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The best method of heat supply on the planet
By Tibor Kovacs
Our human comfort relies just as much on radiant heat transfer as it does on air temperature, yet the majority of heating and air-conditioning professionals think only in terms of air temperature. As a result, we are missing out on a truly comfortable living environment in our own homes or places of business. By controlling both the air temperature and the radiant transfer, radiant panel systems (hydronics) deliver a comfort that is unsurpassed.
The concept of circulating hot water for heat, as opposed to forced air systems, is becoming increasingly popular. In a forced air system we gather a lot of air, blow it through a fire box and use large diameter metal channels to distribute the hot air everywhere in the building. Air, however, is a very light substance with no thermal mass to it. What this means is that it carries a small amount of heat energy and loses it very quickly. In order to carry substantial heat around this way we need to move a lot of air, which requires a fan that may generate noise, through a distribution system (ductwork). The air may cool substantially before it gets to where it was intended to go. Then we recirculate it and heat it up again. The result is lots of hot air when the system is on followed by a cool down when it is not. The obvious result is temperature fluctuations.
In radiant hydronic systems water is run through a boiler and small diameter piping distributes heat throughout the building. Water has the best capability to absorb and move heat. It soaks up the heat, holds on to it and releases it slowly to the floor. The surface warms up and radiates in the room as soon as there is the smallest temperature difference. To sum up, hydronic systems use less transfer material, provide even heat distribution (due to the thermal mass) and reduce system noise.
Floor heating creates a temperature profile where the warmest part is the floor and the coolest is the ceiling. The vertical temperature distribution is practically even, with the exception of the few inches above the floor surface, which may be warmer. Remember, radiant heating does not heat the air. Heat transfer only happens when the radiant heat waves encounter dense material. This heating method heats up objects such as walls, furniture and the area’s occupants.
It is a direct heat transfer to the body, similar to the sun shining on you in winter, so we are not reliant on the surrounding air temperature. Because the walls are warm there is less heat drawn out from the body. Our feeling of warmth is mostly due to the constant low-density heat radiation with minimal changes. The temperature fluctuation in a radiantly heated room is about 0.5 degrees. Occupants get used to the constant conditions very quickly.
After the water is heated by the boiler, or geothermal heat pump, it is circulated through the piping to heat emitters. Today the most popular methods of radiant heat emitters are baseboards, radiators and radiant floors, walls and even ceilings. There are a number of differences in the technologies.
Radiant baseboard is commonly a length of metal tubing with fins on it. Hot water runs through it, the metal heats up and transfers the energy to the air around it. The air heats up and starts travelling up, cooler air flows in then heats up, and so on. The main effect is the high temperature tube heats up the air and creates warm convective air currents, which eventually fill the room. There is not a lot of direct radiation coming off it.
Radiators, as the name implies, are the real thing. The hot water flowing through the radiator heats up the body of the radiator and that in turn radiates heat into the room. The surface area of a radiator is much larger than the baseboard and this creates the difference. The air around the unit heats up and starts rising and creates convective currents distributing the heat. There are two main heat transfers here, direct radiation of the surface and convection heating using the air. The main improvement is more even, stable heat distribution. The size and thermal mass of the radiator stores the heat and releases it slowly, evening out the distribution.
In-Floor delivers an even, continuous heat supply. As radiation is a direct function of the surface area it makes sense that the bigger the warmed surface the more heat we get out of it. Compared to the other two delivery methods the surface area available is large. In order to get the same interior warmth, much lower surface temperatures are required to the extent that the floor surface temperature varies between 21C to 29C, most commonly 22C to 23C. This is not a hot floor but is all we need to heat the space to our comfortable temperature. Sometimes it is barely warm, but a properly designed and operating system is never cold. A heated floor normally feels neutral. Its surface temperature is usually less than our body temperature, although the overall sensation is one of comfort. The exception is when maximum heat output is required and the floor may actually feel warm.
Low heat density and warm surfaces, two unique features of radiant floor heating, are two important factors in creating human comfort. The third is even, constant temperature. Lots of research has been done trying to identify the conditions in which the human body feels the most comfortable. We like to be surrounded by warm surfaces that do not draw heat away from us and we love even and constant temperature.
Top Five Customer Questions
1. Can you heat the entire house with radiant floor?
Yes you can and you should. This is the best way of creating maximum comfort.
2. What if the pipe breaks?
The pipe does not break on its own. It only happens if somebody damages the pipe. When that happens a qualified installer should repair the damage.
3. Why is it more efficient?
There are a number of factors adding up to the increased efficiency. Radiant heat does not overheat the air, so the heat loss through the building envelope is less. Radiant heat provides direct heat input into the human body, so the air temperature is less important.
Usually lower thermostat settings result in the same level of comfort.
4. What fuel source can be used?
Any fuel that is available may be used, including natural gas, propane, oil, wood, and electricity. The systems are also well suited to geothermal applications.
5. What happens to cooling?
Ductless or mini split air conditioning systems are becoming increasingly popular. In some situations, they offer the opportunity to “zone cool” resulting in an efficient use of energy.
To see more advanced technical articles related to hydronic heating, please go to www.hpacmag.com and choose “hydronics” in the “search by topic” field.
Tibor Kovacs is the owner of Hydronic Panel Systems/Hydronic Comfort Systems Inc. He can be reached at firstname.lastname@example.org.