Is my slab a heat sucker?

crowbar

I have a concrete block building sitting on top of a large slab that extends 12' on 2 sides...like 2 big patios.
I installed radiant heat with sleepers, rigid foam between, then PECS, then filled the space with sand.
It took a huge amount of energy to heat the place.
My question is: is the outside part of the slab acting like a heat sink, sucking heat away from the structure?
Would it help to cut a trench in the slab close to the house to disconnect the inside of the slab from the outside?
Or any suggestions for warming this place would be welcome!

kcopp

Why did you install sand? That is a really poor heat transfer medium. You would have been much better off Installing heat transfer plates.
What size tubing?
How long are the loops?
What temps are you running?
What is your temperature going out to the "panel" and what is the temp returning?
How well is the area insulated?
How many windows?
Was there a "heatloss" / heat calculation done?

hot_rod

Is there any thermal break from the radiant to the concrete block? 2- 4" of insulation between the radiant and cold block wall would be nice to have. Really for the same reason you insulate any exterior exposure wall
Known as edge loss it can be a big "sucker"

An infrared camera a will show you a visual of where the energy is going. Even in summer months you can fire up a slab and see the heat picture.

Unless the pex touches the floor somehow, you have a joist warming system, which isn't the most powerful, but should work to some extent.

The infrared camera will also show how the floor is warming up, and you could calculate the floor output by recording the surface temperatures.

Sand inside like that can become a haven for ants, a winter home

Have you run this system through a winter yet? If so, did it heat adequately? Large fuel bills?
Insulation is the gift that keeps on giving.

crowbar

We didn't use plates because there's no access to the floor from underneath.
It's on an existing slab. 
We used 3/4" PECS...I think.
Loops are under 300'.
Now I have to remember...I think temps going out were a bit over 140 and maybe 134 returning.
Although we kaid rigid foam down first, we neglected to put any "upright" along the wall. Unfortunate rookie mistake, I guess.
We have 2" rigid foam on the outside.
We did use it this past winter.
My elderly mother lives there and keeps the temp quite high...maybe 75-76.
When you're old... you're cold.
We are using a high efficiency electric furnace... it's not technically an on-demand because a little water stays in it, but it's close.
Energy use was VERY high.
We were really bummed because it was incredibly expensive to run.
What I'm really trying to figure out is if it would be worthwhile to cut a trench and disconnect the outside part of the slab from the inside.
From working with metal, I often notice how heat travels and how quickly something cools when one uses a "heat sink".

crowbar

As for the "joist warming system", we put no small amount of effort into leveling the sand with the tops of the joists. I suppose there are some voids, but for the most part, the sand should be in contact with the sub floor.

Jamie Hall

Well... it would be very helpful to have some idea as to the power demand -- the heat loss -- of the structure before we got too dismayed about figuring out ways to give you a better thermal break. Depending on where you are, if you are using electric resistance heat as you are, the actual bill can be astronomical even with an otherwise reasonable heat loss and a good setup. So that would be the first step. To give you some idea, where I am I have a building which costs about $8,000 a year to heat with oil. To heat the same building with electric resistance would cost about $34,000.

crowbar

I'm in Massachusetts.
The company that sold me the system talked me into electric, claiming that the efficiency of the high-tech unit would be comparable to gas or oil.
What do you mean when you say giving you an idea of the power demand/heat loss?

hot_rod

If you look over a years worth of electric bills you could get a good idea on how much is for heating.

What are your electric rates there.

hot_rod

crowbar said:

As for the "joist warming system", we put no small amount of effort into leveling the sand with the tops of the joists. I suppose there are some voids, but for the most part, the sand should be in contact with the sub floor.

Go to beach on a hot summer day. The surface of the sand could be unbearable hot. Wiggle your toes down an inch or two.

Sand is a very poor conductor of heat. I suspect the output would be the about the same without the sand. And certainly the sand cannot increase the output from the tube, at best it gives you some heat storage. How close together are the tubes?
If the sleepers are 16” on center there should be two tubes per bay to get the best output.

crowbar

Yes. Sleepers 16" and a loop in each section, so 2 tubes.
Nobody can weigh in on whether the slab extending out is sucking heat?

crowbar

Our electric rates are about 11c/kWh.
They briefly surged to 33 (!!) but I think there was pandemonium and maybe even a lawsuit. 
That month the electric bill was around $2k.

kcopp

Design is less than desirable.
Your temp difference from supply to return is only 6*F. Typically you should have about 10-20*. Its not transferring the heat out to the floor well.
So no its not sucking heat....
Extended run times w/ a electric boiler will give you huge power bills.
What is on the floor for coverings? Tile? Carpet?

PC7060

crowbar said:

Yes. Sleepers 16" and a loop in each section, so 2 tubes.
Nobody can weigh in on whether the slab extending out is sucking heat?

Yes.   A thermal break would help. 

hot_rod

Heat  travels to cold,always 

The rate at which hot goes to cold depends on the temperature difference

insulation slows heat transfer

So any where a warm surface or space is in contact with a colder
space heat energy will travel

If the floor space us 120f and outdoor is 20 degrees the insulation between the two slows transfer

A typical exterior 2x4 wall is R 19  You have what for an R value between?
 

GroundUp

Heat seeks cold. If you have a warm surface physically touching a cooler surface, conduction will occur and heat will be lost to the cooler surface. A thermal break is a must to eliminate that, however I think the root of your issue here is much deeper than that side thermal break. Do you have any photos of the installation or final product? Maybe I'm just dense, but I can't seem to picture what you're talking about with the whole sleeper/foam/sand ordeal.

crowbar

@groundUp, it's a concrete slab with sleepers on it. 
The foam went on the slab between the sleepers.
The pecs went on the foam.
All the remaining space was filled with sand.
On top of that went 1/2" ply sub floor and 3/4" cherry flooring on top.

crowbar

Right now the place is all finished...wainscot, sheetrock, flooring....and my elderly mother is living there. 
I'm trying to do what I can without tearing it apart and starting from scratch.

GroundUp

How much sand? An inch? 4 inches? How much foam below? Are the "patio" areas simply an extension of the slab under the heating system in question? So they are not physically touching the sand bed aside from a little thermal creep through the sleepers?

kcopp

To get it to work right you will need to start again.
The sand is insulating the heat from getting to the floor.
Who did this design??
An online company?
It may be simpler to install a gas space heater like a Rinnai EX series.

MikeAmann

You have nothing to lose and everything to gain by, like you said, cutting a trench in the slab close to the house to disconnect the inside of the slab from the outside. How much this will help is anyone's guess.

PC7060

1/2” plywood subfloor?  3/4” is the standard and 5/8” lowest acceptable. Did you rated tongue and grove subfloor? 

Does the floor feel soft?

hot_rod

You don’t have a lot of good option

The idea you mention is probably the least destructive, worth a try.

crowbar

GroundUp said:

How much sand? An inch? 4 inches? How much foam below? Are the "patio" areas simply an extension of the slab under the heating system in question? So they are not physically touching the sand bed aside from a little thermal creep through the sleepers?

GroundUp said:

How much sand? An inch? 4 inches? How much foam below? Are the "patio" areas simply an extension of the slab under the heating system in question? So they are not physically touching the sand bed aside from a little thermal creep through the sleepers?

The sleepers are 2x6.
There's 2" of foam, so sand thickness would be 3 1/2" to as much as maybe 5 1/2" because the structure had been made for drainage in the center.
The "patio" is part of the slab the building is on. 
Imagine (random numbers) a 40x40 slab with a 20x20 structure smack in the middle.
Concrete block is laid directly on the slab.

crowbar

PC7060 said:

1/2” plywood subfloor?  3/4” is the standard and 5/8” lowest acceptable. Did you rated tongue and grove subfloor? 

Does the floor feel soft?

I actually don't remember the thickness of the sub floor.
It may be 1/2".
The sand is right up to the top of the sleepers, so the sub floor is completely supported...resting on sleepers and sand
It feels very solid.

crowbar

kcopp said:

To get it to work right you will need to start again. The sand is insulating the heat from getting to the floor. Who did this design?? An online company? It may be simpler to install a gas space heater like a Rinnai EX series.

The company who designed the system is a reputable one that I found through research.
They reccommended sand because the concrete slab was already there, so I couldn't put pipes in it.
They thought sand would have good heat transfer properties.
There is a thermostat sensor in the sand, and the sand gets to around 90 degrees.

crowbar

kcopp said:

Design is less than desirable. Your temp difference from supply to return is only 6*F. Typically you should have about 10-20*. Its not transferring the heat out to the floor well. So no its not sucking heat.... Extended run times w/ a electric boiler will give you huge power bills. What is on the floor for coverings? Tile? Carpet?

I'll chec the send/return temps.
My memory could be wrong.
I'm pretty sure the send is 140F and the temp sensor in the sand reads around 90F.

crowbar

MikeAmann said:

You have nothing to lose and everything to gain by, like you said, cutting a trench in the slab close to the house to disconnect the inside of the slab from the outside. How much this will help is anyone's guess.

Well... 2 1'x1' trenches 40' long in concrete looks like a BIG job to me.

GroundUp

So is the warm sand touching the cold concrete/block or not? If not, then no you don't have an issue there.