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

On 03/12/2014 20:23, 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? 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.

In article ,
newshound wrote:
On 03/12/2014 20:23, 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? 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.

I haven't done the calculation, but did you include the latent heat
of condensation as well? That is clearly more significant than
that of freezing from liquid.


Regards,
Nick Maclaren.

On 03/12/2014 21:02, Nick Maclaren wrote:
In article ,
newshound wrote:
On 03/12/2014 20:23, 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? 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.

I haven't done the calculation, but did you include the latent heat
of condensation as well? That is clearly more significant than
that of freezing from liquid.


Regards,
Nick Maclaren.

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).

On 03/12/2014 20:23, 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? Thanks

IMHO using a paraffin heater is a double-edged sword. Yes, you will get
heat to protect frost-sensitive plants, but the more moisture you put in
the air, the worse the problem of fungal attack. In fact, because air at
low temperature holds less water vapour than warm air, the relative
humidity in a greenhouse at 1°C will be considerably lower than one at
5°C. So if the plants will withstand the lower temperature, the problem
of fungal attack is lessened when compared with the higher temperature.
It will also mean lower heating costs, of course.

I keep my greenhouse at 1°C (provided by a thermostatically-controlled
fan heater). More importantly, I keep the air moving continuously with a
small oscillating fan.

--

Jeff

In article ,
newshound 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? 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.

I haven't done the calculation, but did you include the latent heat
of condensation as well? That is clearly more significant than
that of freezing from liquid.

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.

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.


Regards,
Nick Maclaren.

"john t west" wrote in message
...
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

Doesn’t say anything about it in
http://www.fao.org/docrep/008/y7223e/y7223e0d.htm
so I doubt it myself.

And
http://www.autogrow.com/general-info/humidity-and-vpd
says that you want to ensure that the humidity doesn’t get
too high by venting which makes it even less plausible.

"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? Thanks


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

--
From the Sofa of Brian Gaff Reply address is active
"john t west" wrote in message
...
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

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.

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


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?

On 04/12/2014 10:23, 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 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'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).


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. 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.

--
Regards,
Martin Brown

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

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

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.

"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.

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.
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.