Correctly calculating heating loss in-slab radiant heating 2

[EnergyProfessional]

I'm designing a slab-on grade radiant system, and haven't done that before. I may be overthinking this, but got deep into calculating the actual heat loss to the ground and outside. The uponor design documents consider "Downward losses"significant, bit refer to their software to actually calculate them. I used THERM to model what is going on. I attached a picture of how the slab looks like 50 ft in, and the outside 50 ft along with the isotherms. The slab will have 4" horizontal XPS, and 2"vertical XPS insulation. The building will be in WI with -15°F OAT design temp and ~55°F indoor temp. I assume soil temp of 50°F in 30'depth.

I use Trace to calculate the load, but doubt it has specific provisions to calculate what i seem to overthink.

With these conditions I calculated U/R values for every 10 ft. of slab. Not surprisingly the R-values get very large as i move past the first 10 ft. The R-value is calculated from the heat loss from the slab. Both ground temperature and OAT are important. near perimeter, OAT is dominant. away from perimeter, I only lose heat to the soil. If i didn't have in-floor radiant, this would not be much (55°F space vs. 50°F ground at 4" insulation). it may not be intuitive, but since I deal with two heat-sinks (ground and outside) my U/R value changes with different temperatures. these R-values i can use to calculate heat loss (regardless of if heat goes outside or to the ground)

My theory is:
- when calculating space load I don't take into account any floor losses since the floor is warmer than space temp. (being conservative, I could just
include it)
- when calculating boiler and system size I need to add the floor losses including the fact the floor will be 100°F (and not just space temp)
- the sizing of the radiant heating (spacing of tubes, flow etc. ) would be done by manufacturer. Unless i come up with a better method

I think what I'm asking for is any advice to really understand and calculate the heat losses, size the system and radiant flooring. If any books are recommended, buying them is not a problem.
Amazon has some, but most seem to be for DIY or for installation.

there doesn't seem much concrete information. Even ASHRAE "HVAC Equipment and systems" chapter 6 doesn't really say much. I think most is written about electric radiant heating, wall and ceiling panels, and radiant cooling. but hydronic in-slab heating seems neglected, even if it is the most common.

Another thing I need to consider if i need to have some sort of hydronic Unit Heaters by the Overhead doors (this is a garage) for when the doors open assuming the radiant heating doesn't react quickly. Comfort is not an issue, but don't want sprinkler pipes to freeze if a door stays open long.

any advice appreciated.

 

[317069]

Drazen
what is the difference between using hot water pipes inside a wall (radiant wall) and flashing this wall by a supply air ( in air duct system) as long as we increase the wall surface temperature and this is what all heating system does not only radiant.
we are fighting heat flow from outside to inside on the wall line not in the center of the space, and that why codes asked to flash exterior walls by heat.
the wall radian is the same as a regular radiator but has larger surface and less water drop temperature.

 

[317069]

oops,sorry, heat flow from inside to outside

 

[Drazen]

317069
i thought you need better explanation on the process, not dispute on it.

i don't know how much did you study convective systems, but there is nothing like flashing the walls to raise their temperature that i ever heard about. convective process is funded on good air mixing within space.

creating sort of air curtain does not serve to raise surface temperature of walls but to prevent cold draft created by surface which is too cold and cannot be improved, like old-type windows.

such a process is completely different compared to radiant heating.

traditional radiators, again, have only about 30% of capacity related to radiation (despite their name), the rest comes from convection. so the issue of covering the back side is minor compared to radiant heating.

i feel as my task within forums to explain when i fell able, but not to defend what is common and public knowledge, and can be found in many countries' codes, calculation procedures, software modules. ignoring it at trying to simplify it by force only for lack of previous experience with it is a bit strange for me. i myself learned many useful things on this forum, but current atmosphere here is quite strange...

 

[317069]

Drazen
I am not disputing the process and it is not about a challenge, it is about comparing between systems.
you say that the wall flashing is to prevent drafting, Ok but how?, I think to prevent a draft you have to play with temperature difference (by heating the surface). so where is the wrong by saying wall flashing is to raise the surface temperature of the wall, it is easy, forget about a wall, just put a cold thermometer the stream of a hot air, does it cause a raise in temperature or not, most code and standard flash exterior walls, why? and why are you nerves?
I don't know either where did you get your study or experience in HVAC but I think it is not a logic to say that your space load calculation will vary by varying the system used
( it looks like saying: you need for your house either 80000Btu/hr boiler or 120000 Btu/hr furnace, does it make sense?)
you said the radiant wall temperature will raise the heat flow, it is right but the wrong is you forgot that the heat flow between the space and the radiant wall be reduced due to the delta T is less than before.(space design set point is constant regardless the walls temperature)I think there is mixing between this two points.
this forum is to exchange information, me, you or anybody else are free to share or not, but you don't have the right to judge people here and say it is strange...etc. people can judge you too. so take it easy.
as an example can you tell us how to calculate heating load for a space that have all his exteriors (floor, walls, and ceiling) are radiant surfaces if you are going to delete every radiant surface from the calculation, and how will you specify the boiler capacity in this case?
if you like to answer please post it with a numeric example
but, if you are going to be nerves, you better not answer.

 

[EnergyProfessional]

317069: I'm not sure if one can calculate it easily, but the type of system affects real life load. Heat/cooling load calcaultions also take into account radiation. whne you have high ceilings with stratification your roof loses more heat etc. Based on past comments, Drazen seems to know his stuff.

 

[317069]

Sure, that why I am asking him.

 

[EnergyProfessional]

Sprinkler 1000: I just got the book you recommended and browsed through it... wow.. it really explains a lot about hydronic heating in general. Most HVAC engineers will know many things, but it really has some things one normally wouldn't know or consider even if in the business for many years. It also is really up-to date (even details those new automatic control circulators, like the Grundfos Magna)
thanks for the hint. Really great book... I wish I would have had it sooner, would have saved me much time in my career.
I'll study that book thoroughly, and do some of my own load calculations, play with that uponor LoopCAD software and then see what happens.

 

[sprinkler1000]

Ok Herr, I am glad to help a little bit in exchange for so much information one can pick up in this forum.

 

[NoHtPp]

HKL:

for garages with newer tight doors and std insulation's:

how close did it come to ~ 20 btuh / sq ft in the book ?

and for 75F out to flooring and near 62 returning in a 55f garage on a zero Northern night?
we found a 440sq ft garage held 45 to 50f , ice-melted the autos well, on just 35,000 btuh.

We have homes always using insulated slabs though.

We have homes in Cleve Ohio (usually ~ 12-13) at 11 btuh/sq-ft with forced air HVAC and ceiling registers throwing -downward , at over 3f/sec the warm air in rooms and basements, as to in the homes radiant out puts at just 9 to 10 btuh per sqft, relative , one to another. [0 to -5 below through -12 below peaks, a night or 2 , usually]