Heat generated by LED lights 2

[MedicineEng]

Hi AlL:

I have to make a HVAC cursory analysis of an area that we are repurposing which will have a big increase on LED lighting quantities and power.
I was trying to get some information on how much actual input power is dissipated as heat in case of LED lighting, but a ggogle search gives me figures between 10% up to 60% heat.

This is a huge range and I'm a little bit confused on which number to follow to check if any upgrade to the HVAC system is required.

Any hints on where can I find accurate information on this?
What are you guys assuming in your projects?

 

[HVAC-Novice]

100% of the electricity becomes heat. Much is conductive (heatsink), some is little is radiating. Part of the radiation energy is typical infrared heat. Part is visible light, but that also becomes heat ones the light "disappears" in tot he dark. So, assuming all light goes into that room, 100% of the energy you put in, will heat up the room.

Obviously with LED having larger efficacy for visible light than fluorescent or HID, that amount of electricity you put in is less. but all of what you put in, becomes heat.

In Trane Trace you can enter radiation/conduction % since the conductive heat goes into the plenum, and the rest into the space. I don't know if they have one for LED yet, but I would guess for LED the radiative % is a bit less than for fluorescent. Definitely less than HID or fluorescent. there also is a difference if the fixture is below the ceiling (all heat goes into space, as opposed to plenum for recessed fixtures)

 

[LittleInch]

Ten % to 60% of what?

Actual power used from a recent test I did showed about 1.5 times the light output in Watts.

So e.g. a "9W" or about 600 lumens actually used 15W of mains power (230V).

But it all becomes heat.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MintJulep]

All of the electricity becomes heat.

Not all of the heat necessarily becomes HVAC load for the room being illuminated.

 

[nuuvox000]

Not all of the heat necessarily becomes HVAC load for the room being illuminated.

Could you give some examples, just curious about this. Maybe the light that leaves through windows? My opinion is that all of the energy going into that light is going to be changed into heat load so I'd love to hear your point of view.

 

[HVAC-Novice]

Light going through windows will be an insignificant % and can be safely ignored. The solar load coming into windows will be much larger.

Consider air movement in the space, and the ceiling construction.

If the fixture is fully inn the space (i.e. suspended fixture) the conductive heat goes to the air around it. If this is a room that exhausts at the ceiling, that warm air doesn't necessarily contribute to cooling load where people are. but the radiated heat from the fixture goes to the people and will be cooling load. In Trane Trace you can enter the % of what is radiation vs. conductive heat for that reason.

Or consider a recessed fixture. the conductive heat mostly goes to the space above the ceiling, not the space itself. If this is a plenum return, this load adds to the AHU (coil) load, but not the space load. Minus the air that the building exhausts and doesn't re-route to the AHU. If this is not plenum return (it is just a large void between ceiling and floor structure above), that heat goes to the ceiling and radiates down, but also to the floor above.

So, it all becomes heat, but not all heat becomes a cooling load. and not all cooling load becomes a cooling load for that space. that is why a load calculation software needs to know what type of system, fixtures and building you have.

You can model and CFD-analyze this to the nth degree. but in practice you use certain % based on typical fixtures and construction and system type. Look into the option your software has. the above just explains why they account for it in different ways.

 

[MintJulep]

Not all of the heat necessarily becomes HVAC load for the room being illuminated.

Could you give some examples, just curious about this.

For example, a space with overhead lights and a dropped ceiling where the void between the ceiling panels and the floor above is not used as the HVAC return plenum.

In this case, heat from the lights will be rejected into the void, causing the void's temperature to rise. That heat will go partly downward through the ceiling tiles to the space being illuminated, and partly upward through the floor to the room above.

 

[MedicineEng]

I never thought of visible light as heat but it makes sense.
Also, I understand if some of the heat has gone to a void that this won't be loading the HVAC system.

Thank you all for your feedback.

 

[SAK9]

While I agree that 100% of the lighting KW will get converted into heat eventually, the amount of heat that goes into room sensible load is the lowest for LED as compared to fluorescent or incandescent lights. This may have an impact on fan kw.

 

[HVAC-Novice]

The heat that goes in the void above a ceiling (recessed fixture, but no plenum return) goes somewhere in the building. Most likely into another conditioned space.

If you use a software like Trane Trace, just correctly tell it what type of structure and fixtures you have, and it will account for it to sufficient degree. Don't overthink it, just be aware all the above issues are an actual thing and there is a reason the load calculation differentiates those.