Hi, all.

I'm building a Hot Box :-)
Now, Now. Settle down there in the back.

This is a wooden box, about the size of a coffin ( say 6 ft x 3 ft by 2
ft ).
It's purpose is to hold 4 pairs of skis at a temperature of around 50 -55
deg C for several hours ( to aid wax penetration into the base structure ).
It's critical that the temperature does not rise above around 60 degrees (
the p-tex base begins to melt much above that.. ) The temprature may be
permitted to cycle reasonably far below the set-point without any harm.
I'll just be using the heater's integral stat most likely, not any
whizz-bang computerised controll: as I say, I don't mind a reasonable
hysteresis, so long as I can limit the temperature at the peak of the cycle.

This will be used in my garage ( part of the house, not warm, but not cold
either. )

The general advice is to use an oil-filled 'baseboard heater', along with a
circulating fan in the cabinet. The general consensus is that the
oil-filled units produce temperature fluctuation cycles which are more
damped than using a straight fan heater. This helps avoid big temperature
'spikes' when the heater cycles on.

The question I have is: what kind of wattage of heater would perform
reasonably in this role?
It obviously depends on the heat loss from the box.
Assume say an un-insulated 18mm plywood box in a domestic garage. Since
we're only going up to 55 degrees, I'm hoping the temperature gradient from
inside to outside is small enough that the heat loss will be low enough to
permit the use of a fairly low wattage heater.

I suspect the answer is suck-it-and-see :-)

Just looked at my kitchen fan oven. It's 2.5KW, and can ramp the oven up
to over 250 deg C. It's insulated, and smaller, but the heat loss at that
temp will be fairly high.

I'm thinking of trying something in the range of 500W.
Sound reasonable?


--
Ron

Ron Lowe wrote:
Hi, all.

I'm building a Hot Box :-)
Now, Now. Settle down there in the back.

This is a wooden box, about the size of a coffin ( say 6 ft x 3 ft by 2
ft ).
It's purpose is to hold 4 pairs of skis at a temperature of around 50 -55
deg C for several hours ( to aid wax penetration into the base structure ).
It's critical that the temperature does not rise above around 60 degrees (
the p-tex base begins to melt much above that.. ) The temprature may be
permitted to cycle reasonably far below the set-point without any harm.
I'll just be using the heater's integral stat most likely, not any
whizz-bang computerised controll: as I say, I don't mind a reasonable
hysteresis, so long as I can limit the temperature at the peak of the cycle.

This will be used in my garage ( part of the house, not warm, but not cold
either. )

The general advice is to use an oil-filled 'baseboard heater', along with a
circulating fan in the cabinet. The general consensus is that the
oil-filled units produce temperature fluctuation cycles which are more
damped than using a straight fan heater. This helps avoid big temperature
'spikes' when the heater cycles on.

The question I have is: what kind of wattage of heater would perform
reasonably in this role?
It obviously depends on the heat loss from the box.
Assume say an un-insulated 18mm plywood box in a domestic garage. Since
we're only going up to 55 degrees, I'm hoping the temperature gradient from
inside to outside is small enough that the heat loss will be low enough to
permit the use of a fairly low wattage heater.

I suspect the answer is suck-it-and-see :-)

Just looked at my kitchen fan oven. It's 2.5KW, and can ramp the oven up
to over 250 deg C. It's insulated, and smaller, but the heat loss at that
temp will be fairly high.

I'm thinking of trying something in the range of 500W.
Sound reasonable?

I'd try a 200w bulb.

PS baseboard heaters arent oil filled. Extra thermal capacity in the
heater will simply enable/cause the cabinet temp to rise further once
the stat switches the heater off. And they dont normally have an
integral stat - and if you use one that does, its not gonna go to 55C.
You'll need a separate stat that does.


NT

On 21 Apr, 22:30, "Ron Lowe" ronATlowe-famlyDOTmeDOTukSPURIOUS
wrote:

I'm thinking of trying something in the range of 500W.

Really cheap 500W - 700W oil-filled radiators are on clearance this
week!. That should be excess power, but the thermostat means it isn't
a problem. You might have to work on the thermostat to see 55°C
though. I'd also watch it closely around the over-pressure vent first
few times I went that high.

If you want something tiny for money, someone like TLC sells packaged
"rod" heaters that are simple, robust and have a bit of thermal mass
to them. You can add a thermostat to the inside top of the cabinet.
Thermal inertia shouldn't be an issue (i.e. oil vs. fan) provided that
you're measuring the ski temperature and not just a fast-changing
heater.

Power rating depends on the steady-state conditions for a box of known
surface area at 55° inside a room that's colder (presumably Winter
garage), with a thermal resistance through each wall depending on how
thick the Celotex offcuts you lagged it with are. Find some web
tutorial on U and R values to work out the rest, but I expect that
bare plywood would need a bigger heater than you'd like to get to
those temperatures.

Don't use a lightbulb - too fragile, too low power.

On Wed, 22 Apr 2009 10:35:41 -0700 (PDT), Andy Dingley wrote:

Don't use a lightbulb - too fragile, too low power.

OK for smaller boxen: had a propogator about 40cmx50cm, ally plate with
Perspex cover, box underneath. Used a 60W bulb and it blew in about a month
(even with a 'stat).
Wired the 'stat to 2x100W in series, dull orange glow, lasted for years.
--
Peter.
You don't understand Newton's Third Law of Motion?
It's not rocket science, you know.

PeterC wrote:
On Wed, 22 Apr 2009 10:35:41 -0700 (PDT), Andy Dingley wrote:

Don't use a lightbulb - too fragile, too low power.

OK for smaller boxen: had a propogator about 40cmx50cm, ally plate with
Perspex cover, box underneath. Used a 60W bulb and it blew in about a month
(even with a 'stat).
Wired the 'stat to 2x100W in series, dull orange glow, lasted for years.

If you want to make them last forever without dropping the power
significantly, use a series L or C to knock 20v off the supply.

Fragility isnt a problem with 100 watters, 200s have much thicker
filaments. Its when you get down to 25w that they get rather fragile.


NT

On 23 Apr, 00:44, wrote:

Fragility isnt a problem with 100 watters,

They'll still break if you hit them with a ski. A rod heater can be
fastened to the bottom of the box, a bulb (actually a series pair, as
the easiest way) needs you to make some sort of protective enclosure
for it.

Andy Dingley wrote:
On 23 Apr, 00:44, wrote:

Fragility isnt a problem with 100 watters,

They'll still break if you hit them with a ski. A rod heater can be
fastened to the bottom of the box, a bulb (actually a series pair, as
the easiest way) needs you to make some sort of protective enclosure
for it.

You dont want to enclose a lightbulb. A rod or bar over the top is
most likely enough in this case. It struck me as a lot quicker and
easier than going and finding a tubular heater.

I wouldnt worry too much about series-ing them, just do whats
convenient.


NT

On Apr 23, 8:02*pm, wrote:

You dont want to enclose a lightbulb.
[...]

My Dad had a 'heat box' for his photographic processing chemicals
which was just a light bulb inside a wooden frame of low height, with
Aluminum plates covering the top & bottom to seal it. You put the
trays of developer, fixer etc. on the top and it kept them at (approx)
the right temperature.

He must have built it in the fifties and it was still going strong
with the same lightbulb thirty years later.

I agree probably not for this application though.

J^n

Not sure if it helps or not, but checkout.

http://www.gb-online.co.uk/incubating.html