Hot florescent lights

[Nick Nicholas]

Hello all

I have three 3 feet long florescent light fixtures in a recessed cavity in the kitchen ceiling. The cavity is 3 feet by 3 feet by 10 inch deep made from drywall. The cavity is covered by thin plastic florescent light covers made for T-bar ceilings. Above the cavity is the attic with about 10 inch blown in insulation. These fixtures are regular Home Depot fixtures and are paralel to each other about 10 inches apart. They are all supplied from the same junction box also in the cavity.

Problem is that they get so hot that the thermal safety in the fixtures trips and shuts off the fixtures. This happens after a year of constant daily use. First time it happened, I replaced all three fixtures because the ballast were leaking out. It is starting to happen again. This time, the ballast are intact, but the lights still shut off if left on too long.

Is this to be expected given the way they are installed? The one weird thing that has me concerned is that the junction box supplying all the fixtures have an unused red wire. The lights are connected to the white and black wires. This was the way I found it when I first replaced the original leaking ballast fixtures. Is there an electrical problem?

What are the possible solutions? Venting the heat into the attic is not an option since that would bring other problems in the winter.

Any and all input most welcome. Thanks.

nn

[RBM]

[Terry]

Sounds unsafe; despite the lower heat dissipation of fluorescents than incandescent bulbs I understand there are codes for not putting unvented light fixtures into areas surrounded by insulation. As the OP says there is heat build-up and potential for fire! Probably a 'good' thing that the heat cut-outs worked as they did! Coincidentally; fire in ceiling of a small commercial building here, today. May have started in the 'office' area. Some plastic stock then caught fire and local schools and some office buildings were evacuated because of toxic smoke! Fortunately no deaths. Several events scheduled in that area tonight were moved to other locations.

[nicksanspam]

And it gets too hot and the ballasts burn out. If you don't want to make any holes in the top, you might open some knockouts in the fixtures near the ballasts and add a small fan. If the fixtures make 1000 Btu/h of heat, Grainger's 11 watt 36 dBa $37 3LE76 78 cfm fan will limit the box temp to about 83 F in a 70 F room.

Nick

[m Ransley]

Sure nick pay for fans, hardware, installation, and electricity to run them, and he still has junk ballasts that are not up to the job. Now wouldn't it really be better if he got quality ballasts made for the job ! I mean this is standard stuff, enclosed fixtures, or didn`t you know that.

[nicksanspam]

You know little of heatflow, m. It sounds like the OP has about 3x80 = 240 W ie 820 Btu/h in a 3'x3'x10" box with 10" of insulation above it and glazing below it, and for all we know, the sides of the box may be insulated as well. If it only loses heat by radiation, 0.1741x10^-8((460+T)^4-(460+70)^4) = 820 would make T = 401 F. What kind of ballast would survive that?

Nick

[m Ransley]

Nick you know nothing about lights, that is the issue.

[nicksanspam]

Would you have any evidence for this article of faith?

Nick

[TKM]

Two suggestions:

1. Replace the solid plastic panel with the open "cube louver" type. Get the half by half by half (1/2 in. in all cube dimensions) for best glare control. There will be plenty of natural air flow to keep the cavity cool. The louver is not expensive and you'll like the improved lighting effect. When the time comes, you can wash the louver by sloshing it in the bath tub with dish detergent.

1. If/when the ballasts fail, replace them with commercial units. Check to make sure they are UL and "Class P" labeled. HD carries such replacements. I prefer "instant start" to "rapid start" types for home use since the light comes on, well, instantly.

TKM

[nicksanspam]

Because warm air falls? :-)

Nick

[m Ransley]

No Nick it is called air circulation and the release of radiant heat. And your 401f internal temp just shows you need some hands on experience, that calculator of yours is no replacement for logic and knowledge.

[nicksanspam]

Warm air rises, and the OP said there was 10" of insulation over the box, with no holes. The 401 F was from a radiant heat calc.

How does air circulate in this case? Where are your numbers?

Nick