RADIANT SPACE HEATING

For purposes of solar, when we talk about radiant space heating we are referring to using solar energy to heat the fluid running through the in-floor tubing of a radiant heated structure. It can also apply to radiant wall panels, free standing radiators, or radiant wall baseboard units. If you haven't looked at the Hot Water Heating page you may want to take a look at it because much of its content applies to solar radiant space heating as well. The one big difference between solar domestic hot water and solar radiant is that with the latter there's more of it--more collector panels and more water storage tanks.

How much of the space heating is a practical goal for solar? With domestic hot water we said that 70 to 80% of the annual requirement is realistically achievable in Southwestern Colorado. Up to 50% annually for retrofits and on up to 70% for new construction planned out in advance is probably realistic for space heating. It is not practical to provide 100%. The dead of winter has the shortest days for accumulating solar heat, but as we move away from the winter solstice toward spring and fall the days are progressively longer and warmer with solar radiant providing closer to 100% of the heat demand. If we were to size a solar system to provide 100% of the heat on the coldest and shortest winter days we would have too much heat at other times. If we size the system to provide the heat for a sunny average winter day, then over the length of the entire heating season our solar system will have still provided a significant amount of the heating requirement without overheating the system.

Can a solar radiant and domestic hot water sytem be combined? They most certainly can and are sometimes called combi or hybrid systems. The two combined should be cheaper than the two summed separately, a good reason to do both.

Can a solar radiant system only be installed with new construction? No, if you already have a radiant heating system you can retrofit the solar portion of the system. Think of solar radiant heating as having two sections. One is the part installed by the plumbing or hvac company with its typical boiler, floor tubing, zone valves and manifolds, etc. The other section is the solar loop with its collectors, storage tanks and heat exchanger. The integration of the two is the most important and usually complex part of the job. The solar section is, of course, installed by the solar company. If its a remodel or new construction we at Solar Today and Tomorrow can install both sides of the system and will gladly bid the radiant side of the job if it includes a solar system. We especially like the radiant designs by Caleffi since they are so efficient, economical, and compatible with solar. Our first choice in controls for the interface or integration of the solar and radiant sytem is to use Tekmar products since the Tekmar 363 control device is well suited to optimizing solar radiant systems.

What does a solar radiant system cost? maybe two to three times the cost of a simple domestic hot water system as discussed on the Hot Water Heating page with a median cost of $9500. It's like buying a new car, the big difference being that this "car" earns or saves a lot of money over time. There are several variables that determine the cost, the most important being your home's heat loss in btus, space available for heat storage and collectors, and finally your budget. After your site analysis, we will work up a bid/proposal along with a financial analysis. The 30% federal tax credit also applies to residential solar radiant just as it does to solar domestic hot water. With commercial there is also the MACR depreciation over 5 years for a total tax equivalency of about 50% off the cost.

What if I don't have enough room on a south facing roof for all the solar collectors needed to do the job? If you have a suitable location on your property you might want to consider a ground mount. In our climate the structure should be elevated on concrete columns a suitable height above snow accumulation (2 or 3 feet) and if damage from deer, elk, or cattle is a possibility then higher yet (maybe 4 or 5 feet). An underground trench is dug, insulated piping is installed, and the heated fluid is piped back to your home. While a roof mount might seem ideal, some roofs and attics make for such a challenging installation that a ground mount is actually cheaper and easier to install. Snow can also be quickly and easily removed after a snowfall.

Are all solar thermal collectors the same? No, there are two types. One type is the flat-plate collectors that look like roof skylights. Evacuated tube collectors are the other type. These are cylindrical tubes of glass with the air removed to create a vacuum around the heat absorber, a metal plate and tube running down the center of the glass tube. Because of the vacuum very little heat is lost from the absorber back into the atmosphere as is the case with the flat-plate collectors. Primarily for this reason evacuated tubes are more efficient than flat-plate collectors during the winter and might be the better choice for radiant heating. They can also be used for solar domestic hot water. There are pros and cons to both and we would be pleased to discuss these with you if you want to learn more.

How does solar radiant compare with ETS (Electric Thermal Storage) heating units? These are popular in this area and are available through LPEA. They have bricks in them that are heated with electricity at LPEA's off peak rate under its TOU (time of use) program, currently (2010) 4.4¢ per KWH. The bricks then become the heat storage material as opposed to solar radiant's water in a tank. The heated bricks can then exchange their greater heat with the heat of the fluid in the radiant tubing to keep it at the desired temperature. Since the regular electric rate is 10.4¢, then an electric heating rate of 4.4¢ will seem pretty good. But it's a little more complicated than this because many other electrical activities in the home will probably be powered at the on peak rate of 14.6¢ (4¢ higher than the 10.4¢ charged to those who don't opt into the TOU program). This means that about 44% of the total energy consumption has to be at the off peak rate just to break even with those who don't go with the time of use program. So it's really about the average cost per kwh. An off peak percentage of 67% of the annual total is an average cost of 7.77 cents per kwh for TOU compared to 10.6 cents per kwh. For a house 2000 sf to 2500 sf a large ETS unit hooked up to the radiant system can cost $10,000 installed. How does this compare to the cost of a solar radiant system? If you had a combisystem (domestic hot water and solar radiant combined) that cost $20,000, the difference is $10,000. Now subtract the 30% federal solar thermal tax credit to arrive at $7000 (the actual up front additional cost of solar over ETS). With the ETS unit you now pay out 7.77¢ for every single kwh while the solar system could easily be saving you $1000 a year! In less than 7 years the price difference in the two systems would be paid for with solar system savings and now its free money from then on with the solar system. However, with the ETS units after year 7, you still just pay and pay albeit at a lower rate. Also because of the greater annual savings with solar over ETS the home equity increase with solar is greater. Clearly solar radiant is the better choice if its a solar suitable property.