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Heat Your Home Office for 8p a Day. Part 3 - It Takes Time

Dylan in his original YouTube video made the point that the tea lights only burn for about 4 hours and that he replaces them after the morning burn for an afternoon warm up session. At 1p per candle, 1p/~35W, we shouldn’t lose sight of how cost effective this heating method is. There is some doubt as to the actual power dissipation for each candle but this approach is scalable, if a 4 candle arrangement isn’t enough, then just consider using 2 or 3 such candle+flower pot arrangements! Let’s continue to compare and contrast the effect of using the flower pots, or not, in the context of how warm the room air gets after 4 hours.

It was with a certain sense of arrogance that I had assumed that the flower pot configuration, as it captures more of the power dissipated by the 4 candles and provides that back to the air, would result in the room air heating up more quickly. One advantage of simulation is that it helps you learn from your mistakes (is there any other way?). Starting with the room at a very chilly 0 degC I simulated (this time with the more appropriate FloVENT) the increase in temperature in the room over a 4 hour period. Specifically at a series of points, half way between the candles and the wall, from ceiling to floor.

Without the flower pots the air temperature in the room establishes itself relatively quickly (graph on the right). Within about 20 minutes the temperature stratification settles down, hotter at the ceiling, getting colder as it reaches the floor as hot air rises (…and HeatSinks, an electronics cooling ‘joke’ and title of the excellent book by Tony Kordyban, sorry getting a bit off topic). From then the air temperature rises slowly as the wall temperature rises slowly due to the radiative heating direct from the candle flames.

With the flower pots the room air heats up more slowly. Sure, after a while the room air gets hotter as more power is provided directly to the air, but it takes time. The reason is due to the thermal capacitance of the pots. They are dense. It takes quite a few Joules of energy to soak the ceramic material to the extent where it then passes those Joules back to the air. [Analogies of soaking, sponges, water etc. are often useful when creating mental constructs to perceive heat transfer processes]. At the end of the 4 hours the air is hotter in the top of the room compared to the naked candles, about the same near the floor.

However, in both instances, the air temperature achieves only about 75% of the maximum temperature rise were the candles to burn indefinitely. This thermal soaking period of the flower pots, that delays the increase in air temperature, could be overcome by say pre-heating the pots in an oven, maybe at the same time as you bake your morning loaf of bread?

In terms of how warm you feel as a human occupant, air temperature is only one factor. There are other factors that need to be taken into account when judging thermal comfort. I’ve touched on these before and I believe that when taking them into consideration in this study, the thermal comfort benefits of the flower pot heater will become more pronounced (O dear, more arrogance).

27th November 2013, Hampton Court.

FloVENT, Heat Your Home Office for 8p a Day, CFD, thermal comfort

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About Robin Bornoff Follow on Twitter

Robin BornoffRobin Bornoff achieved a Mechanical Engineering Degree from Brunel University in 1992 followed by a PhD in 1995 for CFD research. He then joined Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd) as an application and support engineer, specializing in the application of CFD to electronics cooling and the design of the built environment. He is now the Product Marketing Manager responsible for the FloTHERM and FloVENT softwares. Visit Robin Bornoff's blog

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Comments 6

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so does this model include convection - which is really how it works

dylan
6:12 PM Nov 27, 2013

Dylan, yep, the model predicts convective heat transfer (moving air due to buoyancy) as well as radiative heat loss. And of course conductive heat transfer as well.

Robin Bornoff
10:16 AM Nov 28, 2013

I read an ASHRAE paper that concluded the strongest factor in room comfort was to give the test subject access to the temperature control knob (even if the knob was not connected to anything.) When people feel in control of the thermostat, they stop complaining about the temperature. So maybe we don't need to study CFD, we need to study OCD. (Obsessive Compulsive Disorder.)

Tony Kordyban
9:47 PM Nov 28, 2013

It would be interesting to compare the two systems after the candles have burnt out, to see how good the flower pots are as "storage heaters"

David Roberts
4:57 PM Nov 29, 2013

I am no modeller or mathemetician but I think what is going on here is that the device moves a bigger volume of slightly cooler air around the room - it certainly feels to me as though my feet and hands are warmer than if I just have a plate of candles - certainly on the boat you tend to get a pool of cold air down around your feet - the hull is in contact with a massive thermal sink - the water - and the flower pot trick stops that heat from pooling around your head. I guess some-one should place some temperature sensors around the room. Of course, as pointed out above because I think it works then it probably does make me feel wearmer - smoke and mirrors

dylan winter
7:57 PM Dec 1, 2013

Dylan, it's not just air temperature that makes you feel warm. What I also hope to demonstrate in another part of this series is that the radiated heat is an important contributor to how warm you feel (so long as there is a clear line of sight between you and the flowerpots). But sure, Tony's comment about the ASHRAE study does hint at a psychological aspect to the perception of comfort and how it can be enhanced if the occupant is 'in control' of the room's thermal management.

Robin Bornoff
4:29 PM Dec 2, 2013

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