Radiant systems

Chris_32

We are loking at a radiant retro project and can not use the floor easily. We are considering a wall or cieling system. Our concerns are that there seems to be a consensous of about 2 btu/h/sqft/f from a floor but that the numbers from a wall or cieling are all over the place with as much as .5btu/sqft/h difference from one manufacturers literature to another and almost none of them agree with ashrae as near as I can figure. What is the deal with these systems. Any help is appreciated.

Chris

RB_2

For your bucket of knowledge...

Chris,

The 2 Btu/hr sf F is called a heat transfer coefficient or HTC.

The HTC is a “nominal value” i.e.: it is not fixed – its variable.

The value of 2 is actually the combination of radiant and convective (natural) heat transfer.

In a radiant heated floor, it is typically assumed by most designers to be 50% radiant and 50% convective but there are many things, which can influence this split.

For example the more inefficient the home the greater the convective transfer. This is because in poorly build homes the airs buoyancy is fairly aggressive – meaning as cold air contracts it becomes denser, drops to the floor where it is heated – expands, rises and thus the convective current begins - which figuratively scrubs the Btu’s out of the floor. The greater the temperature differences between the heated floor and unheated walls and ceilings the more aggressive the air currents.

Very efficient homes have very little natural convective thus a greater percentage of radiant heat transfer.

By definition, a radiant system must have over 50% if its heat transfer by radiant but there are in fact many systems where the convective component is higher and though they might have heated floors, by definition they are not truly a “radiant heating system”.

In a radiant heated ceiling, the HTC is almost 100% radiant and virtually no convective transfer. Conversely, a radiant cooled floor is almost all radiant and no convective. It is possible to increase the output of a heated ceiling or chilled floor by encouraging the air to move across the surface – think ceiling fan.

By the way walls operate closer to the performance of floors. (appx. 1.5 to 1.6 Btu/hr sf F for heating.)

One reason why the values you may have found in ASHRAE differ from those in manufacturers manuals could be due to the way the ceiling is heated. Ceilings constructed with radiant panels, which do not cover the entire surface, have higher convective outputs thus a higher HTC. The manufacturers published HTC’s could also be based on a specific surface temperature…

Short strokes - the differences are explainable.

My hunting buddy Tim Doran from Wirsbo is out for opening day with his kids but he usually shows up every few days…he can help you out further.

rb

GMcD

Radiant heating or cooling or both?

For ceiling systems you have a choice of Karo capillary tubes (www.naturalcooling.com) or suspended (or surface mounted) aluminum or steel radiant panels. Another alternative is to use 1/2" PEX and apply it to the back of the ceiling surface with 4"-6" loop centres, and cover the back of it with 3" dense insulation. Heating outputs for ceiling systems are generally based on an average of 180F average water temperature and the heating output is over 250 btuh/sq.ft. of panel surface area at those temperatures. Output is directly related to the radiant emitter surface area and temperature - remember the Stefan/Boltzmann law - temperature to the fourth power.

You could use small radiant surface areas at high temperatures, or you could use large radiant surface areas at lower temperatures to get the same heating effect.

Check the Twa panels site at:
http://www.h2ohtg.com/TWA/Products/Hyrdronic Radiant Linear Panels/output-imperial.html for heating outputs, and this link for radiant cooling outputs:
http://doas-radiant.psu.edu/panel_ATE_2004.pdf

Generally the lowest radiant cooling temperature you can run is limited by the ambient dewpoint in the space, typically 64F, and that will yield a radiant cooling capacity of around 25-27Btuh/Sq.Ft. of radiant panel surface area from the ceiling.

RB_2

Hi Geoff...whats you thoughts on...

Geoff,

Whats your thoughts on radiant temperature asymmetry defined by Standard 55 and panel output.

ie: the limitation on heated ceiling asymmetry is listed as a nominal 41 deg F...this would limit the surface temperature of the ceiling nes pa?

RB

GMcD

Radiant asymmetry

Yes, that is true. Typically suspended radiant panels have been used for heating only systems running off the usual 180-200F heating water. The comfort isn't compromised too much, and there are thousands of these systems installed here in Canada where we have some real perimeter zone challenges in winter with the crappy glass most architects specify. What ASHRAE 55 is saying is to keep the radiant surface temperature asymmetry below certain levels, which matches a lot of human comfort studies. It also depends on the distance away from the radiant surface too. The best comfort control system has very large radiant surface areas operating at surface temperatures within 10F-15F of the desired room/comfort temperature setpoint (operative or resultant temperature).

RB_2

Another thought...

Geoff,

I was just running the numbers and the HTC at 250 Btu/hr sf F comes out around 1.3…with a 70 F AUST it puts those panel temps up over 200 F…if the floor surface was heated from 70 to 84 maximum recommendation by Std 55 the asymmetry grossly exceeds the 41 F listed in the Standard…

What am I missing?

rb

RB_2

Hair trigger...

Should have waited for your reply...

I live in Calgary, Sit on TC 65, ALI liason etc... et al, we're always looking for new recruits - are ya interested?

By the way read your article in PM on cooling - good stuff.

RB

GMcD

Not sure what you're getting at

Yes, running small surface area high temperature radiant ceiling panels doesn't necessarily jive with ASHRAE's guidelines, but, it does work, and does not comprimise comfort too badly- ask the many occupants of schools, arenas and office buildings that have small high intensity radiant heaters, with corresponding cold surface temperatures at perimeter areas (glass/windows as radiant cooling panels in winter). It's more about the average mean radiant temps in the space. There are numbers and then there is human comfort. Human comfort doesn't always follow the numbers, ask any HVAC engineer how many "secretary's thermostats" he's used.

GMcD

Got the t-shirt

Did the Edmonton/Calgary shuffle in my misspent youth learning the trade through the early '80s. Having too much fun on the Wet Coast doing radiant slab buildings. I was thinking of getting on TC6.5 but the Company wants billable hours, hence little time for out of office experiences other than presentations and articles.

RB_2

...if you happen...

to find yourself on the way to Florida next Feb. - do drop by - Dale and Ackermann from U of A occasionally show up and maybe we'll have the boys from NRCan as well.

Was in Ottawa last month and had dinner with Kamel (NRCan) re radiant research...I had no idea the Federalies had so much going on - particularly as it relates to modeling.

GMcD

Dr. Dale from U of Eh?

I took heat transfer from a Professor Dale in second or third year engineering back in the mid-70's at the U of A. No trips to Fla. planned, just Winterpeg and Portland in the near future.

Maybe we ought to take this offline- contact me at geoffmcdonell@hotmail.com