Check my math please

[quicksandfarmer]

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?

You need to know the outside temperature too. It's the temperature difference that matters, not the temperature, that's just a number on an arbitrary scale.

What heating degree-days tells you is essentially your average outdoor temperature during the heating season, multiplied by the length of your heating season.

 

[bigtiller]

You need to know the outside temperature too. It's the temperature difference that matters, not the temperature, that's just a number on an arbitrary scale.

What heating degree-days tells you is essentially your average outdoor temperature during the heating season, multiplied by the length of your heating season.

I'm not so sure about that. Yesterdays heating degree days number was 46 but the high for the day was 24. Total for the heating season is 641

 

[dave1949]

You need to know the outside temperature too. It's the temperature difference that matters, not the temperature, that's just a number on an arbitrary scale.

What heating degree-days tells you is essentially your average outdoor temperature during the heating season, multiplied by the length of your heating season.

Isn't that information (heating degree days annual average) already captured in the original Manual J calculation? How would a heating designer predict the change in fuel consumption for a range of indoor temperatures? That is what BigTiller is looking for. Or is the J calculation based strictly on an assumed indoor ideal like 72F?

He already knows roughly how much fuel is required to maintain 68F over a heating season. Assuming his heating degree day annual average, and his heat losses inherent to his house, are known constants that are already wrapped into his fuel consumption @ 68F. He cannot predict future heating degree days on an annual basis, so I think he has to use the historical average.

In other words, he already has an actual Manual J equivalent result given his known propane usage under existing conditions. Whatever answer a calculated Manual J result returns can be compared to that.

Or, it's entirely possible I understand none of this. :laughing:

 

[Danno1]

...radiative heat transfer,... 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.

A few years ago the house next door to me got above 300F for an hour or so! :shocked:



.

 

[txdon]

Don't forget:

1. Add All sources of exhaust because when the exhaust are working they are drawing in cold air.
a. Dryer
b. Stove vent hood
c. Bathroom exhaust
d. Central vacuum (exterior vented)
e. Fireplaces/wood burning stoves - with no separate intake vent

2. Subtract all sources of heat:
a. People
b. pets
c. stove and oven
d. all other heat sources in kitchen - toaster, bread machines etc?
e. refrigerator (minus the times it's open and cold air spills out.)
f. TV

All of these need to be factored by minutes used times air displacement or heat generated times minutes on.

To do it right we have to come up with a formula that incorporates everything. I can see this formula taking up an entire chalk board.

 

[Kyle_in_Tex]

I need to go to the bathroom and practice my natural logarithms.

Your asymptotes are showing.

zz was correct in that it was not a linear situation. A supercomputer with all the information like Don posted including the number of times the outside door is opened, and what temp it was outside at that moment, how hard the wind was blowing, how long it was open.... I'm already tired of thinking about it. No matter what anyone says, it would be an educated guess. Thermal dynamics is a &%@##***.

I hope the op gets the picture. Just like your electric bill and propane bill varies from year to year. I think it will be close to zz's prediction.

 

[zzvyb6]

Just don't forget to add in the effect of the number of spotlight, wireless cameras, wireless phones, cell phone and laptop chargers left on. And, FWIW, your new DVR has been found to be the biggest heater in an average house. Heaven forbid that TBN would become a Discovery Channel series. My house would heat itself just with U-Verse...

In today's show, we're going to show how to use a box blade and install a hydraulic top link. Be sure to tune in tomorrow on how to pick the best front mounted snow removal device. Then on to picking the best surveillance system once your barn has already been broken into. Stay tuned.

If TBN went on TV, I'd never get anything done around here.

 

[quicksandfarmer]

It's easier to show than explain.

I went to the NOAA website(NCDC: U.S. Climate Normals -) and selected Iowa. I downloaded the historical normals for Iowa. I don't know where in Iowa BigTiller is, his profile says "Central Iowa," so a picked a weather station in the middle of the state, Ankeny.
On page 17 it has heating degree-days by month. This is the line for Ankeny:

009 ANKENY HDD 1450 1148 894 497 199 26 6 23 120 431 866 1301 6961

The numbers after HDD are the months, 1450 is January, 1301 is December, 6961 is the total for the year. I'm going to assume that the heating season is October 15 to April 15, 182 days. I'll also assume that all of the HDD in October and April are during the heating season. The total HDD for the heating season is 6123. That's for an indoor temperature of 65F. With a heating season of 182 days, each extra degree of indoor temperature is 182 extra HDD, so an indoor temperature of 68F means 6669 HDD. Raising that to 70F means 7033 HDD.

The difference between an indoor temperature of 68F and 70F is 364 HDD, or 5.5%. If 68F means you need 1000 gallons, 70F would need 5.5% more, or 1055 gallons.

It's a simplistic model to assume that energy consumed is directly proportional to difference in temperature, but if your heating system was engineered, it's the model the designer used.

 

[dave1949]

It's easier to show than explain.

I went to the NOAA website(NCDC: U.S. Climate Normals -) and selected Iowa. I downloaded the historical normals for Iowa. I don't know where in Iowa BigTiller is, his profile says "Central Iowa," so a picked a weather station in the middle of the state, Ankeny.
On page 17 it has heating degree-days by month. This is the line for Ankeny:

009 ANKENY HDD 1450 1148 894 497 199 26 6 23 120 431 866 1301 6961

The numbers after HDD are the months, 1450 is January, 1301 is December, 6961 is the total for the year. I'm going to assume that the heating season is October 15 to April 15, 182 days. I'll also assume that all of the HDD in October and April are during the heating season. The total HDD for the heating season is 6123. That's for an indoor temperature of 65F. With a heating season of 182 days, each extra degree of indoor temperature is 182 extra HDD, so an indoor temperature of 68F means 6669 HDD. Raising that to 70F means 7033 HDD.

The difference between an indoor temperature of 68F and 70F is 364 HDD, or 5.5%. If 68F means you need 1000 gallons, 70F would need 5.5% more, or 1055 gallons.

It's a simplistic model to assume that energy consumed is directly proportional to difference in temperature, but if your heating system was engineered, it's the model the designer used.

Thanks, now I understand what you meant by the indoor-outdoor difference is what matters.

 

[bigtiller]

It's easier to show than explain.

I went to the NOAA website(NCDC: U.S. Climate Normals -) and selected Iowa. I downloaded the historical normals for Iowa. I don't know where in Iowa BigTiller is, his profile says "Central Iowa," so a picked a weather station in the middle of the state, Ankeny.
On page 17 it has heating degree-days by month. This is the line for Ankeny:

009 ANKENY HDD 1450 1148 894 497 199 26 6 23 120 431 866 1301 6961

The numbers after HDD are the months, 1450 is January, 1301 is December, 6961 is the total for the year. I'm going to assume that the heating season is October 15 to April 15, 182 days. I'll also assume that all of the HDD in October and April are during the heating season. The total HDD for the heating season is 6123. That's for an indoor temperature of 65F. With a heating season of 182 days, each extra degree of indoor temperature is 182 extra HDD, so an indoor temperature of 68F means 6669 HDD. Raising that to 70F means 7033 HDD.

The difference between an indoor temperature of 68F and 70F is 364 HDD, or 5.5%. If 68F means you need 1000 gallons, 70F would need 5.5% more, or 1055 gallons.

It's a simplistic model to assume that energy consumed is directly proportional to difference in temperature, but if your heating system was engineered, it's the model the designer used.

I could put an extra 55 gallons in the budget easy enough but we had a complication Friday. After all this discussion, Mrs bigtiller went outside Friday afternoon to cool off. She said it's too darn hot in here. So now our 70 degrees are limited to evening TV watching.