Pex emissivity

Glen

I use .85 when checking tubing temps - but that is just a gut felling towards accuracy. I'm really not sure. But I must comment on your install thoughts. 10 years ago the big WOW factor was pex suspended under the floor - and I have now gone back to every home and added plates. The best bang for the radiant buck is concrete overpour, period. Then either roth or warmboard panels, and I think warmboard is just as good as or even better in some construction techniques. I now consider any sort of suspended tubing as floor "warming" rather than a heating system. JMO

cool in michigan

pex emissivity?

Hi,

Does anyone have the emissivity value for PEX? I am considering installing an underfloor (and also possibly in- wall) heating system that relies on infrared emission from the PEX tubing to warm the floor (with separated reflecting barrier and insulation below)as opposed to conduction (i.e. contact between tubing and flooring etc. will be minimized).

jp_2

why oh why????

why greatly reduce the efficiency of the system and force it to use high water temps.

I doubt you'll see much difference if you use .7 ,.8 or 1 for emissivity? reason being your surface area in the direction of where you want heat is so darn small, roughly 1/4 of the tubing.

added:

don't know for sure, but I think you get more heat from convection than radiation in the hanging piece of pex??

cool in michigan

Thanks for your question, but what I really need is the number (if anyone has it). I'm not sure if this addresses your question, but the tubing will be suspended by clips, meaning that >90% of the tubing surface will be available for emission (not 1/4 of the tubing) -hope I didn't misinterpret. It is true that emission is dependent on temperature -but if I have the emissivity I should be able to calculate how much tubing area is required per square foot per degree to meet my BTU needs, without having a very high water temperature. (BTW, the best emissivity number I can find for now is 0.92 for black high density polyethylene -I don't have confidence in using this number).

jp_2

going in the wrong direction.

hate to say it, but probably 98% of the responses you get will advise against this type of installation.

why install a radiant system that needs 150-170F, when a conduction system can run 90-110F? i have a gypcrete system running ave 85F.

what do you get when you use .7 or 1 for that matter.

also, you are using a round surface, 90% of the radiant heat will need to be reflected more than once to be reflected back to the floor surface, not true 90%. since you are not installation silver mirrors underneath, your reflection losses will be quite high.

why build an inefficient system?

lower, middle, upper, very upper michigan?

added: can't find much, but what I have suggests plastic in the range of .9 emissivity.

GMcD

Emissivity won't matter

In a case like you've described- that is about the least efficient and poorest use of PEX tubing for creating a radiant surface possible. You have to conduct the heat from the fluid in the PEX tube, through the wall of the PEX tubing, and transfer that heat to your desired radiant surface. Conduction is the "strongest" form of heat transfer there is. So you need to conduct that PEX tube wall via direct contact to the inner side of the surface you want warmed, which will then act as your radiant heat emitter to the humans in that room.

Trying to get heat from your warm fluid inside a PEX tube via radiation heat transfer through more insulating layers is just a waste of good pex and energy. In order to get the best heat transfer from your PEX tubing to the wall/floor/ceiling you need as much direct contact between the pex tube wall and your desired radiant surface. That's why most folks use the heat conduction plates for more efficient heat transfer, better comfort and lowest energy use.

Mike T., Swampeast MO

I've searched numerous times for an emissivity value or PEX with no success. Most non-metal coated plastics (like most construction materials) have fairly high emissivity. If I had to guess, I would say the emissivity is about 0.85

I have attempted to calculate heat transfer via radiation from suspended tubes in a joist bay. Not only is this extremely difficult when the tubes are precisely placed; it is essentially impossible when you consider the high variability of tubing placement in service. What I can say however is that given the small area of any reasonable amount of tube and temperature limitations of the PEX, that heat transfer via radiation in such a construction is far, far, far lower percentage wise than convection. So low in fact as to be virtually inconsequential.

The reflective barrier below will [probably] help a bit initially, but again there just isn't much radiation and the effectiveness of the reflector will diminish rapidly as the inevitable occurs and it is coated with dust.

Bare-tube suspended applications can work for floor heating, but in general your load must be very low (think a highly efficient shell), the floor itself must have a very low insulation value (forget carpet) and your supply temperature will be significantly higher than with other methods (greatest difference when comparing to tube in heavy extruded aluminum plates secured to the floor.) Since such a construction heats almost exclusively by convection, I would not consider suspending tubes in walls. Not only will heat transfer be poor, but natural convection inside the stud cavities will ensure that the warmest air rises to the very top and the walls will heat very unevenly as a result.

If you must use suspended tube, I would suggest that you look into Ultra Fin. A HIGHLY regarded regular here has used and tested Ultra Fin and finds it acceptable--especially for higher temp systems using conventional boilers.

If your heat source will be a condensing/modulating boiler, you really, really, really need to get as much conduction as possible to allow the use of the lowest possible supply temps. Thermofin by Radiant Engineering of Bozeman, MT is highly regarded. If you want serious conduction, use copper tube in the Thermofin...

zeke

You do it that way and you really WILL be cool in Michigan.

Even with emissivity of 1, your thermal conductance between tube and floor will only be about 2.2 for 140 deg water and 1 to the floor below(convection) an order of magnitude less than with the standard method of direct contact to the floor where the much higher thermal conductance is in the water- to- inside tube; so the overall conductance between a person standing above the floor and the water inside the coil would be at least twice as good vs your design. And with deterioration of that "reflector" you will eventually lose 1/2 the heat to that wall and so eventually, your thermal conduction efficiency will be 1/4 of the standard method. and your thermal efficiency will be 1/2.

Not only a bad idea from the standpoint of surface efficiency but also very costly in terms of heat loss.

jp_2

save the infared

for the surfaces directly 'seeing' the occupants, otherwise use conduction.

Tom Manton

look

at it this way though, your return water temps will be nice and high

eluv8

it can be done but why

Uponor/Wirsbo has a software program and design for such an install. I have installed this way and it can work. The design is more complicated, water temps are higher,flooring choices are limited,and the insulation below must be good and all holes and spaces must be sealed to eleminated all draft. The savings in install material is not worth the liability and time to make it work. When there are much better alternatives. Everything has to be just right, and heating loads low. If using a cast Iron or non condensing boiler a btu is a btu. But with savings that are available with mod con boilers achieving 94+ afue in low temp applications the long term savings and design flexiblity outway most suspended tube installations.