RADIANT SPACE HEATING

"Winter grey and falling rain, we'll see summer come again...gonna happen every time."---Grateful Dead, "Weather Report Suite: Part One" (1974)

But for now we need heat.

For purposes of solar, when we talk about radiant space heating we are refering 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 yet read the Hot Water Heating page you may want to take a look at it first because much of its content applies to solar radiant space heating as well. For the most part, the only real difference between solar domestic hot water and solar radiant is that with the latter there's just more of it--more collector panels and more water storage tanks.

If you are contemplating a solar radiant system the place to start is to first budget money for a well insulated scructure. If you are thinking of installing a radiant sytem into a drafty, poorly insulated structure with old leaky windows you are better advised to first spend your money to get your structure as well insulated and airtight as possible. That is where you will always get the most bang for your buck. After that we can think about solar space heating.

How much of the space heating is a practical goal for solar? With domestic hot water we said that 70% of the annual requirement is realistically achievable in Southwestern Colorado. About 50% annually for retrofits and on up to 70% for new construction planned out in advance is probably realistic for space heating. It would be rare to provide 100% because there are too many cloudy days during the winter making it impractical to have sufficient heat storage (water tanks) to do the job. The dead of winter also has the shortest time periods for accumulating solar heat, but as we move away from the center point of winter 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 winter day, we would have too much heat at other times and seriously overheat some components of the system. 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. It should be noted here that there are two types of solar thermal systems for freezing climates. One is called Drainback and the other is an Antifreeze system employing a mixture of water and propylene glycol. The overheating issues discussed here only apply to the latter, so why not use a Drainback system and go all the way for 100% of the heating requirement? It is a matter of diminishing returns. If it costs "X" money to achieve 70%, it could easily cost twice that or more to get to 100%, not good economics.

Can a solar radiant and domestic hot water sytem be combined? They most certainly can and are sometimes called combisystems. 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. Then the integration of the two involves a few minor components. The solar section is, of course, installed by the solar company. There are even some companies, albeit just a few, that install it all. Solar Today and Tomorrow does the solar loop only. Obviously the boiler, or a hot water tank in some instances, is the electric, natural gas, or propane backup for the portion of the heat that the solar loop does not provide.

What does a solar radiant system cost? This is not an easy question to answer--maybe two to four times the cost of a simple domestic hot water system as discussed on the Hot Water Heating page with a median cost of $7500. There are several variables, the most important being your heat loss in btus, space available for heat storage and collectors, and finally your budget. After your site analysis, your solar technician will then plug the necessary data into a software spreadsheet and show you the possibilities. The 30% federal tax credit capped at $2000 also applies to residential solar radiant just as it does to solar domestic hot water. With commercial there is no cap and MACR depreciation over 5 years also applies.

What if I don't have enough room on a southerly 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 (below frostline), insulated piping is installed, and the heated fluid is piped back to your home. While a roof mount is ideal, ground mounts are more common than you might think. I have even seem them on Aspen--Snowmass McMansion properties.

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. 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 are also 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, currently (2008) 4.4 cents 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. If you think back to the Hot Water Heating page and to its cost analysis where the regular electric heating rate of 10.1 cents is used, then an electric heating rate of 4.4 cents will seem pretty good. But it's a little more complicated than this because everything else in the home will probably be powered at the on peak rate of 14.6 cents (4.5 cents higher than the 10.1 cents charged to those who don't opt into the on peak, off peak time of use program). This means that about 44% of the total energy consumption has to be at the off peak rate to break even with those who don't go with the time of use program. If the ETS unit's energy consumption is above 44%, then the cost of the electricity is below the 10.1 cent normal rate. To estimate the actual percentage would require looking at someone's utility bills who has a large ETS unit for a home comparable to yours. The off peak percentage might be as high as 85% in Jan. and 5% in July. If the off peak percentage were as high as 67% (2/3) of the annual total, this is an average cost of 7.77 cents for on and off peak combined compared to 10.1 cents per KWH. For a house 2000 sf to 2500 sf the ETS unit 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, subtract the $10,000 cost of the ETS unit to get $10,000. Now subtract the $2000 federal solar thermal tax credit to arrive at $8000. If you followed the math on the Hot Water Heating page you will realize that even with our generous allowance of 67% for ETS off peak energy consumption for an average cost of 7.77 cents per KWH that the solar system would easily save $1000 and possibly much more in electricity costs per year. Now divide $8000 by $1000 to get 8 years for the "payback" time. Additionally there is that pesky 5% annual inflationary increase compounded for the cost of electricity that is going to knock the payback down to 7 years. Then there is the 20 to 1 ratio discussed on the Hot Water Heating page that increases your home value by $20,000 ($1000 minimum saved energy per year times 20), completely knocking the payback period down to zero. In our math we have been conservative with solar and generous with ETS. Clearly solar radiant is the better deal.

Below is a photograph of a ground mount, evacuated tube system provided by its manufacturer. It is 16 tubes, generally the ideal number for a domestic hot water, family of four application. More evacuated tubes would be required for radiant heating.