information about how warm the solar collectors can make the water from Blue Pacific Solar, a California vendor.  

A solar collector consists of a network of pipes through which water is heated. Collectors come in various sizes, with 4 by 8 feet the most common. On a typical summer day (sunny and warm), the fluid in the collectors reaches 140°F to 180°F. On a clear winter day (sunny and cold), it can reach 120°F to 150°F. When it´s cloudy and warm, collectors can reach 70°F to 90°F, and when it´s cloudy and cold, 50°F to 60°F. As long as the temperature in the collector is greater than that of your incoming cold water (usually about 50°F), your solar hot water system is saving you energy.

I hope they can finish the installation tomorrow.