Check my math please

[zzvyb6]

'e' is the basis for natural logarithms.

Natural logarithm - Wikipedia, the free encyclopedia

so the thermal constant times the temperature desired is the power that 'e' is raised to.

The solution to many so called first order differential equations involves 'e' or its converse, the 'ln' function.

It's usefulness is in the multiplication of numbers by adding their logarithms. If you ever used a slide rule back in the day, that's how a slide rule works. You add the 'lengths' of a numbers on the rule to multiply them. When something cools down or heats up or if you let the air out of a tank or fill it, the pressure or temperature at any instant in time is governed by this math property.
In most cases, the value of a new vehicle vs. time or mileage is also well defined by the curve shape that this function has: the fastest drop occurs at the beginning and finally settles down to a worth that's just above the junk yard value. For some vehicles the 'time' constant is swift. For others, it's pretty slow. There's actually a brand factored into this time constant for some products.

 

[newbury]

"We" decided ehh? Ok, sure, if that's the story you're sticking to. :laughing: (p.s. Your wife isn't going to be swayed to turn it back down to 68 no matter how much fancy math or thermodynamic equations you impress her with. )

GAD!! 68?

We keep the house at 63 now and wear flannel shirts if we get chilly. Right now it's 63 and I'm comfortable in a T-shirt because I was just "exercising" unloading a trailer an hour ago.

Which brings up an old guy's story.

Back in '85 SWMBO, her 7 yr old son and I had just moved into a house in Alexandria, Va after getting married on Feb 29th, 1984. She and her son hailed from Mississippi and it was their first full year in Virginia. The house was poorly insulated and no storm windows. Energy was cheap, but so was I.

I'd set the temp back to about the mid 50's or lower for the night and up to 65 for when people were up and about.

One cold winter morn I was sitting in the library browsing the bulletin boards on my Commodore 64 and heard the son coming down the stairs making a breathing out sound (woosh) every few steps. He came into the library, looked at me with wide eyes in amazement and said "I've NEVER seen my breath inside before!!"

Now after 7.5 anniversaries she's comfortable with cool temps and warm clothes. And the now 37 year old son is comfortable running his house in Memphis cool in the winter. Based on comparing heating bills with local friends with similar sized houses SWMBO and I have probably saved about $400 a year in heating bills.

The adapting to cool temps with clothing seems right and natural to me. Now letting the house heat up and preserving some modesty in the summer is different. I can only take off so much I drew the line at 80 degrees for the summer, and that crept down to 78 recently.

 

[MossRoad]

60's - YIKES! We keep our house at 72 minimum with the furnace thermostat, but it rarely comes on because the wood burner keeps it at 74-76 most of the time. Its nice sitting around in light weight pajamas in the middle of winter in a warm, dry house.

 

[CurlyDave]

So far, I have not seen any correct answers.

The original question has left out an important piece of data--the average outside temperature over the time period to be considered. Most HVAC men would call the energy required to maintain the difference between the average outside temperature and the inside temperature for a day one degree-day. If you just take the temperature in Fahrenheit and use that in the equation, you are assuming that the average outside temperature for the entire heating season is 0 degrees F, which I don't think would be true in Iowa.

If the average outside temperature is greater than 0 degrees F, the propane required will be greater than that calculated in the early posts.

* * * * * *

In zzvyb6's post we are getting into the equations for radiative heat transfer, which is typically very small compared to the usual convective heat transfer from a house to the environment. The usual practice is to express temperature in degrees Kelvin, which is (degrees Celsius + 273). This component of heat loss from a house is small enough so that most engineers ignore it for typical temperatures. It starts to become significant somewhere around 300 degrees F, and at high enough temperatures is the dominant heat transfer mechanism.

But, your house is never going to in a environment where radiative heat transfer will be significant compared to convective.

 

[CurlyDave]

So far, I have not seen any correct answers.

The original question has left out an important piece of data--the average outside temperature over the time period to be considered. Most HVAC men would call the energy required to maintain the difference between the average outside temperature and the inside temperature for a day one degree-day. If you just take the temperature in Fahrenheit and use that in the equation, you are assuming that the average outside temperature for the entire heating season is 0 degrees F, which I don't think would be true in Iowa.

If the average outside temperature is greater than 0 degrees F, the propane required will be greater than that calculated in the early posts.

* * * * * *

In zzvyb6's post we are getting into the equations for radiative heat transfer, which is typically very small compared to the usual convective heat transfer from a house to the environment. The usual practice is to express temperature in degrees Kelvin, which is (degrees Celsius + 273). This component of heat loss from a house is small enough so that most engineers ignore it for typical temperatures. It starts to become significant somewhere around 300 degrees F, and at high enough temperatures is the dominant heat transfer mechanism.

But, your house is never going to in a environment where radiative heat transfer will be significant compared to convective.

 

[buckeyefarmer]

There is no "I" in team, but there is a "Me" if you want to jumble it up a bit...

Actually there is an I in TEAM.... Just look at the A hole.

 

[quicksandfarmer]

Curly Dave has it.

People in the heating business use degree-days. You can read about them here:
Degree day - Wikipedia, the free encyclopedia

You can get the degree-day history for your area here:

NCDC: U.S. Climate Normals -

You also need to know how many days are in your heating season.

The annual degree-days for your area gives a baseline of how much heating is required to maintain an inside temperature of 65F. For each degree higher, add one degree-day for each day in your heating season.

For the math nerds in the crowd, the underlying assumption is that the amount of heating required is directly proportional to the difference between the inside temperature and outside temperature. That assumption seems unreasonable on its face, but it is the basis of the Manual J calculation that is used to design just about every residential HVAC system.

 

[dave1949]

<snip>

For the math nerds in the crowd, the underlying assumption is that the amount of heating required is directly proportional to the difference between the inside temperature and outside temperature. That assumption seems unreasonable on its face, but it is the basis of the Manual J calculation that is used to design just about every residential HVAC system.

Since the OP has a baseline for how much propane is used for 68*, he could run the calculations for that and see how well it matches his actual consumption. Any difference might be useful as a % correction factor to apply to other temperature targets?

 

[txdon]

Math is like love – a simple idea but it can get complicated
so
when it gets empty fill it.

 

[dave1949]

Math is like love – a simple idea but it can get complicated
so
when it gets empty fill it.

No No No. It's better to insist that according to the TBN Braintrust, that tank can't be empty. :laughing: