Heating Effect in Baseboard ratings?

[Deleted User]

FUGETDABOUTIT.

It's a false number without basis.

I like Brads explanation. It's about the only one that KINDA makes sense. However it is riddled with a lot of "what if's"...

I was told by an early mentor of mine that it was a number that was arbitrarily thrown in to make baseboard more attractive than its clunky cast iron upright cousins. He's joined the ranks of Dead Men now, so I guess I may never know if he was yanking my chain or not. He was a certified I=B=R instructor tho...

We always discount the 15%. I was told by a knowledgeable mechancial engineer that we are also supposed to be derating the output of baseboard at altitude (5,280' ASL) by around 20% as well due to air density issues... I've never done that, and never had any issues.

So, if I am below seal level (Death Valley) can I INCREASE the output numbers based on altitudinal atmpospheric density increases???

Wandering minds...

ME

Chuck_17

Baseboard ratings include 15% for heating effect. What is the heating effect?
It may be explained elsewhere on this site, but I could not find it.

Brad White

It is

a factor applied by I=B=R back in the day to account for radiation installed along outside walls.

By creating a warm film of air between you and the otherwise cold outside wall, it reduced the difference in mean radiant temperature between you and the outside wall. In effect, it allows you to install less radiation to meet the heat loss. More specifically, it is built-in to the ratings.

If you were to install the radiation on an interior wall and not on the exterior, you would need 15% more element to compensate.

I am sure that they arrived at the number with some science behind it but I suspect it is also rounded up for convenience. It also gives parity across a number of manufacturers using independent ratings.

EDIT: I should have mentioned, as Mark E. did, that I usually back out the 15% anyway and assume a lesser output. That extra 15% effect if I ever see it for real, is a bonus. But at least I know that I am covered and if a surplus of radiation, I can use a lower design water temperature across the board. I was only trying to answer the "where did it come from" question, but a "how to apply it to a design" is a fitting follow-up question.

Mike T., Swampeast MO

Watch out. I believe the 15% "heating effect" addition assumes that the exterior walls are nearly completely covered by the baseboard.

If this is a well-insulated and tight structure requiring that only a small fraction of the wall have active baseboard element, then I would forget about the "heating effect".

Jed_2

\"small fraction\"

Mike, what kind of room layout are you picturing? Here, in the northeast, the ratio of exposed wall vs. room area and volume can vary greatly. Given a room (let's say Living) of 15x20, with 8' ceilings and typically insulated to Code standards, with two exposed walls and no unusual window/door area. It might take about 18' of standard baseboard. This is using a 90^DT ( I doubt you have anything near that in your area). So, the heating effect of 15% would be about 3(2.7) feet of baseboard. If the heat loss is around 10,400 BTU/H, and we ignored the heating effect, you would put in 15'. But the HO's want a quick response from their radiation(we're not talking constant circulation, here). The cost of those 3' of additonal active element is peanuts. I'm just saying with copper finned tube baseboard, it doesn't make sense to "micro-engineer" the "perfect heat balance" system you so passionately strive to achieve. There is much forgiveness in it. And most Mainers appreciate that.

Jed

Mike T., Swampeast MO

What type of room layout are you picturing?

Granted this is an extreme example, but at least it's in far northern Maine.

Large master bedroom on the highest heated floor. About 40' of exposed walls on two sides and fairly generous window area. So freaking well insulated and weatherized (literally a shell constructed inside of another shell and attic insulation so thick that it was damned near the point of no additional return by adding more) that about 5' of conventional baseboard at 180F average temp and 20F delta-t would meet Manual J design at -20F outside.

Two-pipe, home run system with TRVs.

Instead I designed for 150F @ 20F delta-t with all expectation that 130F actual supply temp will prove adequate. 5' of double-sided "pedestal" Myson b/b in the room. I could not in any conscience include the 15% "heating effect" in such a space even if the b/b is located under a bank of triple-paned "super" windows that face west towards a lake and "teriffic" winter wind.

Again, an extreme example, but once we connect mod-cons to baseboard in even reasonably insulated structures I personally would not include the "heating effect" addition for new construction.

ralman

factor

The modern hydronic heating book, 2nd edition, states that it is an allowance for baseboard convectors placed near floor level rather than the height of a typical standing cast iron radiator. They allow the manufacturer to rate baseboard output 15% higher than the measured heat output established by laboratory testing as long as they put a note indicating the factor was added in the rating table. The author(John Siegenthaler) recommends to correct the factor(divide output by 1.15) before selecting baseboard length.

GW

how about convective venturi effect?

I'm just a regular heating dude, but I was wondering how much "radiant" actually emits from a baseboard anyway. Maybe I misunderstood; perhaps we're discussing the cold radiant from the wall/glass?

Secondly, as the warm air is passing by the windows, isn't it possible for the baseboard heat to induce a venturi effect on the non-air tight window framing? That is, sucking in cold air.

When we do panel rad jobs, that question comes up all the time and I dismiss the notion that heat must go on the outside wall.

However, I realize if some person with sparse hair reclining up against a cold window will have the heat sucked out of him; there are always several comments to go along with the many heating topics out there.


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GW

secondly

Secondly, how many BB systems are running all the time? That film you discuss (I've heard the word air curtain)only works when the heat is on.

In an effort to achieve maximum comfort, the good heat dudes will install constant circ. I know you know this, just commenting or the masses.

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Brad White

Good discussion points, Gary.

"Regular Heating Dude"? Hardly, Gary. I think you are exceptional, not that you asked.

The "radiant" portions that I was referring to indeed was the "cold radiant" or MRT portion of the comfort spectrum.

To illustrate:

Older heating systems, for example, commonly hot air was supplied from the interior core of a house (late 1800's, early 1900's construction). This was an off-shoot of the old coal stoves in rooms venting into the central chimney. "That is where the heat went in", seemed to be the operating philosophy, or at least that is where there was chase space for the ductwork.

Return air, added to that, was optional. But those houses (I have lived in some and owned one), always felt cold on the exterior side of one's body. That exterior wall could never get warm for it always saw "the end of the heating chain".

Once the walls are insulated at least, that effect can be minimized but not eliminated. I favor panel radiators below the windows to counter the MRT negative effects.

Forever there is that issue of 110-120 degree air against the exterior wall (increasing delta-T) but this mitigates quickly as the air rises and mixes. Should be an interesting study out there somewhere!

Take your infra-red gun and measure wall temperatures to see the difference between one heated on the perimeter versus one heated from an adjacent wall or from the interior, both insulated and not insulated. Compare that to the 85 degree or so bare skin body temperatures of the occupants. One degree difference is perceptible to most people, imagine a room wall at 72 degrees versus one at 58 degrees.

I agree with your underlying premise that the baseboard system works far better with constant circulation, but even intermittently, once you get that convective film going, you have a "block" between your radiant body and the cold outdoors.

If the "venturi effect" of rising air inducing more outside air into the space through the windows is an issue, I see that as a window problem. The motive force of venturi action in a baseboard system is very small. If the windows leak that much, no point in trying to heat it until that is fixed, in my opinion.

I also see that even with low-temperature baseboard applications, the convection is accelerated during initial warm-up, when the room is coldest and the delta-T to the element is greatest. Naturally this narrows as the room comes to temperature. The convective output I would have to think is about 85-90% and the remainder radiant but have no data to back this up.

I would think that if you IR-gunned a baseboard cover and assumed a "two BTU per SF per degree difference" over the cover area, you could come up with some idea of the radiant portion.

Sorry to ramble, you got me thinking....

Brad

Glenn Sossin_2

Interesting concept

I like rambling - some new ideas/approaches seem to creep out from within.

Brad - you said "By creating a warm film of air between you and the otherwise cold outside wall, it reduced the difference in mean radiant temperature between you and the outside wall. In effect, it allows you to install less radiation to meet the heat loss."

I can understand the reduced temp on the interior surface of the outside wall, but I'm having trouble accepting the idea that less baseboard would be needed. This film of air loses it's heat energy to the colder outside wall, this energy has to be continually replaced to maintain this "curtain effect". Where is the savings? It seems like it flies in the face of common logic.

The radiant portion of heat transfer is dependent on distance, mass, and temperature difference. The air has comparatively no mass so I don't understand how this curtain of air could have such a substantial affect on the energy transfer between a more substantial mass like a body and a cold wall.

The closer a heated mass is to a cooler mass, the higher the radiant energy transfer component. We are talking about putting the heated baseboard directly up against the cold outside wall. I see more heat energy lost this way than if the board was placed on an inside wall.

The idea seems to be, that by creating this warm air curtain, and reducing the temperature difference between interior objects and the (curtain)interior surface of the exterior wall, there will be less radiant energy transferred. What about the increase in conduction energy transfer between the much higher temperature / higher mass baseboard being placed in direct contact with the cooler outside wall? Wouldn't this resulting heat energy loss be higher than any reduction in radiant losses between other surfaces/masses?

Why is the baseboard any different than a person? It's a heated mass too and as such releases energy to cold surfaces/masses.

I'm not suggesting this, but by installing radiation on an interior wall, we would virtually eliminate this direct absorption to the outside wall. We would still be heating the air in the room through convection and there would be a reduction in the direct transfer of energy between the heated baseboard and the cold wall.

What portion of a room heated with baseboard is accomplished with radiant energy? Unless it's cast iron, I would think its minimal.

I must be missing something here - can you clarify this concept for me.

GW

yes

good points Brad. I never considered the film/curtain effect with space heating. I will keep that in mind. If we have a choice, we always go for the windows, but i there's a challenge or an obsticle, we're not going to "insist" we make the route happen (likely to lose some sales in that process getting to some of the upper floor locations).

Once at a Unico training I learned of this concept with a wealthy rock and roll star who wanted an opening to the outdoors (no door, or door always open; can't remember). The mini-ducts would be placed over the door, creating a curtain. Of course this is mechanically induced, where as this thread is discussing convection.

Always a pleasure, Gary

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Brad White

Hi Glenn

You asked, Brad - you said "By creating a warm film of air between you and the otherwise cold outside wall, it reduced the difference in mean radiant temperature between you and the outside wall. In effect, it allows you to install less radiation to meet the heat loss."

Yes, if you accept the 15% factor or at least accept the effectiveness of the "barrier effect" of a film of warm air or warmer surface between you and the outdoors, you could install less radiation. More to the point, you would be getting a much better use out of the radiation you have.

I am not advocating reducing the amount of radiation as I would rather have a surplus. So long as the radiation is sufficient to take on the heat loss and is, even if over-sized at a given temperature, proportional room to room, I am a happy camper.

Agreed on the principles of radiation, but this goes further than that and in a simpler way as I see it. Once you negate the outside wall heat losses, (by that film of warm air between you and an otherwise colder wall), you no longer have transmission losses. (Infiltration being the other portion). Does that make sense?

I have heated large warehouses using surface sensors on the outside walls. So long as I kept those walls at room temperature, that portion of the heat loss was done.