An example of hydronic radiant heat from Warmboard -- heated water snakes through the tubes.
Radiant Heat Options
Almost every one of my clients has at least asked me about radiant heat, and most projects where we add heat involve radiant. In this post I will describe different options for radiant heating, but first it will help to understand other common types of heating.
With heating it is important to distinguish between the type of heat transfer and the fuel used to produce the heat. Someone will say they heat with gas, but this does not describe the way the heat warms the home, just that gas is burned to do it. For instance, natural gas or propane can be burned in a boiler that heats hot water on its way to radiators, or it can be used to heat air that is delivered to rooms via ductwork. It is important information, it just doesn't tell you everything you need to know, you also need to name the system type.
The most common type of heater in the U.S. is forced air. In this system you'll find register grilles in all rooms, and at least one large return. The return is a grille, usually on the first floor, that allows the air to circulate through the system. Since cold air sinks, it helps to have it low.
Radiators can be heated with steam, hot water or electricity. The radiator heats the air around it which then rises and circulates air through the room. There is less air movement in this type of system than forced air, but the method of heat transfer is still primarily one of convection. Moving air distributes dust and allergens, which is one of the reasons why people are more and more interested in radiant heat.
Though you can heat walls or ceilings, the most common form of radiant heat involves heating the floor. Just like the sun warms the earth through radiation, a radiant heated floor delivers heat to objects in the room by giving off waves. Ideally, right above the heat source will be a flooring material with a lot of thermal mass, like concrete or tile. These materials absorb the heat and release it slowly over time. If the floor is heated, heat will also be transferred through conduction, straight to your feet, and if your feet feel warm, you do not need to heat the room to as high a temperature. The main difference between a heated floor and a hot radiator in a room is that the whole floor is heated (except under cabinets, toilets, etc) so the temperature of the floor doesn't need to be as hot as the radiator. These factors combine to make radiant an efficient choice in general. Remember heat rises, so starting as low in a room as possible is a good idea.Electric Systems
The question of efficiency gets us back to the energy converted to heat that we started this discussion with, and it also brings us to the main difference between radiant systems. Radiant floors are either electric or hydronic (water). In general, a hydronic system will be more efficient, but an electric system involves lower up front cost, especially if the area heated is small, like a bathroom for instance. Each room with electric radiant has its own thermostat, and with greater control you can save money. I had one client whose home was so well insulated that he only heated the bathroom floor with electric radiant and didn't use his other heat through the Philadelphia winter.
There are different ways to install an electric system, but ultimately they all come down to a wire that snakes back and forth and gives off heat. It will need to be embedded in thinset, the material used to set tile. Some come in pre-made mats, sometimes the wire is routed and staples to the subfloor by the contractor. If you are going to be heating more than a couple rooms though, I do not recommend electric. It makes sense to invest in a hydronic system.Hydronic System
In a hydronic system, a boiler heats water to a temperature lower than that used in radiators - so if you want to combine radiant and radiator heat you will need to have a mixing valve that mixes some cold water in with the hot radiator water. In a hydronic system the heat snakes back and forth under the flooring in PEX plastic piping. It may be embedded in concrete, stapled up under the subfloor in between the floor joists or snapped into special sub-flooring (like Warmboard, as shown in the photo at the top of this post) with grooves. Though tile is best, it is possible to install hardwood flooring over radiant, just be sure to check with the manufacturer to make sure you are not voiding the warranty. One boiler can heat the water for multiple zones, and it is best to make remote parts of the house and separate floors separate zones, each with its own programmable thermostat. To get a house up to temperature will depend on how much of a difference in temperature is needed, but once you are heating the house, you should not notice spikes or drops in temperature. By the time the heat held in the floor has started to dissipate, more hot water will be circulating in the floor to keep it at temperature.
To sum up:
If you're looking for a bit of luxury, heat your bathroom floor with electric radiant.
If you're building in an area that has a good number of heating days and your flooring choices are compatible with it, use a zoned hydronic system for the whole house and you'll have efficient, even heating. Everyone I've installed radiant for loves it, and that's not something I've ever heard someone say about forced air heat.