The greater the temperature differential, the greater the heat transfer rate

[Kaleva]

There was an interesting OP here earlier about heating a room using clay pots and candles. I'm skeptical if these would work if the temps outside are near zero an you are trying to maintain something like 60 degrees or higher. Because the greater the difference in temperature between the outside areas an the inside one you are heating, the greater the heat transfer rate or loss. It would take a great deal of BTUs to make up the loss when the temperature difference is something like 50-60 degrees or greater. Far more then what I believe a candle or candles could put out even if using clay pots.

On the other hand, if it was zero outside and you were just trying to keep the temp in the room above freezing, it may work as the differential is much lower and thus the heat transfer rate if lower and thus you won't need as many BTUs to maintain the temp in the room above freezing.

This is something to bear in mind if purchasing or making your own (such as clay pots with candles) portable space heater. If you test it out in a room on a nice sunny day when the temperature differential may be close to zero, you may think the heater kicks ass and really warms up the room. But then in the winter with subzero temperature, you may find you have to be quite close to the heater to get warm because it doesn't have the capacity to overcome the heat loss and raise the actual temperature in the room to where you desire it to be.

I have the materials down in the basement to make a clay pot heat to try it out. Just need the time to do it. After I post this, I'm back to house cleaning before bedtime as I'll be babysitting till 8 or 9 pm tomorrow. Maybe this Sunday I'll get it together.

[NutmegYankee]

1. The science isn't there.

I don't even see the need for the clay pot. If you are trying to heat the room, the naked candle flame is putting all of the heat it is going to put out into the air already. The clay pot might take some of that heat and release it over more time, but the heating effect is no different. What would work far better is a metric crapload* of candles.


*more than 20 minimum.

[exboyfil]

2. Enough energy to warm you

also needs a method to vent the CO. That is the situation we have faced since we inhabited the caves. I wonder how many times our forebearers blocked the entrance to stay warm only to die from the CO.

[NutmegYankee]

3. It depends on the space. A candle puts out about the same heat as a 100W incandescent light bulb.

I know from delightful experience that when the furnace craps out the heat from the fridge, computers, lights, receiver, etc don't do much to stop the drop. I turn to the 23,000 BTU Dyna-Glo kerosene heater, which is equal to roughly 288 candles. That works!

[Happy Hoosier]

19. Actually it can help.

Engineer here. You are correct that adding the pot does not add heat. The candles make all the heat there will be. BUT, what the pot can do is concentrate the heat, which can lead to a perception of greater warmth because it will be warmer near the pot. The heat will disburse slower. It will not raise the average temperature of the room as a whole, and in fact it might actually be a little lower due to to inefficiencies of heat transfer.

[NutmegYankee]

22. I'm a Mechanical Engineer.

I understand the perception and the effect it might have on a person, but the laws of thermodynamics are still robbing heat from the room and person. The real answer is choose a method that can actually increase the air temporarily enough for comfort. My backup is kerosene.

[Happy Hoosier]

23. Absolutely right....

My BS was in ME too! (fist bump)

Yes, any process will lose heat because of efficiency, 'cause 2nd Law of Thermo.

It's more about concentrating the heat in a smaller area and limited the dispersal of the heat. We 100% agree. If you want more heat, you need more heat.

[ProfessorGAC]

24. Nothing To Add, But...

...just chiming in as a retired scientist (physical organic chemistry) with the other folks talking thermodynamics!

[Miguelito Loveless]

4. The solution to heating problems

is ground source hear pumps (geothermal). Once you get 18-24 inches underground, the temp is a constant 55. Great for Summer cooling, and not too bad for heating. Energy efficient beyond any above ground system. It is expensive, because so few people do it. Start mandating it for new construction, and economies of scale bring the cost down quickly, making it a cheaper retrofit.

[Hermit-The-Prog]

7. The underground temperature is an average of the local extremes.

The depth is dependent on those extremes.

[Miguelito Loveless]

9. True,

below the frost line is the standard. But again, mass deployment would drive down cost.

[pansypoo53219]

5. setting a fire outside + putting hot coals in pots under pots of snow water might help.

+ lots of layers.

[Hermit-The-Prog]

6. One neat $1 trick makes your car get 200 mpg!

You can pile clay pots from floor to ceiling but they won't add any energy to what that candle puts out.

You might be able to insulate well enough that the candle can heat the house, but it might get a bit uncomfortable at times.

[HAB911]

8. what many use here in Florida

not cheap but clean and ventless. We use denatured alcohol

https://www.thebioflame.com/13-grate-kit-ethanol-wood-burning-conversion/?item_index=1

[Scruffy1]

10. All carbon based fuels, including alcohol, burn oxygen and give off CO2 and water.

Many years ago I put an unvented heater in a garage for occasional use and would never do it again. The CO2 level was ok but the humidity was not acceptable unless you like rusty tools and such. I have propane heaters for emergency use to keep houses from freezing up but I wouldn't run them in an inhabited place indoors without a CO2 monitor. If the air in the house gets below 17% oxygen you have maybe 2 minutes to live.

[LeftInTX]

12. CO or CO2?

CO2 is harmless, carbon dioxide
CO is carbon monoxide and not harmless

[HAB911]

17. yep

[HAB911]

16. understand

but here in Tampa, it is rare the humidity is below 80% anyway, everything is rusty, inside and out. The only time it is actually dry is with a cold front when it can drop to 40% or lower. Of course central air keeps it in the 40-50 range during the summer.

I would never attempt to heat a 2300 sq ft house by alcohol, it's more the aesthetics of a fire, which you can actually back up to, and looks great, and in a pinch could make life bearable. Plus keeping the flue closed retains all the heat. I'll never burn another piece of wood unless it's to survive. My electronics keep the family room at ~70 degrees unless we are below freezing for days on end, which happens very rarely. So when we just want the fire for aesthetics we have to crack the door to maintain and moderate the temp. My wife likes it much colder than I do. I spent a year on the DMZ and promised myself warmth for the rest of my life, hence Florida, lol.

However, several years ago we had "rolling blackouts" that were supposed to be 1 hour on, 1 hour off, but in reality was 5 hours off, 10 minutes on, for three days. Wish I had this thing then.

[Kaleva]

21. Humidity is usually very low in the winter

In the house right now, it's about 20% RH. Ideally, it ought to be somewhere between 35% to 50%.

[NickB79]

11. There are a set number of BTU's in a candle

Adding clay pots doesn't add BTU's

[ProfessorGAC]

25. You Just Don't Understand!

These are magic clay pots! Not sure how you didn't figure that out!

[LeftInTX]

13. One of my FB friends did that clay pot candle thing

She raved about it, but despite the dire stories, our homes did not get below 40 degrees. (My bubble jacket, hat, wool socks and boots were all I needed)

I thought it might have worked because it forced some of the heat to go down instead of up.

[Kaleva]

14. I made one yesterday and am going to test it out later today.

Your comment:

"I thought it might have worked because it forced some of the heat to go down instead of up."

The clay pots make the heat emitted from the candle(s) more useful.

[krispos42]

15. If I recall correctly it's a cube function

Doubling the temperature difference increases the energy looks by a factor of 8.

[Kaleva]

18. Which means the heat source has to put out a lot more BTUs to make up for that extra loss.

[krispos42]

20. Yup. It's worse than driving faster

Going from "A" to "B" in your car at 100mph requires 4x the total energy and 8x the engine power compared to going 50mph.


Maintaining 60° above outside temp takes 8x the BTUs as maintaining 30° about outside temp. That's why moving the thermostat a couple of degrees can make a noticeable difference in power consumption.

If it's 30° outside, then maintaining a 40° difference (thermostat set to 70°) will require 37% more power versus a 36° difference (thermostat set to 66°).

Obviously it's more complicated than that in the real world because you have a lot of solar heating during the daytime. At my job I have to turn the AC on after lunch to keep the temp in spec if it's sunny out and above about 32°. If it's below freezing, or cloudy, I can just leave the heat on.

[ProfessorGAC]

26. Sorry, But No

You might be thinking of BTU demand change due to increasing volume.
This is the heat transfer equation:
The following equation relates to the heat transferred from one system to another

Q=c×m×?T

Where

Q = Heat supplied to the system

m = mass of the system

c = Specific heat capacity of the system and

?T = Change in temperature of the system.

There aren't any exponential factors. Since the surface area in any given situation is a constant, what changes is the rate of heat flow. That's a measure of velocity, which is linear.

But, if we talk about a space heater for a bathroom vs. a living room, then your cubic function applies.
A thousand BTU heater would maintain a bathroom temperature, but we'd need much more heating for the living room.
Even then, it's not quite cubic because the heat flow is a function of surface area, which is a squared function.
We could do the math to do it exactly, of course, but we could easily say that if a room is 3x the smaller one, we'd need more than 9 but less than 27 times the heat.