CFL usage in cold weather

I've noticed that most CFL bulbs seem to have a warning that they should not be used at temperatures below 10 or 20 degrees F. It was my understanding that this is because they are harder to start up and may require a lot of time to come to full brightness in colder temps. But, if the bulb was left on 24/7, would it matter then? Once it gets going does the cold temp have any negative effect on it? Or am I off base in my assumptions?

LMU

I think it would depend on how much wind and how cold the weather is. If the bulb was in a sealed fixture that could retain heat well when it gets colder or blowing wind/rain it may be fine but at a certain point the bulb may not be able to generate enough heat to replace what is lost and get cooler to the point it effects the light.

That's what I was wondering too. I thought the low temps were only a factor on startup, and once the thing got going it would be OK?

low temps are a factor when operating also I think as fluorescent lights arc through a gas and the gas is affected by cold making the atoms slow down the colder it gets thus less light. If the bulb is unable to get the gas inside to a temp that allows electricity to flow well through it it will struggle and it could be possibly draw more current causing problems. As I stated before you have to be able to maintain a certain temp for fluoros to work well if the temp drops it will struggle, and as fluoros run cooler than incans there is less heat wasted that could keep things warm.

A compact fluorescent lamp is essentially a low pressure mercury discharge lamp, with a fluorescent phosphor coating applied to the inner wall of the discharge tube. As a result there are two factors determined by temperature which would affect the light output.

1. Mercury vapour pressure. There is a certain amount of elemental mercury in any fluorescent tube (though the amount is really quite small), in an inert gas atmosphere (usually argon). A certain amount of the mercury dose is evapourated when the lamp is at operating temperature - the electrical discharge though this vapour is what creates the shortwave UV radiation which excites the phosphor on the lamp walls - which gives your visible light. The vapour pressure is determined by the temperature of the lamp wall at the coldest point (which is where the mercury dose will usually sit), many compact fluorescent lamps (Most notably Philips as I recall) have specially designed mercury amalgams - cold points where the mercury remains primarily in its liquid state. Being able to control the temperature of this cold spot means that the vapour pressure can be controlled very accurately. If the cold spot temperature reduces, the mercury vapour pressure will decrease, leading to a decrease in light output. If it increases too much, this will increase the voltage required to sustain current though the lamp - the precise results of this will vary depending on the type of ballast, but generally the result will again, be a reduction in light output beyond a certain point.

2. Phosphor temperature. This is the main culpret for poor performance at low temperatures as I understand it. The phosphor coating on the tube is picky. Really, really picky. If the temperature of the phosphor layer increases or decreases by a really, really small amount - it can have a drastic effect on light output. If it gets higher than it should, this will also have a huge effect on the phosphor's ability to actually convert UV into visible light - this is why fluorescent lamps dim notably throughout their life also.

I've had no problems with any recent CFL's starting at down to -25°C (Our freezer), though they all start dim and usually purplish at this temperature, taking a fair time to run up, but always getting there eventually. So long as the lamp isn't directly affected by strong winds, you shouldn't have any problems with them in outdoor fixtures so long as they're running to start with - just bear in mind they'll take a while to get up to temperature when turned on.

The types of lamps with an outer (usually standard PS60 or A23 shaped) bulb are probably better, as this will afford some level of thermal insulation for the tube itself, shielding it from draughts anyway.

Hope this helps!

The regular twisties will drop quite a bit in their light output at cold temps. OTOH, the CFLs that have a glass globe over the twistie, such as the 'decorative' or torpedo-shaped bulbs fair very well outside, even in non-enclosed fixtures. Surprisingly, even the very low-wattage (3 watt) bulbs do OK. They do take a few minutes to warm-up, though.

I am with brickbat. I used to have regular or twisties outside. They actually do get really dim running at 0F or below. The ones with the glass globes to make them look like a "regular" bulb help a lot. They do take a while to come up to full brightness, as did the other ones, but even at -10F they get pretty darn bright, not as bright as 70F, but more then enough to snow blow or shovel by.