"David" wrote in message
...
On 04/12/2014 20:54, Rod Speed wrote:


"Caecilius" wrote in message
...
On Wed, 03 Dec 2014 20:23:32 +0000, 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.

Just a guess, but could it be similar to the way smudge pots help
protect orchards from frost: not from the heat, but by reducing
radiative heat loss.

Could the paraffin burners help by steaming up the inside of the
greenhouse,

Not with the TINY FLAME he is talking about.

and the steamed up windows reduce IR transmission through the glass?

Can't see that either given that most greenhouses
are likely to have condensation on the inside of
the glass in winter when there is a real frost risk.

Like I said: it's just a guess; I've never actually used greenhouse
heaters.

I wonder if this idea stems from the early days of polythene cladding of
greenhouses.
The idea was that a film of moisture on the inside of the plastic changed
it's properties regarding the retention of short wave and long wave
radiation

That's not correct. They work fine without any film of moisture
and are MUCH cheaper than the traditional glass green houses.