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

On 04/12/2014 21:13, David wrote:

snip


What a load of codswallop.

yes

This is the most ridiculous topic we have had for a long time.

yes

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,

except for 1.4 times as much water vapour as the paraffin they consume

it's a
yellow flame that gives you carbon that can coat everything and will
also have a sulfur element

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

"David" wrote in message
...
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.

We'll see...

This is the most ridiculous topic we have had for a long time.

Bullshit.

If humidity would keep out the frost

No one ever said that.

then a misting unit using warm water would do the job without the risk of
pollution by a badly set flame.

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

Blue flames on a paraffin heater give virtually no pollution,

So there is no real risk.

it's a yellow flame that gives you carbon that can coat everything

And that is trivially easy to avoid.

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.

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

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.

Ditto.

"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.

In article ,
john t west writes:
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

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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 05/12/2014 22:13, Andrew Gabriel wrote:
In article ,
john t west writes:
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

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.

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.

--
Roger Chapman