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.

 

[Drazen]

Heat loss model through ground is complex, as your sketch is showing - equations which describe it are as non-linear as your isotherms are.

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.

 

[Drazen]

Yep, DIN 4725 and DIN 1264 series cover radiant heating design in details.

 

[sprinkler1000]

There is a very good book by John Siegenthaler, of Hydronic Heating.

I think that Trace and Hap are more air conditioning oriented, and are not useful for radiant heating load calculation.

 

[EnergyProfessional]

Main motivation to look into in-floor heating is that Ceiling height will be high (~35 ft and I would hope to be lower the space temp with a warm floor. If i can go from 55°F to 50°F the energy savings with ventilations etc. would be great. it also would get everything out of the way. unit heaters typically are some where in the way if you mount them the height you need to.
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)

 

[sprinkler1000]

The book is worth the money and very technical (no rocket science).

By the way very good site that of Uponor.

 

[GMcD]

Way too much overthinking going on here. Radiant floor heating in a concrete slab has been done for decades in high bay aircraft hangers, bus and truck maintenance barns, and all kinds of similar installations and works very well for a low energy system using low temperature heating water, condensing boilers, and an ability to maintain comfortable conditions in cooler ambient air temperatures.

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.

 

[Drazen]

gmcd, is you despise engineering standards, maybe there are other places where you would be welcome more, while i also participate in one construction-oriented forum, but almost never hear a word against engineers.

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.

 

[EnergyProfessional]

GMcD: you probably are right that I'm over-thinking this a bit. But with a large facility like this I'd like to know exactly what I'm doing and even a small % error can be huge.
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.

 

[willard3]

The building is a control volume and any heat that passes anywhere is your heating load, including perimeter and slab losses.
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

 

[GMcD]

Drazen- I'm not sure how my post indicated that I despise engineering standards, or that I may be a contractor. I've designed. specified, presented to ASHRAE, and written many design articles on radiant systems design, so all I was trying to indicate was that it's sometimes a bit of an effort to re-invent the wheel, especially when radiant floor slab heating has been a fairly mainstream system application in North America.

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

http://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.

 

[Drazen]

gmcd, i am sorry if i misunderstood something, but your previous post is almost the same kind of story which some contractors are trying to sell to clients, while i have to fight it.

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.

 

[317069]

Drazen:
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

 

[Drazen]

317069

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.

 

[317069]

Drazen
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.

 

[EnergyProfessional]

I think 317069 summarized my question well. I think the radiant heating floor needs to dissipate 80,000 Btu/h (Wall+Ceiling) to the space. but it will disspiate more than 10,000 btu to the soil, since the 10,000 was calcaulted assuming the space being at lower tem whne it didn't have radiant (warmer) floor.

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.

 

[Drazen]

For radiant heating you would calculate radiant floor model using space load of 80000.

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.

 

[Drazen]

yep, to be more precise (rush makes me as concise as possible), in calculation of floor in question, only u-value of part of floor between radiant pipes and the outdoor goes into equation. This part carries most of insulation value, yet for larger magnitude it can make some difference.

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

 

[317069]

Drazen
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.

 

[Drazen]

in another case we are not installing 80000, 80000 is only effective load radiant floor brings to space, while boiler load could be either 92000 (80000 + 12000) or 88000 as described above.

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%.