Radiant in slab new garage build

Here's a graph that shows what my heated floor has been doing for the last 48 hrs. It is 36x24 ,4 loops @ approx 150' 1/2" hepex on 9" centres. The insulation underneath the 6" slab is 2" high density styrofoam boards. I didnt use any on the sides. when the concrete dries it shrinks and there is a 1/4" air gap around the perimeter. Also we dont get frost below the surface here very often.. It heats roughly 1800 sq ft.

Hondaracer,

I don't like to bash other's opinions, but have to opine here.

leon said:

The thing is that heat is going to leave your garage the minute an overhead door is open and it will take a long time for the slab to recover.

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leon said:

If you use the steel or cast iron radiators you will have less work and
the garage will stay warmer longer simply because of the radiators
shedding heat.

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The misdirection in those statement proves that he's never had the fortune to work in a shop with a heated floor. Find me one person that's installed a properly working in floor heat system and regretted it, and I'll mail you $100.


Lets do some high school math:
What is the highest mass element in your shed: the concrete floor by a factor of 10. Your 6" slab would weigh 250,000#
A 28x40x14 shed contains 1160# of air. If you left your OH door open long enough to exchange every molecule of inside air with the outside, you would have lost an 'astounding' 11,000 BTU of heat in the air. BIG WHOOP. That takes 2# of wood to burn to replace. Assuming it gets inserted via your floor it lowers it's temp by 0.044 degrees.

A shop with a heated floor returns to a comfortable workplace in almost a mystical fashion the instant the door is closed.

I'll offer some advice - don't pony up $ too soon for the Modine to bump the shed temp up from 50* to 60* on occasion. You will never use it. Scouts Honor. Heat your shed with the floor to 55* instead and you will be much happier and take very little more energy. Spend the modine money towards the $2000 it would take to spray foam the walls 1".

Hydronic floor heat and spray foam are an unbeatable combo.

UM let say me say first that I never insult anyone or their opinions on this forum
nor do I feel the need to. I was not misdirecting him either.

He also stated he was not going to heat the building all the time at a higher temperature
and a steel or cast iron radiator used for heating could be set with a manual thermostat
on each radiator and keep the building at a comfortable temperature effectively,

Heat goes to cold that is simple physics. You can cool a building down by opening an
overhead door. Your heat source has to have enough hydronic heat energy available to heat the
open area back to the desired temperature.

by properly insulating it to an R32 or higher the garage will not require much heat and number radiators
would heat the building easily.

The more heavily an out building or for that matter any building is insulated the better it is
at retaining heat and it loses less heat with standby losses due to windows or doors or overhead doors.
AND the faster it will come back the desired temperature.

I said using steel radiators of any size was much less work and much less plumbing
to fuss with that was my main point. A slab that is insulated from frost will hold plenty
of heat from radiators or a Zurn hot water to air garage heater.

I also mentioned the apron to the garages doors is ideally where he would want tubing anyway to
eliminate ice buildups as that is where there is going to be a lot of ice anyway if read what I posted.

He has not mentioned tubing spacing and whether he is going to use the so called thermal concrete
either that adds to the cost which is something I explained extensively in my other posts.

A lot of buildings with overhead doors are heated with hanging steam radiators or steel or cast iron radiators and
I also mentioned that by installing aluminum foam board insulation behind the radiators will push even more heat into
the building more effectively.

A simple pipe run is all that is needed with hot water to air garage heater if desired for his use as he is not heating living space and we do not know his budget either or even if he has considered the tubing spacing nor has he mentioned it.


It would be better for him to obtain the books Classic Hydronics and Pumping Away before he spends any money on this.

OH and by by the way you can find a leak much easier with radiators than you can with in floor heating which requires you to shut the lights off pull down the shades and put a cat in there and it will lay down on the warmest spot in the garage or the room or run around with a stethescope to find it unless you buy the very expensive pex with a wire tracer in it. then you have to dig it up....................

Your heat source has to have enough hydronic heat energy available to heat the
open area back to the desired temperature.

That is true. But you will need a LOT of radiators to match the heat energy (thermal mass) of a heated slab, all running a lot warmer water temps than a slab would.

Hello Maple One,


The BTU per gallon issue again rears its ugly head and
since water cannot carry more than 10K BTU per gallon
no matter the temperature, that is the major issue.
The larger the radiator in square feet the greater the amount of
heat it can provide and they can provide 150 BTU per square foot
of area or more.
A long low radiator can provide the user with lots of heat quickly,
especially with steam or hot water with manual thermostat
mounted on the radiator OF ANY SIZE.

leon said:

Hello Maple One,


The BTU per gallon issue again rears its ugly head and
since water cannot carry more than 10K BTU per gallon
no matter the temperature, that is the major issue.
The larger the radiator in square feet the greater the amount of
heat it can provide and they can provide 150 BTU per square foot
of area or more.
A long low radiator can provide the user with lots of heat quickly,
especially with steam or hot water with manual thermostat
mounted on the radiator OF ANY SIZE.

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One gallon per minute at a 20 degree drop yields 10,000 btu per hour. if you can operate at a wider temperature drop, you have more heat available. 40 degrees drop=20,000 btu per GPM.

you are correct, the larger the radiator, the more heat it can provide. one the size of the slab will therefore be the best. (at low water temperature)
and with the heat stored in the slab, it will recover more quickly without additional heat input.

a long low radiator cannot be "of any size". it has to be sized based on the available water temperature and heat load, which would include: heat lost by opening the door, bringing in an 8000 lb truck that is below zeroF and covered in ice and snow, and bring it to room temperature. a slab will excel at that.

Heat is also stored in everything else within the heated area including tools, machines and everything else that's heavier than air making the heat lost in the air exchange a non issue. That 8000 lbs. of cold steel will radiate somewhat but that's where in slab shines because that heat you're standing on overcomes or at least neutralizes that radiation. It's not temperature, it's comfort.

Karl_northwind said:

One gallon per minute at a 20 degree drop yields 10,000 btu per hour. if you can operate at a wider temperature drop, you have more heat available. 40 degrees drop=20,000 btu per GPM.

you are correct, the larger the radiator, the more heat it can provide. one the size of the slab will therefore be the best. (at low water temperature)
and with the heat stored in the slab, it will recover more quickly without additional heat input.

a long low radiator cannot be "of any size". it has to be sized based on the available water temperature and heat load, which would include: heat lost by opening the door, bringing in an 8000 lb truck that is below zeroF and covered in ice and snow, and bring it to room temperature. a slab will excel at that.

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What I ment and glossed over was the fact that the garage he wants is 28 by 40 and is 1,120 square feet in area and
a seven tube radiator that is 20 inches tall will have 4.25 square feet of radiation per section and that equals 264 sections of radiation equivalent to his proposed garages square area. He can line the three walls with surplus cleaned and tested radiators and accomplish the same thing and reuse perfectly good radiators that would end up at the smelter otherwise.

If he skimps on insulating he saves nothing by installing floor heat. A small zurn heater wil provide plenty of heat to go along with salvaged hot water radiators at less total cost

The plumbing is simple and faster as one continuous loop of galvanized or black iron pipe can be used to his advantage to the floor connection to his radiators across the door header then to his other radiators and the hanging heater and then to his apron and then to the forest eater.

Why make it more expensive than one needs to? lots of garages have hot water radiators and Zurn water to air heaters.

I'm not sure it would be that much or any more expensive. Four rolls of pex & 2 manifolds, vs. an uncertain number of radiators and an overhead heater plus the plumbing for all of it? I see used rads here for sale quite often, but they're certainly not giving them away.

not to mention all the wall space that is not available for things now. Laying 1200 feet of pipe for a simple rectangular slab will take 2 guys a couple hours to lay out and put down. and if you use cheap 02 per you can spend about $800 for the materials for that and be done. you will spend that and more for enough radiators not to mention the time to clean, test, etc.
it also does a nice job of drying out the floor.

Hello Maple1, Karl_Northwind,

It all boils down to an economics/opportunity cost problem.


minuses

1. buying thermally enhanced concrete
2. plastic anchor sheets for the Pex
3. Oxygen barrier pex
4. stone mix
5. concrete shrinkage during and after curing if the slab is not saturated and kept wet
using heavy canvas tarp and a thick layer of straw applied to hold tarp down and to
hold moisture to increase slab strength during the curing process

Assuming: in both instances:
slab on grade construction stone backfill to below grade elevation

1. slab is properly insulated against frost
2. foundation footers are dug to proper depth
3. top soil is stripped away to actual depth of slab
4. crushed stone is laid in footer trench to support perimeter footer
5. rebar is installed to increase strength of footer foundation.
6. foil backed insulation board is installed prior to pouring concrete in footers and for slab pour
7. 6000 Lb. sand mix with rebar tied into concrete mesh

Not knowing what spacing is desired for the in floor heat loops or the inside diameter of the
oxygen barrier PEX is another issue entirely.


Plusses:

1.surplus salvaged and tested radiators are used.
2. all plumbing connections are exposed
3. less work as simple single pipe run is used with two connections(in/out)
to slab apron and water to air Zurn heater at door way
4. each radiator will have a manual thermostat allowing the shop to stay at a low
temperature when not in use.

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All those minuses should be done anyway, no matter how you heat the garage - except that if not putting pex in the floor, you don't need numbers 2 & 3, or course. And 'thermally enhanced concrete'? I don't know what that is.

Then if doing pex, the only ones I've seen done didn't use 'anchor sheets' (also not sure what those are even), they just tied the pex to the mesh - and used the same concrete mix as if not putting pex in. So the only added cost to the slab was the pex, and the time to lay it out & tie to the mesh. Slab should be insulated either way.

I've even been involved in a hockey rink build where the opposite was done - the floor was cooled. They just laid the pipe out & tied to the mesh. Piping was bigger than for infloor heat, but otherwise done the same way as heated slabs I have seen done. We have one heated slab in a small building here, also done same way.

'Surplus' still costs - can't find used cast rads around here for less than $100 each, if that. Not tested, just pulled from old houses. If they are less, they are leakers & look like they should go for scrap.

Also, how do you control the heat in each rad with its own thermostat if it's all in one run/zone?

leon said:

It all boils down to an economics/opportunity cost problem.

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I disagree. It comes down to preferences, comfort, and usability of the building.

The whole purpose of in slab in great heat without hearing and no visual appearance. Maybe rads are your thing but I don't want them taking up valuable wall space in my workshop/man cave/garage/party center.

It appears Leon in incapable of understanding that the original poster has already decided to install in floor heat and is looking for some advice in detailing his thermal breaks.
Leon please start your own post on "the many advantages of wall hung used cast iron radiators over in-floor heat" and we can all leave Hondaracer alone.

Hello Maple one,

Each radiator could have its own thermostatic radiator control valve that is connected to the lower tapping and the water flow would continue on to the next radiator or one radiators thermostatic control valve could control an entire wall of radiators if each wall had its own loop and had reflective foam board insulation to push the heat back into the open area rather than towards the exterior wall.

A cast iron radiator can be heated using lower temperature water and for than matter will soak up the heat from the daylight coming through a window and you can run them at 100 degrees water temperature very efficiently using smaller piping.

As far as lost usable space goes the area above a radiator lends itself well to shelving does it not it?

We still do not know what size PEX he wants to use or the spacing or what his desired EDR "Equivalent direct radiation" is for what whatever he plans so were spitting in the wind here and for that matter he may just be trolling for information.



To each his or her own

@ HondaRacer2oo4, I'm pouring my building next week and have decided to put 2" foam directly under where the overhead door comes in contact with the floor. I'm going to leave about 1" concretrete on top and use the expansion joint (when I pour the approach)for the thermal break for the top 1" of concrete.

This is the best solution I can come up. I'll try and post some pics once I form the doors.

I would worry that the bricks that you use for spacers would weaken the concrete and invite cracking. The best way to space the mesh away from the insulation is to hook it and pull it up into the mix while pouring. This has always been the method for bringing the mesh up whether there is pex or not. There are concrete hoes made especially for this with a hook welded to the back. If you don't have one you can use a potato hook. You just must be careful not to pull it up too far. Take your time. There are some drivers that will rush you and some that will work at the pace you demand.

Also, don't forget to order their highest pressure floor mix.

Fred61 said:

Also, don't forget to order their highest pressure floor mix.

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Have any examples? 4,000psi?

I used 3000 on my last pour and didn't get any cracking on my 6 inch slab but 4000 would give you a good margin of safety.

Honda, here is what I've come up with so far. I think it's going to work out pretty well. Still need to backfill and put a string on the front of the 2x4 to get it straight.

This summer I'm pour the approach. Until the approach is poured I'll use 2x8's and plywood to protect the edge from breaking out. I will not be driving on it until the approach is poured.

Fred61 said:

I used 3000 on my last pour and didn't get any cracking on my 6 inch slab but 4000 would give you a good margin of safety.

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My uncle(who is also doing the pour) said 4,000 psi with fiber mesh is the norm for floors like mine.

I was trying for 1.5" but things didn't quite work out that way. I still think it's going to work OK.