Solar Hot Water System

At the end of June 2007 we installed a solar hot water system from Butler Sun Solutions . It was a closed loop antifreeze system that heats water in our existing propane hot water heater using a Solar Wand heat exchanger. We have four Fireball 1001 collectors from ACR Solar International for a total of about 40 square feet of collector area. On the left side of the collectors is a small 21 W Uni-Solar photovoltaic module that powers the Ivan Labs EL-SID pump, which moves the antifreeze fluid through the collector loop.

The system was largely installed by Lucas Christy of Solar Gain Services (719-588-3044) with help from Frank. Luke did a very professional job. Later we insulated the hot water tank with an R11 fiberglass blanket and some Thermax rigid insulation.

We had a slow decline in performance due to dust in the collectors. During windstorms dust had blown in the collector openings around the pipes and had gotten into the air spaces in the polycarbonate glazing. ACR Solar International has since changed their manufacturing process to prevent dust or moisture buildup in the glazing, and they generously offered us new polycarbonate glazing for the cost of shipping. On 2011-01-28 we took the collectors off the roof, removed the old glazing, cleaned the dust off the black collector surfaces, sealed the new glazing in the collectors, and reinstalled the collectors on the roof.

We continued to have a substantial drop in solar hot water performance, characterized by higher collector return temperatures and lower hot water temperature gain (even on sunny days). At first the good performance could be restored by flushing the collector loop with water and adding new antifreeze solution. By Spring 2013 these flushings no longer worked. Butler Sun Solutions told us to remove the solar wand and cut part of the end of the center tube in the solar wand to restore the flow. We did not have head room to remove the solar wand without tipping the hot water tank, which we didn't want to do. Finally we learned that we could pull out the center tube of the solar wand by bending it. In early October 2013 we did that, and the solar hot water performance greatly improved. We fired the propane in the tank for 26.5 hrs in 2013 due to this problem.

Slowly the hot water system performance declined. We got tired of needing to flush the anti-freeze loop about three times a year. Fundamentally the problem with the Butler system is that the solar wand has very limited heat exchange area and efficiency, which leads to high anti-freeze temperatures (usually above 210F midday for our system), which causes a partial breakdown of the recommended Sierra antifreeze solution, which clogs the very small flow path in the solar wand. The over-pressure system of a radiator, and radiator cap for overflow into an external tank, does not work well for a system like ours with a large collector area.

We asked Luke Chisty of Solar Gain Services, LLC to design a solar hot water system upgrade that would use our existing collectors, antifreeze solution tubing, and PV module powered collector-side pump. On 2016-04-07 Luke added a 3"x3"x8" external heat exchanger, an AC potable water circulator pump, a new thermostatic mixing valve, a small expansion tank, and a Resol DeltaSol controller for the circulator pump. He removed the Butler over-pressure system. Our solar hot water system immediately started operating much better. The collector return temperatures are much lower, and the tank temperatures increase much more for the same amount of sun. Only about an hour of sun midday is required to produce enough hot water.

We are monitoring the performance of our solar hot water system by measuring temperatures of the top of the hot water tank and of the collector return loop. The temperature probe on the top of the tank is on the outside of the inner tank under some insulation, but it presumably is still cooler than the actual water temperature. The temperatures are measured every five minutes (along with the solar basement temperatures). The solar hot water temperatures for the past three days may be viewed on this graph . From the nightly tank cooling rates we determined that the Newton's Law of Cooling time constant is more than four days. The additional tank insulation is important for keeping the water hot through cloudy days.

The hot water tank temperatures are summarized with this daily average graph:

Click on the graph for a larger version.

Solar hot water temperatures for 2007-2008
Solar hot water temperatures for 2008-2009
Solar hot water temperatures for 2009-2010
Solar hot water temperatures for 2010-2011
Solar hot water temperatures for 2011-2012
Solar hot water temperatures for 2012-2013
Solar hot water temperatures for 2013-2014
Solar hot water temperatures for 2014-2015
Solar hot water temperatures for 2015-2016
Solar hot water temperatures for 2016-2017
Solar hot water temperatures for 2017-2018
Solar hot water temperatures for 2018-2019
Solar hot water temperatures for 2019-2020
Solar hot water temperatures for 2020-2021
Solar hot water temperatures for 2021-2022

The graph also shows the daily mean outside temperature and solar transmission as measured by our weather station. The top of tank temperatures are consistently well above the 100oF needed for adequately hot water. Our hot water has been nearly 100% heated by the Sun because we turned off the hot water heater pilot light. For the 2021-2022 season backup propane heating was not used.

The hot water tank broke and was replaced in October 2019. The new hot water tank has more insulation, so we no longer have an insulating blanket wrapped around it or extra insulation on top. As a result, the top of tank temperatures are lower than before for the same hot water temperature. From April 18 to May 5 and July 8 to September 25, 2020 the solar collector loop circulation was not working due to a loose joint, which let air into the collector loop. During this time we burned propane in the hot water tank for 20.4 hours (our solar installer was quite busy that summer).