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

[David Delaney]

At 11:04 AM 21/04/2007, Christa Carpenter wrote:
Dear David,

Thank you for you thoughtful reply to me question..... <snip>

Hi Christa

It is unlikely that you are using too much thermal mass 
unless you are going to real extremes. But sometimes people 
do.What exactly are you doing? How much extra mass (what 
volume, weight) are you considering including inside the 
heated space of your house? The much more common problem is 
south and west windows  that are too big and will overheat 
your house.

I did not mean that a lot of thermal mass was a bad 
idea because it was cheaper to use purchased energy.   I 
meant that past a certain point money spent on thermal mass 
in a direct gain house was wasted -- did no almost no good at 
all even if it cost nothing, which it certainly does not.

By the way. in my view it is almost impossible to insulate a 
home too well. I think you will be very lucky if you don't 
see the cost of home heating in New Brunswick (and 
everywhere else) double several times in the next 10 years.

It is actually a bit tricky to design a balance of thermal 
mass and window size to maximize solar fraction for a direct 
gain house. So much so that few people try. They resort to 
rules of thumb that give less than maximum solar fraction, 
because getting it wrong results in severe overheating, even 
in houses that have a very large amount of thermal mass. 
More thermal mass than you can use is not really harmful, 
just wasteful, but too much window is both harmful 
(produces a _smaller_ solar fraction) and wasteful. There 
are people on this list who know more about direct gain 
houses than I do. One of them should be able to point you 
toward a good source for those rules of thumb.  (I am 
more interested in _indirect gain_ houses, because they can 
have much larger solar fractions.)

A large thermal mass in the heated living space is very good 
at keeping the house warm at night, but cannot easily absorb 
the sun's energy, with the result that either the house must 
be grossly overheated to charge the mass, or the mass cannot 
be charged by the sun. The result is a waste of costly 
windows and thermal mass, and a _smaller_ solar 
fraction than might have been achieved with smaller windows 
and a smaller thermal mass.  The reason:  big windows 
capable of charging the big thermal mass lose a lot of heat when 
the sun is not shining. They don't pay their way 
energetically because you cannot stand to let them make the 
house hot enough when the sun shines to charge your thermal 
mass.

Indirect gain permits higher solar fractions than direct 
gain. Here's an example:  You can ease the direct gain 
mass-glass trade-off, and get a higher solar fraction by 
having a low-thermal-mass sunspace on your south wall which 
you don't mind letting get very hot or very cold. The 
sunspace has very big windows to the out-of-doors, but only 
modest windows between the sunspace and the house. You let 
heated air from the sunspace into the house when the 
sunspace is hot and you need the heat in the rest of the 
house. You shut the sunspace off from the house when the 
sunspace gets too hot or too cold. At night, for example. 
You can use a fan and dampers -- or just motorized dampers 
-- and a differential thermometer to automate the process, 
but manually operated doors and windows between the sunspace 
and the rest of the house will work very well in an 
emergency or when conditions are such that you want to move 
freely back and forth between the sunspace and the house. If 
the sunspace is very hot on a sunny day you can 
overventilate it, and/or overventilate the whole house to 
balance heat-in vs. heat out, if you want. When the sun goes 
in , you shut off the sunspace. It's large windows will make 
it very cold, but you don't care because you are snug in 
your living space because it does not have very large 
windows that lose too much heat.

On cloudy days with periods of sun, a low-thermal-mass 
sunspace will get warm very quickly during those sunny 
periods, exactly because it has little thermal mass.  Even 
during fairly cloudy days the sunspace will have periods 
when it is warmer than the rest of the house. A 
thermostatically controlled fan and/or motorized dampers 
will transfer much heat into the house during these periods. 
This is why an indirect gain sunspace can help you achieve a 
higher solar fraction than any direct gain house. This 
possibility is often screwed up by putting thermal mass into 
the sunspace to keep it warm late into the evening.  The 
thermal mass will do that on a sunny day all right, but it 
also slows down the rate at which the sunspace heats up, and 
keeps you from getting much heat from it into the house 
during cloudy days. Think very carefully about what you are 
trying to do, what you really want, before putting any 
thermal mass into a sunspace.  To get the highest solar 
fraction, the sunspace should have as little thermal mass as 
possible.

The whole strategy of using the sunspace to get a large 
solar fraction can be screwed up because people decide to operate 
their sunspace as a fully heated space.  If you do that, you are 
right back to an overheating and overcooling house that gets 
a smaller space heating benefit from the sun than if it had 
smaller windows..

David Delaney, Ottawa