# Slab as a heat storage

Posted By Beno, Feb 10, 2008 at 4:01 AM

Not open for further replies.
1. #1

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
Hi there,

We designed our new ICF house, box shape (30ft x 60ft), 2 floors, having concrete floors for slab on grade and also for the second floor (with Hambro joists), no basement. The house will have a heat loss of 80KBtu. Location: ON, Canada.
The plan is to have a wood boiler like EKO25 with a 15KW electric boiler for backup (in parallel), an indirect (40-60 gal) for DHW, in-slab radiant heating for both floors.
Now, please feel free to correct me...
WIth the 4" slab, the total volume of concrete in the 2 slabs will be 30 x 60 x 4 x 2 / 12 = 1200 cu ft.
I found a formula to convert from cu ft to gallon: 1200 x 7.481 = 8977.2 gal
If we consider that the thermal mass of the concrete is half of water's, this is equivalent to 8977.2 / 2 = 4488.6 gallon of water.
Conclusion: the thermal storage of the 2 slabs is equivalent to that of a 4488 gallon tank with water.
Now, I don't mind having some temperature variations of the slab or having to make 3 fires a day in the wood boiler instead of one. But, at this stage, I'd like not to have a storage tank for heat. I will burn mainly softwood, from our lot.
Can you please review the numbers (and the idea in general)? Will this work? I mean that the EKO will not have to idle, but rather increase the slab temperature with few degrees.

Thanks,
Beno

2. #2

### ericjeeper New Member 2. ```NULL ```

Dec 28, 2007
85
0
Loc:
central Indiana
I live on a slab house 3200 square feet. If you plan on storing any extra heat.. you will be leaving windows open. I have a 600 gallon storage tank.. My heat loss is 26k. If I get slab temps above 75 I get room temps above 75. So no room to store excess heat. I consider it a bank. as the center on the home is on manufactured sand.(crushed limestone) It will hold heat for a long time.. Usually stores it the longest when 20 people come over for dinner at holidays. My great room is 22x40. starting out at 72 within 2 hours of company we have hit 82 and have the windows open.
Is there any way that you can get your heatloss down going in? triple pane windows?Lo-e Krypton gas filled.
Walls with icynene foam? 2x6 walls?
Do not worry about the big numbers.. Get the little numbers up.Doors and windows are you big dollar losers. Triple pane windows will offer anywhere between R-8 and R-10. where as most double panes are only R-1andR-5. Good insulated doors. Trick with lumberyard doors. Some of the doors have a double pane clear glass window. If you find one with a leaded cut glass, It is usually sandwhiched in a double to give you a triple pane with a slightly better R value. Good luck

3. #3

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
Your heat loss is very small. Our walls will be with ICF (concrete walls, with 2.5" foam on both sides), 2" insulation under slab, energy star windows/doors for Arctic zone (triple pane), R50 roof, HRV. I guess my higher heat loss comes from colder temperatures in the winter. The ICF walls should perfom better than a stick wall, some say an effective R35 for our area. Passive solar design. The best design I could come up with.
Even in your scenario, if you accept a delta of slab temperature of 7 degrees, this will give you a lot of heat storage.

4. #4

### Nofossil Moderator Emeritus 2. ```NULL ```

Oct 4, 2007
3,603
68
Loc:
Major fallacy - one pound of concrete does not store anywhere near the same amount of energy as one pound of water. In fact, despite the density, one cubic foot of concrete does not store as much energy as one cubic foot of water.

If water were really expensive, it would still be used for heat storage, at least in high end systems, because nothing else other than phase-change compounds even comes close.

Water takes one BTU to change the temperature of one pound by one degree. Concrete takes about 0.2 BTU. The ratio is the same in SI units :-(

5. #5

### SciGuy Member 2. ```NULL ```

Aug 17, 2007
25
0
Loc:
Marion NY
Beno,

My math pretty much agrees with yours on the storage capacity of your slabs. Here is my quick version.

The two slabs have a volume of ~1200 cubic feet. Normal concrete has a density of 150lb per cubic foot so your total weight of concrete equals 180,000lb. Since concrete's specific heat equals ~0.2 BTUs per pound per degree F. , 180,000lb. times 0.2 equals 36,000 BTUs per degree of temperature change. Since water's specifc heat is 1.0BTU/lb/degreeF and it has a weight of ~8.3lb /gallon the equivalent water storage would be 36,000/8.3 = ~4337 gallons.

I do find it hard to imagine your heat loss at 80,000 BTUs. My wife and I live in a geodesic dome near Rochester NY that we built 26 years ago. I clocked our little Dunkirk XEB-2 boiler for the last week and found that we used 20 therms gross or about 17 therms net to provide heat. The house is about 2400sf heated by ~ 300 sf of radiant floor in the kitchen area and we maintain a temperature of 70F with no setbacks. I hate the carbon emissions we create by burning natural gas but haven't found a way to justify going to a slick gasifier boiler when we only use ~ 475 therms per year.

A 7 degree delta T for you slab will create comfort mayhem. In order to make your heating load in your super insulated house, your floors will need to run at a very low temperature. They will not seem warm to the touch. For comfort's sake, I would suggest that you target a few specific area of the house for the radiant heat. Perhaps you might consider storing heat in an insulated layer below the lower slab as some have done with solar designs.

Best Regards,

Hugh

6. #6

### SciGuy Member 2. ```NULL ```

Aug 17, 2007
25
0
Loc:
Marion NY
Yes but his math is fine for the total storage per delta T. Since the density of concrete is ~ 2.4 times as much as water's, that changes things a good bit when you look at storage per unit volume. X times 2.4 / 0.2 = .48 . Concrete does store ~ .48 times the BTUs per unit volume that water does. It doesn't run through a circulator pump worth a darn but that is a different issue.

I like math.

Hugh

7. #7

### Jersey Bill Member 2. ```NULL ```

Jan 21, 2008
132
0
Loc:
Central NJ
In my house, I have 6" slabs on the lower level and 1.5" slabs everywhere else.
I thought I was going to store heat in these also, and they do, only not the way that I wanted.

The floor temperature is closely related to the room temperature. In the off season, I have to leave the heating switch OFF until the outdoor temperature goes below 60 f, or so, otherwise we overshoot by a lot. The mixing valve has a hard time delivering very little heat. On low load days i have a problem getting the loop temperature down to set point. I believe that the pump heat is really heating the house and not the boiler.

I have a "small" boiler for the house size and practically no storage. My wood boiler is maybe 80%-90% of the peak heat load for the house.

I don't fire the boiler unless the outdoor temperature is below 40 f. If I do, by the time that the boiler heats up and begins to burn efficiently, it has to shut down because the loop temp is already maxed out. To get a longer run, I will wait until evening, when the outdoor temperature is decreasing. As the outdoor temp decreases, the water loop temp that heats the slabs is increasing. It takes a few hours to bring up the floor temperature so I have a constant load for the wood boiler for at least a few hours.
If the outdoor temp is between 17 and 30 f. I can adjust the boiler output to match the load. below 17 the gas boiler comes on from time to time. Above 30 I have to "sleep" the boiler for hours at a time. I usually just turn the fan off manually when the loop temp gets above 180. I check it from time to time, and wake up the boiler when the loop gets down to 160, usually 2-4 hrs. This will give me a 1-2 hour run. With the fan off, the boiler is at a really low, stinky, burn. The cresote builds up in the lower chamber and in the chimney. Outside you can small the stinky burn, luckily some of the neighbors burn wood, so its hard to trace back to me. I claim high efficiency and "no smoke" from my gassifier, but its only partially true.

Beno, you said that you dont mind lighting 3 fires a day, but you will have to sleep. My boiler will go for 2 hours at full output before starting to decline. Believe me that it will get old- quick.

A wood gassifier is the way to go for efficiency, but there has to be storage so you can sleep without getting up many times during the night to load wood. This is assuming thet the electric boiler is the backup as you mentioned- otherwise the wood boiler would be your backup.

I am involved in a project where we are going to put a 1000 gallon square storage tank (pre-cast septic) under the stairhall floor slab on grade. We will insulate w/ 4" foam. any heat that escapes up from the storage will go into the house. The tank access will be from the closet under the stairs. This could work for you also.

Good luck!

8. #8

### Nofossil Moderator Emeritus 2. ```NULL ```

Oct 4, 2007
3,603
68
Loc:
Yup - my bad - din't follow the math. Concrete is effectively half the 'thermal mass' on a per-cubic-foot basis.

9. #9

### solarguy New Member 2. ```NULL ```

Jan 14, 2008
147
0
Loc:
southern, nh
They are out of vermont & I believe Bob Starr has done a couple of things
with slabs for heat storage on solar systems that might be helpful for you

10. #10

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
I forgot to mention the solar gain from the passive solar design, which, at least during the sunny winter days, will provide some heat too. Also, a wood stove on the first level.
If I take the average 20 BTU/sq.ft for a well insulated house, I come with 70Kbtu heat loss.

If I understand correctly your comments, I can't "efficiently" use a wood boiler in my application w/o the storage tank. Can you recommend a solution w/o the storage tank?
(I'll have a look at http://www.radiantec.com/systems-sources/systems.php, later today)

Thanks everybody!

11. #11

### Nofossil Moderator Emeritus 2. ```NULL ```

Oct 4, 2007
3,603
68
Loc:
So - one other difference between a slab and a tank of water: The tank of water has a usable temperature range that goes from somewhere around 100 degrees to 180 degrees. I doubt that a floor slab can be used over as wide a range of temperatures.....

12. #12

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
I thought that if the slab is cold, I can send very hot water (up to 180 F) for, let's say, 2 hours, thru the PEX tubes, and this will slowly raise the slab's temperature to 80 F. Then leave the slab to cool down a lot, and restart the process. Like a masonry heater.

13. #13

### SciGuy Member 2. ```NULL ```

Aug 17, 2007
25
0
Loc:
Marion NY
Beno,

If you heated the slabs of your extremely well insulated house to 80 degrees, you would find the house temperatures hovering near 80 as well. I would bet that you will need a floor temp only slightly above 70 degrees F. in order to maintain a 70 degree interior temperature dur9ing even the coldest parts of the winter.

Hugh

14. #14

### BrownianHeatingTech Minister of Fire 2. ```NULL ```

Oct 5, 2007
1,253
0
Loc:
Northwood, NH
Let's say that you can let your slab swing from 65 to 75 without discomfort. So, multiply the equivalent thermal mass of the slab (ie, what it would weigh, if it were a thermally-equivalent amount of water) by 10 degrees for the btus that it can store.

The number is quite small, compared to water that can swing by 80 degrees.

There's also the loading rate of the concrete. In order to actually get the heat into the concrete, water needs to heat up pex, which needs to heat up the concrete in direct contact with the pex. That heat needs to transfer by conduction through the concrete. Which will be a relatively slow process, at typical tube spacing. Radiant slabs don't heat up quickly.

Water, on the other hand, will convect. Once the heat exchanger heats the water near it, that water will flow away and be replaced by cooler water, which will get heated and flow away... You can "load" and "unload" a fluid-based thermal storage system much more rapidly than a solid thermal storage system.

Joe

15. #15

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
If the 2 slabs are equivalent to 4400 gal water, for 10 degrees delta T it will store 44 KBtu.
Now, if I add to this also the indirect for DHW, with 60 gal, that will store an additional 4.8 KBtu. Total: 44 + 4.8 = 48.8 KBtu.
If I take a water tank of 600 gal, for 80 degrees delta T it will store 48 KBtu.
Assuming that all this is correct, I can burn the EKO25 once a day full speed, and get out of it 48.8 KBtu. If I need more heat, I can start the wood stove. Can you please check again my numbers?

Another question, for this to work, is how "smart" is the mixing valve? If the mixing valve will constantly output water at 75 F, then my solution will not work. The demand for hot water is low and long. If the mixing valve can be somehow "programmed" to give 180 F output when the slab is 65 F until the slab is 75 F, and then stops, then it might work, there will be a shorter demand for hot water, when EKO25 will burn at full speed.

Thanks!

16. #16

### Nofossil Moderator Emeritus 2. ```NULL ```

Oct 4, 2007
3,603
68
Loc:
I follow you to this point, but I think unit conversion problems have slipped in.

4400 gallons is right, but 1 BTU is 1 pound of water by one degree. 8.3 pounds per gallon (gotta love our units) id 368,000 BTU for 10 degrees. The 600 gallon tank is another 400,000 BTU.

Finally, the EKO 25 is rated at 80,000 BTU/hr, and actually can deliver a sustained 60,000 BTU/hr in the real world.

17. #17

### ISeeDeadBTUs Guest 2. ```NULL ```

Though I like math, I stick with the stuff I get paid to do. So right off the bat I will warn you that the 'tech guys' here bantering about numbers know this stuff better than I.

Having said that, I can tell you what I experiance. My basement slab is about 1,200 [] with pex. Joe is correct, it takes forever to heat up, and puts a hurtin on the boiler when it does. We didn't go with slab sensors, just standard room temp stats. Yesterday, since I knew the temp outside was going to drop, I moved the room temp of the basement up. This takes many hours. But when I went to bed at 10 PM last night, I had the basement up to 74*. I then set the room stat back to the normal 68*. As of 5Am, that zone had not come on. And the basement is 1/2 walk-out, so it's not 'super-insulated'.

Last winter we went away for a weekend, and I got the basement up to 77*. We left very early on a Saturday morning. When we got back late Sunday night, there was still a Piece of hard Maple in the GW. And the house was warm. Priceless, as they say.

If I had it to do over, I would have put in a 6' (yes, foot) slab. Yes, I know water is a more efficient storage.

A HUGE plus (for me and the dog, maybe not for most readers here) is the summer. Since we have radient heat - and this is upstate NY - we did not put central air in this house. The slab helps termendously here. Engineers may tell you that heat 'moves', not cold (lack of heat) but that slab will store the coolness too.

18. #18

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
Shall I understand from the last couple of answers that my solution might work?

19. #19

### slowzuki Minister of Fire 2. ```NULL ```

Feb 1, 2007
508
9
Loc:
I have 4" slab in my house with passive solar. The sun can get the slab up to about 80 F when it is quite sunny and hot inside the house. It lasts for about 4 hours from when the sun goes down, then the room temperature tanks and it takes a bit of work to get it back up to temp in there.

20. #20

### Nofossil Moderator Emeritus 2. ```NULL ```

Oct 4, 2007
3,603
68
Loc:
I think the answer is 'yes'. Probably should do a plumbing diagram. You'll want to limit the temp of water going into the slab. Don't want to make it uncomfortable, and you don't want to plan on changing the slab temp too quickly. With storage, should be good. Don't remember your heat loss budget, but with over a half million BTU available you should be warm for a good while after the boiler has gone out.

I have a pretty good sized house, and I do OK with a max of about 400,000 BTU of storage.

21. #21

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
My open question is still about the mixing valve, can it be controled to give a high temperature for few hours, instead of 75 F for the whole day? With such a huge concrete mass, I guess the slab's temperature will not change too quicly.

22. #22

### Nofossil Moderator Emeritus 2. ```NULL ```

Oct 4, 2007
3,603
68
Loc:
There might be valves like that, but it might be easier and cheaper to just manually adjust it for those situations. With the system that you're envisioning, I think that you could keep the slab virtually constant and just build a fire to recharge the storage.

23. #23

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
It seems that, with a proper combination of solar gain, wood stove and wood boiler I can have a good comfort. For small heat loads solar gain and wood stove will be probably enough, for colder days, burn few hours the wood boiler, to heat the slab. Thanks everybody, and let me know if you have other ideas/opinions.

24. #24

### SE Iowa New Member 2. ```NULL ```

Jan 17, 2008
212
0
Loc:
SE Iowa
I live in a 4600 sf icf home on 3 levels. Basement is ~1700sf, main floor ~1700sf and 2nd floor ~1200sf. We have a 5" slab in the basement with a radiant barrier plus 2" HD foam underneath. The other 2 floors have 1.5" of gypcrete (lightweight concrete) that weighs a total of 42000lbs. I don't know how much the basement slab weighs.

From my limited experience, you will NOT want to use your slab as storage. The 1.5" of gypcrete responds suprizingly fast but does not hold the heat as long as you'd expect. The opposite is true for the basement. It is sandwiched b/t 2 layers of radiant "zones", one from the slab and one from the ceiling above. In my ICF home the basement zones have NEVER come on except when I bump the thermostats up just to "flush the lines" once a month or so. Admittedly, once I put a finished ceiling on the basement rooms not as much heat will go down so that the basement zones will come on occasionally. But for right now, they are at 68 degrees almost continuosly. If we do have very cold weather, when the 1st floor zones have to heat longer than usual, the temps become unbareable in the basement. Radiant Heat is so much different than force air. I notice that if you have the house at 69-70 degrees it is confortable for doing most household activities. IF you change that temp to 72-73 OR do even light house work you will notice how sweaty you become. I have become so spoiled that I can tell a difference of 1 degree in various rooms in the house. We had a problem with our igniter on our boiler and that is how I could tell that the boiler was off line, the house would cool off by 1 degree.

I think you'd be cooking yourself for part of the day then you'd feel cold when the temps drop back down. It is hard to explain, but I can not handle temp variations since living in this home. Like I said before, I'm spoiled from any other heating and home styles.

25. #25

### Beno New Member 2. ```NULL ```

Feb 26, 2007
175
0
What I thought is, for the start, I'll heat the water for the slab radiant with the electric boiler, and, in the second stage, I'll add the wood boiler if needed.