optimum thickness of radiant heat slab?

mR_Slug

Hi I have read this https://www.hpacmag.com/features/hydronics-radiant-floor-tubing-depth-siegenthaler/ and plan to follow it. but what is the optimum thickness of the slab?

I understand that it can be as low as 1 1/4" in Gypsum applications.

First off this is a 15x15' workshop. Anti-freeze will be used, and 5/8 Pex. Here is how i plan to use it. Wake up, turn on the heat, after breakfast, 1 hour later it is 65 when 32 out. I may include frost protection, as this 1 hour time may not be possible on the coldest days. This 1 hour isn't rigid. I will be playing with the system to keep the water temp down. From what i understand, a thinner slab is better in terms of responsiveness but a thick slab can be used as a thermal store. So how thin should i go? If it were 2" could i use concrete?

How thick would you make the slab?

Other info. 4" of "kingspan 100" (uk product) under slab, 1/2" on sides. R-14 on walls, Im going to start out with R-14 in the roof for cost reasons, this will be increased to try to keep that water temp down.

Thanks

Hot_water_fan

For a 225 sqft building not used all the time, I'd skip the radiant entirely. Keep the cash, install a small propane/electric low mass heater and roll! Might get you better ceiling insulation too.

Jamie Hall

May i ask why you wish to use radiant floors? Radant floor hear is marvelous for applications which need a nice steady temperature -- and the thicker the slab, up to a point, the better. They are very poor for applications which have a varying temperature -- particularly a widely varying temperature, such as you are proposing.

So... what is the logic behind using a radiant slab? As @Hot_water_fan says, that's an almost ideal application for a propane or electric low mass heater.

HomerJSmith

It's not so much a thickness of the slab, but the depth of the tubing in the slab. About 2" is the max depth. The heat emitter is the slab not the tube. Too close to the surface you get banding. Too deep in the slab you get much slower response time. Most important is insulation on the ground side and edge protection of the slab to prevent back loss. Heat energy goes in a straight line to what ever is coldest, up, down, sideways, doesn't matter.

mR_Slug

Thanks for the input.

The radiant heat seems ideal for a workshop, that has cabinets/tools/workbench all around on the walls. It was also recommended by Norm Abram.

The warm up and cool down problem should be less if the heatloss can be reduced, via additional insulation. Particularly the roof. The 2" insulation is going to be PIR. This is what was recommended to me years ago. The attic space may have a thermal store+solar controller once everything is complete and then more insulation eventually. I will have to do some calculations as to warm up time. What I will probably end up with is something on a timer, as I'll use this every day.

Now the heat source for the UFH and the house is predominantly a boiler stove, with gas backup. The fuel is free wood (pallets), but they need to be cut up, in the workshop, and i want warm(ish) feet

I am not really planning to use the slab as a thermal store. Propane/electric is insanely expensive here, i can get free wood.

So the question remains how thin/thick should the slab be? There isn't heavy equipment there.

EdTheHeaterMan

I use a repurposed mobil home heater for my 35' x 22' garage. I maintain about 45° all winter long. When I plan to do a project I will crank it up to 65° and it will get there in less than 20 minutes on the coldest days.

The repurposed mobil home heater. It is sitting on a 14" x 8" duct that runs under the work bench and has 3 outlets. two face the garage door and one faces the other two bays of the 3 car garage. The heat rolling across the floor will keep the feet warm as the rest of the room gets to temperature. And the heater was free from a customer that replaced the oil furnace with a gas furnace. Been chugging away for over 20 years since then.

pecmsg

In floor radiant takes several hours to warm up the slab THEN start heating the room. Best is too set it and forget it. 

Hot_water_fan

Agreed, if you’re set on the in-floor just keep the temperature constant! The fuel is free allegedly! 

Jamie Hall

Well, as several folks have said -- including me -- a radiant slab is the wrong approach for a heat it up and then cool it off type application as you are proposing.

That said, the thinner the slab is the less time it will take to warm it up and to begin heating the space. You could probably get it down to 6 hours or so. So if you want to operate that way, I'd go for a 2 inch slab -- enough to cover the pipes after a fashion. On the other hand, if you wanted to keep the slab warm all the time you'd want a nice thick slab -- say 6 inches or so -- which would hold the heat much more stable...

hot_rod

Consider a radiant ceiling 
3/8 pex in plates, 6” on center. That would be a quick responding system

The Roth radiant panels would add sone R value also

with a slab you beed to get all the mass warmed before you feel the heat. 

Mad Dog_2

I'd almost NEVER talk someone out of doing  radiant, but you're buying a Barrell Racer to do Steeplechasing...Wrong horse... wrong application. You have 3 options;

1) Go with a well insulated slab, 3-4"s of monolithic mass, concrete, mud, et cetera, tube in the middle more or less...AND...A SLAB sensor to maintain a minimum idling temp..

2) Hot Rod's excellent solution of a Radiant ceiling/walls for a faster responding attack.

3) Modine Hot water Unit heater for Quick response, low installation cost and "shop" quality heating.   

Those would be my options to you. I've never regretted radiant in a slab...can't beat it...  Mad Dog 🐕 

hot_rod

High mass slabs I consider to be 4” or more. 4” is plenty for most garages and small shops. 5, 6 or more if heavy equipment or heavy spot loads.
Mid mass might be a 2” concrete over pour. With a small aggregate concrete could go as thin as 1-1/2”, although plan on some road map cracking over the tube.
Low mass to me is a gyp type pour which just barely covers the tube, since it is a sand mix, no stones to consolidate.

Concrete weigh around 4000 lbs per yard, so calculate the weight, then look at the btus required to raise the temperature of that much mass. Now you have the warm up time.

Panel radiators, radiant ceiling and walls are very low mass, the thin metal or Sheetrock on the tube ,so a fairly fast warm up.

I like my shop to be noise free, no blowers running, just the Sirius playing in the background 😁

mR_Slug

I consider myself schooled. Slab thickness seems to be a function of what equipment you want to put on it. Well from a practical standpoint. It reduces the warm-up time, that's the trade-off, but if it's a constant temperature design this isn't really an issue. (Also still following the John Siegenthaler research on depth.)

hot_rod said:

...so calculate the weight, then look at the btus required to raise the temperature of that much mass. Now you have the warm up time....

Slab is say 15' x 15' x 3" = 56.25 cu ft, 150 lbs per cu ft, so, 8437.5 lbs

Specific heat of concrete 0.21 Btu/lb ºF

BTU/hr' Required = Desired temperature rise * Specific heat * weight of the material / time to temperature in hours.

so
65º - 32º = 33º

and
33º * 0.21 Btu/lb ºF * 8437.5 lbs / 1 hour = 58,472 Btu/hr (cough)

33º * 0.21 Btu/lb ºF * 8437.5 lbs / 2 hour = 29,236 Btu/hr (cough)

33º * 0.21 Btu/lb ºF * 8437.5 lbs / 6 hour = 9,745 Btu/hr (ah something manageable)

So lets say a 6,000 BTU/hr heat loss*, 32º out, once the slab is up to temperature, that's 6,000*24 = 144,000 Btu/day to keep the workshop warm all day.

So if you were to turn off the slab on say a timer, it looses 6,000 BTU/hr. As i understand it, If you wanted to turn it on again in 1 hour, it needs 6,000 BTU before it will start to heat the room? Hench why everyone is saying keep it constant temp.
Have I got my math right?
Its interesting what other solutions people have proposed. Yes my wood is free (for the time being). Thanks for all the help. Think i will go with a 3" slab.

*5,102 Btu/hr heatloss (note: used r20 in roof) https://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

6,058 BTU/hr heatloss https://www.calculator.net/btu-calculator.html?roomsize1=225&roomsize1unit=feet&ceilingheight1=8&ceilingheight1unit=feet&people1=1&roomtype1=living&insulation1=good&sunexposure1=normal&climate1=normal&ctype=room&x=66&y=26

hot_rod

58,000 btu/hr into a 225 sq ft slab?
That would require 260 btu/ sq ft. That is a big number even for snowmelt.

Typical, comfortable heating slabs can give you 26- 28 btu/ sq ft.

Rule of thumb 2 btu/ square foot for every degree difference between ambient (slab temperature) and floor surface temperature. So if you can get the slab to 82°F in a 70° ambient you get
82-70 X 2 = 24 btu/ sq ft output. 82-65 ambient= 34 btu if you can work in 65° space?

Call it a 25 btu/ sq ft required input. 225 x 25=5,625 btu/ hr.

What temperature will that slab drop to? Starting from 23°?

I don't think you want to heat it too quickly, if you have enough tube and boiler to drive it.

Dave Carpentier

When I had to choose between a 4" and 6" slab in our 29x18 garage slab I figured; a) it was only 3 yards more, b) a 6" would probably outlast a 4" , and c) the in-floor would cycle less (longer run times) with the larger inertia.

HomerJSmith

All the math is great, but one thing missing is use time. As a work shop, how much time is going to be spent in the shop. 8 hours 24/7, great. One or two days a week, a lot of wasted energy=cost.
It might be better cost analysis to use radiant ceilings or steel panels which gives a faster response time as was suggested.

Radiant infloor is a trade off, comfort vs operational costs. More comfort=higher costs. What is mR_Slug's needs?

GroundUp

To actually answer the question, a 2" concrete pour is adequate for foot traffic and basic tools/machines. Forget the 5/8" pex as well as the 1/2" edge insulation. Drop the tube size to 1/2" and run 2 loops at 6" spacing, 250ft long. Bump your edge insulation up to 2" minimum. Raising your 33* would require approximately 39,000 BTU to do in an hour, which is pretty unreasonable. 2 hours however, 86 BTU/SF is not out of line for a heat-up. With that said, the odds are very slim that it's going to drop to 32* every night so the required BTU to heat up will likely be considerably less than calculated. I've got a 4" slab in my workshop that will drop less than 1 degree per hour with a 0* outdoor temp, and will climb 7-8* per hour with a 77 BTU/SF input. And yes, I do that every day because it only takes 1-2 hours to heat back up.