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

[Corwyn]

On Apr 19, 2007, at 14:54, David Delaney wrote:

> At 03:37 PM 18/04/2007, Corwyn wrote:
>
>> On Apr 18, 2007, at 17:50, David Delaney wrote:
>>
>>> 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.
>
> No. You _will_ have a house that leaks energy, no matter what you do.  
> Wanting a lower temperature at night may be, _is_ in my case, wanting 
> a lower temperature at night. Others might want a lower temperature at 
> night to save energy.   If you reduce the indoor-outdoor temperature 
> difference by 15% for 12 hours a day, your house loses 7.5% less 
> energy to its environment over the whole day than if you don't reduce 
> it.  This will save at least 7.5% of the cost of purchased 
> space-heating energy. (More if most of your purchased energy is used 
> at night.)

'leaks more energy' then.

  15% for 12 hours a day.  Call it 60º average over those 12 hours.  50º 
minimum temperature.  20º drop in temperature spread over 7K BTU/ºF 
thermal mass in the average house (Nick's number)  requires you to lose 
140,000 BTUs every night.  11666 BTU/hr.
For 4096 square feet of surface area that works out to R21.

Same house with R40 insulation, loses 6553 BTU/hr and 11º over those 
same 12 hours. Saving 60,000 BTUs.  minimum temperature 59º.


>> 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.
>
> The above assumes extremely unlikely numbers. Satisfactory ratios of 
> the numbers with direct gain solar as the only source of space heat 
> (solar fraction = 1) are extremely difficult (difficult design) and 
> expensive (impossible?) to achieve for a livable (acceptably limited 
> temperature swing) house in the Northern US and Canada.

They are completely made up numbers.  Not intended to indicate a solar 
fraction of 1 (as we were discussing things to make a standard house 
better, not a full on passive solar house).  So, forget the numbers.  
Two houses, one low mass, one high mass, both have the same amount of 
insulation, both get the same amount of solar gain.

>>> and will cool to a desired minimum temperature by next morning,

If the high thermal mass house is warmer, it is losing more heat 
(through the same insulation).  Therefore it will be in a _better_ 
position to accept the next mornings solar gain.  We care about energy 
here, not temperature.  (yes, I know that warmer surfaces will not 
radiatively gain as much as cooler surfaces but since we are comparing 
them to the temperature of the surface of the sun, the difference is 
negligible).


Thank You Kindly,

Corwyn

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