[GBlist] Re: Separated Thermal Mass

["Bion D. Howard" <bdhoward@ix.netcom.com>]

At 10:32 AM 3/24/02 -0500, you wrote: 
>>>>
> Separated Thermal Mass
> Bion D. Howard:
> In over 40 cases of design support for Air Core hybrid heat storage systems
> that employ pressurization supply of warmed air to the CMU mass channels,
> there have been no moisture complaints that I have been made aware of.

> Me:
> Is the CMU mass channels an essential to your systems?  Is this in the walls?  Would your observation apply to a localized thermal mass, say large gravel in an insulated room in the basement?
<<<<
Avoid gravel bed storage, since this technique is used to return heat to an air stream rather than for re-radiation from a building element (floor or wall) to a room or zone.  Solar air systems that we instrumented and monitored in the "National Solar Data" program in the late 1970's did not perform well due to excessive fan power (operating loss), air leaks and control problems (getting the air dampers to work properly).  

With passive heating of spaces adjacent to mass that is heated up using a controlled air flow many problems are addressed, the chief one being simplicity.

>>>>
>   Bion D. Howard:
> 1)  hybrid blowers activated by dT controller only when supply temp > mass
> temp, plus several degrees
>
> Me:
> Is this in both directions, removing excess heat and supplying heat from rocks or CMU?
<<<<

For the hybrid air core system the solution is to control the blower so it moves heat into the storage sub-system where it is absorbed into the core of the mass. The mass warms through convection and conduction (upward to the building since the underside is insulated) and re-radiates the stored energy to adjacent spaces located near the mass component.

<>  Advantages of hybrid thermal storage:  

lower fan power (only uses electric power to move heat into block bed); 

lower temperatures (rock beds and solar collectors have to be hotter since the losses are higher)

reduced moisture problems since warmer, dryer air compared to the mass conditions is only supplied when a delta-T exists from source to sink - you do not have operation of cool moist air into the storage  sub-system.

>>>>
> Bion D. Howard:
> 3)  totally passive cooling - never intentionally air-cooled where
> temperature in mass could go below the dew-point temperature, and cause
> condensation => droplets => water => mold
>
> Me:
> If I used cooler summertime night time mountain air for the cooling with a fan to aid (could be considered active), would that protect me from cooling the rocks below dew point temperature?  Do I need to study dew point temperatures for summer air to stop cooling the rocks by cutting the fan at some point.  During the hot 90 plus humid days in the summer, it usually gets down the the 50's at night.  Since the house would start out cool from all house fan, the rocks would not be called on to absorb heat until the afternoon when even the insulated rocks will likely have warmed some. 
<<<<

I do not recommend the air-core floor system be used for summer night flush cooling.  

The only exception might be if the air flow to the floor was controlled with a programmable controller so conditions when moist inlet air (night flush air can be cool but have elevated levels of water vapor in it) can load the block with moisture are "programmed out."  Such controllers are expensive and require very careful commissioning.  

In residential buildings maintenance (or lack thereof) becomes a critical concern for their calibration and accuracy over time.  Humidity controllers are notorious for going out of calibration, as well.

If you night flush cool an air-core wall (NCMA has a "Passive Cooling" TEK note on this) and the outdoor cool-air does not blend with conditioned space air then you might avoid problems.  

In your climate a night flush cooling mass (vertical block wall with open cores) should very likely be sealed with a water proofing compound.  If not sealed, then DO NOT mount dry-wall to the night cooled masonry surfaces, to avoid the GWB from possibly absorbing moisture through the cooled block.

> >>>>
>  
> Bion D. Howard:
> 5) destratification duct design can move air into system in swing season,
> albeit under the conditions cited in 1 - 4 above (could provide tempering
> of indoor spaces from overheating)
>
> Me:
> Is this a duct from the high point of a sloped ceiling that would channel heat to separated thermal mass? 
<<<<
Yes - insulated and routed to a medium efficiency filter prior to entering the air-core block bed AHU

>>>>
>  
> Bion D. Howard:
> 6) CMU "block bed" always installed over insulation, vapor retarder, gravel
> drainage layer (well sealed foundations are a cornerstone of good  design
> anyway)
>
> Me:
> Is this below a slab or framing for floor?  If it is a slab, is the slab insulated on from CMU "block bed"?  
<<<<

Not a frame floor, but would finish as a slab on grade.  Typical systems are described in the ASME paper on our web site.

Also the precast concrete hollow core floor planks have been used for this purpose but mainly in commercial buildings.

*** If you want to try a system in one of your projects, I would be glad to provide some project support -- computer analysis, plan review, etc. -- so you can install an Air-Core system.


Typical cost for the residential study is ~ $750 to $1,000 depending on complexity and size (commercial projects are subject to review and individual proposals).

- Bion Howard





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