It's up to you. Some people change their bulbs every year to have the optimum performance as the bulb's output diminishes with time. Are you not getting the results you did when the bulbs were new?

24000 hours is just an estimate for how long the bulb should last before burning out

I would use them until they start flickering.

Results seem the same so I'll save my money for now.
Thank you both.

lifespan is also determined how often they are turned off and on. if you are running on for 16 hours off for 8 hours, they will have more than 24000 hours of life. also, the website should show maintained lumens. you are using a pretty wimpy t8 lamp: I use lamps that are rated at 32,000 lumens - so when you replace them, get yourself some XHL lumen lamps or other similar high output, just search on the internet, and you will see that many companies produce very high output t8 that will grow lettuce faster. only do this according to your budget - pm.

Thanks overdrive,

How often do you replace them?

Hi Fred,

Manufacturers of fluorescent lights usually report two lumen ratings, 'Initial' and 'Means'. Your bulbs are rated at 2800 initial lumens (not 28000) :-) and 2520 means. The means rating is always based on the lumens tested at 40% of the tube's rated hours of life. So, if your bulb is rated at 24000 hours, then the manufacturer is saying that when the bulb is new, it will put out 2800 lumens. When it has been used for 9600 hours (40% of 24000), it should put out about 2520 lumens.

Your bulbs have been in use for 11000 hours so, theoretically, the light output is probably somewhere around 2480 lumens (instead of the initial 2800). That's about 88.5 percent of the original light output. If you don't mind losing a little energy efficiency, that's not really too bad.

I use a light meter and usually replace my T8 fluorescent bulbs when they get down to about 85% of their initial lumens. Not sure if that's right or wrong, it's just what I've always done :-)

Hope something here is helpful,

Art

This post was edited by art33 on Fri, Nov 8, 13 at 16:55

Art,

Very helpful! Just what I was looking for. I did buy a box of 30 of the same bulbs at HD and I'll install them on a cabin fever day in a few months.

Thank you,
Fred

Fred,

You're very welcome, glad I could help. Keep in mind that by reporting the mean rating of their bulbs, the manufacturers are not necessarily suggesting that you replace the bulbs at that point. They're simply giving you an idea of how to estimate the lumen depreciation of that bulb.

Personally, I've never heard of a 'proper time' to replace a fluorescent bulb :-) I think its kind of like slimak said, "it's up to you". If your lettuce is growing very well with the old bulbs, you might want to wait awhile before replacing the bulbs.

While on the subject of growing lettuce. I was just wondering about the 4100K color temperature of your bulbs. I know that blue light is supposed to induce vegetative growth and the red part of the spectrum tends to induce flowering. Would you be better off using bulbs rated at 5000K or above, which would provide more blue light to the lettuce seedlings? Just wondering, as I said I'm not a lettuce grower :-)

Art