Reflective sheets?

rcrit

Does adding a reflective sheet behind a radiator help at all? I've seen some references to these, mostly in the UK, and it seems like a reasonable idea assuming the materials aren't too expensive. One post I read suggested wrapping heavy-duty aluminum foil around a styrofoam board as a cheap solution.

jpf321

reflective sheets...

although I don't have any real information regarding whether they help or not, I do have behind one of my rads a foil covered piece of cardboard .. this was not a homemade device, looks like it was specifically made with the foil. these guys sell 3/16" foil laminated foamboard .. and that's about the thickness of the one I have. http://store.foamboardsource.com/foil-laminated-foamboard-flfb010.html .. but very expensive .. perhaps you can find it by the single sheet at an art supply store.



i know that the big-box home stores sell foil laminated insulation board .. but i think the thinnest they have is going to be 1" ..



keep in mind a couple of things .. 1) rads should be about 2.5" away from the "wall" to assist with convective flow. 2) only a small portion of the heat from a rad is provided as actual radiation (which is what the foil board will assist with) most heat is provided by convection. therefore, if you add foam board but remove necessary convective flow, I think you are in a losing situation.



what will certainly help is removing enclosures if you have them (and that's free) long before a foil board.

David Nadle

I totally disagree

A radiator at greater than room temperature will give its heat away to reach equilibrium. In the absence of convection 100% of the heat transfer will be radiated.



Whatever fraction of heat that is radiated in a given room/rad/enclosure scenario, and I think the fraction is not small, but typically close to 50%, half of that will be radiated towards the outside wall and then, depending on the quality of insulation, conducted out of the house.  It makes perfect sense to have something IR reflective against the wall behind the radiator. Foil or aluminized mylar on a piece of foamcore or 1/8" hardboard should do it.



It'd be great if one of the Wallies with the new thermal imaging Christmas presents would take a shot from outside a house, with and without a reflector.

jpf321

true .. but

it's true that all things tend toward equal .. however at what rate?



According to this .. and several other sources .. most "radiator" heat is transferred through convection .. [url=http://www.diyfixit.co.uk/central-heating/radiators.html]http://www.diyfixit.co.uk/central-heating/radiators.html if the convection can transfer it's heat at a great rate, then the effect of radiation is diminished.



If you read the info about rad enclosures in Dan's Library .. you will find that simply placing a shelf 3" above the rad which overhangs the rad by 150% of the raddepth+2" of the rad, you will reduce the Heat Emission by 35% (see attached PDF) ..that's without even covering at all the face of the rad.



Certainly in the absence of convection a rad will attain equilibrium through other means, but it may take all day or longer to expel through radiation means vs. the flow of air passing over the hot metal and capturing the heat.



some reference material:

http://www.g9toengineering.com/resources/heattransfer.htm - explains that radiation is the least efficient form of heat transfer.

http://www.engineeringtoolbox.com/radiation-heat-transfer-d_431.html shows the formula for the rate of radiation heat transfer (loss)

there are numerous places to find the rate of transfer using convection, in fact it's the "standard" formula.  http://en.wikipedia.org/wiki/Heat_transfer_coefficient

JH_2

my limited experience...

I have installed panels made from "reflectix" foil faced bubble wrap on a bunch of my radiators on outside walls.  I did it only because I had the stuff. I really doubt any savings in fuel would offset the cost of materials otherwise.



I can tell you that when the heat is on, if I peel back the panel and feel the wall its ice cold.  If I then walk to a rad without one and feel the wall right behind the wall is quite hot. Now since my house is 200years old and many walls are uninsulated this may be saving me a buck or two.  If your walls are well insulated I doubt it makes a difference you would notice.



This is just one humble homeowners experience with no science involved, so take it with a grain of salt.



-Jeremy

David Nadle

Effective vs. efficient

JP, "Most" can mean 51%. Just sayin'... In the worst case (as far as those 1930's examples are concerned), an certain enclosure or shelf reduces emission or effectiveness by 35%. That's like having a radiator with 35% less surface area, or an equivalent heater with 35% less wattage. It does not mean the radiator is 35% less efficient. Efficiency is a dimensionless ratio of useable heat output to heat input, not power per unit area.



So, you're way off base in saying it would take "all day or longer" to transfer heat by radiation. if it requires X Joules of heat to bring a room from T0 to Tcomfortable, and you've got an enclosed radiator at 0.65 * Punenclosed, then it will take 1.5x longer to deliver the heat. Thirty minutes vs. twenty minutes.



Enclosed or not, a good part of the portion of heat radiated into a poorly insulated wall is lost to the room and therefore a reflector can increase both efficiency and effectiveness.   

jpf321

thanks David

thanks David .. i wish I had the brainpower to work out those rate equations for radiation vs. convection... but I know that I have heard it hear, from Dan or somewhere before that most i.e. 80%+ of the radiator heat is transferred through convection .. and I was only saying that if you eliminate the convective currents (part of the 80%+), you may be doing more harm than good by reflecting some of the remaining 20% ..



i wish also that someone would back me up on this 80%+ figure Dan? a book reference .. nevermind, I'll find it somewhere ..



there is no doubt that reflecting radiation otherwise lost into the room is useful .. i was only trying to say not to do it at the expensive of what i believe to be a bigger portion of the pie....



WAIT...I found it .. pg. 113 of We Got Steam Heat .. 60% by convection. so I stand corrected .. i'll call than an even split (50%/50%) .. so placing a reflector to eliminate rearward radiation would be saving potentially 20% of the rad's heat... a reasonable figure if my math is reasonable.



my apologies David .. thanks for pushing.

David Nadle

Deadmen from 1915

This article from 1915 points out that convection of hot air behind the radiator also warms the wall behind it and is conducted outside.



[url=http://books.google.com/books?id=OuMGAAAAYAAJ&lpg=RA6-PA22&ots=cpWZ9bMXbF&dq=radiator%20conducting%20heat%20to%20exterior&pg=RA6-PA22#v=onepage]http://books.google.com/books?id=OuMGAAAAYAAJ&lpg=RA6-PA22&ots=cpWZ9bMXbF&dq=radiator%20conducting%20heat%20to%20exterior&pg=RA6-PA22#v=onepage

jpf321

touche .. however ..

excellent find! I especially got a chuckle of the phrase "A brick spongy in character .. "



you will note at bottom of column 1 top of column 2 on that same article on page 20 ...



"The common acceptance of the term radiator or of the action of radiation as applied to heat transfer, needs correction. The term is a misnomer as applied to heating apparatus commonly known as a "radiator," since the heat radiated from such low temperature surfaces is a minor part of the work of heat transfer which they effect. They should br more properly termed "convectors,'' since by means of air-motion over their surfaces and physical transmission o: heat by contact with it, they effect the major part of their work.



Available information upon the subject of radiated heat, its laws, its methods and its comparative effects. is extremely deficient."

jpf321

there's also this ..

a fantastic device to measure the radiant heat of a gas radiator .. unfortunately .. "matters of pressing importance" prevented them from actually doing the measurements .. but it's a cool apparatus .. Heating and Ventilation Magazine May 1915



.. in that same issue is an article on the "difficulty of measuring heat" by Barker and Brendal .. where they say:



"We can easily conceive an enclosed room heated altogether by convected

currents of warm air, indeed this is one of the standard methods of

heating a room. It is not by any means easy to conceive a room heated

altogether by radiant heat. "



Anyhow .. below is the radiant heat measuring device.

Hollis

Years ago

I saw this in a magazine,... title  now long forgotten but probably Popular Science or Mechanics as I subscribed to them and they were showing this "new" technology of Infrared cameras ( which is actually older than most think as it took a while before it was in much use)

It showed a pic of a house from the exterior that had very clearly showing all its radiators glowing. What impressed me was that showed up pretty square shaped,..(I don't know what or if it had insulation)

I would have thought that by the time the heat came though the wall , whatever it had for insulation or not, it would have shown up as a diffuse spot.

lutorm

No way

Just because 40% of the heat leaves the radiator as radiation does NOT mean that you'll save 20% by putting a reflective sheet behind the radiator:



1. The wall already reflects some thermal radiation.

2. As the wall warms up, it also radiates thermal radiation back into the room.



The exact heat loss will depend on the emissivity of the wall in relation to the thermal conductivity to the outside.



With little insulation, the wall never warms up enough to radiate back as most heat is conducted to the outside. With a lot of insulation, the wall will quickly heat up and because radiation emission increases much faster with temperature than conduction (T^4 as opposed to T), this means the heat loss is smaller. This can be measured: if the wall is really well insulated, the surface temperature of the wall right behind the radiator should be the same as the radiator temperature itself (though getting a true measurement of the surface temp of the wall without getting biased by the radiation from the radiator is not so easy.)