by Jack Dulaney, licensed master mechanical contractor, State of Michigan.
There are dozens of systems out there and a lot of different schemes to make them cheaper. A common theme seems to be eliminating components by employing various tricked out gadgets that supposedly do what two or more components do.
One such gimmick is to get rid of the "solar storage tank" and heat the water directly in the existing hot water heater. This supposedly eliminates a major component and frees up the floor space.
This scheme has so many flaws in it that it's difficult to believe their actually being sold.
The first problem is that for there to be a significant temperature gain from the solar unit, the water in the existing water heater must be cold when the sun is shinning which is ridiculous and just unacceptable to most people. Even then the existing water heater is in a race with the solar collector to heat the water.
Even large solar collectors are no match for the utilities and very little solar gain is possible. If part of the water heater is disabled or the thermostat turned down this loss of performance will cause wild temperature gyrations in the hot water service and still very little solar gain because again the water must be cold in the existing water heater for there to be significant solar gain
Even if cold showers are acceptable hot water heaters in most homes are already at or near the correct temperature when the sun is shinning. It is the early mourning and evening when most hot water is used in a typical home.
The only way this scheme can give significant solar gain and avoid wild temperature gyrations is to raise the water temperature drastically higher than the recommended 125 degrees. There are systems out there that do just that and they have serious problems as well. A significant gain from a solar system should be in the neighborhood of 60 degrees.
That then puts the water temperature in the existing water heater far above the third degree burn level, a term lawyers like to use.
The plan is then to mix the dangerously hot water with cold water as it is being used with a device called a "tempering valve" ( or mixing valve). As with all mechanical devices it will fail sooner or later. Think about it next time you are taking a shower.
The author of this manual has experience building devices used in the chemical industry that functions similar to a tempering valve. They are reliable and will fail safe. The basic model costs $3,500.00 The type of tempering valve used in solar applications being discussed here costs $50.00 or less. You get what you pay for...
There is yet another problem with heating water in the existing water heater above 125 degrees and that is collector efficiency. Most systems have an excess of capacity in the summertime, it's the winter and it's lack of daylight and cold temperatures that will define a systems performance limits.
It is a phenomenon of all solar collectors, especially the flat plate type, that the efficiency goes down as the temperature difference between the fluid being circulated and the ambient air goes up. To put it another way heat radiates out of the collector faster when it is cold outside and the hotter the fluid that is being circulated through the collector is the faster it is going to radiate. Going to higher temperatures with the potable water necessitates higher temperatures in the collector, which will in turn lower the system efficiency when you need it the most, during cold and cloudy weather.
No matter how these systems that eliminate the 40 gallon potable tank are configured they have serious problems and none of them work very well. We took a different approach with the Solar Patriot.
Instead of heating a little water very hot or heating a lot of water a little hot we decided to heat all the water to the correct temperature which keeps the operating temperatures down and the efficiency at it's highest. All of the potable water heated by the Solar patriot is heated in a separate tank which operates independently of the existing water heater. There are added benefits to doing it this way.
First is the safety and liability. No part of our system gets much hotter than 125 degrees. If there is a component failure nobody is going to be hurt.
Second is reliability and lifespan of the system. The hotter the system operates the shorter its lifespan is going to be. This is true for every component in the system, especially pumps.
Lastly it provides a constant source of hot water at a specified temperature without interruptions, without temperature variations, and without manual interventions, which is what we believe the vast majority of homeowners demand.
As you might expect there is a catch. Thermosiphoning systems have to be carefully setup to insure that convection currents flow freely within a wide range of conditions. Some of these conditions vary from home to home and can be difficult to predict and can lead to ghostly problems. Given the very low power consumption of the 1/12 horsepower pump used on the solar patriot we felt that the potential problems were not worth the gain.
A serious problem inherent with every type of solar water heating system is thermal shock and temperature cycling. The fluid being circulated to the solar collector, weather it is water or glycol can go through large temperature swings very quickly which is very hard on every component of the system. It is this temperature cycling and thermal shock that largely determines system lifespan in many cases. Pumps seem to be effected the most, but every component including the pipe joints will fail sooner because of it.
On the Solar Patriot the water coming from the solar collector goes directly to the 12- gallon tank where it is mixed before it is circulated to the rest of the system. This buffers the wide temperature swings and greatly extends the system life. There are many systems out there that do not do this or make the solar tank so small as to be useless.
Be sure to look for this feature before you buy a system. All closed loop glycol systems use a small pressure tank to compensate for expansion of the glycol, which is shown in fig 24 of the Drainback Installation Manual, however the fluid in this tank is not being circulated and does not give any buffering action what so ever.
The Solar patriot has two industrial grade controls with integrated digital temperature displays which are very accurate (fig 27) and a sightglass that allow the owner to verify the system is operating properly. This is a important point. There are systems out there that have no intelligent means to verify weather they are working or not. Others have installed their gauges in such way as to give a misleading impression of performance and many use cheap, low quality dial type gauges that are very inaccurate and fail quickly.
All of the controls on the Solar Patriot have been preprogrammed and no adjustments are necessary. If the control settings are tampered with or reprogrammed incorrectly it will not result in a unsafe condition. This is not the case with all solar water heaters.
We have found that our customers do not want to baby sit their water heater or do they want to give up having a reliable source of hot water. What they all seem to want is to plug it in and forget about it. A question often asked is what happens if the weather doesn't cooperate. There are going to be days that there is no solar gain possible, no matter what type of system is used.
It is important that under these worse case conditions that the hot water service is not interrupted or that it go through temperature gyrations. With the solar patriot there are no modifications to the existing hot water heating system and it will come on as it has always had and make up for any deficiency. If desired the Solar Patriot can be turned off without loss of hot water service or it can be left on for extended periods of time without using hot water.
We feel that we have met these requirements and have even exceeded them in some ways. We do not say that our competitors have not met some of these requirements as well, they have but we have yet to see one that has met them all. There are cheaper systems out there and if cost is a factor that will make the difference of weather a system is purchased or not, one of our competitors system may be your logical choice. But if you are looking for something that really works and is worth the money we feel confident that the more you learn about our system and our competitors as well, the more likely it is that you will buy ours.
Articles from various trade publications, especially the excellent Home Power magazine and manufactures specifications have been included with this manual. We urge the customer to read them to gain a further understanding of how solar hot water heaters work and what are some of the issues concerning component selection . Caveat Emptor.
8/01/04