There are lots of words buzzing around about “new” hydronic heating technologies — radiant, water to air, solar thermal, geothermal and modcon boilers. Do you have to pick just one, or can you mix and match? Could you even use all of them together? Yes — you can do any or all of that. The question then becomes how and what are the advantages? Are any of them worth the trouble of learning the new technologies?

A quick review

Hydronic heating means using a fluid to transfer heat from the heat source to the space to be heated. Let's look at the technologies in question to see how they fit into the heating process. They might be transfer methods, or they might be heat sources. Transfer methods move heat that is created at heat sources. Radiant and water-to-air are heat transfer methods that move heat from the water to the living space. Solar thermal, geothermal and modcon boilers are heat sources. So, either radiant or water-to-air can be used to transfer the heat that is collected or created by solar thermal, geothermal or a modcon boiler.

Radiant usually means running tubing through or under a floor. Hot water heats the floor, and the floor heats the air in the living space. Heat moves from the water in the tubing to the air in the living space.

Water to air is short for water-to-air heat exchanger. A heat exchanger contains a coil that hot water moves through. At the same time, air is blown across the coil. The air is warmed when it picks up the heat from the water coil. The heated air is then distributed throughout the living space by heating ducts.

Solar thermal, geothermal and modcon boilers are all heat sources. Solar thermal simply means using the heat of the sun. The challenge is how to collect and store this free energy until it can be used for heating a living space. Here's where water comes into the picture again.

Typically, solar collector panels are used to collect the heat from the sun. Panels can be placed anywhere that the sun can regularly shine on them — typically they are mounted on a roof or on the ground. Piping goes through the panels, and through it runs a fluid, which we often call water, which is often a mixture of antifreeze, so it does not freeze. After the water is circulated through the solar collectors and becomes hot, it is often stored in storage tanks until it is needed to supply heat to the living space.

Geothermal works the same way, but with an additional step. About 95% of the solar energy that reaches the earth is stored in the ground. The result is that ground temperature below the frost line is a uniform and year-round 45°F-55°F. Compared to winter air temperatures, this is a heat source. A means is needed for retrieving that heat from the ground, and that is liquid circulated in piping called loops. This liquid is then put in storage tanks until it is needed for heating.

Even with these alternative heat sources, a standard boiler is almost always required for supplemental or backup heating. “Supplemental” usually means that it is intended for frequent use. Water of 55°F from the ground is not enough to be a sole source of heat. But, heating 55°F water takes less boiler energy than heating cooler water.

A modcon boiler is a modulating condensing boiler. Modulating refers to the type of gas valve. Most gas valves are like a light switch — they are either on or off with nothing in between. Modulating valves change their output and gas consumption based on the demand for heat. So, when there is a call for just a little heat, they produce just a little, using just a little gas.

The term condensing applies to what the boiler does with flue gases. In older technology boilers, flue gases must be kept hot enough to rise up the flue. These gases contain products of combustion that if cooled will corrode the inside of conventional boilers. However, letting them escape when they are that hot is a waste of heat and energy. Condensing boilers can withstand those products of combustion, therefore, increasing efficiency by allowing little heat to escape up the flue.

System requirements

The requirement for using any of these technologies together is for the system designer and the installer to understand what they are doing. More than ever it's essential to know what the energy requirements are for the various parts of the building. This is determined by performing a simple yet extensive analysis called a load calculation.

Once the building's heating needs are determined, it's a matter of identifying the capacity of various technologies to address the needs. For example, a high heat-loss area such as a room with three outside walls over a garage might require a high-temperature heat source, so solar thermal and geothermal alone would not be adequate. But, in-floor radiant heating with the right water temperature could address the problem of cold floors.

Sizing the heat source remains the same challenge as always. How many Btus do you need for that area, and what size equipment does it take to get that?

Regardless of the technologies you choose, you can count on increased energy efficiency with all of these technologies. Add them together, and you'll have the efficiency of the total project.

Carol Fey is a technical trainer and the author of books and a DVD on electricity, troubleshooting and controls. You can e-mail her at or visit her website at You can also visit her blog at: