Radiator Placement

Aidan (UK)

I think the radiation meeting the glass is either absorbed, reflected back into the room or transmitted straight out into space. Most of the energy passes straight through.

This is all a sweeping generalization about the theory behind why radiators are usually placed under the windows. I couldn't estimate any numbers. I think the calculations involved emissivity ratios, Stefan-Boltzmann constants, aspect ratios, etc. There would be some very tedious number crunching involved. I would think the cold draught from the glazing would be eliminated by a radiant floor as it passed over it, but the draught might still be uncomfortable at some distance from the window.

The total heat emission from underfloor heating will have convective, radiant and conduction components. I think the radiant proportion is larger for the radiant floor than for radiators or baseboard, but again, I don't know the actual numbers involved.

Mike T., Swampeast MO

In this quest of mine to maximize, utilize and understand radiation in space heating I may have hit on something that is quite contrary to "conventional" wisdom.

Can't find much on the web, but I'm NOT the only one. Link below is really quite good--pay particular attention fairly far down to the questioning of conventional radiator placement (under windows/on exposed walls).

http://www.risingtide.org.uk/tyh/themes/heatwat.html

The "suggested" placement for radiators (iron or panels) has long been either under windows or against exposed walls. The logic is that this is the area were most cold air enters a space and such placement helps negate drafts by WARMING THE COLD AIR ENTERING THE SPACE.

As we all know, home construction in general is MUCH tighter than in the days when this "rule" was made. Lifestyles have changed as well and it doesn't seem as common to leave windows cracked open even in the dead of winter.

Most of the radiators on the 2nd floor of my house (built 1903) are on INSIDE walls; most on the 1st floor are on OUTSIDE walls. The home is reasonably and recently insulated and the windows (while original) have VERY little infiltration due to good storms and effective, long-lived infiltration control.

The radiators on the INSIDE walls tend to run relatively cool with a very high delta-t (as measured at supply/return connections to the rads). Those on the OUTSIDE walls tend to run much warmer, but with much lower delta-t. (Please remember that I'm considering MAINTENANCE conditions with TRVs and constant circulation here!)

Why? I BELIEVE THAT THE PLACEMENT IS AFFECTING THE AMOUNT OF RADIATION DELIVERED.

Those radiators on outside walls or under windows have the best "view" of the warmest portions of the space--the INTERNAL walls. Since radiation is COMPLETELY driven by difference in temperature these radiators are delivering a higher proportion of their output CONVECTIVELY as there is not as much "cool area" in view of the radiator to "suck" the radiation.

The radiators on the INSIDE walls however, have a wonderful "view" of the COLDEST portions of the space--the exposed walls and windows. Consequently, more heat in the form of radiation is "sucked out" of the radiator by these cold areas.

I've noticed that most radiant wall PANELS tend to be on INSIDE partions. Have you done this for freeze protection or to make the coldest areas have the best "view"?

Is this yet another piece of the puzzle?

The only time I notice what could be considered an objectional draft in the spaces heated by radiators on INSIDE walls is when a set-back TRV has been raised. Bare feet are definitely able to detect cold air moving across the floor. Once maintenance is achieved however, it seems no greater than any unheated hardwood floor in any space.

If we can agree that space "a" ALWAYS looses "x" number of BTUs with a specific set of conditions, it seems to me that by merely placing radiators where they have the best "view" of the coldest portions of the space that we can enhance the radiant portion of the rads output.

Aidan (UK)

radiator position

The theory does not involve cold external draughts entering the room, but generating indoor convection currents.

The window will act like a plate heat exchanger, which cools the air adjacent to the glass inside the room. The cold air will drop towards the floor. If the radiator is on the opposite internal wall, it will create a convection current of rising air. This can create a fierce convection draught, with cold air moving across the floor from the window to the radiator and a warm draught moving across the ceiling from the radiator to the window. People are very sensitive to ankle-level cold draughts.

Commercial buildings with forced air heating will often have convectors under the windows to counter the cold draught the glass can generate. When some heaters failed, I've watched people trying to find the (non-existent) gap where they thought the cold draught was blowing through the sealed, non-opening window.

I've got to agree that, in terms of efficiently getting heat into a room, an external wall is not the ideal position.

Mike T., Swampeast MO

Is this a fair representation?

Typical rad placement under largest glass area in space with two exposures.

Aidan (UK)

Radiator location

As I understand it, the window is generally a large cold area and will generate a cold, downwards convection air current. The radiator will generate a warm upwards convection air current.

The convection generated by the radiator is usually more powerful than that produced by the window, because there’s more heat emitted.

If you put the radiator on the internal wall, the two air currents will add to one another and generate a ‘conveyor belt’ type movement of air in the room. There will be a cold draught flowing across the floor and many people, especially women with exposed ankles, can find this uncomfortable. This doesn’t require any draughts from leaky windows, but they would add to the effect.

If the radiator is under the window, then the two convective air currents will oppose each other. The warm draught from the radiator will overwhelm the cold draught from the glass. The resultant air current will be a warm upwards draught which will disperse into the room. However, the radiator will also heat the external wall, so the heat losses will be greater.

You’d need thermal modelling software to estimate the air velocities produced. Double-glazing, draught-proofing and insulation would reduce the effect. Two friends have put rads on internal walls and say they haven’t noticed any draughts. The draughts from the sealed windows I mentioned in the previous post were generated by large (probably 12 feet square) single glazed windows in steel frames.

Boilerpro_3

Ahh, but I think you are only looking convective heating,.....

not radiant heating. If the radiator can "see" the cold surface of the window it will heat the window surface directly via radiant energy, warming the surface of the window. If the radiator is placed on the inside wall, it can then radiate heat to the cooler surfaces of the outside wall. Together, I bet these both would reduce the cold draughts that overwise would occur if you were using only convection heating, not a radiator that produces some of both.

Unknown

Ok, estimate for me,

> not radiant heating. If the radiator can "see"

> the cold surface of the window it will heat the

> window surface directly via radiant energy,

> warming the surface of the window. If the

> radiator is placed on the inside wall, it can

> then radiate heat to the cooler surfaces of the

> outside wall. Together, I bet these both would

> reduce the cold draughts that overwise would

> occur if you were using only convection heating,

> not a radiator that produces some of both.

Unknown

Ok, estimate for me,

What will be the surface temperature of the glass in both cases, at design conditions? The surface temperature controls the convection part. I don't think clear glass "traps" much radiant energy.

I'm trying to figure out how much of a difference it really makes.

Noel

Mike T., Swampeast MO

Bare ankles, droughts & radiation

I've worked in enough office environments to know that the typically bare ankles of women can be a serious concern to them. Alas, all were convective environments--either ducted air or unit heaters.

It seems worst on hard surface floors and I know more than a few women who keep a small electric heater under their desks in the winter time--most seem to prefer the RADIANT type.

I wonder how much of this problem has its roots in STEAM heat--remember that steam rads give LOADS of convective output when "steaming" but since the iron holds so much heat they tend to radiator as they cool--kind of an off-hand way of achieving "balance" but it seems to work reasonably. Steam rads certainly have the power to make objectionable air currents.

Perhaps we've just continued with the "norm" even though much of the reason it became that norm is now missing... NOT an uncommon occurrance by the way.

Mike T., Swampeast MO

Agreed that the glass doesn't "trap" much radiant energy but it does? "draw" it because of its relatively low temperature. Since it's quite transparent to most/much radiation (including IR I believe) I have ZERO idea how (or even if) radiation could contribute to staying somewhat warmer than the air on the other side.

I use light-blocking roller blinds in my office with the rad on the interior walls. I generally find it more comfortable to keep the blinds open during the day in winter and closed at night. VERY HARD to differentiate temperature comfort from lighting/viewing "comfort" though...

The exposed WALLS are a completely different matter!

Some emissivity values (closer to one the more radiation is received and transmitted):

Glass .89
Ice .98
Skin .98

Unknown

I think that way, too.

My point is only this, the convection currents falling from cold glass being met by convective currents from the heat emmitter cancel (to a point) each other out.

Does radiant energy added to the glass itself do the same job, with the radiator moved to an interior wall?

I feel the way Aidan does, here.

Noel

ScottMP

I remember

A few months back some one posted the percentage a Radiator actually "radiates", as compared to its convection output. It was a small amount of its output that was radiant.

I am not sure how much of the window would be effected ?

Scott

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Mike T., Swampeast MO

Thoughts regarding glass from a bit more research:

It seems you essentially have to forget that you can see through it! Like any other object it is radiating to and receiving radiation from its environment.

Glass has relatively high emissivity--thus it is quite good at receiving and giving radiation. If cold glass is view of a warm object like a radiator, it will be receiving more radiation than other (warmer or less emissive) objects in a room.

Forget that it's transparent, but remember that it is a good conductor and VERY thin. The OTHER side of the glass is in "view" of the great outdoors. As soon as the glass is warmer than outdoors it is a net radiator OUT--just like walls, ceilings and everything else. BUT since it's a good conductor and very thin, much of the energy it receives is immediately lost. This is why multiple glazing is quite effective. Not only does it slow convection greatly (by trying to make "dead air") but slows radiation as well because the final outside surface can be quite cold (little net radiation leaving) with the final inside surface significantly warmer. Low-e(missivity) glass COATINGS "work" by changing the radiation potential on ONE side of the glass.

As to a "sweeping generalization" as to why rads are generally placed the way they are, I completely agree. It's just that the "sweeping generalization" (at least from EVERY source I've read) is that rads are placed in the area with the coldest AIR.

Mike T., Swampeast MO

Remember that MANY supposed scholarly and highly "professional" sources INSIST that even with radiant floor heat in a concrete slab of low temperature that the radiation/convection proportion is about 50/50!!

I cannot "prove" nor "disprove" this. REMEMBER though that convection is a form of CONDUCTION--just between a solid and a non-solid. Convection IS conduction--it just happens more rapidly than if both objects were solids.

The extremely low floor/ceiling air temperature differential with "pure" radiant panel heat leads me to believe that the convection that is occurring is EXTREMELY slow--much more similar to conduction as we know it than convection as we generally "feel" it.

Placing a radiator along an exposed wall or under a window puts the radiator in an environment intended to enhance convection--those cold walls/windows are heated primiarily by the moving air in that situation.

Place the radiator FACING the cold surfaces and you are trying to enhance the heating of the cold surfaces by RADIATION! You can make this task MUCH "easier" and EVER more effective if that radiator is controlled proportionally and putting out just the amount of heat required by the space.

In a space loosing a given number of BTUs with a radiator of given area IN FULL VIEW OF AN EXPOSED WALL:

YOU MUST ASSUME YOU ARE MAINTAINING SPACE TEMPERATURE PROPORTIONALLY, i.e. the radiator is giving out just the number of BTUs required to maintain the current state.

Compute your heat loss of that wall at current outside temperature. Measure the temperature of the wall in a few places and do your best to get an "average".

Assume the radiator is quite hot, say 170° and compute its radiation using some constant (say 40% of EDR) for the rads' "real" radiating surface area. Find the proportion of radiation of the total heat loss of that wall.

Do the same at lower temperature, say 100° and re-compute keeping the rest of the numbers the same.

You will find that the PROPORTION of radiation INCREASES as the temperature of the radiator DECREASES!

I have done this in different ways and at different times even trying to do it with all the surfaces in the room. The results are ALWAYS the same. The proportion of radiation increases as the temperature of the radiator decreases. The "real" numbers are impossible to compute--it is the PROPORTIONALITY that matters.

Now go back to my contention that in a given space being maintained in temperature:

Place a radiator against an exposed wall or under a window and that radiator will run at a higher temperature, with a lower delta t.

Place that radiator in VIEW of the exposed walls and that radiator will run at a lower temperature and higher delta t.

What has been maximized??? What has been minimized??? If air infiltration into the space is reasonably low which do you think will be more comfortable?

Eric Taylor_35

Me Again.

Hi Mike,

I'm wondering how you arrive at the conclusion that the proportion of radiation to convection increases as the temp of the rad drops. My reading shows that the convection heat transfer coefficient is not a strong function of temperature while the radiation heat transfer coefficient is. That leads me to believe the exact opposite of your conclusion if all other conditions are the same.

I do believe that a rad placed on an interior wall will radiate more due to it "seeing" cooler surfaces (the radiation coefficient is increased), but by heating the windows and exterior walls via the increased radiation you are in fact increasing the heat loss of the space and the load on the rad. This would account for the rads placed against the interior walls running cooler than those placed against exterior walls--they work harder so the delta T across the rad is higher.

Eric

Mike T., Swampeast MO

Hi Eric

You saw and I believe agreed to the ACTUAL equations I used in our previous (off wall) writings. Check your archives.

Eric Taylor_35

New PC

Mike,

My disk crashed a few months ago and I had been neglecting my backups. My recolection is that we were talking about finding the real proportion of radiation to convection for a given set of conditions and not the change in proportion due to a change in those conditions. I think this topic is different and I'm curious how you arrived at your conclusion.

Eric

Mike T., Swampeast MO

The true purpose in that testing on those cold, overcast, temperature stable days/nights that winter was to find out how that radiator in my office manages to heat the space while it stays at such low temperature.

At about 95° average temp, that rad heats a 200 sq.ft. or so space to about 68° when in the low TEENS outside!

Many published iron radiator output tables show this is impossible! According to them, the output of a radiator at that temperature isn't low, it is ZERO!!!

The "tissue paper test" revealed that the convective current coming from my computer monitor seemed greater in magnitude that that from the radiator!

So, I set out to try to find out the convective/radiative proportion of the radiator as compared to a space that I could heat nearly purely by radiation (radiant floor panel) or nearly purely by convection (electric baseboard). While that task proved WAY beyond me, what I seemed to find EVERYWHERE was proportionality--proportionality that could be PREDICTED.

In general, things that can be predicted can also be MANIPULATED.

The only "real" result that came from the experiment was what appears a "rule"--that an object heating a space at MAINTENANCE will be at the lowest possible temperature with the highest possible radiation. FOR THAT DEVICE, IN THAT SPACE!

BUT, something STILL seemed wrong. Radiators of a similar degree of oversizing in a similar space but a RELATIVELY DIFFERENT LOCATION IN THAT SPACE seem to run hotter. Why? How can that be?

If I'm correct, it is because "AT THAT LOCATION" should be added to the above "rule."

Unknown

another factor, Mike

What is the temperature of the wall directly behind the radiator. In a maintenance period, how do you feel that this will effect the water temperature, and do you think this effects the VOLUME of BTUHs at each water temperature. I think the wall temp behind the radiator would become much higher at the interior wall, and reduce the radiant output total, which would cause the same output to the room to occur at a higher water temperature. Did that make sense? Higher temp, same output?

Noel

Mike T., Swampeast MO

As far as I can tell (regarding standing iron against interior partition) it's not much warmer (less than 10°)directly behind the rad. VERY hard place to get a reasonably accurate measurement... The differential drops RAPIDLY with degree of separation from the rad. My guess is that it primarily a convectively generated differential.

It is often stated that you can increase radiation from a rad by putting a reflective surface on the wall behind. I haven't tried to test that...

Guy_5

Radiator Placement

Having been in this industry for many years, I can certainly appreciate all of the thought that has gone into this issue. In my own (previous) old home I really believe that the upper floor radiators were placed on the inside walls because that was where the chase way was. Right along side the old chimney flue. Central fireplaces were the central heat. I realize that this is a less than scientific approach, but back then I think that they were very happy to have a heated room.

Eric Taylor_35

We Know

Anytime there is a temperature difference heat is exchanged. So any emmiter that is above ambient temp will reject heat to the space, so any chart thet shows zero output at even a small temperature gradient is flawed from the gitgo.

Why does your rad heat the office so well at such a small operating temp? The simple answer is that it is grossly oversized. A more complex answer is that the room is tight, insulated, in contact with other rooms and heat sources, the rad is perfectly controlled and constantly emmitting the right amount of heat into the space so operating temp is minimized. (It's not oversized if you DESIGNED it that way :-P)

The tissue test and your conclusion about the magnitude of the convective currents is dead on! But magnitude and energy are not the same thing. The computer monitor is operating at a higher temp then the rad, so it will put out a small volume of heat at high magnitude (temperature), BUT the rad is way bigger so it will put out large volume of heat at low magnitude (temperature). Measuring small amounts of anything is difficult to say the least.

I still don't see anything to support proportionately more radiation at lower operating temps.

Eric

Mike T., Swampeast MO

I just "resurrected" the post with the numbers. I was wrong, it wasn't in "off-wall" communication, it was "on-wall" in the week of Christmas, 2002.

Mike T., Swampeast MO

I'm not claiming that the dead man who designed this system placed the rads on inside walls to maximize radiation--he most likely considered it a "concession" required by the pre-existing structure. He may have even thrown in a bit of extra iron to compensate...

Guy_5

Just look at all of the thought that one Dead Man has provoked here. We all have a collective education that stemmed from his work. Whatever the reason for the radiators being where they are, we are all now better for it! Thank You.