Lightbulb Sauna: Incandescent Is 100% Efficient When Used for Heat

Turned a meat locker into a sauna using incandescent lightbulbs which are 100% efficient when used for heating purposes.

The brushed aluminium walls and ceiling reflect the warm glow of incandescent light bulbs, to create an infinity mirror effect for the heat rays, giving rise to a new kind of artistic effect.

Running the bulbs at half voltage gives a nice warm glow, as if gathering around a campfire.

A "lightbulb moment"!

This is an experimental research prototype so safety first: like gathering around a campfire, don't touch the bulbs, but unlike the campfire, don't pour water on the bulbs... this is a dry sauna. Observe all safety precautions as you would with live wires, fragile glass, and extreme heat. Never bathe alone, and know your limit and stay withinit. As with natural sunlight and campfires, don't stare at the bulb. Look away from the light as your natural reflexes tell you to, unless you're wearing a Shade 12 or darker welding glass for the bulb or a Shade 15 or darker when looking at the sun.

Lightbulbs and sockets, high-temperature wire, amplifier, controller, or control circuits, metal boxes for safety and grounding, CGFI circuit protectors for safety, and a suitable place to build the sauna.

Lightbulbs are readily available at thrift stores and the like, and nearly any wattage will do. Simply use more bulbs of lesser wattage or fewer bulbs of greater wattage. I experimented with a linear strip of 100 watt bulbs like in a theatre marquee, as well as with one or two 1500 watt bulbs. I kind of like the larger bulbs for aesthetic reasons, but a row of smaller bulbs can also look nice, especially if surrounding the entire perimeter of the room. A two-dimensional lattice of bulbs can also work, in which case lower wattages like 60 or 25 or 15 watts make sense when the number of bulbs increases greatly.

For best results the interior should be of smooth shiny metal surfaces that can withstand great amounts of heat while reflecting both visible and infrared energy like a hall of infinity mirrors. Brushed metal works best as it provides good shiny reflections but you probably don't want metal that is polished to a mirror finish as this will cause annoying glare. A brushed metal finish is a good compromise between matte finish and mirror finish.

The sauna room should be well insulated. I used an abandoned (no longer needed) walk-in meat locker (walk-in freezer or cooler) that was pre-existing. Rather than demolish and remove it, I simply turned it into a sauna. The effect is wonderful and I really love the way the light bounces around inside the room, and the multiple reflections produce a warm glow without excessive glare. Spend a good amount of time polishing the metal (as shown in the above photo) until you have something that looks really wonderful!

Safety first! Always check materials for heat capacity, fire retarding properties, and safety overall. Never bathe alone and never leave the apparatus running unattended. All electric circuits must absolutely be protected by RCCB (often incorrectly called "GFCI" or "GFI").

Unlike a traditional rocks-based (ballasted) sauna, the light bulb sauna heats instantly with wonderful radiant infrared energy on-demand. And unlike an infrared sauna, there is a wonderful optical aspect to the incandescent lights that mimics natural sunlight.

The key element here is a design for the thermal, optical, visual, and overall aesthetic of the light bulbs. Like the old EZ-Bake Oven (light bulb oven) but for humans!

To turn the walk-in cooler (meat locker) into a sauna, the cooling equipment was removed. Rather than using cedar, the aluminum walls and ceiling were polished, to give a nice design aesthetic that would work well with the warm glow of incandescent bulbs, reflecting the rays of optical heat, like a hall of infinity mirrors.

What remained was to decide on the color temperature (operating point) of the bulbs.

We wish to have a magnificent warm glow, mostly heat with some light, like the light from the sun, where you can feel the heat when your eyes are closed, but there's also a good balance between heat and light, with a warm reddish-yellow amber glow. This happens very nicely when the lamp operates at about half its intended voltage. Interestingly the lifespan of an incandescent bulb varies as the 12th exponent of the supply voltage. So running a 1000-hour bulb at half its design voltage brings the lifespan to about 4096*1000 hours = 4,096,000 hours, which is just over 427 years if left running continuously.

Here I have a test setup to determine operating power at half voltage. I've wired the lamps in 2s 2p (two parallel pairs of two bulbs in series), so each bulb runs at 60 volts when connected to a 120 volt line.

With purely resistive loads when connected in series, the power is reduced to 1/4 for each, i.e. total of 1/2. Thus two 1000 watt resistive heaters in series each operate at 250 watts for 500 watts total.

But with light bulbs, the resistance varies with heat. Light bulbs are "self regulating" in a way, e.g. as the voltage increases, the resistance increases, and thus the power only increases modestly.

Conversely, at reduced voltage, the power only decreases modestly.

A 1000 watt light bulb at half voltage does not drop to 250 watts, but instead drops to about 350 watts, i.e. about 35% of its normal power. Thus two 1000 watt light bulbs in series consume about 700 watts.

I determined this experimentally and made a small table of amperage as a function of voltage in increments, as follows: two in series (60 volts each), 3 in series (40 volts each), and 4 in series (30 volts each).

I decided to operate the lamps at 60 volts as this seemed to provide the best balance between color temperature and heat. This is an artistic design decision that could change with preference.

With series pairs there will be an even number of bulbs connected 2s (2 in series), and any other integer number in parallel.

Here we see four bulbs, each of which is about 300 watts normally, reduced to each being around 100 watts when paired (60 volts), giving a total of about 400 watts for the 4 bulbs.

I also experimented with larger bulbs, 1500 watts each normally, which run at around 500 watts each when paired (60 volts rather than 120 volts).

Six of these 1500 watt bulbs thus produce a total of 3000 watts which is good for the size of sauna, which has an approximate 10-person capacity, considering the roughly R20 wall insulation and R60 ceiling insulation. Note that each person puts out roughly 100 watts of body heat, and calculate accordingly.

There is also quite a bit of leeway, depending on how quickly we wish the sauna to heat up, etc..

Safety first: never try this unless you have lots of experience with this sort of thing, and never leave it running unattended. Obviously don't touch the bulbs or put water on them!

Bulbs are wired for reduced voltage, for being supplied with approximately half their normal voltage. In the initial testing this was done by pairing in series. Voltage control was alternatively done by supplying the bulbs with a reduced voltage supply, such as output of an amplifier (with a nice slowly varying waveform, musical input so the bulbs "sing", biofeedback input for brainwave-based biofeedback, or the like).

I built three amplifiers each with a capacity of 2000 watts, so that each amplifier could drive a pair of 1500 watt bulbs in series (total of about 1/3 * 1500 * 2 = 500 * 2 = 1000 watts). Thus each amplifier was running at a nice 50% capacity.

The setup runs from a triplex outlet (3 single phase outlets) taken from a 3 phase electrical supply.

For safety, all three of the circuits are balance fault interrupt protected by RCCB, somewhat erroneously called "GFCI" or "GFI" or "CGFI" (I think it should be called "balance fault interruptor" since it will trip not only on ground fault but also on fault to another electrical line somewhere else, or by any other condition in which the circuit flow across the two terminals is unbalanced).

If you don't want to bother with fancy electrical control circuits and systems, you can simply connect pairs of bulbs in series to reduce the voltage to 1/2.

The above photo shows a simple bulb sculpture demonstration I made where you can see the wiring of the bulbs. I used thick bare wire so it shows up well in the picture.

Don't try this at home!

Sparks fly when the wires are connected live, so eye protection, which is a good practice in any electrical lab or workshop, is required.

Here two 1500 watt bulbs are connected in series. If they were resistors they'd together consume 750 watts, but because the lightbulb filaments have lower resistance when cooler, each consumes about 500 watts and the pair in series consume 1000 watts total.

With a sauna, winter becomes fun! Running barefoot on a blanket of fresh snow, and jumping into an icewater lake does wonders for health and wellbeing. Safety first! Know your limit and stay withinit.

Thanks to Scott, Perry, and everyone else at SwimOP who helped with this project.

To learn more about our group, join SwimOP = Swim at Ontario Place, facebook.com/groups/swimop

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