Re: Ceiling mass air heater options

On 3 Dec 2003 14:51:19 -0500,
[EMAIL PROTECTED] (Nick Pine) wrote:

>David Delaney  <[EMAIL PROTECTED]> 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?
Yes
>
>>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. 

OK. Classic misreading due to my preoccupation
with finding a way around HRVs. I thought the
ventilation slot connected the air heater to the
*outside*.
>
>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...
Yep
>
>>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.
Yes, that would do the trick. 
>
>Nick