Correctly calculating heating loss in-slab radiant heating
Correctly calculating heating loss in-slab radiant heating
(OP)
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.
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.





RE: Correctly calculating heating loss in-slab radiant heating
Different norms cover it differently, while I do not know any instance where outdoor air temperature is taken into account other than model related to use "in pair" with CLTD, where all losses are assumed to be taking place through perimeter only.
That is more scientific than engineering issue, and if you would not use Uponor software's calculations, than all other calculations you use with that software will be invalid, so you should do manual calculations which makes very little sense.
When calculating space heat load, you must not include losses through radiant surfaces. These losses are taken up by radiant calculations, and load is directly transferred to radiant surface, so if you include it once more you are effectively doubling it, which is always so heavy oversize that you can even come to fake figure that your radiant floor cannot cover load.
What you ultimate have to calculate is U-value at the back of radiant surfaces exposed to outdoor. Having too high U-value renders complete radiant systems senseless. When designing radiant heat system, you are not designing only pipe layout, diameter, water flow, flow temperature, but physical properties of backing wall as well. You have to enforce it on architect.
Radiant heating on non-occupied space makes sense only if there are specific demands to permanently dry out construction or similar, otherwise it makes no sense. The main purpose of radiant heating is to provide additional comfort by radiating heat directly to bodies of occupants, and if there are no occupants, main purpose is lost.
If you plan to use other kind of heating terminals, take care to separate it from radiant circles all along to the source, especially if there are fan coils, as totally different thermal inertia will create mess if connected to common distribution network.
RE: Correctly calculating heating loss in-slab radiant heating
RE: Correctly calculating heating loss in-slab radiant heating
I think that Trace and Hap are more air conditioning oriented, and are not useful for radiant heating load calculation.
RE: Correctly calculating heating loss in-slab radiant heating
With in-floor radiant I also could get lower boiler temp, increasing boiler efficiency. Obviously I may lose more heat to the ground when the floor is warm. So I'm not sure if the overall efficiency is better with radiant. The radiant heating itself is not that expensive. But the insulation is expensive (especially in vehicle garage where I need the 60 or 100 psi type)
I found uponor has a LoopCAD software. but it seems it is designed for that software to calculate the heating load (then it also takes into account ground temp). If you enter your own load data, you can't chose a location (inc. ground temp) anymore. In addition I always think I want to be able to calculate things myself before I trust a software. i saw this software assumes 42°F ground temp for my location, while I assumed 50°F, this really is getting complicated...
Sprinkler 1000: this book looks good. I just ordered it... at $200 it better has everything I'll ever need :). i read the reviews and the only complaints people had is that it has too much science :)
Drazen: DIN probably has it right since Germany has a lot of radiant floor heating. Problem is each the standard cost over $150, and I'm not even sure if it is available in the US.
Maybe I'm not 100% sure anymore if I want radiant heating, but still want to know how to properly account floor losses. I think modeling with THERM may be my best bet. I tried it with a basement and got similar results as in th ASHRAE fundamentals. (well, when I only looked at wall and floor independently - with both floor and wall transferring heat, the floor losses were smaller)
RE: Correctly calculating heating loss in-slab radiant heating
By the way very good site that of Uponor.
RE: Correctly calculating heating loss in-slab radiant heating
With R-10 slab edge insulation and R-20 under the slab (4" of type 4 or better Styrofoam), your losses to the ground and edges can be considered negligible. Haven't you ever heard the joke about the difference between a mathematician and an engineer? The punchline is that engineer enthusiastically advances to the finish line because he can get close enough for all practical purposes.
The intent is to control the slab surface temperature as your heating emitter and the heating "output" of the slab is based on Btuh/sf, which is what you try to match up with the heat loss of the envelope above the slab. Basically a surface at 85F will have an "output" of about 30 Btuh/sf, so if your heat loss from the space being served is less than 30 Btuh/sf, you are golden. If the heat losses are greater than 30 Btuh/sf, beat up the architect some more, or add supplemental heat for the peak winter periods. There are a lot of radiant design guides out there that can help. I've been doing radiant slabs, walls, ceilings with a variety of systems since the early '80's, and once you understand the basics, it's pretty straightforward.
RE: Correctly calculating heating loss in-slab radiant heating
your kind of story is exactly what i explain to clients to be inevitable waste of funds, when i have chance to approach them.
contractor will "specify heat output by unit surface output only because he is not familiar with calculations. calculations are by no means dry theory, but combination of theory and practice.
in 90% of situations i am able to save lot of funds to client by making good calculation, by variations 3-4 parameters to get optimum output. it is actually the easiest way to get new clients if i only have chance to challenge costly oversimplifications.
and, from what i can see by more than 20 years of observation, clients who rely on engineers have much longer business life than those who rely on contractors who like to simplify things only for lack of sufficient knowledge on specific subject.
RE: Correctly calculating heating loss in-slab radiant heating
In addition, for regular heating systems (not radiant) I believe the floor load is not always calculated properly.
you are right, the amount of insulation i want to use will minimize the error even more.
Part of my job is to fix systems that don't provide enough heating, or systems that are way over-sized and hard to operate because of that (designed by consultants). So, yes using some more thought upfront is important to me :).
I was involved in smaller scale radiant heating systems and during construction the designer and contractor just decided on whim (literally) to increase tube spacing from 6" to 12". So this is a scary world.
BTW, does someone have a good way to install the radiant tubes elevated in the slab when no steel-mesh is used (like to use the fiber reinforcement since salt etc. will cause corrosion)? When just laying and stapling on the insulation, the tubes are low and have less heat output, and the vapor barrier gets pierced.
RE: Correctly calculating heating loss in-slab radiant heating
First law of thermo predicts that, if you offset your heat losses by adding btu's, the building will maintain temperature.
Second law requires a little extra heat to offset entropy
This is not a complicated problem
RE: Correctly calculating heating loss in-slab radiant heating
HerrLaKeun- your simulation and design issues are real, and yes, worth knowing, but the reality is that even with an R-10 insulation layer under the whole slab, and the insulated slab edges, there is very little heat loss to the ground compared to the emitted heat radiated off the surface of the slab to the space being conditioned.
There are a lot of FEA and design information papers, and technical design materials to read at www.healthyheating.com that basically pave the way for your review. I agree that there is quite a bit of "art" related to how to space the tubes, water temperatures, and how to control the slab. I prefer the tried and true in-slab temperature sensor, coupled to the control valve at either the main manifold or individual tube loop control valves depending on the size of tube loop zones being used - generally on larger applications like maintenance garages I'd be using 3/4" dia tube for cost-effectiveness and larger area coverage. Depth of the tube matters when you need to specify the operating water temperatures for your desired slab surface temperature.
Installing the tubes in the slab without a mesh: Depends on how much reinforcing bars there are to clip the tubes to - if there is a decent mesh of rebar for a load bearing slab on grade (bus and truck maintenance, aircraft hangers) there might be enough to be able to clip the tubes to the rebar which are held up in the slab by plastic chairs, or where we have a contractor interested in cost-effective solutions, old bricks, and sometimes rocks! The trick is to make sure that the tubes are below the saw-cuts used for the control joints and for any future anchors or things that want to be drilled into the slab. Sometimes it may be better to keep the tubes down near the bottom of the slab to allow for that future flexibility, and make sure the structural engineer is involved. Typically a heavy mass radiant slab wants to be operated with a constant flow, variable temperature system since the slab reaction time to a change in temperature is so slow. There are ways of controlling an intermittent flow duty cycling control depending on the hydronic fluid deadband you can tolerate for your application. Google some of the Thermally Active Slab papers by Bjarne Olesen and there are a few that describe some energy efficient slab temperature control methods. Tekmar Controls also had a couple of good radiant floor controls papers on their website - mainly aimed at residential and small commercial applications, but the theory is the same.
RE: Correctly calculating heating loss in-slab radiant heating
i strongly believe that it is completely unacceptable to fix output of radiant heating per area, as it is actually the place where engineering work can provide savings. such fixing allows use of simple tables, while variations of all possible factors mentioned above requires either lot of manual iterative work of use of software, and many reputable suppliers provide their product-based software.
saving can be sensible. you can play with piping distance, diameter and supply temperature until reaching possible best combination. things are getting complicated if client want to use floors for partial cooling as well, but with adequate concept again best solution can be found.
it is very similar to any other hvac-related issue. contractors sometimes convince client to not waste time on engineering work, than double-oversize all equipment. surface temperature limitation of radiant floors makes such decisions only clumsier.
RE: Correctly calculating heating loss in-slab radiant heating
you said (calculating space heat load, you must not include losses through radiant surfaces. These losses are taken up by radiant calculations, and load is directly transferred to radiant surface)
could you please explain it a little more
RE: Correctly calculating heating loss in-slab radiant heating
when calculating radiant heating surface, you are actually calculating heat balance equation, so in stationary state radiant surface is emitting heat in both directions, toward the heated space and toward the outside, if it is placed at floor or wall (or ceiling) exposed to the outside. therefore, heating surface is directly taking up heat loss of the floor/wall in question and you have to subtract it from "classic" space heat loss calculation to avoid counting it twice.
so you actually have two heat balance equation, the one is balance of surface in question, the other is space balance where your surface in question is not losing but contributing the heat.
in total, your overall plant load, summation of two balance equations, would be higher than with classic calculation IF you would have typical insulation of floor/wall in question for simple reason that temperature difference of that surface to the outside is higher than temperature difference between room design temperature and outdoor temperature.
that is why you ultimately have to have higher than normal (whatever is deemed normal) insulation on the outside of radiant surface exposed to the outdoor, so that all makes sense.
RE: Correctly calculating heating loss in-slab radiant heating
Thank you for your response. now I have a question:
let say we have a space on grade has 4 exterior walls, concrete slab floor, a ceiling
and we need to use a hydronic heating (radiators).
we ran a load calculation, it say
- ceiling losses 50000 Btu/hr.
- walls 30000 Btu/h
- floor 10000 Btu/hr
we selected a 90000 Btu/hr boiler to stand this load.
now if we switch to use radiant in floor, would we still need 90000 Btu/hr boiler or less.
RE: Correctly calculating heating loss in-slab radiant heating
the bilwer itself needs to provide this new (higher) floor loss + the wall and ceiling losses.
Obvioulsy this will depend on isnulation since the original calcualtion likley didn't have 4"of slab insulation.
RE: Correctly calculating heating loss in-slab radiant heating
In model floor losses will be calculated depending on radiant floor temperute chosen by designer and U-value.
If you do not change u-value of floor compared to radiators alternative, loss through floor will be say 12000 Btu/hr. Than you would alter u-value of floor using recommendation from standards. In general the point is that loss always become less that "standard" as you are using that floor as a heating surface. So, let's assume that after you use recommendations, you will get something like 7-8000 Btu/h.
I hope with this description it is clearer (my English is not perfect) why you need to separate calculation of surface in question from the rest for the space.
If you follow engineering practice widely developed in few European countries, you are actually designing both heating pipes AND u-value of the floor, that is your word has to have more weight than architect's.
Best software I use currently disallows me to set floor U-value higher that maximum, which is always lower than "normal". In most of countries around it is also included in local codes, so there is maximum permissible u-value for "normal" floor, and the other maximum figure for radiant floor which is always sensibly lower.
In Mediterranean area where I currently live, roughly said "normal" floor insulation would be 4-5 cm thick, and the one for radiant floor would be 6-7 cm at least. In other climatic zones there could be differences, but in practice noone will insulate floors higher than needed as cost difference is very sensible.
RE: Correctly calculating heating loss in-slab radiant heating
anyhow, i hope that i managed to describe why it is important to take out "classic" calculation of that floor. if i didn't, that would be likely for my bad english
RE: Correctly calculating heating loss in-slab radiant heating
your English is way better than mine, but it seems a little confusion here to me.
why we connect between the space load itself and the type of system used, I mean why I have to install a 90000 boiler in the first case (radiators), and 80000 boiler in radiant floor case.
our goal is to keep the space heating set point, the load calculation tell us the amount of energy required to keep this set point regardless of the system type used.
if we can use a smaller boiler just because of floor radian then we could make floor, walls, and ceiling as a radiant the we could use a smaller boiler than before, it does not make sense to me.
my point is the separation between load calculations and system type used.
RE: Correctly calculating heating loss in-slab radiant heating
the point is that 10000 calculated as loss through floor without radiant heating will certainly not hold if radiant floor is installed as radiant floor has higher temperature than space, so temperature difference is not the same as when calculating wall losses within space load calculation, and piping is installed say in the middle of the floor so only outside part of floor makes resistance to heat losses toward the outside.
if you neglect that, you are neglecting physical process that actually takes place - room is not losing heat through radiant floor but is receiving it, while radiant pipes directly lose energy through that very same floor. The difference can go to 5-10%.
RE: Correctly calculating heating loss in-slab radiant heating
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.
RE: Correctly calculating heating loss in-slab radiant heating
RE: Correctly calculating heating loss in-slab radiant heating
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...
RE: Correctly calculating heating loss in-slab radiant heating
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.
RE: Correctly calculating heating loss in-slab radiant heating
RE: Correctly calculating heating loss in-slab radiant heating
RE: Correctly calculating heating loss in-slab radiant heating
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.
RE: Correctly calculating heating loss in-slab radiant heating
RE: Correctly calculating heating loss in-slab radiant heating
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]