With a tea light and an inverted flower pot, you can heat a (small) room. That’s what a multitude of sites and videos claim. The Californian Doyle Doss sells his Kandle Heeter (heater) even as a kit.
My father in law knew the principle, and claimed years ago that formerly the kitchen was heated by placing an inverted flower pot on the gas stove. (Yes, the fire had to be lit.) At the time I thought a boiler with water would be more efficient, but I never used the idee. (This is true for heat storage, not for the (instantaneous) utilization.)

bloempotkachelIt seemed to me that a candle would heat as much, whether or not 1, 2 or 3 flower pots were hung over it. But this turns out to be not quite right. Scientific studies on this apparently are still missing, but from various forums, blogs and sites some wisdom can be extracted. What kind of heat (transfer) do we discus?

Heat is a form of energy exchange between systems that are not in thermal equilibrium.
Heat transfer takes place by conduction, convection and / or radiation.

Conduction (guidance through direct contact): energy transportation from high- energy particles (which itself stand motionless) to particles with low energy. In solids (walls..) heat transfer occurs only by conduction.
It also occurs in liquids and gases. The heat flux is dependent on the thickness of the object, the coefficient of thermal conductivity (λ) and the temperature difference. Applications: iron, cold hands around a warm mug, hand warmers, a Hot Water Bottle, a pan on a stove plate...

Convection (meaning "move together", flow) : energy transport of solids to liquids or gases that are flowing more or less in the same direction along them. There is natural convection, but also forced convection (fan, bown pipes). The convection process consists among other things of conduction.
Heat does not rise, warm air does. Heat air, as opposed to conduction and radiant heat, is an indirect form of heat transfer through the air as a transporter. Air is a poor conductor, and will need to warm up quit a time. (We even use stationery air as an insulator!)
The heat flow depends on gas properties (thermal conductivity, specific heat, density, viscosity), volume, temperature difference (dT), speed of the medium, and above all the heat transfer coefficient.

Benefits compared to radiant heat are that the space is heated faster and more uniform, and the temperature differences in the room are smaller.
An air heating system is almost entirely based on convection.
This requires high hot water temperature and ditto energy needed.
In radiators the rate at which heat is released is depending on the surface that is in contact with the air flow. Convectors have blades as cooling fins on the pipes in order to increase the contact surface, so that many convection occurs there.

Just like in a sauna room temperature rise with the height. Between floor and ceiling is 10°C difference. So you get a hothead and yet cold feet.

Radiation: energy transfer by electromagnetic radiation without ((in) direct) contact.
On a sunny spring day with 15°C it is pretty warm in the sun, and immediately cold as a cloud slides infront. The air temperature is however not suddenly plummeted. Especially water vapor blocks infrared light. You feel that clouds block the calorific IR rays.

The heat flux dependents on gas properties, temperature difference, temperature, emissivity and surface area.

Radiation with a high frequency is associated with a short wavelength and high energy, and temperature. Radiation is classified in groups (from high to low): gamma, x - rays, UV, visible light, infrared, microwave, radio waves. People only see a small part (without tools). Some animals can sens UV or IR.

Heat is thermal radiation, caused by the movement of molecules and atoms. Every object above absolute zero (-273.15° K, then there is no atomic / molecular motion more) emits infrared. Glowing charcoal more than an ice cube. Infrared Optics make this visible.
The more atomic and molecular motion, the higher the temperature and the more infrared radiation there is.

In addition to transfer heat through infrared radiation: ultraviolet, microwaves and radio waves / induction also is possible. UV energy is richer and hotter. Also more dangerous. Our skin may turn red or burn through much UV. (Infrared contains no UV.)

An infrared sauna is actually not just a sauna but a (radiant) heat cabin. The infrared spectrum is divided into three areas : short (IR -A, 0.76 and 1.5 microns), medium and long wave (IR - C, 5.6 to 1000 microns). The principle remains: high T° = short wavelengths. Infrared heat penetrates deeper into the skin than warm air. So you sweat more intense and at a lower temperature than a traditional sauna.

Radiant heat is direct heating whose energy is converted into heat when the electromagnetic waves are falling on a surface. This heat moves independent of air movement.
Because there is not first (like convection systems and water) heated air for heat transfer, radiation can heat with less energy more effective and with less wasted heat to stratification (layering).

Radiant heat gives the same comfortable feeling as an air temperature of conventional systems at 3°C higher. The experienced temperature feels (3°C) higher.
It would be healthier than convection heat because you have less hot air and breathe less dust.
So even better for people with sensitive airways or asthma.

Floor, wall and ceiling heating primarily use radiant heat in which the heating water works at a lower temperature regime. There is a lower boiler room temperature and it requires less distribution losses and energy consumption, but a slower operation (time) than in convection devices.
We know the principle of (soap) stone stoves. The strength of the heat radiation decreases with the distance.

The return is positively influenced: much less heat is stored in the room air, and therefore less lost in high altitudes and ventilation. The difference in temperature between floor and ceiling is usually only 2 to 4°C.
Heat transfer can also be caused by evaporation and condensation. This condensation causes local heat. Through evaporation, heat is extracted. (The principle that keeps the food cool in the ‘zeer’.)

The radiant heat from a campfire, a winter fire pit or barbecue are delicious, but little comfortable as your back freezes. For rooms are convection and radiation (preferably uniformly) combined.
Convection heaters produce a minimum of 50% of hot air, radiation devices more than 50% radiation heat. This is easily recognizable in electric heaters : with fan or red glowing heaters.

The air temperature in a room rise 3°C per meter. To watch TV so you ‘d better put the seat 1.5 meters higher (and the thermostat lower).

Natural convection is not bringing warm air where you want it. Put a Ecofan on your stove and make so without additional power a desired (or forced) convection.
You can preheat combustion air. Or with a fan propel warm air from the ceiling down.

Thermal conductivity is the ability of a material to transmit the (added) heat. Especially metal conduct good. From less to more: lead, steel, iron, nickel, zinc, aluminum, gold, copper. Copper is a good choice for soldering and pans. (Copper oxide is toxic, so clean it well!)
Use boilers with a thick bottom, and a stove with a thick, preferably cast iron plate. On a warped plate is much less contact, and therefore less of heat conduction.

It makes little sense to produce optimum heat and spread, if you then let it go to waste.
To insulate conduction heat use bad conductors: air, textiles, cork, glass, wood, straw, plastic, bone and wool, felt.
To insulate radiant heat you use reflective barriers, such as aluminum foil behind a radiator, stainless steel mirror behind a heating element.
With convection heat you can also use bad conductors in which much air is trapped between as insulation. (Fibrous) clothing in layers, double glazing, bubble wrap, snow, newspapers...
This works better as the medium and the encased gas are dry. Otherwise conduction occers, and even cooling by evaporation.

Of course, it is also important to control the air circulation. Drafts, ventilation and opening doors cool the air a lot. But since combustion also consumes oxygen ventilation is absolutely necessary. You can connect the burner with a supply pipe directly to outdoor air, so that no (or less) is sucked through cracks across the room. Many modern boilers have in or against the flue also an input tube. Additional benefit : the combustion air is preheated a little. You can also supply air preheating in a spiral behind (or under) the stove.
A pelmet (almost closed box, corniche) above the curtains can prevent (or reduce) a cold air in front of the window.
An (little) entrance hall, airlock or curtain behind the door reduces the intake of cold air. Door Brushes and weather strips can close gaps.

Tea lights
Can we come with good insulation and efficient heating through the winter with a tealight stove? You will need to have a passive house.
A tealight works great for warming plates on the table, a teapot, and even a (preheated) cheese- or chocolate fondue.

A candle produces, depending on composition, wick, speed of combustion, metrology (mass consumed, delivered heat ..) 40 to 77 W (+ or minus 9W) of which a small portion is converted into light. It contains 14g paraffin with a calorific value of 46 MJ / kg. This yields 0.64 MJ / candle (4 hours at 100% combustion efficiency). For convenience, we assume 60W of heat.
Four candles deliver about 240 watts. An electric heater usually has 2000 watts. If you wantto emulate the warmth of 1000 Watts would therefore 16.6 candles be needed. Seems feasible to keep a small room to temperature, but not to warm it.

Candles with 70 grams of parafin will use 150 liters of oxygen, and produce 100 l of CO2 and 100 l water vapor (26 ml of water). Outside air is 400 ppm CO2, measured between 800 and 3.000ppm, with 1,800 already called a poor air quality condition. Ventilation so is certainly needed. What the heat effect drastically decreases.

If you use a lot of tea lights, realize that this "fire" from candles works on petroleum as raw material. To keep indoor air healthy, you need a chimney. By witch 90 percent of the heat disappears. So you do need 10 times as much tea lights to get the same temperature.

Flower Pot Stove
17% of the heat of the candle flame is radiated to the surroundings. 83% flows through convection to the ceiling. So we feel no effect. Like a fire compared to a heater, we can enhance the effect by using a case to produce more infrared radiation (and less convection) and exploit it. That can do one reverse flowerpot. Or more.
You can hang a heavy metal bolt into the hole, or screw 2 to 3 pots together with it. As a good conductor this mass is rapidly heated up, and gives heat smoothly (conduction) to the pot(s).
A part of the heat is required to gasify paraffin for combustion. Under the already warm roof will therefore be (miniscule) more heat left for heating. This effect can still improve by not putting the candle(s) on the (cold) ground but somewhat higher, or on insulating material, or in an extra tin (low, not higher than the wick). The candle will burn harder (and faster).

This also works with any other heat source: radiator, camp fire, kerosene heater...
And other heat -accumulating and radiating objects: brick on or around the stove,...

Given radiant heat is experienced more effective this thus is a workable idea, in a not too big and not too cold or drafty room, with ventilation. If you sit close to the flower pot.

2 additional tips :
Given the produced heat you can instead of a 3 tea light stove also invite as many guests. (Although there is a chance that they consume more.)

Heat rises, so you can also put a flowerpot on your head.

To be clear: it’s a great example of placebo heating.