Radiant in ceiling

singh

Radiant ceilings can heat satisfy the entire load, with out supplement. There are quite a few old systems out there in our area.
The installers back then did their math. If you perform a heat loss calc. first you would find most ceiling heat systems out peform the typical rubber tube staple up job with the same supply temps to both, Radiant ceilings cost effective to install to boot.
A draw back, may be what is called shadowing, as radiant energy can not see under , let's say a desk. Speaking of radiant energy a ceiling system is almost all radiant output , not conductive.
As with all systems, look at the thermal envelope, was there enough insulation behind the radiant panel, in the attic.
Tell him , he should be proud to own such a fine method of heating in his home.



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john_83

Radiant in ceiling for primary heat

Went to a job today for plumbing problem people just moved in and the gentleman asking if his heat was suffient asked what he had he said radiant i said very, but then he said its in the ceiling. Come to find out their is only attic above this and the old owner said thats how he heated the house. I've heated with radiant in walls and ceiling but only as supplement. Did'nt really have much to tell him. Wondering if anyone has any insight

Gordy

Ceilings

John if the initial design was good they are a wonderful thing. My house has radiant ceilings from the 50's. The house has seen winters to -25 plus while maintaining 75* interior temps. Grammy liked it warm.

I believe if you go to the library on this site there is some literature from chase copper on radiant ceilings.

Gordy

Unknown

many

John,

Not only have I serviced many systems with radiant ceiling but i installed it in my own bedroom. If you stay in the parimeters you have not problem.

110F supply equats to a 100F surface temp, or 40 BTU/sq foot. Stay below the 100F surface temp or you feel it on your head, unless your ceiling is higher the 8 feet.

The Wirsbo CDAM manual has a nice section on radiant ceiling.

Designed right, radiant ceiling is wonderful!!

steve

zeke

Personally, I think it is fundamentally a bad idea, since at water temps about 180 deg, the radiation heat transferred is only about 1/2 the total heat output, the remainder is convection heat which is not very useful at the ceiling level, since it remains up high. Unless you muse a fan to blow it downward, not the best way of heating a room.

Ed_26

What??

Please, you must read Seigenthaler's "Modern Hydronic Heating". You statement is completely wrong. Ceiling heat works very well.

zeke

I haven't read it. My observation is based on fundamental reasoning. You get the radiative out directly,so a person below will get that but the convective heat transfer which is about 1/2 the total is removed in the vicinity of the panel which, in this case is close to the ceiling and that heat will collect up high and will not do much good.

I would be interested if your reference disputes that.

ScottMP

Zeke

Where did you get the information that radiant is half conductive ? I'd always assumed there is some conductivity going on but never thought of it as being half. Can you validate that remark ?

IF, floor heat is 1/2 conductive would'nt cieling radiant be less, as the movement of heat from the floor to cieling would create a draft and increase transfer ? At the cieling there would be very little movement ?

I designed a number of them including my own bathroom. Not totally cieling radiant but going across the floor AND accross the cieling. We do shower walls now also. ME showed me how you can out the stuff ANYWHERE !

Scott


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Unknown

radiant 1/2 convective?

Zeke,

You better go back to the books and study.Or at least take a class on radiant heat. I think you may have misunderstood ME.

Radiant heat waves travel like light waves. The waves move in the direction of least resistance, in a straight line. Up, down, horizontal does not matter

Three types of heat transfer, (1) Radiation, (2) Conduction and (3) Convection. (The following are close approximations) Forced air 100% convection, baseboard fin tube heat 80% convection 20% radiation, cast iron baseboard or ci rads 60% convection 40% radiation and Radaint Heat 90% radiation 10% convection. (NOTE: Conduction phase on all of these is the heat transfer from water to the heat medium)

I did however come up with two theories to support your statement, (these are tongue in cheek and please take them as humor) if radiation was as much convection as you stated then (A) the earth would have to always be above the sun to get heat and the world is flat OR (B) if the earth was above the sun we would melt.

Just theory,

Steve

Gordy

Zeke

180* water temps?? Where did you get this number?

My system only sees 115* tops water temp. The temperature of the actual ceiling surface is about 85 to 90 degrees tops.



Gordy

Unknown

There is also a reduction in effectiveness due to distance. The rule of thumb for radiant output is 2 BTUs/sq ft per degree of surface temperature differential to room temp. For a ceiling, it's 1.4 or 1.6 BTUs.. I forget which off the top of my head. That takes into account the loss of convection and distance for a typical ceiling, I believe.

However, then you get to correct for the reduction in resistance to heat transfer (piece of drywall vs. finish floor and possibly a subfloor)... and voila, you've got your output back.

Ceiling is great. Though I have two areas (one, over a cold slab, the other, unheated space below the floor and possibly less than specified tubing coverage) where it didn't keep up. I'm not sure what's up with the over cold slab one.. I suspect I have a cold radiant plane fighting a hot one, and the cold one is winning... but the slab has never gotten up to temperature. I came in late.. it's uninsulated... I fear that it's just drawing all the heat into the rocky new hampshire ground.

Luckily we can insulated over it before finish floor goes down, but it has shaken me out of the "high output ceiling" complacency I had when dreaming of those 40BTU/sq ft output ceilings.

anyway. Ceiling is great.. it's not magic, darn it... but it is great.

I have one other situation where someone wasn't happy too. They have high output radiant floors in most of the cost, and some high ceiling radiant ceiling areas. It's just too much of a transition for them to go from their very warm floors to unheated, room temp floors. They said, "It would be fine if it were all ceiling", but the shock of the transition is just too much. I speculate it would probably also be fine if they were separated spaces (1st fl, 2nd fl) but they are adjacent rooms in this case.

Just things to think about.

zeke

Scott and Steve,

You guys are really piling on.

Let me explain myself, if I may.

Don't misunderstand me. All I said was that ceiling radiation is not as efficient as, say wall or floor; it's still is pretty good and has some advantages over purely convective heat.

The data I have is from an old "bible" on heat transfer, " Heat Transmission",`McAdams,McGraw Hill, 1942. It had collected numerous correlations from investigastors for plates heated and in various orientations ( and by the way considerable dust since I last used it)

For a heated horizontal plate facing downward, our case:

h=0.2*(delta T)^.25

If delta T is, say 100-70, then:

h=0.2*30^.25=0.47 BTUH/ft^2 deg/ft

The radiation emitted for a black body, one square foot is
e*@*(T1^4-T2^4)

where e = 1 for a black body

@= Stefan Boltzman constant .173*10^-8

For T1=100 +460=560 Rankine ( must use absolute Temp)
and T2=70+460=530, I get

q=34 BTUH/ ft^2. Then

h=q/delta T=34/30=1.1

And the total conductance is
h=1.1+.47=1.73

So, I am mistaken, and the convective heat transfer is about 1/3 of the total and you don't get much benefit out of the convective part and FYI, the floor numbers are:

h=.38*(delta T)^.25=.9 and the total conductance is

h=1.1+.9=2

So for floor heating the convective part is almost equal to the radiative and all of the convective is utilized. That was the case I was trying to make. Moreover, the heat there would be more uniform and since the person is closer to the floor, and the socalled "view factor" is higher and more uniform; hence the effect of the radiative component would be uniformly much stronger.

Needless to say, my vote is for floor radiation for both efficiency and comfort.

Unknown

That's for equal surface temps though. That does not take into account what it takes to achieve an equal surface temp in each case, which in practice seems to just about even out the loss of convection. The rest, and you'd also have to get over the psychological aspect of heating.. the people may have been able to set a lower thermostat due to more localized radiant transfer with a floor system... but they probably won't.

Plus, a ceiling system can easily be 1/2 or 1/4 the cost of an equivalent radiant floor method. That can be quite a cost hurdle to make up with an incremental efficiency benefit, or even to get the comfort in. And if you get the floor in... then you need to make sure no one throws a big thick throw rug over it, or your efficiency increase goes out the window as you have to jack up your water temperatures further.

You're right that floor is a 1st choice. But Ceiling is certainly an excellent choice and has a strong place in the hydronics toolbox.

Dave_4

Convection requires air motion

I guess the millions of sq. meters of radiant ceilings installed all over the world aren't enough to convince Zeke. And where does this talk of "convection" come into a radiant ceiling discussion? Convection requires air movement to convect the heat energy - where is the convection in a room with purely a radiant heated ceiling? Radiant is radiant, period. If you haven't got any air moving in the room, then there is NO Convection from a radiant ceiling and the radiant heat exchange from a ceiling vs a floor, when both surfaces are at the same surface temperature, is the same. Re-read the radiant equations again. The calculations listed above are based on exposed heating panels that have an air space around them that will create local air convection, just like the baseboard and steam radiators combined heating discussion in the other post here. That is simply not the same as a flat radiant ceiling surface.

The only reason that floors are the preferred radiant "heating" surface is because of the Conduction of heat to your feet, so you get a combined Conduction plus Radiant heat exchange. A ceiling radiant heating (and cooling) surface is strictly radiant only.

What have human beings evolved with? A large radiant heater above our heads (the sun). So where do all the myths about radiant heating from ceilings come from? A radiant heating ceiling is more "natural" than a heated floor if you want to split hairs.

jp_2

geoff

two ways to remove convection.

1.) remove the medium, air

2.) remove gravity

otherwise unless ALL is at the same temp, convection will be at work.

Dave_4

Yes, but here's the reasons

Not when you are dealing with large flat surfaces and still air in a closed room. Given that the radiant floor temperatures are not more than an 85F surface, that isn't enough to create enough bouyant air movement to provide any significant convection patterns. Similarly, with a large horizontal flat radiant ceiling in still air, even at 115F surface temps, you will not get any convective air patterns because the boundary layer of air next to the ceiling surface is the warm bouyant air, which has nowhere to go, it's already as high as it can be. So, no air physically moving, therefore no convective heat exchange. There would be some Conduction heat exchange from the warm ceiling surface to the air in contact with that surface, however.

In reality, a radiant floor surface will likely move more air by bouyant convection patterns, IF there are sources of cooler air that move onto the warm floor to be heated up and then convect up from bouyancy. This is the principle of displacement ventilation - warm objects in a space will create thermal air plumes, and the air has to come from "somehwere" to move into the lower region of the warm object to make up air for the air being drawn up in the convective plume.

Unknown

Interesting kinda debate guys,

let me ask this,,if your on a camping trip sitting beside an open-fire(no wind), of course the flames are shooting straight-up, but sitting directly beside you feel the heat. Is this radiation or convection?
BTW- No long drawn-out formulas are required, a simple yes or no will suffice.

Dave

Weezbo

Dave, i consider that ..

cooked on one side freezing on the other
only problem is the radiant would bake you before it heated your thermal mass all the way through to the side away from the fire *~/:)

John,

Radiant ceilings are wonderful things.
they are all over America.
radiant ceilings in a tight house with plenty insulation..needs, controlled ventilation.

Radiant floors are wonderful things... think of heating anterior rooms entering your home...
maybe you make a radiant bench...
*~/:)

radiant : floors, walls, ceilings, beams, posts ,benches all good

i like radiant Beams
it works as good as it sounds
*~/:)

Dave_4

Radiation

What you get from that fire in still air conditions is radiant heat. Although the reality is that the fire will be creating localized air movement as it draws cool air from around it into the bouyant hot air plume rising upward with the flame. In a still air environment, you will feel the cool breeze hiting your back as the air is drawn to the fire, but the heat you feel is going to be predominately the radiant heat from the fire.

G.Kaske_2

Geoff

Usually when I sit around a camp fire, Murphys Law says smoke will blow in my face .

Gordy

Gary Hayden_3

Excellent Paper Comparing Radiant Ceiling to Floors

http://www.byv.kth.se/avd/byte/reykjavik/pdf/art_089.pdf

G.Kaske_2

Excellent

Paper, and only reinforces what all the Pro ceiling radiant people have been trying to convey.

Gordy

zeke

Wow

What a wonderful paper, Gary. Thanks for bringing it to our attention.

I read it carefully and came to the folowing conclusions:

1) As we all suspected, there is a significant temperature gradient problem for the ceiling heat system at the floor level where, from fig 7 it is seen that the differential between room average and the floor is 3.5 deg C or 6 deg F, so the comfort where you need it most is compromised. This cannot be a quantitative generalization, only a qualitative observation since a lot depends on the floor insulation.

2)The ceiling convection is markedly reduced from the usual model owing to the increased localized temp there so my previous estimate of convection there is too high.

3 Surprisingly,the thermal efficiencies of the floor vs. ceiling heating are about the same.

4)The ceiling installation is cheaper and its susceptibility to damage is much less.

jp_2

but then what

heat the air, radiation or convection? you don't always start at 68F.

what do you mean by ""bouyant convection patterns"""

doesn't "bouyant convection" translate to "convective convection"?

G.Kaske_2

Zeke

I find the 6* temperature gradient from floor to ceiling a bit high. Of course it all depends at what point you measure the gradient(during heat call or at steady state).

I find only about a 2* difference about my home at steady state. The steady state floor reading is what you will see more than a reading during a call for heat when the ceiling panel will be at its highest temp.

One other benifit to a ceiling panel system is the fact that it is not restricted by floor coverings, and furniture about the room.

You could call shadowing a disadvantage but then how long does one sit at the kichen table or a desk. When one is relaxing how often are your feet on the floor to enjoy the benifits of floor radiant.

Gordy

Kal Row

it comes down to comfort...

because the efficiency differences can be debated until we are blue in the face – they are close enough, that it wont matter much on the bill, and your body cant feel the numbers, what it can feel however, is the warm tosies from floor radiant and the freezing tosies of legs under a table with ceiling radiant, or the burning sensation on your face when you are lying in bed and being bathed by infrared light from the ceiling, you might have an easier time getting your woman going with ceiling radiant but you will probably pass out from heat exhaustion long before she is done!!!

however, ceiling radiant has the huge advantage of being able to be used for cooling also (provided you have efficient staged low speed ac to remove moisture, and mold resistant ceiling materials), - but more important is, the radiant temp rise/fall speed and control – high mass radiant in floors, drives lots of people crazy as it doesn’t respond fast enough to rapid outdoor temp changes – whereas ceiling is almost always low mass and responds faster to demand changes – this is a problem even with fancy systems that have a slab sensor at each zone – I have seen houses with blown insulation on outer walls and between floors can take days to cool down, I have taken to insisting on supplemental electric radiant in baths so that people are comfortable even when the system is in Warm Weather ShutDown mode or the ac is keeping the house cool and dry and they don’t want to freeze in the bath – my systems are a real hit with the ladies!!!

!!!!SELL COMFORT!!!!

It’s a lot easier to deliver than efficiency – remember a man drives a car with 24” rims, so many cubic inches, and on and on…, a woman drives a “red one” with “nice music” --- political correctness doesn’t pay the bills, know your customer!!! – and know that a some point you will have to compete with DAIKIN multi head heat pump system which allows 8 heads on one outdoor unit and allows heat in one room, ac in another, dehumidification in a third, so on, and all at the same time, even well below 32 degrees as it moves unneeded heat around before it kicks in the electric heat boost, such systems coupled with electric radiant in kit/baths, are tough to sell against and they are damned near silent – the “scorched air” phrase is getting you nowhere with that customer – neither is efficiency!!!

Dave_4

Conduction heats the air

The air that is in contact with the warm radiant surface is heated by Conduction (contact). Convective bouyancy is warm air being lighter than cooler air and the warmer air will will rise, and conversely, cool air introduced into a room will sink in a body of warmer air - that's why the cold draft falls down the cold windows in wintertime and moves along the floor. Moving air (or any fluid in motion) creates Convection heat exchange. Convection is heat exchange via one cooler substance (usually a fluid or gas) moving across a warm surface and taking away heat from the warm surface from the fast-moving molecules passing across the warm surface. It is like "fast-acting conduction".

Unknown

ceiling over slab

Rob,

It is a matter of MASS. Radiant ceiling can not over cpome the slab mass. In experiments that we did, to over come this, we put a heavey pad and carpet over the slab and PRESTO it worked GREAT.

As for my radiant ceiling, I went that way because we had existing heavy pad and carpet. My surface temps we actually higher on the floor then if I had put in CFin in the joist bay!!

This stuff is fun to play with!

Unknown

In short hand, you can't get downward convective heat transfer. Cooling, yes, but not heating. Convection causes rising air, so if you heat from above, it can't rise any further. That's known as "dead air" if you're in a well enclosed space like a joist cavity. In a room, it's just not useful for heat transfer.

you'll heat some air, and you'll stratify a bit, but you will not get convective heat transfer in any meaningful sense.

Unknown

If you are getting a "burning sensation" from a radiant ceiling, you didn't do a very good job with the system.

Remember if a house is properly built in the first place, a radiant floor won't even be noticeably warm in the first place. In those homes, ceiling really "shines".

Unknown

Yeah, but I don't get that Steve. I would expect the mass to, at some point, even if it is days or weeks later, heat up. But apparently it was losing heat faster than we could supply it, because it never, ever did.

In a FHA home, if it's well insulated and the heat is run consistently, the floor will eventually warm up to something near room temp. But in this case I was surprised that the slab stayed as cold as it did. Now, it was not insulated. But still I was surprised at how much of a load it presented to us.

I hear the carpet/pad thing though.

Unknown

we tried

NRT, I had a tough time understanding it too until I started to think on the mass vs heat ratio.

The only way I understood it was to have a match trying to heat a pot of water. Even if the match couild burn forever you could never really overcome the loss fron the water.

We tried eveything including jacking up the supply temp (ie surface temp) to an unbearable high and just could not do anythinig with the massive downward losses.

We nvested in the carpet and pad and presto, all problems gone AND at a 100F ceiling and 78F floor surface temps.

GMcD

Downward convective heat transfer

Yes you can-that's how chilled beams work - by creating cool air convection - cooler air is denser than warmer air and will "fall" down in a room. A flat radiant cooling ceiling normally doesn't operate at a cool enough temperature to cool off the boundary layer of air near the ceiling surface to cause it to separate and fall. But if you look at chilled beams, they DO need to operate at cooler temperatures than a radiant cooling ceiling, and will cause boundary layer separation, and convective cool drafts falling down in the room.

Unknown

Yes, and that's cooling, not heating... as I stated

Kal Row

Just you “splain dat”....

to the woman that equates “radiant” to being able to walk around barefoot comfortably after being on heels all day – let me know when and where – I’ll take video
I go crazy with this, taking the heat loss of each room and hallway location, and placing more tubing where she is likely to walk and stand and spread it out everywhere else so that the total btu is the same but she has the warm floor effect at the same time –

Notice that women are always cold and men are always hot, that’s because the creator designed the male to go out into the cold and hunt it, and the female to stand over the hot fire and cook it, ah “BIOLOGY”, the great equalizer, puts the “post”, into post-modern feminism!!!

Unknown

yeah, but that's what bugs me. You can crank out 30+ BTUs/sq ft with ceiling. It's got to go somewhere! If the slab loss isn't that high, then eventually it should heat up... but, it doesn't.

So either slab loss can be pretty darn high, or there is an element involved here I haven't wrapped my head around successfully yet.

Unknown

I do, every day. If they are willing to shell out the bucks to micro engineer a warm strip floor, assuming they can even use an installation method where that is possible, then so be it. Of course if you're doing a slab (the only method I can think of where that is even feasible), there is no reason to use ceiling in most cases. Slab is as cheap as it gets.

But for suspended floors, if they have done due diligence on the envelope, which we *always* recommend they do before we do anything with a heating system... then floor and ceiling aren't all that different. Granted there is often a room or two where it's just not possible. But really, when you're at 12 BTUs/sq ft or below especially, the difference can be very small. You just have to talk "comfort", instead of "warm toes". And the ceiling can save big dollars to buy all that insulation and good window tech.

But if you want to play the "splain that" game, how about explaining to the owners of this proud new radiant system why they can't use oriental rugs in their dining room. That happens a lot too.

jp_2

radiant & convection, little conduction

in theory I agree with you Geoff.

but that conduction area is maybe a few angstroms deep and air will spend fractions of a nanosecond there.

so in practice, you have much more convection going on than conduction, when considering the room as a whole.

so then the question falls back to, "how much 'air' heating happens by radiantion?" some air? no air?

not really talking about 'forced convection' here.

GMcD

What is the force that is moving the air?

If you have a room with no air going into it, or moving out of it, what is going to make the air move and set up convective air movement patterns?

A warm floor will set up convective air movement patterns, especially if there are cooler walls and ceilings that have cooler boundary layers of air against them that will start the convective air movement patterns.

A warm ceiling cannot create any convective air movement patterns beacause the warm bouyant air at the boundary layer, that has been heated by Conduction, has nowhere to go.

GMcD

Just quibbling with ya.

"Convection" doesn't just apply to heated bouyant air. Convection is any motion of a fluid in direct contact with a hot or cold surface. Convection can even be between two fluids of differing density- consider how cooling towers work-the heat transfer from warm water to cooler air is accomplished through loss of heat energy from evaporation, as well as the cooler air convecting heat away from the warmer water droplets as they drop through the air. Evaporation is a dominant heat transfer mechanism in a cooling tower, but convection also fills in the rest of the heat transfer total.

A hydronic heating coil or cooling coil in an airstream are examples of convective heat transfer as well.

Unknown

Understood, I was imprecise in my use of the term.