Concrete Slab Radiant Flooring for small church

Chuck Becker

Our small church is building a 34' diameter, 15' high round structure to hold weekly services in Maryland (and one or two meetings during the week, such as choir practice, etc.). It has been suggested to use concrete slab radiant flooring for the heating, based on the idea that radiant flooring is a comfortable, efficient form of heat.

I'm certainly not a heating professional, but playing around with some basic heating equations, I became concerned that given our application of a couple of hours use, a few times a week, that radiant floor heat would not be efficient for our usage at all, that heating up 19 tons of concrete in order to warm the meeting space was too much wasted effort. While we are concerned about comfort, we do not have a lot of spare cash and also don't wish to burn up more fossil fuels than is necessary to achieve a comfortable meeting space.

My basic questions are these:

1) As an expert, would you recommend concrete radiant floor heating for our application (more details below)?

2) If we use a point-source heating method instead, such as a stove or propane wall-mounted radiant heaters, can we effectively distribute the heat with upward-facing ceiling fans without creating an uncomfortable draft (assuming we kept the indoor temperature high enough)?

Here are some details of the structure. I'm new to this forum so I'm not sure how much detail is appropriate here. If anyone wants to, I would also appreciate any HVAC engineer volunteers who offered to look over my calculations and see if they're valid (in an excel spreadsheet link below), but of course I appreciate any advice that could be given.

DETAILS

- Concrete slab will be 34' diameter, 4 inches thick, or about 38,000 lbs. Interior area is 850 ft2, somewhere around 10,000 ft3 volume.

- The radiant floor installer has estimated the heat load of the structure to be 35,000 BTU/hr on a 10 degree F day.

- The Avg. temperature in this part of Maryland for the October to April season is 43.7 degrees F.

- My attempts to model this situation are at: http://home.comcast.net/~krisnachuck/HeatCalculations.xls.

Again, thank you in advance for any help you can offer.

-Chuck Becker

Ken_8

I believe your concerns are legit!

However, maintaining the mininum temperature desired by radiant means AND fully zoned and/or modulated heating via HWBB (hot water baseboard), you might just wind up with the ultimate solution.

With a condensing boiler capable of 92+ AFUE, radiant in the sanctuary's concrete slab AND perimiter HWBB you may have your cake and eat it too!

The sanctuary must be left at some minimum temperature. If that temperature is say 55 degrees - there is no better way to get there and stay there other than radiant.

During services, the HWBB would be timed via programmable thermostat(s) - depending on zones needed, and the boost done with minimal fuel expense. If the congregation is large, remember, each modestly active participant will put out at least 400 BTU's! Between the "natural heating" of church attendees, radiant and zoned HWBB, you could save a forune is wasted energy.

Up front costs would be moderate. The concrete slab floor will be poured with or without the tubing. Just make sure the mason contractor puts down at least 1" thick rigid insulation under the sanctuary (or any other part of the pour) beforehand. Otherwise you'll be sending your energy dollars into some house on the other side of the planet!

Perimeter insulation is also a must. Those of us who own homes without either have flowers budding in January from the escaping perimeter heat "leaking from the slab edges into the dirt - faking out the daffodils.



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Chuck Becker

Thank you so much for your help!

I hear what you're saying: that any constant temperature, even below "comfort-level," is well-met by a radiant method. In our case, though, there is to be no plumbing in the meeting-space building at all (there is an existing structure--currently a house--that would serve as church office space, restroom facilities, children's nursery, etc.). Since there's nothing to freeze, we were intending not to have to keep it heated all week, even at a minimum temperature. We were told the fluid going through the slab wouldn't freeze, for example. Perhaps this brings up a secondary question as to whether that's a good idea (we think the sound equipment and other church objects will be OK in the cold). Any thoughts on that?

Assuming that we can let it get cold, however, does that put us back to floor radiant being an inefficient fit? BTW, can hot "water" baseboard be easily refitted to use a nonfreezing fluid?

Thanks for your thoughts on the importance of under- and around-slab insulation in a radiant floor application. The radiant installer recommends Insultarp (I just saw the recent thread in this forum discussing Insultarp -- sounds like there's some controversy over that vs. rigid). If we weren't going to do radiant floor, should we keep that anyway, considering the inside space won't be warm long enough to heat the concrete much?

Thanks again for your advice!

hr

Or

while I like Kens idea to idle and temper with the slab, public spaces with large loading like that may need some air movement, and fresh air blend.

If the building goes from zero to completly full in a short span of time, then doors close... Odors happen

Codes generally require some air changes per hour in public spaces, (ASHRAE standards)especially with tight construction.

A small air handler with a hydronic coil or a Life Breath HVR type of unit with some heat may be another option. It could also provide some cooling/ dehumidification component, depending on your climate.

hot rod

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rich pickering

For what it's worth,in my father's church they setback the temps from 70 to 60 and got complaints about the equipment and the instruments.It now stays at 70 - 72 year round.

Uni R

I'm not a professional...

And even to me it seems pretty obvious that you should not want to use a high mass (pardon the pun) heating system to heat a building that is used sporadically.

Floyd_5

Chuck, I ain't an injuneer...

however I can tell you from frist hand experience that you can , with a properly insulated slab, keep your building at a comfortable temp. year a round and still heat it for les than with any other heating system running just on weekends. PLUS you will have a MUCH more comfortable building!!!!!
Ken does have some very valid points about the HWBB and that should work very well. Don't try to over engineer tis thing..keep it simple....install the radiant tubing in the insulated concret pad, install a mod. con. boiler and enjoy the comfort and savings!!!!!

BTW, I hav found that good quality foil/bubble/bubble/poly works as well as anything else out there and is more economical for the under slab insulation.

Also, as far as letting a building go to low temps. during the week and heatind it up for the weekend.... bad idea, from many stand points......drywall ain't gonna like it, wood ain't gonna like it, sound equipment and instruments, no like it.....and you's a gonna feel like you sittin' in the freezer with all the cold stuff aroung you radiating the cold inside it to your body.....not exactly the way to feel all warm and fuzzy on a sunday morning :-)

Floyd

Chuck Becker

Me neither, but...

...But I can't see how 24*7 heat can be achieved more economically than an hour on a Sunday. I would appreciate if someone could explain a bit of the math that could make that work. All the other concerns about the cold instruments, walls, etc. make perfect sense to me.

My (admittedly simplified) calculations tell me that for an average week, which in this part of Maryland, Oct-Apr is supposedly 43.7 degrees, I need about 2.5 million BTUs to heat 24*7 with the concrete slab radiant, and only about 48,000 BTUs to heat once a week with, say, a wall-mounted propane burner. That's about 50 times as much propane, which does keep the building warm for about 100 times the number of hours, but all that is time we really don't care that it's warm.

Here's the super-abbreviated version of the math reasoning. Again I'd go into more detail if it's appropriate in this forum, or by email if someone is willing (or you can muddle thorugh the excel spreadsheet at
http://home.comcast.net/~krisnachuck/HeatCalculations.xls
). If anyone can correct it or explain the reasoning to show how the 24*7 radiant floor heat can use less fuel, I would ecstatically appreciate it.

I'm going by the 35,000 BTU/hr estimate that the installer gave (assuming all the insulation he thought necessary for radiant floor), which was based on a 10-degree day, or a 58 degree difference between inside the building and out. A 43.7 degree day would mean a 24.3 degree difference, so by proportion the heat loss on the avg. day ought to be about 14,664 BTU/hr. Multiply that by 168 hours in a week and you get 2.46 million BTUs to keep it at 68 degrees if it was always 43.7 degrees outside (that would be nice, wouldn't it :-)

Meanwhile, if you figure you need to maintain say 70 degrees to stay comfortable with a non-radiant floor method (because we're not getting the extra comfort effects of that Mean Radiant Temperature effect), that gives you a difference from inside to outside of 26.3 degrees, which by proportion again means you ought to lose heat at about 15,871 BTU/hr. I don't know the heat capactiy of the building, but I figured it's safe to assume I can raise the inside space 26.3 degrees with the equivalent of two hours of the day's heat load (because I'm not heating the concrete slab!). Someone please tell me if there's a better way to be figuring that, BTW (the installer did not give us that number). Anyway, an hour-long service plus the 2 hours worth to heat it up gives 47,600 BTU.

Having more frequent meetings makes the concrete radiant floor option more attractive compared to the propane wall-mounts, but even if we had a 1-hour meeting 7 times a week (each time using 3 hours worth of fuel), that still only gets us to about 333,000 BTU by propane wall-heater method, compared to 2.5 million BTU by 24*7 concrete slab.

Ken's suggestion to keep the slab at 50-55 degrees sounds better, especially if you have no choice but to keep it at some level to avoid freezing and mess up the instruments. A congregation member had actually made a similar suggestion (to 60 degrees), and so we tried to figure that out. 43.7 to 60 degrees is 16.3 degrees, which by proportion, at that temperature, the loss rate should be 9,800 BTU/hr, times 168 is still 1.65 million BTU per week, and that's before you have to raise the temp that last 10 degrees when the meetings occur. Ken's suggestion to keep it at 50 degrees ought to be 638,000 BTU in the avg 43.7 degree week, but the closer you get to that Oct-Apr. average, the less you can use the average temp. as an overall indicator, because you do get so many days that are colder (for example, if we went one step further and said let's keep it at a base of 40 degrees, we couldn't say "hey we can do that for no fuel," even though you could in this theoretical always 43.7 degree winter).

So if anyone can show how something about radiant floor makes it not work like the above (such that it's cheaper fuel-wise to use it and thereby get all the nice comfort things that we'd like too), you could make our little church so happy and have our fervent gratitude.

---
BTW, if you look at the Excel sheet, you'll see where I'm also figuring fuel usage for situations (in addition to those scenarios keeping the concrete slab at a constant temp) where I turn off the heat to the concrete floor in between uses. This of course uses less energy than keeping it all the way up all the time (since the rate of heat loss is proportional to the difference in temperature, keeping it high results in higher heat loss). Because of the huge thermal mass, the temperature stays warm for awhile even without putting in more fuel energy, although of course it still loses heat energy proportionate to the difference in temperature, just like anything else, so depending on how long it's been since it was last heated, it's easier to heat it up the next time.

By looking up the specific heat of concrete and multiplying by the 38,000 lbs that I've got to heat, I get that the slab should gain a degree every time I put in 29,800 BTUs into it from the boiler, or lose a degree every time 29,800 BTUs radiate/conduct/convect out of it into the bulding and into the atmosphere (they tell us the underground insulation - Insultarp - is really good. I don't know how good and it'd be too much math for me anyway, so since I'm really trying to give radiant flooring the benefit of the doubt, I use the same heat loss rate in the radiant floor scenarios as in the propane wall burner sceanrios, essentially assuming no heat energy goes out of the slab into the ground in the radiant floor scenario). I use that to figure how hot the slab is after it loses a certain number of BTUs in the first hour, then I can use that new temperature to figure by proportion how many BTUs it should lose in the next hour, and on and on iteratively (Excel is a godsend for this).

Again, not to go into too much detail (and sorry if I already have), but doing this, even though it saves energy over keeping the slab at the constant 68 degrees, I am still getting that it is more inefficient than the propane wall-burner. In the church service only once-a-week scenario, it still comes out to using 783,000 BTU per average temp. week (compared to 48,000 from wall-mount burner, or 16 times more fuel). Even in the 7 meetings a week scenario, it comes out to 2 million BTUs per avg. week (compared to 333,000 BTUs from wall-burner, or 6 times more fuel).

Unless someone can explain where I'm going so terribly wrong, it seems to me it's a lot cheaper NOT to use radiant floor heat, despite the fact that it's also hard on instruments, maybe less comfortable, etc.

Again, any guidance (or re-education) is appreciated!

Thanks so much,
Chuck

S Davis

Cold Concrete

I would be concernd about the cold slab, radiant works both way's, when you stand next to a window on a very cold day you can feel the heat being pulled off your body.
If you let the building go cold and then try to bring up the temp with all that ice cold concrete that slab will absorb a lot of heat.
Standing on 30-40 degree concrete is not very comfortable.

JMHO

S Davis

Chuck Becker

Sure is

Yes, we have some folks who know that cold concrete feeling firsthand. We would need to use some of the money saved to put down a bamboo floor (pretty much the same physical properties of hardwood). If my rough calculations aren't wildly wrong, we still easily save money.

Of course heatwise, a recently-cold wood floor is nowhere near as comfortable as a heat-GIVING floor, but hopefully it won't suck out the heat as much as icy concrete, especially since, as it's not a residence, we'd expect everyone to be wearing pretty good shoes in the cold months. At least that's what I'm hoping, since I'd think with wood, the very topmost surface (which as I understand is the important part when thinking radiant effect or calculating MRT) ought to warm up a little just from the warm air and the radiance from the propane burners, without that heat immediately conducting away like it would with bare concrete.

I haven't done calculations -- those are just impressions I've gotten talking with people. If anyone has any thoughts or figures as to whether we can or can't expect the wood/bamboo floor to lessen the effect, or how much to expect, I'd love to hear that too.

Thanks to everyone so far for pitching in!

Floyd_5

Chuck,

Way too many intangibles....... that you will never put a number on with any kind of simplified spread sheet.
Where's Mike Swampy when I need him??????

Anyway I'll give it a whirl......
just a couple of things that I can see wrong right off the bat...

26 degreee rise from a cold start may work on paper, but it'll never work in real life...... the building will suck up the heat with the mass of the building like it's going outta style.... the heat will rise to the top of the building and sit there like a dog on a dead ground hog.
It will take many hours to build the heat to the floor.
The concrete will be cold enough to cause the feet of an Eskimo to shatter!!!!! You'll be starin' them propane bunson burners up Sat. night and still have whinin' and complainin' on Sun. morn.!!!!

That 35k BTU is figured for RADIANT heat.... there IS a difference!!!!!! 35K with hot air ain't gonna do it!!!!
They are two TOTALLY different animals and the heat loss of the radiant system ain't gonna compare with the hot air system. Gotta trust me here......like I said Swampy's better at explaining this stuff than me......

I know you wanna go cheap and it seems as though you already got your mind made up so........

please........


don't make me have to say.......



I TOLD you so!!!!!!!!


Go radiant, bite the bullet, and the warmth you will feel as your worshipping will warm your soul as you know and feel that you did the right thing.

Floyd

Weezbo

hello again

last week i was tempted by the evil voice within me to ask are you limiting Gods blessings Already? and then again i was tempted to ask you,what about when the little church becomes the new childrens school or new administrative offices? and now ,today, i am ,unworthy wretch that i am Tempted, once again to ask are you the vision of the church................?
We are at heart good people and like the warmth and comfort thing...so,mostly what you hear, here ,is get ready for the time when God picks you up and puts your feet on higher ground. laying in the radiant now,with an eye to the future will ensure that you have well insulated concrete floors,well insulated walls and ceilings,a straight direction to follow when you are ready to continue the good work that you have started.for now you may consider the operational costs of the heating plant this is fine however a bit short sighted.while you use the space more and more for family meetings and sunday school and the attendance grows you may find the intangible benifits of radiant heat ,hrv and radiant cooling..to more than outweigh the Cost. here is an example...right now you think of the nose on your face too much focus on it Blurrs the Vision...its ok everyone does it. just lets look through another window for a momment...why a window? well it will provide some light...well wont it be a heat loss?.... yes especially when you want to let 26000 +another couple hours of congregating Heat:)Oops... Out:) even a window may be argued to be a benifit rather than a loss.it isnt an hrv however it certainly will allow some small change in the comfort level...our laws are changing everyday, the government would put a meter on the sun if they could and charge us for that,however we must all live under the law,all of us must live under the laws of physics, God s laws dont change.he gives us a free will.for you he is supplying you with good council so you can excersize it in making your decisions. has anyone suggested zoning?plan for it now then you may use the space to its optimum in the future.forgive me if i didnt say things right I havent attended a church since my good girl died.

Chuck Becker

Chuck Becker

Thanks !

Seems like most of you agree that a radiant floor is still best for us. I'm glad I was pointed to this site. Your ideas and suggestions are much appreciated, and it's great that there are folks like you willing to help.

Thanks again,

Chuck

DaveGateway

Just a thought

Does the church have AC? Maybe a hydro coil addition would be good for the pre-service warmup and allow the radiant to maintain min temps.

SRP

regarding this radiant floor heating

Dear Mr. Chuck Becker,
May be you should (in fact we all should) check this topics from the US Department of Energy's website (I submited their website address below). You can calculate how long it will take to heat up those 19 ton of concrete mass. This website is definitely without any influence of any manufacturer.
http://www.eere.energy.gov/consumerinfo/refbriefs/bc2.html

Mike T., Swampeast MO

Heat Loss Calculations

You've forgotten something VERY important in your calculations. Heat loss is expressed in terms of temperature MAINTENANCE!

That 35,000 btu/hr loss statement means this: the structure will loose approximately 35,000 btu/hr at the outside design temperature and AT THE CALCULATED INSIDE TEMPERATURE. It is the amount of heat required to MAINTAIN the desired indoor temperature, not the amount of heat required to obtain that temperature!

In your "occasional use" calculation you have essentially assumed that the air AND the structure itself are ALREADY WARM!

--------------------------------------------------

Regardless of whether you use a radiant panel (presumably the floor) or unit heaters (presumably forced air), BOTH have to raise the temperature of the building and the objects it contains to achieve reasonable human comfort. Forced air heats the air which in turn heats the structure and contents. Radiant heats the structure and contents which in turn heats the air. Radiant heat can use significantly less energy to achieve comfort because it achieves a higher mean radiant temperature at a lower air temperature. Unless the mean radiant temperature is reasonably high, people will not be comfortable regardless of the air temperature. For forced air to achieve a comfortable mean radiant temperature the air temperature will be higher than with radiant panel heat.

If you let the building sit without heat for days in cold weather you're going to have lots of problems. The floor is going to be very cold and the structure and contents are going to be EXTREMELY cold. You better have heated pews!

If you utterly insist on leaving the heat completely off for days, don't use a radiant slab. But, don't expect the forced air system to have the place even close to comfortable in any short length of time--say a couple hours before services or worse the choir meets for practice! The hot air will zoom to the ceiling where it very rapidly cools because of contact with all of the cold surfaces. If you think, "I'll just oversize the forced air system and crank the thermostat", think again. You'll have the ability to make the air quite warm quite rapidly, but the structure and contents will still be VERY cold and people will be TERRIBLY uncomfortable. The air will be too warm for heavy clothing and the structure will be to cold for light clothing.

If you think that putting a wood floor over the slab will help, think again. The floor will STILL be very cold and if it's insulated against the slab it may well be even colder than the slab below! [Installation over sleepers "counts" as insulation because of the air gap by the way.] A friend of mine lives in an upstairs apartment of a 1½ story home. Little if any insulation and terribly leaky. In the winter, they typically hang a heavy blanket across the top of the stairwell to help combat the draft. I happened to be there on a really cold evening this winter. The place was terribly uncomfortable. Furnace was running and there were a couple of space heaters to boot. T-stat read 70°. I felt the floor--it was nearly ice cold. Downstairs was vacant and they had a key and said house owner told them to set the furnace downstairs as low as it could go--they did so. I told them, "There's NO heat down there--I promise." We went in and guess what, wireless t-stat with dead batteries! A cup of water sitting on kitchen counter was frozen solid. Again--air temp in their apartment was "fine" and their equipment was operating properly--but it was nearly unbearably cold feeling. Wood floors, two layers thick. The floors in your sanctuary will be similarly cold (and occupants similarly uncomfortable) unless you keep a reasonable amount of heat in the place AT ALL TIMES.

While I love hydronic heat--particularly with as much radiation as possible, forced air would probably use less fuel--particularly with extremely deep setbacks and very occasional use. You will though use a LOT more fuel than you imagined to achieve any reasonable degree of comfort. The choir will likely complain BITTERLY and even refuse to practice.

Sorry, but the calculations to determine the difference [fuel use] are beyond me. Even if I was intimately aware of the construction and knew the mass of the structure and contents, the calculations would still be nothing but an estimate with reasonble accuracy only for a VERY specific set of indoor/outdoor circumstance and degree of setback.

I do though believe that you should still consider radiant heat.

Aesthetically it will be MUCH more pleasing as you won't have ductwork or ugly hanging heaters to deal with. As long as you keep setbacks to a reasonable degree, people will be very comfortable and fuel consumption will be reduced. Never forget that people ARE radiators and when the place fills on Sunday you really don't need a very high air temperature to achieve comfort.

Floyd_5

Thanks!!!, Mike

I knew you'd come to my rescue sooner or later and back me up :-)