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re: ceiling mass air heater options
3 dec 2003
david delaney   wrote:

>>consider 2 steady-state 8' r10 cubes with 64 ft^2 of r2 glazing with 80%
>>solar transmission, after a long string of 30 f average january days in
>>phila, with 1000 btu/ft^2 of sun on a south wall...
>>
>>1.  --------------
>>   |fs|    tc     |       cube 1 has a flow separator and a transpired
>>   | .|           | r10   mesh collector. we might assume the air hear the 
>>   | .|           |       glazing is about the same as the ceiling temp tc. 
>>s  | .|   70 f    | 30 f  this sunspace might be uncomfortably hot, but the
>>   | .|           |       amount of airflow is not limited by the cube temp.
>>   | .|           |       how does the ceiling keep the cube 70 f? 
>>   | .|           |        
>>    --------------        
>>
>>2.  --------------
>>   | .     tc     |       cube 2 has a ventilation slot at the top and 
>>   | .|           | r10   a transpired mesh collector and a potentially-
>>   | .|           |       unreliable motorized damper at the bottom which
>>s  | .|   70 f    | 30 f  is controlled by a thermostat that keeps the 
>>   | .|           |       cube air 70 f during the day. we might assume
>>   | .|           |       the air near the glazing is about 70 f. we might
>>   | .d           |       need another layer of glazing south of the mesh
>>    --------------        to ensure this. the ceiling might keep the cube
>>                          70 f with a thermostat and a slow ceiling fan.

>i'm not sure i understand this. here's my
>interpretation:

of cube 2?

>i assume the glazing is being kept cool by fresh
>air falling from the ventilation slot...

the ventilation slot lets hot air pass from the sunspace to the cube ceiling. 

here's my interpretation: house air enters the glazing cavity via the damper,
rises up to the south of the transpired mesh, passes north through the mesh,
where it gets heated, then rises up through the vent slot and slides along
the underside of the ceiling, heating the ceiling mass to about 100 f. when
the bulk of the cube reaches 70 f, the damper begins to close and the airflow
lessens and the air near the ceiling becomes warmer. this probably makes solar
collection less efficient, with warmer air near the glazing...

>the fresh air mixes with the house air at the bottom of the
>air heater, (if the damper is open)...

no fresh outdoor air. just house air...

>the mixture rises as it is heated. at the top of the air
>heater, some of the hot air enters the house and a quantity
>equal to the fresh air inflow has to leave the house through
>the same ventilation opening.

through the damper. this might work better with another duct
inside the house... 

>presumably you would have a few inches of inverted hot air trap above
>the top of the ventilation slot, with the house slot being at the top
>of the hot air trap, to prevent entry of cold air at night.

seems to me that's not a problem, with the damper closed and
an external insulated lip that terminates under the vent slot.

nick




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