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).
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
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.
Sounds a bit sus to me, cos a by product of the combustion of paraffin is water vapour. Brian
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.
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.
Would be interesting to see the results graphs. A small dehumidifier would surely work much better. They're only about £3
0 now.NT
AFAIK, none work below 12C, which is the domain of an outbuilding in the winter. It's not that they couldn't in theory, it's just that they are not designed for low temperature use where the evaporator should be only a few degrees below the air intake. I have wondered about pre-heating the air inlet - if that worked it might be slightly more efficient than just heating the air to raise the RH. However, I don't seem to need that much power to achieve results by just heating the air. Significantly increasing the air flow rate might also reduce the temperature drop of the evaporator, but the ones I have don't have variable speed fans, and they run with high airflow anyway.
In an earlier experiment, I used a standard dehumidifier on a short duty cycle - 5 mins on, followed by 3 minutes just fan (to defrost the eveporator). It did sort of work, but I wasn't too keen on such repeated short cycling.
I have a bog standard compressor dehumidifier and it works fine to below
5C. If it does freeze up it just turns the compressor off and runs the fan until it needs to run the compressor again. It will only stay frozen if the temp is below zero. It still dehumidifies if there is ice, the latent heat released by the condensation just speeds up the defrosting.A desiccant one may well remove more water at 5C but do you really need to as the water content is very low at those temps anyway and you only need less than ~60% to avoid rust.
For more than a year now most of my downstairs furniture has been out in my unheated barn/garage/workshop which is attached to my house. The barn has damp wall and several minor roof leaks most of which don't get past the upper floor. Tools do go rusty over time so I thought I would check the current RH. Surprised to find it was only a fraction over 70%. Outside is currently 89% and has mostly been in that region for more than a month. Unfortunately I have no previous data so can't tell whether that just happens to be an abnormally low reading.
The main reason why the furniture has been out there so long is that what started off as a relatively minor house refurbishment the principal part of which was replacing a damp concrete floor has now turned into a major financial disaster the root cause of which was the unexpected necessity of replacing all the roof timbers, and I do mean all, and then installing a loft where there was no loft before so that the roof could be insulated to something like modern standards. And yes I do have a completion certificate for the roof.
The spare bedroom that I used to use as a drying room was always a little damp and had a dehumidifier to keep the RH down to 70% was at 50% RH when I moved the meter to the barn this morning. Oh yes and the bad smell which I always thought was due to the unwashed clothes that also lurked in there has gone as well. That end wall was obviously a lot damper than I had previously thought even though only about 1 foot projects above the attached barn's roof line.
Anyone know where I can dispose of some really manky old oak beams for money? They must still have some strength left as they were supporting
20 tons of Yorkshire stone slates.
Sounds like firewood.
NT
Bonfire material perhaps. Only a mug would try and cut up iron hard 200 year old oak for firewood. What is left of the sap wood is so soft it just scrapes off and what is largely left is the heartwood.
Dessicant based ones heat as well, so are probably more suitable. (less efficient than compressor ones hence less desirable for normal use).
Not my experience of 200 year old oak at all.
Which is?
30 years ago we would sell old beams from barn demolition to an architectural antiques firm in Dunsfold for restorations, got the same money as veneer quality oak in the round.
AJH
There are greenhouse heaters and greenhouse heaters. Some are toys. Serious ones give off serious heat. When we were poorer we used to keep one on the landing during the cold weather. An Aladdin model istr. No fumes or smells once the wick was kept trimmed The ones looking like flying saucers remind me of the heaters once available to place under the sump of your car to ke ep it warm in cold weather. About as useful as a chocolate tea pot
HomeOwnersHub website 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.