Posted by *dan* on November 11, 2008, 7:30 am

Hello everyone.

Through a combination of passive solar heating during the afternoon,

and a wood fireplace insert from late afternoon into mid evening, I am

able to maintain an early evening living room temperature of 72F with

ease. As the fire burns out overnight, I typically find the living

room temperature around 61F in the morning.

So we are experiencing at a drop of 11F (72F-61F) over roughly 8 hours

(10 pm to 6 am).

My goal is to reduce the overnight temperature swings. A minimum of

around 65F in mid November would suffice. I estimate this would

equate to living room minimum temp around 61F in late January, which I

can certainly live with.

My initial plan is to add 40 lbs of water storage in the vicinity of

the fireplace insert. Not close enough to melt the plastic of course,

but maybe 5 feet away off to the side.

I'd appreciate some help in estimating the amount of difference this

would make in compressing the overnight swings, and if I may need to

add more thermal mass. Thank you in advance!

Posted by *dan* on November 11, 2008, 10:44 am

Correction...I meant to say 40 gallons of water (332 lbs)

Posted by *nicksanspam* on November 11, 2008, 11:26 am

*>Through a combination of passive solar heating during the afternoon,*

*>and a wood fireplace insert from late afternoon into mid evening, I am*

*>able to maintain an early evening living room temperature of 72F with*

*>ease. As the fire burns out overnight, I typically find the living*

*>room temperature around 61F in the morning.*

*>So we are experiencing at a drop of 11F (72F-61F) over roughly 8 hours*

*>(10 pm to 6 am).*

*>My goal is to reduce the overnight temperature swings. A minimum of*

*>around 65F in mid November would suffice. I estimate this would*

*>equate to living room minimum temp around 61F in late January, which I*

*>can certainly live with.*

*>My initial plan is to add 40 lbs of water storage in the vicinity of*

*>the fireplace insert. Not close enough to melt the plastic of course,*

*>but maybe 5 feet away off to the side.*

*>I'd appreciate some help in estimating the amount of difference this*

*>would make in compressing the overnight swings, and if I may need to*

*>add more thermal mass.*

A pound of water releases 1 Btu of energy as it cools 1 F, so 40 pounds

cooling from (say) 120 to 70 would store (120-70)40 = 2000 Btu. Burning

a pound of dry wood at (say) 60% efficiency releases about 0.6x10K = 6K

Btu, so the water might store the heat equivalent of 1/3 pound of wood.

Nick

Posted by *daestrom* on November 11, 2008, 8:17 pm

dan@cortlandfootball.com wrote:

*> Hello everyone.*

*> Through a combination of passive solar heating during the afternoon,*

*> and a wood fireplace insert from late afternoon into mid evening, I am*

*> able to maintain an early evening living room temperature of 72F with*

*> ease. As the fire burns out overnight, I typically find the living*

*> room temperature around 61F in the morning.*

*> So we are experiencing at a drop of 11F (72F-61F) over roughly 8 hours*

*> (10 pm to 6 am).*

*> My goal is to reduce the overnight temperature swings. A minimum of*

*> around 65F in mid November would suffice. I estimate this would*

*> equate to living room minimum temp around 61F in late January, which I*

*> can certainly live with.*

*> My initial plan is to add 40 lbs of water storage in the vicinity of*

*> the fireplace insert. Not close enough to melt the plastic of course,*

*> but maybe 5 feet away off to the side.*

*> I'd appreciate some help in estimating the amount of difference this*

*> would make in compressing the overnight swings, and if I may need to*

*> add more thermal mass. Thank you in advance!*

You know how much the temperature drops (11F) over a given time (8 hours).

If we knew what the outside temperature was on such a night, we could

calculate a time-constant for your current situation.

But that's not really enough. Similar to an electronic time-constant, a

thermal time constant is the product of two terms, the thermal capacitance

of the heated space and the thermal resistance between the heated space and

ambient. To get your particular time constant, you could have a wide range

of thermal capacitances, each one would simply be associated with a

different thermal resistance. Think of it like finding two factors that

when multiplied give you 8. They could be 2 and 4, 4 and 2, 1 and 8, or

even 0.5 and 16. If we can't peg at least one factor, we have no idea the

'ballpark' of the other. And hence, no way to estimate the effects of

adding to one of the terms.

You can get an estimate of one of these factors, the thermal resistance, by

noting how much energy you use to keep the place warm for some period of

time. If you can estimate how much wood you burn over a week (along with

the average outdoor temperature), we can figure out what the capacitance is

(once the time-constant itself is determined).

Chances are, adding a few pounds of water won't materially affect the

overnight temperature drop. You might be better off looking for some more

places to insulate or seal out the cold air. This would have the additional

benefit of lowering how much wood you need everyday.

daestrom

Posted by *dan* on November 11, 2008, 11:21 pm

Thanks guys. I guessed there would be too many factors involved to

come up with a strong estimate, so I'll just experiment and track the

max/mins downstairs. I'll start with 20 gallons of water (166 lbs)

heated down wind of the fireplace, and look to add another 10-20

gallons in front of our main solar collection area (large southwest

window).

>Through a combination of passive solar heating during the afternoon,>and a wood fireplace insert from late afternoon into mid evening, I am>able to maintain an early evening living room temperature of 72F with>ease. As the fire burns out overnight, I typically find the living>room temperature around 61F in the morning.>So we are experiencing at a drop of 11F (72F-61F) over roughly 8 hours>(10 pm to 6 am).>My goal is to reduce the overnight temperature swings. A minimum of>around 65F in mid November would suffice. I estimate this would>equate to living room minimum temp around 61F in late January, which I>can certainly live with.>My initial plan is to add 40 lbs of water storage in the vicinity of>the fireplace insert. Not close enough to melt the plastic of course,>but maybe 5 feet away off to the side.>I'd appreciate some help in estimating the amount of difference this>would make in compressing the overnight swings, and if I may need to>add more thermal mass.