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? Thanks
Well having high humidity in the greenhouse from the paraffin heater
means that as frost forms on the inside of the glass latent heat is
released, so that probably helps to maintain the internal air
temperature. Of course when it melts it promptly sucks all the heat back.
My back of envelope sums suggest that a kg of paraffin gives about 46 kJ
of energy when burned, while the potential latent heat of freezing of
the water produced would be about 26 kJ. So I would say that the heating
effect is more important.
No I didn't, good point. I suppose the net heat of combustion which I
quoted from Wikipedia assumes the water is in the vapour phase. If so,
you get 46 + 122 = 168 kJ from the burn, once the water has condensed.
So the "protection" from freezing this moisture is proportionately much
less than the "heating" term.
(Relying on memory for the latent heats and J/cal conversion, 50 years
since A-level physics).
Except that you could include both the condensation and solidification
as part of the 'humidity' aspect, so you get 46 KJ for the heating
and 148 for the 'humidity'. That may be what they mean.
Jeff Layman is, of course, right, too. I have read in several places
that professional horticulturists regard paraffin heaters for frost
prevention as a disaster. The old technology was a coke heater,
venting to outside, and the modern one is electricity.
Doesn’t say anything about it in
so I doubt it myself.
says that you want to ensure that the humidity doesn’t get
too high by venting which makes it even less plausible.
Reading the replies, would insulating the greenhouse inside with bubble wrap
not stop a lot of condensation anyway. It's usually the first line of
defence against frost in a small domestic greenhouse. Our new little purpose
made house is now lined (much to my wife's disgust) and I use an electric
fan heater with a separate thermostat.
Regards. Bob Hobden.
Posted to this Newsgroup from the W of London, UK
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.
I can't see that a very small flame burning paraffin
is going to make any measurable difference at all.
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).
The humble nightlight/candle is good for ~100W I'd guess a paraffin
heater would be ~200-400W minimum. And if you have a too big paraffin
lamp flame it will cover everything in soot. Same if you don't allow
some ventilation and your greenhouse is too well sealed so that it gets
low on oxygen (also very bad for both you and the plants).
The heat is also important. Provided that you can replace most of the
overnight losses 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.
What a load of codswallop.
This is the most ridiculous topic we have had for a long time.
If humidity would keep out the frost then a misting unit using warm
water would do the job without the risk of pollution by a badly set flame.
Blue flames on a paraffin heater give virtually no pollution, it's a
yellow flame that gives you carbon that can coat everything 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.
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.
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, and the steamed up windows reduce IR transmission through
Like I said: it's just a guess; I've never actually used greenhouse
I wonder if this idea stems from the early days of polythene cladding of
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
In a slightly different context, someone I know has a large garage
they want to use for storage. As expected for an unheated detached
building, anything left in it gets damp and rusty, even though the
building is watertight and the building fabric itself isn't damp.
I have been running an experiment for over a month now, recording
the internal temperature and humidity, and the outside temperature
and humidity, to try and understand why things get damp in an
outbuilding. I have also added some heating, and what surprised me
was how little heating is required to reduce the relative humidity
inside the garage by quite a bit. I found it quite simple to control
a heater to limit the internal relative humidity to, say, 80%, but
I don't know if this is low enough to prevent timber and furnishings
getting to smell damp, or steel from rusting (or even if controlling
the max humidity is the right thing to do).
When I have more data and understand more of the relationships
between the various parameters, I'll write a blog on it.
The heater I use is a 1kW oil filled electric radiator (what I had
to hand), reduced to 500W by half-wave rectifying the supply to it.
I haven't analysed the figures in detail, but at a quick glance,
it's running with a 30% duty cycle when outdoor humidity is almost
100%RH, which is going to be around 150W equivalent. In comparison,
a gas pilot light is about 250W, and I'm guessing the parafin flame
is going to be in this same ball-park.
[email address is not usable -- followup in the newsgroup]
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.