Preventing halogen lights overheating under Rockwool

... but why have you got them in upturned pots? If it doesn't make them run cooler what's it doing for the lamps or anything else?

Reply to
usenet
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I'm entirely unconvinced that the flower pot makes any difference then. The lamp outputs the same amount of heat whatever you put around it. This heat must escape to the surroundings which will thus get just as hot whether you put a flower pot over it or not. In fact it may produce hotter hotspots simply because you have reduced the airflow somewhat.

The only gain *might* be that more of the heat goes down into the room below but I'm not all that convinced that this will happen.

I meant that the hole in the ceiling, unless dealt with somehow, reduces the fire barrier created by a plasterboard ceiling.

Reply to
usenet

It does nothing for the lamps,other than keep the insulation at bay. the only way to make them run cooler would be to attach heatsinks to them, or have some forced air cooling but that is totally unecccessary.

by having upturned pots around them you

  1. prevent the insulation coming into direct contact with the lamp
  2. have a convenient hole in the base of the pot to take the lead out of and so keep the wiring on top of the insulation
  3. have a cover which can potentially be sealed to minimise warm humid air entering the loft space.

yes the pots will heat up.

without pots : the lamp heats up , then this heats the air. (assuming loft insulation still held back)

with pots: lamp heats up, this heats up the air, the air heats up the pots, the pots get warm and warm up the surrounding air in the loft.

There may be a slight increase in the lamp[ temperature over scenario 1 but I didn't notice any after installing the pots, however before I put them over our fittings I certainly did notice some condensation in the loft after installing 4 spots in the ceiling. **

Since using the pots over the spots this has not been evident, an extractor fan was also added at a later date.

** the spots were installed in mid january , and the conditions in the loft checked a couple of days later, after fairly heavy use of the bath and shower. the extractor fan had not been wired up at this point which would obviously affect the situation as well.

regards

David

regards

David

Reply to
David

In article , snipped-for-privacy@isbd.co.uk writes

Chris, the lamp does not need airflow to keep it cool, that's the point The pot keeps the insulation away (and creates a larger heatsink if you really think that's necessary) but still maintains a sealed ceiling. I have felt my pots on many occasions (ooer missus) and even after prolonged use they are only warm to the touch as opposed to the bloody hot that the lamps themselves get

So the open tube/chimney idea makes this worse as it gives a path for the flames as opposed to the sealed pot

Reply to
David

In message , snipped-for-privacy@chaplehouse.demon.co.uk writes

As a builder this point has worried me for some time. Everyone seems to want down lighters in their new houses/extensions. Installed in floor void they make holes in the half hour fire protection (plasterboard) and I have been told that a study (not seen it) claimed that the blow lamp effect through the holes might actually accelerate the destruction of the floor structure. For this reason I have been making boxes around the down lighters from a fireproof board sealed up with intumescent mastic; so far so good.

In a loft space they make holes in the insulation and make a mockery of the regulations brought in regarding cold bridging through timber joists. For this reason we have been making plasterboard boxes to give a foot square space around the fitting to allow the heat to dissipate while putting insulation over the box to complete the necessary insulation requirement. I am aware that the insulation probably mucks up the heat dissipation but again; so far so good.

Foot square box; too small? I've been asked to install them in much smaller available spaces between floor joists so I don't know... I'd be very interested in other peoples comments and experiences as I was on the verge of refusing to install them for all the above reasons and I was worried about being sued :)

Reply to
mark

I never suggested it did, it was the OP asked the question "Preventing halogen lights overheating under Rockwool". Lots of people answered saying "put your lamps under a flowerpot". You agree totally with me, putting a flowerpot over the lamp "won't" keep it any cooler.

Whether one needs to keep the lamp cooler is another matter altogether.

Since nothing except mineral wool insulation will touch the lamp if the flower pot isn't there it doesn't really matter does it.

Yes, absolutely. We're mostly in agreement you know! :-)

I'm just questioning whether/what the flowerpot does for you as opposed to nothing.

I think it may well improve the 'fireproofness' of the ceiling if you have a flowerpot over the lamp but I'm not totally convinced, it's hardly well tested. Other than that I can see no point whatsoever in putting the flowerpot there except that it makes things look tidier (nothing wrong with that either).

Reply to
usenet

But Chris

putting a pot over it and preventing insulation coming into direct contact will obviously keep it much cooler than if the insualtion was in direct contact with the lamp. That is the primary goal in doing it.

regards

David

Reply to
David

Hi,

Anyone tried measuring the temperature around the fitting with a remote thermometer?

cheers, Pete.

Reply to
Pete C

Now that I disagree with, I don't believe the flowerpot will keep the lamp cooler. How does it do that? With a flowerpot over the lamp there will be *less* air circulation than without, how is that going to keep the lamp cooler?

The flowerpot may make things tidier and/or more fireproof but I really don't believe it will keep the lamp cooler.

Reply to
usenet

If you compare a lamp with insulation over it and one with a pot over it and eth insualtion held back 10-15 cm then the one with the pot will be cooler in comaprison.

regards

David

Reply to
David

Have you measured that? I really don't see how this can be true, do you have any explanation as to why it should be cooler? There is absolutely nowhere additional for the heat to go in the case where the lamp is in a flowerpot so how can it be cooler?

Reply to
usenet

The lamp will warm up the air around it and the warm air will rise.

As the warm air rises up it comes into contact with flower pot and the flower pot then heats up.

the flower pot now gets warm and warms the air around it.

and so some heat from around the lamp is removed

Have i measured this with lamps. No I've no desire or need to do so.

I have looked the heatsinking requirements and thermal characteristics of semiconductors? yes on quite a few occasions.

Lets say I have a component that must dissipate a fair amount of power, a few watts say. Now if this just sits on a PCB board it gets pretty hot, the internal temp may easily be about 125C, If I tightly pack other components around it (i.e provide some insualtion) then it heats up to 175C and fails. If on the other hand I add some heatsinking then its operating temp will be much lower.

If you surround anything in insulation then obviously it will retain more heat than if there is any sort of air gap.

Given that this is the whole point of the insulation it's something that just seems common sense, I can't really fathom your problem with it. You do not need to have an massive air flow to get heat transfered. As above you will have heat transfered from lamp to air to pot to air. Not the most effiecient heatsink in the world but when compared to 150mm of rockwool there will be a difference.

regards

David

Reply to
David

Chris, David -

neither of you seem to have bothered stating what you're assuming happens

*around* the flowerpot. And it seems from the side as if one of you thinks it's "totally obvious" that the flowerpot is *instead* of the rockwool, i.e. that the top and sides of the inverted flowerpot are open to the surrounding air - in which case the lamp will indeed run a lot cooler, since the pot itself will be able to dissipate a fair proportion of the heat produced by the lamp to the free air around it by both radiation and convection. The other of you seems to assume the pot is covered/surrounded by the rockwool, and under this assumption quite correctly points out that once the air around the lamp and the pot have risen to pretty much the temperature of the lamp fitting (which they will, since the surorunding rockwool prevents nearly all the heat from going anywhere useful), the temperature of the lamp under the pot under the rockwool will be no different from the temperature of the lamp directly under rockwool.

Have I got the source of the divergent views right? (And will I regret piping up at all? ;-)

Stefek

Reply to
stefek.zaba

While we are at it, we've not even touched on transformers, a typical spec for which says not to put under insulation. Presumably they will have to be screwed onto the roof framing well above the joists and all the layers of Rockwool?

Chris

Reply to
Chris J Dixon

well I'm half way in between

the pot is in a hole in the insualtion but the top is not covered with insulation.

cheers

David

Reply to
David

Then you'll get radiation losses from the top of the pot, but not much in the way of convection, since there's not much chance of air getting to the bottom of the pot to start an updraught. An old lath-n-plaster ceiling will provide more in the way of airgaps for convection than a nice new plasterboarded one - to get convection going there you'd want to stand the pot on some spacers, or nibble some gaps into the rim (as if the little bleeders won't crack if you try ;-)

Stefek

Reply to
stefek.zaba

Exactly, as long as you have some exposed surfaces you will have losses.

I'm not too sure about your radiative/convective loss assessment though.

Given that the top of the pot is pretty much sealed it doesn't matter if you get any air down at the bottom of the pot, by whatever means

As the loft space is going to be considerably colder than the pot, (it is well insulated after all), and there should be decnt ventilation, then there will be a reasonable air flow above the pot. So most of the heat may well be removed from the top of the pot by convection.

cheers

David

Reply to
David

Ah, sorry, there's another unstated assumption of mine - that the pot has a Hole in the middle of the "base" (base when used as pot, top when used as light cover) - 'cuz all the pottery pots I've seen have this kind of a drainage hole in them. Thuz, the conditions are right for a useful convective flow from the bottom of the pot to the top, if air can get in at the bottom. If there's no such 'ole, then of course there isn't going to be nearly the same air flow, and a higher proportion of the (smaller) losses will be by radiation (assert): though you're right that a fair bit of the losses even from an "unholed" pot will be from convection currents around the outer surface of the pot.

(Have we worked this one to death yet? ;-)

Stefek

Reply to
stefek.zaba

Is that not the whole point of a decent thread on usenet? :-) There is always another avenue that you can veer off down if you really desire, but whether anyone else follows is another matter.

This thread could just be in its infancy compared with an Andy Hall/IMM tit for tat match :-)

cheers

David

Reply to
David

Yes, I think you may well be right. I'm assuming the flowerpot is buried in the mineral wool insulation, otherwise what is the point?

Reply to
usenet

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