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Heat gain from fluorescent lights

I have a job that has 64 34 watt bulbs. Non electronic ballasts. Can anyone tell me how much they put out?

Thank you



  • don_185
    don_185 Member Posts: 312

    about 9000 btus.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Don is correct

    Total Wattage and a 125% factor (1.25 multiplier) for ballast losses (heat gains).

    Now, if you have a plenum return ceiling, the excess ballast load and part of the lamp load does not go to the space as sensible heat but rather it goes directly to the unit load (raises the return air temperature).

    If the ceiling is not a plenum return there are factors to account for the increased temperature in the ceiling cavity and indirect gain from that. In such cases, the load eventually all goes to the space for all practical purposes but you have to account for the time lag, time of day the load occurs relative to say, late afternoon solar gains, early evening transmission gains from stored heat in the walls, all that lovely stuff...

    I mention this in case you do add the ballast load as sensible space gain, you would wind up tending to over-size your primary system based on a too-conservative sensible load.

    Overall too it matters how long the lights are on. If on-off for an hour here and there versus on all day, there are factors that account for the gain over time. More likely it is the latter for that many lights if one space.
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592


    Are you saying the the ballasts are consuming an additional 25% more energy than the bulbs themselves? If so, does that mean the lights as a composite unit, are not very energy efficient? I always through flourescents were more energy efficient than conventional lighting.

    Clearly I'm missing something.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    It does seem counter intuitive, does it not?

    If you can get your hands on a copy of the ASHRAE Handbook of Fundamentals (Page 29.4 of the 2001 edition), there is a good paragraph about that.

    I learned initially in my career from the 1977 edition but the core data is similar, only the technology has changed. The 1.25 was a rough number from the earlier days but now the 2001 edition says 1.21... but with a range.

    They call this a "Special Allowance Factor". Page 29.5 of the 2001 edition has a table with these factors for magnetic and electronic ballasted fixtures. The factors range from Compact Fluorescents (1) 5W lamp with a 1.80 factor so it emits 9W of heat if on all the time.

    This 1.80 factor varies; many of the factors are in the 0.88 range (the lowest I could see for twin 48 T-12 lamps with electronic ballasts) and most in the 1.1 to 1.2's ranges.

    Part of it, from what I gather, is that the lumens from the lamps have partial IR components which have to travel (at the speed of light don't you know) and be absorbed and re-radiated by the surfaces they touch. Thus the spaces themselves are kept cooler than if incandescents are in the space.

    The IR components of fluorescents are less, you can tell, because the bulbs are cool to the touch compared to a 100 Watt incandescent bulb. Ouch.

    The ballasts are capacitors and thus are warm and ready... like resistive heaters. But that load as I noted and as ASHRAE makes clear, may well be relegated to the ceiling cavity or better the return air plenum.
  • mtfallsmikey
    mtfallsmikey Member Posts: 765
    I'm confused too, Brad

    Yes, I can personally vouch for light fixtures contributing to increased return temps. in a ceiling plenum, as well as a lot of P.C.'s, and other electronic equipment in my buildings that give off a lot of heat, but when my system is in the unoccupied mode (12 hrs. daily, overnight)we are seeing a dramatic increase in space temps, since all fans (A.C. units, bath ex.,fresh air, general ex.) are off. Even with using optimal start on my EMS, space temps are usually in the high 70's to low 80's when things kick off in the morning.By the way, is the same calculation used for P.C.'s and other electronic equipment as well?
  • don_185
    don_185 Member Posts: 312
    Hey Mikey

    Hey Mike, sounds like by not maintaining a positive pressure on the envelope...all the unwanted stuff is coming in.

    The heat goes to cold thing if you will.

  • Brad White_9
    Brad White_9 Member Posts: 2,440

    There was a study done by some fellows (Fellows?) at ASHRAE some years ago on heat gains from office equipment both radiative and convective.

    It is known as ASHRAE Research Project 822 (sounds like "Area 51" doesn't it? Sort of like, "you should not ask about such things." :) The authors were Hosni and Jones in two separate contributions both in 1998. Other previous studies were corroborated by their methods (Wilkins & McGaffin, 1994, Komor in 1997.

    Full disclosure: I had to look these up should you want to research further. I keep my brain full of daily useful names such as Holohan, Sweeney, Wilsey, Rohr, Milne, Hunt, Cook, etc. etc. :)

    Essentially what they found is that, on average and office wide, the nameplate data of all equipment should be taken for heat gain with a factor of 25-50 percent.

    50 percent is conservative, 25% far more realistic. Our office default diverstity is 30-35% of nameplate absent better information.

    Naturally among the types, a copier has standby gains and much higher operating gains but for relatively short periods in an open office. A copy center of course would be more constant. PC's average about 25% with monitors near 80% as heat. Printers are similar, nameplate to actual output.

    You can see the wide range of situations!
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Time Lag, Mikey

    Separate post for sanity's sake:

    If you were to plot out the time lag in a building with any appreciable mass, you will find a lag of several hours between load introduction and heat gain to a space, Mikey.

    Often the real gains only happen AFTER 6 PM when the AC is off. The cooling stops but the space is now cool-ish and the re-radiation from the slabs and mass begins. By 3 AM you could be at peak temperature, even without solar gains. I wish I could post some of the ASHRAE charts on this. It is interesting to see the lag effect and the benefit of pre-cooling or nighttime economizer... All buildings are different. Ever wonder why we rely on computer load programs for our calculations? We used to do them by hand back in the day... and probably missed a lot of savings opportunities.
  • Weezbo
    Weezbo Member Posts: 6,232
    my thought....

    T-5... we have some lights that are like LEDs except they look like fluorescent... no humm and buzz of ballasts..

    check this site out there are some really remarkable lights these days...

    it basically eliminates any sensible heat gain from the fixtures. *~/:)
  • mtfallsmikey
    mtfallsmikey Member Posts: 765
    Good info

    Should be a separate post, but I figured, what the heck, throw it out there while we were on the subject. What I did not mention is that our tenant's electronic equipment runs 24/7, P.C.'s are never turned off, copiers do go into a sleep mode. Cannot run the fresh air fan during the winter months, causes my units to trip on low temp, even had a sprinkler head freeze and bust in one of my mech. rooms (ouch!)HVAC systems in both buildings are bare bones, I believe to be undersized for the job.Thanks for all of the input!
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