Why is in-floor heating described as 'radiant' in the US? Surely the heat transfer to the air is by convection?
Pete
Terrific question Pete and
thank you so very much for asking. A favorite subject of mine!
Now we can get into a discussion involving physics. Chris is going to go wild with that on
"The Test"!!!!
Radiant vrs Convection Heat
A kilowatt of radiant heat and a kilowatt of convection heat do not have the same heat transfer properties. While both take a kilowatt of energy to produce, their heat transfer properties are markedly different, implying dramatic differences in the amount of heat capacity you need to install and the length of time you need to run them. So as far as heating your home, office or work space goes, there is a considerable energy difference at stake.
Convection:
Convection is the transfer of heat by movement of liquids or gases. Its properties are:
* Convection currents naturally rise as the hot medium (air in this case) expands and decreases in density and as the cool air increases in density and sinks. Convection in a central heating context therefore implies warm air rising to the ceiling and then circulating gradually to lower levels in the room, being at its coldest near the floor.
*This air movement cannot be controlled and heat transfer always works from hot to cold which you cannot control in the air. If a door to a cold corridor is opened, drafts exist, etc, the warm air will naturally flow there.
*You cannot feel a convection current to the side of a convecting surface (any heat you feel would be radiant) only above it.
*Air absorbs heat poorly and transfers it back out to other objects poorly.
*Air is not “zone-able” and rapidly cools when the thermostat switches off (i.e. the heater is only effective when on).
Radiant Heat:
Radiant heat has
considerable advantages over convected heat for comfort heating because:
*Radiant heat has a higher “flux” (watts output per meter per degree centigrade of the heater) than convection heating. You require less radiant heat for less time to do the job of more convected heat for more time.
* A Radiant heater
directly heats objects in an environment,
not the air in between. So you are heating surface area of objects in an environment which warm up and turn the environment into a 360° radiator. This effect is felt less with heated air which transfers heat into objects poorly.
* Objects retain heat better than air, so residual energy maintains temperature in the environment for longer e.g. if a door is opened to a colder room, or when the thermostat turns off the heat source.
*You can manage the heater by a thermostat set to a lower air temperature because it is the environment that heats up first, not the air.
Different Effects of the Same Kilowatt :
You'd think that is “A kilowatt of heat is a kilowatt of heat: you can’t get more out of one kilowatt from one heater than you can out of another”, wouldn't you?
A kilowatt of heat – while indeed being a kilowatt of energy from whatever source it emanates – does not mean the same physical heating effect occurs regardless of the type of heat transfer method. Thinking about a 2kW oven versus an 800W microwave should make this instantly apparent. It’s all a question of what you’re wanting that kilowatt to achieve and employing the correct type of heat for the correct function.
What Does It All Mean?
You can get very different physical properties out of heaters each rated 1kW. But some heaters are evidently optimal in certain situations where others are not. And therefore use of that kilowatt really depends what you want it for. Miss-application results in an inefficiency in the use of that energy. As much as heaters that are too hot for a particular application represent wasted energy, heaters that are too cool also represent wasted energy because you need more of them. What you really want is the optimum heater for the correct application. In terms of comfort heating a room, when comparing one kilowatt of Far Infrared** against one kilowatt of Convection, you can experience up to say, a 60% difference in the heated area and running times using 1 kilowatt of radiant heat versus 1 kilowatt of convection.
**Far infrared (FIR) is a region in the infrared spectrum of electromagnetic radiation. Far infrared is often defined as any radiation with a wavelength of 15 micrometers (µm) to 1 mm (corresponding to a range of about 20 THz to 300 GHz), which places far infrared radiation within the CIE IR-B and IR-C bands.
Bottom Line:
Radiant heat, heats objects not air and it just feels good. It's as simple as that.
Thanks again Pete, I hope that was helpful.
Thomas
BTW...
... Count Rumford figured some of this out 3 centuries ago and employed it with his Rumford fireplace design. That's one of the reasons I used it in my fireplace. It relies more on radiant heat not convective heat.
