"David" wrote in message
...
On 04/12/2014 20:52, Rod Speed wrote:
Martin Brown wrote
Rod Speed wrote
Brian Gaff wrote

Sounds a bit sus to me, cos a by product of the combustion of
paraffin is water vapour.

Sounds like yet another urban myth to me.

Surely greenhouses are going to be close to 100%
relative humidity at night in weather cold enough
for frost to happen inside the greenhouse.

It is true that condensing and freezing of water vapour helps to hold
the interior temperature steady (as does having a large bulk of water).

I didn’t mean that, just that a tiny paraffin fuelled flame
is going to make no measurable difference to the RH.

I can't see that a very small flame burning paraffin
is going to make any measurable difference at all.

The humble nightlight/candle is good for ~100W

I don’t believe that. Electric nightlights are nothing like 100W

I'd guess a paraffin heater would be ~200-400W minimum.

He was talking about a tiny flame, don’t buy that either.

And if you have a too big paraffin lamp flame it will cover everything
in soot.

Don’t buy that either. There is no reason why a bigger paraffin
fuelled heater will produce any more soot than a smaller one.

Same if you don't allow some ventilation and your greenhouse is too
well sealed so that it gets low on oxygen

Can't see that either with the tiny flame he is talking about.

(also very bad for both you and the plants).

Not with the tiny flame he is talking about in a
greenhouse that will inevitably leak quite a bit.


john t west wrote

Looking at how some people are protecting their Green Houses against
the frost, i was surprised how tiny the flames were on the paraffin
heaters in the green houses. They were hardly giving off any heat at
all.

I was told its not about the 'Heat', but the fact that the flame
changes the 'relative humidity' in the air'.

Could anyone explain fairly simply, how this actually works?

The heat is also important.

Not with the tiny flame he is talking about.

Provided that you can replace most of the overnight losses

You can't with a tiny flame.

then it will keep it above freezing. A layer of bubblewrap on the
glass helps keep the heat in a lot better.

I prefer to keep mine about 4C on an electric thermostatic heater.
Cacti do not like the humidity that comes with paraffin.

But the greenhouse will inevitably be close to 100% RH at night in
winter.

What a load of codswallop.

We'll see...

This is the most ridiculous topic we have had for a long time.

Bullshit.

If humidity would keep out the frost

No one ever said that.

then a misting unit using warm water would do the job without the risk of
pollution by a badly set flame.

Its easy enough to check if the flame is polluting or not.

Blue flames on a paraffin heater give virtually no pollution,

So there is no real risk.

it's a yellow flame that gives you carbon that can coat everything

And that is trivially easy to avoid.

and will also have a sulfur element .

In the dim and distant past you would use a paraffin sump heater (designed
to fit under the car sump to stop it getting to cold in severe frosts) as
frost protection in a very small greenhouse, also cover plants at night
with sheets of newspaper to keep the frost off.

That's all a separate issue to the original claim being
discussed, that a tiny flame works by increasing the RH.

Then there was low voltage soil warming using Galvanized fencing wire, the
idea being that if you kept the soil to around 50f then the air temp
around the plants would keep frost off with minimal heating.

Ditto.