Radiant heating (PEX tubing embedded in the floor), solar heat, and high efficiency modulating/condensing boilers are the perfect combination to provide the highest efficiency heating systems available at a reasonable cost. Radiant heating also happens to provide the absolute highest comfort available in a modern heating system. In other words, there are no "tradeoffs" involved between comfort and utility costs for these systems. An explanation for why this is the case follows.

Radiant floor systems operate on the principle of a large radiant area (the floor surface) being heated to a comfortable temperature of approximately 80 degrees, give or take a few degrees. A heating fluid temperature of approximately 85-120 degrees is needed, depending on building heat load, flooring materials, etc. Given the large radiant area, this lower temperature is adequate to supply the heat load. Other types of heat supply devices, such as radiators, baseboard, and fan coils, require the heating fluid to be supplied at a much higher temperature, say, 160 - 180 degrees. Since the fluid temperature is much lower than for radiators/baseboard, radiant is a great match for the lower temperatures provided by solar and "modulating/condensing" (mod/con) boilers.

Here's why mod/con boilers are so much more efficient: A traditional cast iron boiler operates with an output temperature of approximately 165-185 degrees. This temperature is required for two reasons. First, the heat radiating devices require the high temperatures because of their relatively small area (as compared with a large radiant floor surface). Secondly, and less well understood, the RETURN temperature from the radiator/baseboard to the boiler is kept above 140 degrees to prevent condensation of flue gases (the byproduct of natural gas/propane combustion) on the exterior of the cast iron or steel heat exchanger. If condensation of flue gases were to occur, then the heat exchanger would rust out quickly, thus destroying the boiler. So what we have with a traditional boiler (and why it's not such a good match for radiant systems or solar), is a firing range of 165-180 degrees, and the consequent higher usage of fuel. If used with a radiant floor system, the conventional boiler's high temperature output must be "mixed down" (mixed with cool return water from the radiant floor) to the desired range of 85-120 degrees. This is not the most efficient use of fuel.

By contrast, the mod/con boiler acheives it's high efficiency as follows: First (the "con" part), it fires in a much lower temperature range. They can be programmed to fire from 50 to 185 degrees, but generally fire from 85-125 degrees (the perfect range for the radiant floor system). When firing in this range (below 130 degrees), the cool returning fluid from the radiant floor intentionally causes flue gas condenstation on the heat exchanger. In the case of the mod/con boiler, the heat exchanger is made of stainless steel (not cast iron), and the condensate drains out of the boiler into a special condensate drain. The nice thing about this condensation is that the heat exchanger thus captures the "latent heat of vaporization" from the condensation, and extracts approximately 10% more heat from the flue gasses before they exit the exhaust! This is why effciencies in the 90 percent range are obtainable from mod/con boilers (when they are configured correctly!). Secondly (the "mod" part), these boilers "modulate" the burner to match the seasonal heat load. If it's fairly warm outside, say 40 degrees, the boiler will fire at a lower rate than if it's 10 degrees. Operational efficiency of mod/con boilers can be increased by designing the radiant floor so that it requires the lowest temperatures possible to still satisfy the given heat load. That is why an in-depth heat analysis must be performed to provide for optimal efficiency.

The solar storage tank also fits well in the radiant heating scenario, since it "operates" in the range of, say, 90-170 degrees. Regardless of how high the temperature in the tank rises, it can be "mixed down" to the required range before being circulated through the floor. If, for instance, the solar tank reaches 160 degrees, the radiant mixing device will "moderate " the temperature (by mixing it with cooler fluid returning from the floor) so that it will be , say, 115 degrees as it is delivered to the floor. As the tank temperature is drawn down (loses heat), a fairly constant heat supply can thus be provided to the floor. Heat delivery occurs until such time as the usable heat is depleted from the tank, and the boiler automatically kicks in to provide the balance of the required heat load. The highest efficiency can be obtained from solar storage if the temperature of the tank can be drawn down as low as possible. In other words, if as much heat as possible can be drawn from the tank as possible before the heating system switches over to the boiler. Although this makes common sense; perhaps less well understood is that, if the tank is as cool as possible the next morning when the sun starts shining again, it will "grab" more heat from the collectors than a warmer tank would. Another way of saying this: the cooler the fluid temperatures going into the solar thermal collector, the higher the collection efficiency.

Although radiant floor is the best match for solar and high efficiency boilers, there are gradations in radiant floor design that should be discussed: First of all, the type of finish floor placed on top of the radiant floor has a very marked effect upon the supply temperature. This is significant because any increase in the supply temperature decreases the efficiency of both the solar and mod/con boiler subsystems. To some extent, this can be avoided by proper choice of finish surfaces. As with anything other technical issue, proper design choices can maximize efficiency in a given situation.

From the efficiency standpoint, a poured concrete floor offers the best efficiency for a radiant panel installation. The reason is that concrete (either thin slab or high mass) has a good heat transfer capability, and transmits the heat directly into the room. Concrete floors can be stained and stamped to produce an aesthetically pleasing surface. Poured gypsum (with a tile or other masonry covering) is a close second in efficiency, and has it's own advantages in some situations (especially when retrofitting in older houses). If a wood floor is contemplated, a thin engineered flooring, such as bamboo or hardwood, won't impede heat flow as much as as, say, a 3/4" tongue and groove floor. When using a wood flooring, heat spreader plates can be used to assist heat tranfer from the pex through the wood flooring. Of course, carpeting/carpet padding have the highest "R" value of all of the flooring surfaces discussed so far, and will require the highest supply temperatures. Consider throw rugs instead of wall-to-wall carpeting!