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

[David Delaney]

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.)


>>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. 

Doubling the thermal mass will double the thermal time constant.    This will reduce the rate of temperature decrease in your otherwise unheated house at night by 50%. Unfortunately, it will not decrease as much the rate of temperature increase when the sun is shining, because you cannot distribute the sun's heat coming through windows throughout a large thermal mass quickly enough to have this effect.  You have to overheat the house in order to get the heat into and distributed throughout its thermal mass. (Or put up with a solar fraction significantly less than 1 for a direct-gain-only house in the Northern US and Canada.)

> 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.  

David Delaney, Ottawa