Preventing Frost damage by changing Air Humidity

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
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On 03/12/2014 20:23, john t west wrote:

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.
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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.
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On 03/12/2014 21:02, Nick Maclaren wrote:

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).
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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.
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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.
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"john t west" wrote

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

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On 04/12/2014 10:23, Rod Speed wrote:

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

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

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

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

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

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

Not with the tiny flame he is talking about.

You can't with a tiny flame.

But the greenhouse will inevitably be close to 100% RH at night in winter.
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On 04/12/2014 20:52, Rod Speed wrote:

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.
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On 04/12/2014 21:13, David wrote:
<snip>

yes

yes

except for 1.4 times as much water vapour as the paraffin they consume
it's a

No, you will get exactly the same amount of sulphur dioxide from the blue flame .

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wrote

We'll see...

Bullshit.

No one ever said that.

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

So there is no real risk.

And that is trivially easy to avoid.

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

Ditto.
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On Wed, 03 Dec 2014 20:23:32 +0000, john t west

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.
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wrote:

Not with the TINY FLAME he is talking about.

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.

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On 04/12/2014 20:54, Rod Speed wrote:

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
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wrote:

That's not correct. They work fine without any film of moisture and are MUCH cheaper than the traditional glass green houses.
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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.
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On Friday, December 5, 2014 10:13:58 PM UTC, Andrew Gabriel wrote:

e

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Would be interesting to see the results graphs. A small dehumidifier would surely work much better. They're only about £3 0 now.
NT
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