[Greenbuilding] Fw: Trying to maximize passive solar gain without investing too much....

[Corwyn]

On Apr 18, 2007, at 17:50, David Delaney wrote:

> At 12:03 PM 18/04/2007, Corwyn wrote:
>
>> On Apr 18, 2007, at 15:56, Christa Carpenter wrote:
>>
>>> Can you have too much thermal mass?  I have a book that gives minimum
>>> numbers corresponding to the area of southfacing glazing, but they
>>> don't mention whether or not you can have too much.
>>
>> No.  On a continuously occupied building any thermal mass inside the
>> heating envelope is good.  You can have thermal mass which is too 
>> thick
>> however, so spread it around as much as possible.  It is, of course,
>> possible to have more thermal mass than you can afford.
>
> Hmm...
>
> A very large thermal mass commits you to a fairly constant
> temperature, which would be wasteful of energy if the chosen
> temperature is the desired daytime temperature and you would
> accept, or desire,  a lower temperature at night.

True, for the most part.  But I find that even a low mass house with 
superinsulation commits you to this.  It can be mitigated slightly, by 
having rooms which are less well heated, all the time.  Wanting a lower 
temperature at night is another way of saying you want a house which 
leaks energy.

> If you have chosen to depend on reliably
> available purchased energy, you have little need for thermal
> mass, so any extra expenditure on it is wasted, or is at least only of 
> benefit
> for emergency absences of your chosen fuel, which
> could be provided for also by alternative emergency purchased
> energy -- alternatives which you might evaluate on a cost basis.

Having about 10% of this year without electricity, I am sure glad I 
have reliable fuel (wood).  Many of my neighbors have fuel but no 
electricity to run their furnaces.

> If your space heating is supplied in significant part by direct solar
> gain,  it is desirable to design the windows and
> the thermal time constant of the house together so that the
> house will not overheat during a day of good sun, and will
> cool to a desired minimum temperature by next morning,
> during the main part of the heating season, so as not to
> overheat the next day if that is also a good sun day.

Thermal mass doesn't affect this much.  If you are getting 100,000 BTUs 
in sunlight on a good day, and losing 90,000 BTUs through the 
insulation every day, then you are gaining 10,000 BTUs for every day of 
constant sunshine.  Thermal mass will absorb that, so it translates to 
fewer degrees of temperature increase, thus allowing more days of 
constant sun before windows need to be opened, and allowing more days 
of no sun before you need supplemental heat.  If you are averaging more 
heat in, than heat out, you will need to shed some of that heat, no 
matter what.

Thank You Kindly,

Corwyn



-- 
Corwyn
Kermit didn't know the half of it...
http://www.greenfret.com/
corwyn at greenfret.com