Heating Water With Solar Energy
The two most common types of solar collector. Flat plate (left) and evacuated tube (right)
Use of solar energy to heat domestic hot water and radiant floors is soaring along with the cost of fossil fuels. New, more efficient heating technology, i.e. evacuated tube absorbers, makes solar even more desirable. Below is a description of the most commonly used methods of heating water with solar energy and the advantages and disadvantages of each.
Most people are somewhat familiar with the standard flat plate type solar collector illustrated above. This collector is basically a highly insulated box containing a grid of copper pipes bonded to a flat black copper absorber plate. Special glass enhances solar absorption.
The evacuated tube collectors seen above are an entirely different approach to solar water heating. Instead of many water filled copper pipes, these collectors use multiple vacuum filled glass tubes, each with a tiny amount of antifreeze hermetically sealed within a small central copper pipe. When heated by the sun, this antifreeze converts to steam, rises to the top of the tube, transfers its heat to a collector header, then condenses back into liquid and repeats the process.
Because heat doesn't easily transfer through a vacuum, 92% of the thermal energy hitting the absorber plate stays within the evacuated tube and passes to the collector header. This is a huge advantage because a standard flat plate collector radiates much of its accumulated heat to the surrounding atmosphere like any other hot object.
The evacuated tubes are also completely modular. Although rarely necessary, one or more tubes can be removed and replaced without affecting the other tubes in the array. There is no actual liquid transferred from the evacuated tube to the collector header...just heat. Evacuated tubes also start absorbing heat earlier in the day than flat plates due to their convex design and the tiny amount of antifreeze within the tube is freeze protected down to -50 degrees below zero.
How It All Works
There are three main configurations for solar water heaters. The Open Hot Water/Radiant System uses a heat exchanger and anti-freeze to transfer thermal energy to a storage tank filled with potable water. This approach provides both domestic hot water, radiant floor heat, solves freeze issues, and allows the system to be powered by a very low wattage pump (more on this later).
The drain-back approach also provides heating and domestic hot water and solves the freeze issue by draining all the collector fluid back into a storage tank. The downside is the size of pump needed to make this type of system work. Because the collectors are often located on the roof, 20 or 30 feet above the storage tank, the pump has to be powerful enough to push against gravity. In a "closed" pressurized anti-freeze loop, the weight of water dropping from the return side of the loop, pushes the water up on the supply side. The pump isn't pushing so much as it is simply "stirring" the water around in the loop. As a result, the height of the collectors doesn't matter that much and the pump used is a small 45 watt circulator. By contrast, the drain-back pump needs much more "head" and much more power and costs over 4 times as much energy to run (245 watts).
The Batch Collector is any kind of collector that stores a large amount of water within the collector itself.
We'll describe these three different methods and outline the advantages and disadvantages of each starting with the heat exchanger and antifreeze method plumbed into an OPEN radiant heating system.
The OPEN Solar Hot Water/Radiant System
A radiant heating system is considered "open" any time the same hot water is used for both heating and domestic hot water. This type of system is very efficient because a single heat source (in this case, solar with a fossil fuel back-up) provides for all the home's hot water needs. In other words, the homeowner doesn't need two completely separate systems, many times with overlapping mechanical components, performing separate heating tasks.
Why use solar energy for domestic hot water and heating? Well, besides the obvious advantages of using an unlimited, free (once system components are paid back), renewable heat source, solar water heaters interface well with radiantly heated floors because the large thermal mass common to radiant systems provides an excellent storage medium for the energy generated during the day. At night, this stored thermal energy is slowly released into the living space and a steady, even, and consistent comfort level is maintained.
The following schematic illustrates the components necessary for an open system that uses the external heat exchanger and antifreeze approach.
Solar Set-up With External Heat Exchanger (Schematic #1)
A solar array, either evacuated tubes or flat plates, interfaces with a flat plate heat exchanger. A mixture of antifreeze on the solar side protects the collector from extreme low temperatures, and at the same time, the addition of antifreeze significantly raises the boiling point of the heat transfer fluid. This feature comes in handy during the summer when too much sun and too little hot water usage can become a factor. In other words, the higher boiling point prevents your collectors from making steam and blowing off the pressure relief valve.
The percentage of antifreeze to water is determined by the lowest possible temperature in your region. A 50/50 mix will generally protect down to -29 degrees. As a rule, use as much water in the mix as you can get away with because water transfers heat better than antifreeze. Just remember that the fluid in the collector loop will be siting idle during many long, bitter cold nights and it doesn't have to freeze solid to cause problems. The low wattage circulator pump used in the solar loop won't pump slushy antifreeze.
Electronic and Mechanical Components
If you study the schematic, you'll notice that a differential controller (see detailed discussion at the bottom of this page) activates the system. Two sensors, one at the collector and one at the bottom of the solar storage tank, monitor fluid temperature. When the temperature at the solar collector is 5-20 degrees (you decide which exact temperature works best for you) greater than the temperature at the bottom of the storage tank, pumps on either side of the heat exchanger are activated. One pump (collector) sends heat to the heat exchanger, the other pump (storage tank) draws the heat off and sends it to the storage tank.
Because the storage tank is filled with drinkable (i.e. potable) water, the heat exchanger guarantees that antifreeze and water never mix...only heat is transferred from one fluid to the other.
This type of system uses an external heat exchanger. Other systems use a special solar tank with one or more built-in internal heat exchangers. Obviously, the internal heat exchanger only requires one pump because the water surrounding the submerged heat exchanger coil draws off the heat through natural conduction.
Which is best? As always, many factors come into play. Tanks with internal heat exchangers (see schematic #2 illustration below) are more expensive, around $1,600 for as much storage capacity as the system illustrated in schematic #1. A simple holding tank with an external heat exchanger runs about $800.00, if you factor in the second circulator pump.
Flat plate external heat exchangers are also more efficient. Because the water and antifreeze flow in extremely close proximity between alternating stacks of stainless steel plates (see photo below) a very highly efficient heat transfer results.
Cutaway of Flatplate Heat Exchanger
In contrast, an internal heat exchanger immersed in the solar storage tank, transfers heat at a much slower rate due to the lesser amount of surface area in direct contact with the surrounding water. On the other hand, the heat entering an internal heat exchanger has nowhere to go BUT the surrounding water. An external heat exchanger (if left uninsulated) transfers some of its heat to the surrounding air. Of course, this can be okay if the surrounding space is an area you want to heat.
Solar Setup With Internal Heat Exchanger (Schematic #2)
Back-up Heat Source
Notice that in both applications a back-up heat source is required. In the case of Schematic #1, the external heat exchanger is interfaced directly with an 80 gallon solar storage tank. There is no heating element or burner in this tank. It is simply a large, highly insulated tank with lots of "in" and "out" connections.
These connections are important because most standard water heaters have only a main "cold in" and a main "hot out" connection. This makes it difficult to connect a heat exchanger loop. Of course, you can always "steal" a connection port from another function, i.e. remove the drain valve or P&T valve, plumb them "in-line", then use the free ports for a heat exchanger loop (more on this later). But, depending upon how elaborate your heating system is, you may steal these ports and STILL wish you had more ins and outs. That's why a storage tank is nice...that, and the fact that it provides STORAGE (see more on this topic below).
The options for back-up heating are two-fold. The back-up heater can be a standard tank type water heater and be fueled by gas, oil, or electricity...or a gas fired on-demand water heater. In the case of the solar storage tank with dual internal heat exchangers, the on-demand heater makes the most sense because there's simply no burner in the storage tank. As a result, the on-demand unit is plumbed to one of the two heat exchangers and only fires on days when the solar can't heat the tank sufficiently.
For systems using external heat exchangers, either a tank type or on-demand heat will work. Which is best depends upon the situation. Tank type water heaters used for back-up provide a dramatic increase in storage capacity...usually a plus for solar hot water systems. But like any tank of hot water, unless very well insulated, they suffer from "standby loss", i.e. the heat lost to the surrounding air through radiation. Standby loss can be 10% or more of the heat generated.
On demand back-up heaters don't store any water, so standby loss isn't an issue. But, of course, your solar storage tank must be carefully sized to match your solay array. If the tank is too small, potential solar gain is lost. If it's too large, it rarely or never fully heats up.
All solar hot water systems rely on thermal storage in one way or another. Flat plate panels or arrays of evacuated tubes, or a combination of the two, are sized to generate a given percentage of the home's hot water needs duriring the sunlight hours, and then it's stored in a thermal medium of some sort. Usually, this thermal medium is plain water because, quite simply, water is the best material on Earth for storing heat. A close second would be a dense, solid material like concrete or stone.
For this reason, the solar storage tank is sized differently if radiant heating is involved. In other words, the amount of water needed for storage of the heat generated during the day is much less if that thermal energy can be stored in the radiant floor. So, instead of needing 300 to 500 gallons of water storage, accumulated during the day and used at night for heating, the solar storage tank can be closer to 100 gallons. Since so many BTU's were diverted into the thermal mass of the floor, only what's needed for normal domestic hot water needs remains in the storage tank.
Of course, the specifics of how many tubes/panels and how many tanks/gallons of storage are needed is unique to every situation and the factors used to determine what is best for you will be discussed later.
For the sake of gaining an understanding of how this all works, I'll refer you back to the two schematics.
In schematic #1, a total of 130 gallons of water storage is possible, 80 gallons from the solar storage tank and 50 gallons from the back-up water heater. In an ideal world, i.e. sun every day, perfect solar orientation, no shading elements like trees or eaves getting in the way, super insulation between the collectors and the storage tank, and 24 hours of cloudless daylight during the dead of winter (if anybody ever finds this world, please let me know)...there would be no need for a back-up heater. However, since only Rod Serling lives in the world outlined above, the rest of us need some way to make up the difference between the sun we get and the heating we need.
Put another way, we need a back-up heater.
In schematic #1, that back-up heater is a high efficiency (95%) 50 gallon Polaris water heater. Following the piping in the schematic, you'll notice that the "cold in" to the Polaris is fed by one of the "hot out" ports from the solar storage tank. In this manner, whenever hot water goes to the house fixtures, instead of cold water entering the Polaris to replace it, as would be the case in a normal water heater setup, solar hot water flows from the solar storage tank. As long as that "make-up" water is indeed hot, the burner in the Polaris won't come on. If on the other hand, you DON'T live in the Twilight Zone and a terrible, rainy, sunless day keeps the solar storage tank tepid, the Polaris responds to the drop in water temperature, fires the burner, and maintains the desired tank temperature so that all heating and domestic hot water needs are met.
As an additional plumbing feature, one designed to achieve maximum efficiency and minimize the use of fossil fuels, a circulator pump is plumbed between the two tanks. This pump is controlled by a second differential controller that monitors the temperature between the Polaris (back-up) and the solar storage tank. When the temperature in the solar storage tank is 5 degrees greater than the temperature in the Polaris, i.e. when the solar storage tank has heat to offer the Polaris, the circulator pump activates and mixes the water in the two tanks. This feature greatly enhances the system because now instead of limiting storage to the volume of a single tank, the combined volume of both tanks can be solar heated.
Without this pump stirring the water between the two tanks, the solar heated water from the storage tank would only enter the Polaris when domestic hot water is being used, as outlined above.
The actual make-up water, that is, the cold water that comes from the well or municipal supply to replace any hot water used in the home, is plumbed to the storage tank via the floor (more on this later). In this way, even if the sun refused to shine for days at a time, at the very least, the storage tank would act as a pre-heater to the back-up heater (Polaris)...even if that "pre-heat" is nothing more than room temperature water. But still, room temperature water is better than 45 degree well water.
So, why is the fresh "make-up" water entering the storage tank via the radiant floor? Why not plumb a branch of the house cold supply directly to the tank?
Because this is an "open" radiant system. Fresh water always travels through the floor tubing before entering the heat source in any open system. This is necessary because during the summer months when the heating system is off and no pumps are stirring the water around the floor tubing, the water in the tubing could sit for months and stagnate. This unhealthy stagnation is impossible if the fresh make-up water passes through the floor on its way to the water heater and hot water is used year round.
An important component to any solar hot water system is the mixing valve. This is because, unlike traditional water heaters, solar water heaters can generate savage amounts of hot water during periods of abundant sun and limited hot water usage. A standard fossil fuel water heater simply shuts down the burner when an internal aquastat reaches a given set-point (normally around 125 degrees). In contrast, a solar array will send heat to the storage tank any time the array is 15 degrees hotter than the water in the storage tank. Of course, the differential controller has a "high limit" setting that will turn off the pumps and shut the system down once the high limit temperature is reached. But, that high limit temperature is normally about 210 degrees and water scalds at temperatures above 130 degrees. The mixing valve keeps any super heated water in the storage tank safe for domestic and floor use.
Because the nature of solar water heating is grabbing as much heat as you can, when you can, it's never a good idea to set the high limit switch on the solar controller to much below "max". In other words, if you can store X amount of water at a few degrees below steam, you're getting the most out of your system.
Besides that, it's never a good idea to allow your panels to sit with the sun beating on them, getting hotter and hotter, building pressure, straining joints, and undergoing lots of thermal expansion and contraction. If the sun is available, use it!
The Heat Dump Package
Nevertheless, depending upon summer hot water use, solar orientation, cloud cover or lack of it, there may be times when no amount of storage capacity can absorb the heat produced by the array and the panels enter a state called "stagnation".
Basically, stagnation occurs when the solar storage tank heats up to maximum temperature early in the day; movement through the solar collector stops, and the fluid in the system sits under the sun getting hotter and hotter. The result is a high pressure, high temperature condition that can damage the system over time by subjecting it to extremes of expansion and contraction. In addition, when antifreeze is superheated every day for weeks at a time, it tends to break down and become acidic, thereby transforming into a corrosive substance that circulates through your system slowly damaging its components.
To prevent this, some sort of heat dump should be incorporated into any solar heating system with stagnation potential. In practical terms, any system designed to produce between 80% to 100% of a home's winter heating would likely have stagnation potential because the number of panels required for that much heating makes overheating in the summer virtually inevitable.
So, what exactly constitutes a heat dump?
Basically, any method of diverting waste heat to a hot tub, a swimming pool, a stock watering tank, a water to air heat exchanger of some sort, or an engineered heat dump package (see below). Any approach that prevents the fluid in the system from stagnating will work fine.
If you're inclined toward a low tech approach, covering all or part of your array with tarps or shades can be a perfectly adequate cure for stagnation. The problem with this method is the unpredictability of the weather. During stormy summer days, you'll have to uncover your panels just to meet basic domestic hot water needs. If the array is accessible, and you're home to deal with it, covering and uncovering in rhythm with the weather may not be a bad option. On the other hand, it's frustrating to heat a tank of hot water with fossil fuel after coming home from the lake on an intermittently sunny day and discovering that you'd forgotten to uncover all of your panels.
That's why most people opt for some sort of automated overheating protection.
The Drain-Back System
A drain-back solar system is a non-pressurized volume of water in a closed circuit that, as the name implies, drains back from the panels, down to a storage tank, at the end of every heating cycle.
The advantage of a drain-back system is built-in freeze and overheating protection. Because water only enters the panels after they've heated up, and then drains back when the panels cool, freezing is impossible. During the summer when the solar storage tank is fully heated, no water will be sent to the panels to "stagnate", so no damage can occur from overheating.
However, there are some disadvantages to this type of system. One is the need for a high head pump (and the higher initial cost and higher daily operating costs that go with it) because, unlike a pressurized closed loop system, the pump must be powerful enough to push water from the solar storage tank, against gravity, up to the panels. A pump this size requires 245 watts during operation. As a comparison, a standard solar circulator in a pressurized closed loop system uses just 45 watts.
Also, the solar collector array must be installed at a slight angle. A minimum slope of ¼" per foot must be designed into the support structure to guarantee that all the liquid in the collector drains back to the storage tank. If your solar array is clearly visible from below, this can give your system an "off level" look because, quite simply, the panels are off level. In addition, if you're installing panels in a ground mount application, a drain-back system may lack sufficient fall unless your storage tank is well below ground level.
One other limitation worth mentioning about the drain-back approach involves its usefulness during marginal solar gain periods.
With a standard, non-drainback solar setup, the cold make-up water for the home's domestic hot water supply is fed directly into the solar storage tank. This is because the solar storage tank and the main water heater tank are part of the same pressurized domestic hot water system. The "hot out" from the solar storage tank goes to the "cold in" of the main water heater (remember, the main water heater acts as your back-up system during times of little or no solar gain). This plumbing between the solar storage and the main water heater guarantees that, even during marginal solar days, warm water, or at the very least, room temperature water comes from the solar tank to the main water heater instead of, say, 45 degree water directly from your well. In this way, any heat available from your solar gets used.
With a drain-back system, heat from the non-pressurized solar tank should only be transferred, via an external heat exchanger, to the main pressurized tank when there's enough heat available in the solar tank to make the transfer worthwhile.
In other words, only when the water in the solar tank is hotter than the water in the main water heater (i.e. back-up heater) will heat be transferred over. As a result, there will be times (especially during a cloudy winter) when the solar tank is filled with potentially useful, but only semi-warm water.
In this instance, assuming that the tank is well insulated and can hold its heat overnight, another solar day will be required to raise the water temperature further and trigger a heat transfer. In practical terms this means that some days will pass without any contribution from the solar at all as the collectors struggle to bring the tank up to a useful temperature.
Of course, there's a way to address this problem, but it's expensive.
A pricey solar storage tank with one or more internal heat exchangers can be used in place of the much less costly plain storage tank with an external heat exchanger.
With this approach, the cold water replenishing the main water heater (i.e. when hot water goes to the house, fresh cold water replaces it) passes through an internal heat exchanger and is heated either totally or partially before it leaves the solar tank and enters the main water heater. As a result, any heat available in the solar storage tank is transferred to the incoming cold water instead of sitting in the tank waiting for enough sun to trigger a transfer to the main water heater. This prevents the heat gained during lackluster solar days from simply sitting idly in the tank and radiating uselessly out, over time, to the surrounding air (standby loss).
The Batch Collector
A batch collector can be as simple as a flat black 55 gallon drum mounted to the south facing side of a roof, or as elaborate as a set of black, 4" diameter water filled tubes mounted in a 4' by 8' insulated box (like any other collector) and encased under low emissivity glass. Their defining characteristic is a quantity of water stored within the collector itself, and not necessarily down below in a storage tank. Though in most cases a secondary storage tank is incorporated into the system to act as a back-up.
Another common variation is the "thermosyphon" collector. Essentially a batch collector plumbed to a manifold of copper tubes embedded within a copper absorber plate, the thermosyphon collector requires no electricity to move the water. Instead, it uses the natural tendency of hot water to rise, and cooler water to fall, to create a convective circulation of fluid (see photo below).
The advantage of the batch collector is simplicity. Mechanical components are minimal and no antifreeze is used in the system. That eliminates the need for a heat exchanger (see schematic below). In other words, the water heated and stored in the collector is the same water used directly by the domestic hot water system.
Large, flat black concrete batch collectors are commonly seen in Mexico and other temperate climates, but for obvious reasons (freezing) they're rarely used in latitudes north of the border. When they are, some method of freeze protection is designed into the system because even Phoenix drops below 32 degrees once in a while.
Unfortunately, since the batch collector contains potable water in an insulated tank exposed to the elements, freeze protection usually involves an electric heating element immersed in the tank, or a temperature activated "purge" valve. Both methods are rather clunky. The heating element approach relies on the worst possible method of heating water during a freeze emergency (expensive resistance heating), and the purge valve is nothing more than a solenoid valve that (hopefully) opens at 34 degrees, spewing the chilly water out of the system and replacing it with hot water from the back-up tank. Depending upon how cold the night gets, this purging process may be repeated many times, all the while spending valuable hot water in the service of protecting the collector.
Now, one may argue that in a warm climate like Arizona, the abundance of heat gained during the day will more than offset any heat sacrificed at night to protect the collector. Let's hope so. Because the alternative is the back-up heater, fired with fossil fuels, sending expensive hot water to the collector where it slowly cools down before being dumped down the drain.
Nevertheless, where freezing is not an issue, a batch collector is a simple and effective method of gaining virtually free hot water.
The Differential Controller
Solar thermal systems generally use a special relay called a Differential Controller. As the name implies, this relay activates a pump or pumps when a span (or difference) between two temperatures is achieved. In other words, when the temperature at the solar collector is X degrees hotter than the temperature at the bottom of the solar storage tank, the differential controller activates the necessary pumps and draws that useful heat into the system.
Transferring heat from a hotter to a cooler tank in order to equalize the temperature in both tanks and increase total storage capacity is another common use for a differential controller.
The GL-30 differential controller
Because the differential controller is adjustable, understanding the settings and how the sensors work is helpful. The photo below illustrates the adjustable components of the GL-30.
Differential "turn on" and "hi limit" dials
As indicated on the above photo, two sensors (tank and solar) are required for a proper "differential". One sensor is attached to a pipe near the bottom of the solar storage tank. The second sensor reads the water temperature as it leaves the solar collectors. Both sensors must be insulated (with fiberglass or foam) to prevent ambient temperature from influencing the reading. It should be noted that a sensor clamped to a hot pipe will NOT accurately read the actual water temperature. In fact, the water will generally be 15 to 20 degrees warmer than the sensor indicates.
Fortunately, for the purposes of a well functioning solar hot water system, actual water temperature is not important (unless, of course, it's too tepid for a hot shower). What matters is the difference between the water temperatures at the two locations. After all, if the water is actually hotter than what the sensor indicates, so much the better.
Notice the "turn on" and "Hi limit" dials in the above photo. These allow the user to customize the controller for specific applications.
This dial is the actual differential control. The turn on range is between 8 degrees and 24 degrees. Turn off occurs when the temperature span between the two sensors is 4 degrees. In other words, on the turn-on low range, if the collector temperature is 8 degrees warmer than the solar storage tank, the system will activate and transfer heat until the sensors determines that the differential has dropped down to 4 degrees.
At the turn-on highest setting, a full 24 degree temperature differential must be achieved between the two sensors before the system will initiate. Then, once activated, the system remains on until a 20 degree drop in water temperature tightens the temperature spread to 4 degrees.
So, why the choice? Why not have a preset differential?
Because every installation is unique and some customers prefer flexible electronics that allows each user to fine-tune the controller to match the distinctive nature of their system.
Why a Wide Differential is Generally Best
The "collector loop" is the total length of 3/4" copper pipe, both supply and return, that connects the solar array to the mechanical components, i.e. heat exchanger, storage tank, etc. This loop can be quite short (collectors located on the roof of a garage with mechanicals only fifteen feet below), or quite long (collectors grounded-mounted sixty feet from the house). The pipe in the short loop totals thirty feet (.8 gallons of fluid) . The long loop, one hundred and twenty (3.2 gallons of fluid).
In both these cases, the fluid in the collector loop must be brought up to temperature before the system will "stay on" for any length of time. The reason is because, early in the morning when the sun starts warming the collectors, most of the fluid in the collector loop is still cold. However, once the sun hits the panels, the fluid at the top of the collector, nearest to the collector sensor, warms up quickly and triggers the system. But, as soon as the colder fluid in the loop circulates passed the sensor, it cools down again.
This fosters a completely normal condition known as "short cycle". Expect the solar pump to short cycle until the water in the overall collector loop heats up. If the collector loop is long, and the sun is weak, many gallons of chilly fluid must warm up before any useable heat can be transferred to the storage tank. This may take time.
Rule of thumb: Keep the collector loop short...and insulate it well.
You can see from the above description that a "tight" differential (8 to 15 degrees) increases the short cycle effect. Especially if the collector loop is long and the array is small (i.e. limited heating capacity). The widest possible differential in this situation would minimize the system's tendency to shut off and on every few seconds.
However, if your system is high capacity (many flat plate collectors or more than 48 evacuated tubes), and your collector loop is short, a tighter differential activates the system earlier and more usable heat is gained.
Large heating capacity and short collector loop = tight differential (8 to 15 degrees)
Small heating capacity and long collector loop = wide differential (20 to 24 degrees)
The Hi Limit dial has nothing to do with the temperature at the collectors. It will not affect the differential setting in any way. Instead, it uses the storage tank sensor to monitor high water temperature and shuts down the system when the hi limit setting is reached. Basically, it allows the water at the bottom of the storage tank to reach 200 degrees F. That's plenty hot. And because the tank sensor measures the temperature at the bottom of the tank, the water at the top will be considerably hotter. For that reason, you don't want to max out this setting, but you will want to keep it pretty high.
The idea behind solar is: "get all you can, while you can". If the hi limit is too low, you end up wasting solar potential. And because a mixing valve is always part of a solar hot water system, your home is safe. The faucet temperatures in your domestic water system will be regulated down to the 125 degree range regardless of the storage tank temperature.
Because the evacuated tubes generate such high temperatures, they should be installed at the end of the solar circuit, effectively turning the flat plates into pre-heaters. Plumbing the tubes before the less efficient flat plates could actually cool the fluid coming from the tubes.
Due to the many variables inherent with solar, sizing a solar heating system isn't a simple matter. Latitude, solar orientation, budget, heat loss, type of collector, domestic hot water requirements, esthetics, and performance expectations are all factors needing careful consideration. With unlimited funds, a roof covered with collectors can provide 100% of all hot water needs. More realistically, a modest "starter" system consisting of two or more absorbers can still supply an important boost to the home's conventional heating system. The basic mechanical components (pumps, heat exchanger, controls, etc.) remain the same regardless of how many collectors may be added later.
A realistic, long term view is important with solar. Granted, the sun begins paying back the investment every time it strikes the collector. But sometimes when you really need the heat, it won't be there. Even Arizona, New Mexico, and Puerto Rico have cloudy periods. Even a full day of sun on the winter solstice will provide only a weak, short solar opportunity.
But during the spring, summer and fall, an abundance of energy will flood your collectors. Often during these periods, a modest number of collectors will provide 100% of all heating and domestic hot water needs. In the summer, it's unlikely that any amount of hot water usage could exceed the supply.
So, consider the above factors, discuss your heating needs with one of our technicians, and if solar seems like a viable option, Radiant Floor Company will happily design a system for you.