Solar Domestic Hot Water Pre-Heating

SOLAR DOMESTIC HOT water preheating has been around for centuries. A search of solar collector patents shows patents being applied for back as early as the 1800s. Solar has been used since long before that. Man has always been intrigued with the power of the sun and has been making attempts at harvesting this energy for thousands of years. It has only been in recent times that mankind and technology

SOLAR DOMESTIC HOT water preheating has been around for centuries. A search of solar collector patents shows patents being applied for back as early as the 1800s. Solar has been used since long before that. Man has always been intrigued with the power of the sun and has been making attempts at harvesting this energy for thousands of years. It has only been in recent times that mankind and technology have made a serious effort to use this cleanest of energy for this major consumer of our natural resources.

Back in its heyday (late ’70s, early ’80s) solar energy was subsidized by tax credits that would allow up to 70% of the cost of the system to be written off. While this was a well-deserved shot-in-the-arm for solar enthusiasts, it caused every former vacuum cleaner salesman to become an overnight expert at the design, sales and installation of over-sized solar systems.

At the time, the federal and state investment cap on system costs was $10,000. Oddly enough, it seemed that regardless of how many panels were installed, the going price of most solar systems was $10,000. This is not to say that all solar manufacturers and installers were bad news, but the majority of the people that were out sizing and selling these systems had one thing in mind — a commission check on a $10,000 system.

This caused a lot of problems. The majority of the systems that were installed to perform this task were closed-loop designs. This meant that the solar loop contained some sort of antifreeze compound to avoid freezing and bursting the pipes going to and from the solar collectors and the storage system. The systems worked well during the winter. Generally speaking, the energy absorbed during the day was used at night, keeping the storage tank temperatures in check.

The problems cropped up during the summer months when the only energy being taken from the storage tanks was the energy used for pre-heating the DHW. In most cases, this was a small load in comparison to the actual energy being delivered from the solar collectors. This resulted in a storage tank being topped off at 180°F by 11 a.m., and the collectors being exposed to maximum solar gain for another three to four hours with nowhere to put the heat.

If the consumer were lucky enough to have hooked up with a reputable manufacturer, then the expansion tanks on the loop could take the expanding fluid generated by collectors stagnating at 350°F. One would hope that the fluid could also take those temperatures.

In most cases, however, the expansion tank was not sized to handle the fluid expansion associated with stagnation, and the pressure relief valve serving the solar loop popped off and relieved itself down the floor drain, with no one there to see it. This usually happened during the first summer of operation while the family was in Disneyland on vacation.

The then-critical connection between the top of the system and the bottom of the system was lost, and no additional energy was delivered to the storage tank.

When the homeowners returned from vacation, they had no idea what had transpired, except that the cat acted really skittish whenever it walked past the solar storage tank. They looked at the control lights that the salesman pointed out to them and knew that everything was OK, because the red and green lights were on. If they happened to be sharp enough to evaluate their utility bills, or went to the expense of installing a temperature monitoring control, they may have noticed a discrepancy and called for service. In most cases, the memory of the $7,000 tax credit interfered with their common sense and nothing was ever done about it. Poor solar utilization.

Fortunately, the tax credits came to an end, and the former vacuum cleaner salesmen moved on to new frontiers, such as radiant floor heating systems and lead paint abatement.

But alas, the cost of energy has continued to rise, and the potential for solar utilization has popped back up on the radar screen. DHW solar preheat makes all the sense in the world, if it is properly designed, installed and maintained. It is solar energy use in its second simplest form. DHW loads are a constant, year-round proposition for residential and commercial settings that can be substantially met by simple, closed-loop systems. In the next few articles, we will look at the different types of active solar systems available for DHW preheat and let you make an intelligent decision as to which one best suits your needs. Until then, Happy Solar DHW Preheat Hydronicing!

I would like to take this opportunity to wish CONTRACTOR and the fine folks at Penton Media a Happy 50th Anniversary. May you enjoy another 50 years of positive publications that are beneficial to the trades.

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.