Building with 'thermal mass'. Are the claims bogus?

[s219]

What are his concrete floors sitting on? If on ground level, are they well insulated from below? An uninsulated slab on ground level will want to drive interior temperatures towards the ground temperature, and it can be a very powerful heat sink. I can see that being good in summer, but in winter it seems like you'd be fighting to counteract the effect.

 

[aczlan]

NOW....the benefit of solar mass is when outside temps warm up during the day to the point heat isnt needed. BUT still drop at night where heat is required. All day long while it is warm, you are storing free heat given off by the sun. And at night you may not need heat at all. Whereas without alot of stored heat in the mass, the heat may have to kick on. Thermal mass, like insulation, will balance out the temperature swings of the daytime highs and nighttime lows. But again, when the daytime highs might only reach 30F and drop to 10F at night, thermal mass doesnt save anything. (But it dont cost any extra either).

Correct, where thermal mass helps is where you can setup for passive solar heat when building (ie: south facing windows that let the sun warm your thermal mass), you can get "free" heat to warm your thermal mass and reduce the amount of "paid" heat (be it gas, oil or electric) that has to be added.
You can get a fair amount of heat that way on a sunny day even if its only 30F outside.

Aaron Z

 

[CobyRupert]

Think of it in terms of batteries. If you have one battery, it will only last so long and then require charging. Add a second battery, it will last twice as long. But it will also require twice as long to charge the system back up.

Or think of it in terms of a flywheel.

 

[TnAndy]

I've been using it in my home since 1985. Thick, insulated concrete floors, massive fireplace in the center, double 2x4 exterior walls. In the winter, we can heat the whole place with a small wood stove, lot of south facing glass. In the summer, the temperature stays cooler inside until very late in the afternoon, south glass shaded by good overhang and deciduous trees on that side. Our AC is an 18k BTU mini split in the main living area, and a 12k one in the master bedrm.

My BIL came over to feed animals one winter when we went on a trip for couple weeks, and when we got back, his comment was our house was warmer with no heat on than his trailer was with it going full blast.

Mass works. And has my vote.

 

[CalG]

I can't do much better than those "flywheel" descriptions, and restate that thermal mass does not make heat. THERE IS NO FREE LUNCH!

But if you can find free heat in solar, or in burning efficiency, thermal mass will capture the heat when it is not needed and return it when it is (or vice versa)

The "traditional" Russian stove is just that. Efficient burn technology (no smoldering smoke maker in warm temps) that captures the heat that would other wise go up the smoke pipe. And then releasing that heat over a period of time, at a high degree of comfort. Call it a thermal capacitor.
But if one is heating a home with an already highly efficient heat source like electricity or an oil burner, The only thing thermal mass offers is a high area of radiant warmth . When the mass is properly used in this way, comfort can be enhanced at lower air temperatures. And a lot of energy is lost in heated air.

I've heated with a wood stove in the basement for over 30 years. This past year, I put a couple of old cast iron steam radiator units on top of the stove on a angle metal frame. These "thermal mass units" are plumbed to a second CI radiator in the kitchen, and circulates the water within by thermal syphon. The system works GREAT! A bit less heat in the downstairs room with the stove, and a warm spot to set your butt on when one is feeling a bit chilly.

When we rebuild the chimney, which we will need to do some year soon, I plan to run a third flue up from the downstairs stove all the way to the upstairs ceiling level (20 feet of vert or so) then back down to the normal flue port for the downstairs stove. With a sliding gate, this extra pipe will be my "Russian stove". Now how to build in a sleeping bench.....

Oh! Its 33 degrees outside and the indoor air is 62 degrees F. The wood stove is burning hot on a light load, and the world is a wonderful place to be in. If only the sun were streaming in the windows.

PS, There are no common materials that store heat like water does! It's only drawback is that it's a liquid and must be contained ;-)

cheers

 

[dave1949]

I calculate my solar btu input to be about 70K btu per hour on a sunny day. That's equivalent to a decent size wood burner, plus the heat is automagically distributed, not centralized.

320 btu/sqft/hr is the approximate maximum available at sea level. Our glass is clear. Besides the light transmission rating of the glass, I'm sure the amount of light that makes it inside varies with the sun's angle of incidence on the glass causing refraction. As the seasons change, the sq ft of floor space that receives direct light varies greatly also. The depth of sun penetration into a room varies from 0' in June to 13' in December using a 28" overhang at ~45* N. latitude..

Our slab is insulated underneath. I put in 1" styrofoam around the edges to make a thermal break from the walls too. Below the slab, I put a vapor barrier down on the compacted stone bed first, then styrofoam, then a layer of low-e product called Slab Shield that comes in rolls, then the rebar and radiant heat PEX. I think the vapor barrier is important. Earth is not a very good conductor of heat, but moisture in the earth sure is, and our dirt is always moist here. I think that makes it necessary to isolate the slab from any contact with ground moisture.

There are different opinions about what the insulation below a thick slab actually accomplishes. Some say the insulation stabilizes at the surrounding ambient temperature, which for the bottom of a thick slab will be about the earth temperature. My rear walls and floors in rear rooms (that never get direct sunshine) tend to run around 63* in winter with no radiant heat used. That means there isn't a large temperature differential between the slab and earth, which results in low levels of heat transfer between them. Given a low temperature differential, the insulation doesn't have much to do.

One thing is for sure, you cannot heat or cool the earth below your house to any appreciable extent using a home heating/cooling system.

 

[stephan]

OK so in my other house I have Waffle Pod flooring (concrete slab with polystyrene blocks) and ThemalCell walls (again polystyrene but filled with concrete) with double insulation glass. The north facing internal walls are filled with concrete blocks and I have hydronic (water circulated through the floor) heating off a slow combustion in the kitchen. I think the flywheel is correct in that it takes a little more effort to get it up and running but less effort to maintain. Coming into winter, like we are, I will need to heat the slab over a weekend but then just the evening fire maintains the warmth to a shorts and t-shirt comfort level. The slow combustion fireplace has an 18kW wet back that uses thermosyphon to heat water from a 400l hot water system in the roof. This system was solar powered with a single array of 22 tubes when I bought the house but that just did not produce the energy required. The thermal mass is in the floor and the walls and while hard to measure the actual effectiveness the combination works very well.

I suspect the OP friends probably need to try leaving the heat on for an extended period. It maybe that they are baulking at the initial startup cost and not giving the system sufficient time to go into the maintain phase.

 

[powerpace]

I have been in several Thermal Wall designed houses and most were very energy efficient. I also have slept on a huge sun heated rock mass overnight and been comfortable in air temps in the 40's. Native Americans and many other cultures understand how to live with nature and use the Sun and a mass of natural substance. Rock, clay and timber.

Sometimes engineered structures are not efficient because the preferences of the owner are not optimum for the desired results.

 

[CalG]

Anyone desiring high air temperatures as a metric for comfort, will NOT find thermal mass a useful design.

Most all thermal mass systems take advantage of RADIANT heating of the surroundings.

Spaces not "visible" to the radiant mass will very likely feel cold.

Convection might be part of the design, but that usually just ends up overhead.

Life style and expectations are often at odds with energy utilization.

ps Stephan, I like your choice in tractors!
The Lambo runner / Hurlimann Prince are quite handi units! Have you good parts distribution in NSW? Here in the States, the D-F dealer network is iffy at best.

 

[newbury]

You also have to be able to have a living style that works with nature, not against it.

We heat and cook with natural gas and cool with electricity. the house in Virginia is about 30x30, 3 levels with a 18x30 addition on 2 levels.
A BIG winter gas bill is a little over $200, a BIG summer electric bill about $200. Take off about $30 from gas for cooking and $50 from electric for lights, computers, etc. So our heating cooling bills are on the order of less than $1000/year
We've about 40 or 50 1970 era-windows. The wife keeps on wanting to replace them for energy efficiency. Estimates we were given were a possible savings of 10% - or $100/year.

Part of the reason why is because we flex with the temperature. In summer I try to let it climb to no more than 78, in winter it can drop down to 58 at night, 63 during the day.
We don't come in the house and crank on the a/c or heat. As the seasons change we adapt, what felt cool in August (75 degrees) feels HOT by December. What feels warm now (66 degrees, I just checked, it's a warm day outside) will feel frigid by July.

If there is sickness or visitors exceptions are made.

When Carter introduced his energy policy in the winter of '77 our house was already set at 63 and I refused to turn it up.

Another key thing I learned about 35 years ago is to have the wife pregnant over the winter. Helps with both the heat and a/c bill if you time it right. Of course that is only good a couple of times DON"T have them due in September!