[Greenbuilding] Trying to maximize passive solar gain...

[Corwyn]

On Apr 19, 2007, at 13:58, Nick Pine wrote:

> Corwyn writes:
>
>> Nick wrote:
>>
> You seem to be talking about 24-hour living spaces vs sunspaces, which
> should get cold at night for efficient solar heat collection.

Yes I am, as is Christa, as far as I can tell.

>>> Living space mass can limit night setbacks which save energy and
>>> prolong warmup times after setbacks.
>>
>> Huh?  Night setbacks save energy by not putting heat into the house
>> during the night.
>
> Agreed.
>
>> It doesn't matter from an energy input standpoint
>> whether the house gets cooler or not.
>
> It doesn't? :-)
>
>> Yes, a warmer house will lose slightly more heat per hour
>> than a colder one...
>
> So it does matter...
>
>>> If your house were (say) 32' square and 16' tall with 80 ft^2 of US 
>>> R4
>>> windows upstairs with 50% solar transmission and (say) an R40 ceiling
>>> and R32 walls, eg 8" SIPs or foamboard and fiberglass, the upstairs
>>> would have a thermal conductance of 1024ft^2/R40 = 25.6 Btu/h-F
>>> for the ceiling + 20 for windows + 944/32 = 29.5 for walls. Adding
>>> 15 cfm for air leaks would make the conductance about 90 Btu/h-F.
>>>
>>> With a 60 F average temp (70 day and 50 at night),
>>
>> 70? in the day (say 7:00 - 10:00) and 50? at night (10:00 -7:00) does
>> not average to 60?
>
> It does with 12-hour days and nights...

Most people I know don't sleep for 12 hours.  YMMV.
>
>> And that requires that you drop from 70 to 50 instantly.
>
> It does?

It does.  70 for 12 hours, and 50 for 12 hours  averages to 60.  70 for 
12 hours and cooling to 50 over the night, averages 65.  Unless you 
mean 70 peak at 3:00 PM or so, and peak low of 50 at 6:00 AM.  In which 
case you are asking people to spend their mornings at 50, if that is 
the case, I suspect I will have no problem seducing them to the high 
thermal mass side of the force, where their house varies only a couple 
of degrees over the course of the day.

>> (that makes the cost of keeping it at 70?, 7192 BTUs/day)
>
> 24h(70F-30F)90Btu/F = 86,400 Btu/day, no?

Sorry, I meant, incremental cost over the low mass house.
>
>> you can probably make that up in nothing more than a smaller
>> furnace (i.e. one that doesn't need to bring the temp from 50
>> to 70 in a short time period).
>
> Some people define a solar house as "one with no other form of heat." 
> :-)

Well others define it as any house which derives a significant portion 
of its heat from the sun.  And since that is clearly what we are 
talking about here, what's your point?

But if you insist on the 'no other form of heat' plan, I would say that 
the additional 7000 BTUs could be made up in a slightly lower 
temperature (at the same comfort level) since radiant heat produces a 
warmer sensation than convective heat.

>
>> In order to lose 20 degrees over 9 hours at 90 BTU/h-?F requires 
>> having
>> less than 1620 BTU/?F of thermal mass.
>
> If the house cools from 70 to 50 from dusk to dawn after a 6-hour solar
> collection day with no internal heat gains, 50 = 30 
> +(70-30)exp(-18h/RC)
> makes RC = 26 hours = C/G, and G = 90 makes C = 2377 Btu/F.

You need to define your variables.  But close to the same answer, sure. 
  We can agree that this isn't a reasonable value for a standard built 
house, yes?  So your scenario doesn't work, in the case under 
discussion.

>> The drywall is going to be almost that much, to say nothing of the
>> concrete.
>
> And the furnishings. I seem to recall that a typical house has about 7K
> Btu/F.  Setbacks pay off because a typical house has a lot more 
> conductance and
> air leakage.

>>>  A frugal 600 kWh/mo of indoor electrical use
>>
>> 600 kWh / month is frugal?  yikes.
>
> IIRC, the US national average is 10K kWh/year, ie 833 kWh/mo.

72% of the US national average isn't what I would call frugal, no.  
Even so, I suspect that loses to the outside are more than that.  For 
instance if they have an electric water heater and no GFX or 
equivalent, they are dumping far more than 28% of their electricity 
into the sewer system.

Anyone have a good number for percentage of electric usage which ends 
up as internal heat?

> Don writes:
>
>> Darn, I should've skipped geometry and taken physics. Seriously, I 
>> really
>> would like to learn how to do these calculations. Any suggestions 
>> where I
>> should start?

I would start with _The Passive Solar House_ by James Kachadorain.  It 
goes through the calculations slowly and carefully.


Thank You Kindly,

Corwyn


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