In article ,
jim k writes:
Tim Watts Wrote in message:
On 01/03/18 11:19, Andrew wrote:

My Ebac homedry has a circuit board with a 555 timer.

Once per hour it operates a solenoid valve in the gas stream
reversing the flow of warm refridgerant through the cooling
coils, so the ice crackles, melts and falls off. It does this
for about 3 mins and then reverts to normal operation.

QED, should work at very low temperatures.

I bought it 2nd hand in 1988, it's been around the world, to
the pacific for 2 years, and back. Still going strong.

You can buy special heating cable that is mains rated.

When I wanted about 15m of it, easiest way was to use standard
1mm T&E, short the far end, and feed it with an electronic 12V
halogen transformer. I bought a 250W transformer and connected
it to the cable, using a mains power meter to make sure it was
within the load range (IIRC, I could make it 150W or 230W,
depending if I connected the earth or not). This solution was
orders of magnitude cheaper than mains heating wire. It's taped
to an underground water main which I could only get 500mm deep
rather than the recommened 750mm because the end connections
were nowhere near that deep (and the original lead pipe was only
a few inches deep at one end).

Note you cannot work out the power rating by measuring the
resistance of the wire and using ohms law. An electronic halogen
transformer will be at least 20kHz output, and the skin effect
comes into play, and the impedance will be significantly higher
than the DC resistance, which in this case is a big bonus.

I wonder if anyone still makes a unit like that?

I'd prefer the safety of a refrigerant unit (based on what Andrew said
earlier) but the purpose is to keep a shed dry so it will get cold.


Define "cold"...
At 0 / minus anything, there is relatively little water vapour in
the outdoor atmosphere, so your "shed based damp" should be
migrating happily outside by itself without the need of a
dehumidifier?

I've been controlling the humidity in a large shed for someone for
5 years now. Originally I was thinking I would need a dehumidifier,
but I didn't have a spare one so started with a heater. That works
so well, I stuck with it. It's an old oil-filled 1kW radiator,
running at half power with a half-wave rectifier.

You are right that the absolute humidity (dew point) is low at low
temperature even when the relative humidity is 100%. This means that
a very small temperature rise is required to drop the relative
humidity and protect against condensation.

However, without protection, that moisture is dumped out on the
contents of the shed when the temperature drops any further, and
that's exactly what you don't want.

I keep the relative humidity to no higher than 80%, and the contents
of the shed are on palets and spaced away from the walls so there's
circulation all around (otherwise you will get cold spots which drop
below the dew point). It also has frost protection which kicks in
at 5C (and bumps radiator up to 1kW at 3C, although I just searched
through the logs and that's never happened - coldest shed has ever
got is 4.5C when it was -4.9C outside on Tuesday).

I monitored the costs in the first year, and I think it was around
£50 for the electricity. Previously it was costing £200/month
to store the stuff in a warehouse, so that's an enormous saving.

--
Andrew Gabriel
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