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# Thermal Mass.

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Member Posts: 511
In another topic @hot_rod raises an interesting question regarding thermal mass. He points out that in a very hot location, such as Death Valley, we could be looking at delta T's in the 40 degree plus range.
This raises the question for people who are living in extreme temp areas - hot or cold - Is having a large thermal mass practical for these areas.
At this location, we will see a few days of - 20 and a few days of + 100 each year, and high mass works wonderfully for us.
As @Zman points out if you get a high mass hot, you will have the devil of a time to cool it down.
Your thoughts may help others decide on their building designs.

• Member Posts: 2,646
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Depends on the building envelope. We can see temp swings of 40 degrees in the winter. -30 at night and 10 during the day.

The better the building envelope the better mass seems to work. That said, building with big overhead doors do well too. After the door closes, temps are back to normal in a couple of minutes.

Mass seems to be the enemy with big solar gains, or leaky building where the wind is a big non-linear load.

This is just my experience from climate Zone 7A.
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
• Member Posts: 22,144
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I have run into this mainly in super efficient slab homes. Homes with loads in the single digit BTU/ sq ft range. Which actually is becoming a more common load number in energy efficient buildings.

Remember if that radiant surface is even 1° over ambient temperature there is a heat exchange.

So two things we and the customers discovered.

The slab took some time to ramp up when temperatures fall quickly.

Crunch some numbers a 2000 sq ft home, 4" slab = @25 yards of concrete @ 4000 lbs/ yard= 100,000 lbs of mass to accelerate or decelerate. There is no possible way to drop temperature in that space be a degree or two quickly, or even over several hours by just stopping the heat input.
Forget about setback, it could take 8-10 hours to drop 5°

The other conundrum is the cold floor syndrome. Homeowners expect radiant slabs to feel warm, always:)

Heat Flux= 16,000BTU/ hr. ÷ 2000sq.ft. 8 BTU/ sq. ft at design condition

Take that efficient home with a 8 btu/sq.ft load at design.
The number is about 2 BTU/sq ft/ degree difference.
So a home at 68° set point would need a mere 72° slab surface, at design condition! On a mild day the slab would only be a few degrees above ambient to cover the heat load. Quite possibly internal gains would cover the load or even overheat the space without the heating system running.

Takes around 80F or more for a surface to"feel warm" to bare feet, based on average skin temperature.

So it is virtually impossible to give customers warm radiant slabs in low load, efficient homes. Better to warm some bathrooms, kitchen with radiant and use panel rads for the radiant "feel" elsewhere.
Or 24" on center radiant just in the middle of the rooms

Certainly some good application for high mass systems.

Delta airlines put a large repair shop in SLC years ago with radiant slabs. The accordion doors would open wide and tall enough to swallow a 747. Within minutes of bringing in an ice cold airliner, the space would feel warm again. Millions of lbs of thermal mass allowed that to occur.
Forced air unit heaters would take hours to warm a space like that after the doors opened and closed. So a good use of the "flywheel" effect.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 23,297
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High mass can really work wonders in areas with high day time temperatures but cool nights -- think high desert, for instance. If your structure is designed and built to minimize solar gain on those 100+ days, you can hold the building to close to the mean daily temperature without any trouble at all. For that matter, they will stabilize the temperature to the mean daily temperature anywhere.

In colder climates, adding thermal mass -- however you do it -- is the key to making effective and efficient passive solar houses, too.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22,144
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Proper design is imperative in passive design structures, it is doable. The passive home institute has some incredible cutting edge designs and projects, calculators, tools, www.phius.org
I need to get to one of their conferences some day.

We experienced first hand how concrete slabs can over-heat the space from passive gain when we lived in the mountains of Utah, in a south facing, glass walled home. Even in the winter months a sunny day will run the slab way above a comfortable ambient temperature. A means to control gain is needed, even more so in the summer months. It was the first home we built and didn't really have the knowledge or budget to add all the appropriate control mechanisms.

All thing being equal I would much rather store thermal energy in water. It is much easier to manipulate and move where and when you want it. A large underground well insulated thermal tank, concrete septic tanks for example, for example that do not take valuable floor space.

Then a light, low mass radiant floor, wall or ceiling.
Properly designed in those high, dry desert, radiant cooling becomes a viable option.

https://www.bdmd.com/water-the-ideal-thermal-mass/
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 23,297
edited August 2019
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Quite so on water as the mass for solar -- that's what my father in law used here in New England -- quite successfully -- on a number of passive houses.

And solar gain can be a bear -- he had a nifty patented design for that controlled that for rooves, which worked well. Unfortunately, he's passed away and didn't write the design details down -- and the patent is so opaque that it would take days to reverse engineer his work. Oh well.

I might add that in ancient Persia the locals had even figured out how to make ice with radiant cooling. I'll have to look up the details on that.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 511
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In our case the radiant slab is quite thin, mostly 2" with a few areas that are up to 4" and most south facing windows are shaded by the eves for 4 - 5 months. The bulk of the thermal mass is in interior and exterior walls, the north wall is almost 2 ft thick. We enjoyed warm floors in winter and right now enjoy cool floors when it is hot. So far overshooting has not been an issue, but I can agree with hot_rod that a 4 or 6" slab might be prone to temp over runs. Water of course is the best for heat/cool retention but rock and cement are a very good alternative.
• Member Posts: 22,144
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Certainly mass, low medium or high, can be your friend, properly applied as we mentioned. Simply put it tends to have a mind of it's own and not take kindly to immediate orders.

If everything is dialed in correctly, the load, the weather, your comfort expectations, then all is well.

Options are out there to build the exact system any homeowner desires or expects.

The beauty of HH is the amount of experience with system designs and customers likes and dislikes. What works well and what tends to be problematic.

Hang here for a few years and you see a pattern of which heating system design and installations methods turn people on or off.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream