Math Quiz

[unclebingo]

Please check my math...

If my home losses 80000 BTUh, and I live in a region with 3784 Heating Degree Days, then I use 7,265,280,000 BTU per year.
(BTUh x 3784 x 24)

Which is 72652.8 Therms per year.
(BTU / 100000)

If I pay 113 cents per Therm, then my base cost without factoring AFUE is: $8209.77 per year.
(Cents per Therm x Therms per year / 1000)

This figure seems pretty high to me.

[ampulman]

Please check my math...

If my home losses 80000 BTUh, and I live in a region with 3784 Heating Degree Days, then I use 7,265,280,000 BTU per year.
(BTUh x 3784 x 24)

If you lost 80,000 BTUH every day for a year, you would only lose 700,800,000 BTUH (80,000 BTUH x 24 x 365). Are you at one of the poles?

The figure (80,000) is what you house needs (at design temperature); so you will need to burn more gas to yield that quantity. Sorry.

Perhaps one of the PROs has a formula for approximating usage based on HDDs.

Amp

[beenthere]

I use bin data. HDD's is too misleading.

When its 64° for 24 hours, you will have had 1 HDD. Your not going to loss 80,000BTUs in 24 hours at 64° outdoor temp.

[WhoIsThat?]

If my home losses 80000 BTUh,
at what inside/outside temp, averaged over 24 hours?

[unclebingo]

I've yet to collect the HDD figures, and for the purpose of building the formula I was using default figures from www.hvacopcost.com for both HDD and BTUH loss.

When I can gain access to the house I plan to perform a full Manual J to calculate the BTUH loss, and I'll calculate HDD per "beenthere's" post, i.e.

Base Temp 65f
Monthly 1% DBf
Jan - 54 - 31 days - 341 HDD

I'll only use monthly 1% DB averages so it's going to be an approximation.

If I understand you guys correctly if (subject to ManJ) the furnace has to make up a 80,000 BTUH loss to maintain 70f inside, which totals 700,800,000 BTUH per year. (80,000 x 24 x 365). And wikipedia's formula to calculate annual kWh is correct, i.e.

Area x U-value x HDD x 24/1000

Then do I just multiply the yearly BTUH loss of 700,800,000 (Area x U-value) by the number of HDD's by 24 / 1000 = yearly kWh used

Then I calc my cost per kWh?

[beenthere]

The furnace only has to make up 80,000BTUs when the outdoor temp is at design.
Which probably is only a few hours/days a year.

When its 54 outside(11 HDD), it may only have to make up 13,538 BTUs an hour.
But, even that is not a true statement. HDD does not take into account, solar gain. So at night time, it might loose 13,538 BTUs an hour, but during the day with the sun out. It might only loose 8,000BTUs an hour.

[Carnak]

heat loss times times degree days divided by (indoor design temp-outdoor design temp) x a correction factor, divided by system efficiency

[unclebingo]

heat loss times times degree days divided by (indoor design temp-outdoor design temp) x a correction factor, divided by system efficiency

If I'm using monthly 1% DB data then outdoor design temp varies from month to month. Does this suggest I need to calc each month?

What is the correction factor?

[Carnak]

you use the design temperature difference, usually for a house the 97.5%

the correction factor compensates, depends on your area.

[unclebingo]

So the correction factor is only needed if I adopt a standard 97.5% design temp difference?

I know that the Manual J spreadsheet I have automatically calc's the HTD for me when I enter the climate data. Does that mean I can omit the correction factor from the formula?

[Carnak]

no

The correction factor is sort of like an average of how long the furnace runs per hour over the entire heating system, if the furnace was sized spot on to the design heat loss

So in a deep freeze it runs steady, maybe in mild winter weather it runs 10 minutes per hour

It depends on where you live. Same lattitude you could be in the mountains, on the coast , maybe in the prairies, the geography plays a part


you have to pony up and say where you live

[unclebingo]

Cedar Rapids, Iowa

But understanding the formula is more important to me than just getting you guys to tell me the answer.... I wanna learn!

[ampulman]

You can get an accurate figure for HDDs and other info at:

http://www.wunderground.com/history/airport/KVAY/2008/11/23/CustomHistory.html

Type in your zip code, then follow the 'custom' table.

Amp

[Carnak]

Cedar Rapids, Iowa

But understanding the formula is more important to me than just getting you guys to tell me the answer.... I wanna learn!

I forgot to mention the 24 factor

Indoor temp 70

Outdoor temp -5

Design heat loss at -5 F 80,000

Degree Days 3748 (sounds low like it is metric), maybe more like 3748x1.8=6746. Sioux City is around 6900

maybe the correction factor for your area is 0.65

24x(80000 x 6746)/(70--5)x0.65/100000=1123 therms for a 100% AFUE

For 90% AFUE, divide therms by 0.9, for 80% divide by 0.8, therms get "bigger"

[Carnak]

your heat loss is in Btu/hr so when you use degree days you have to change it to degree hours so you mulitply by 24

Ask guys selling propane or fuel oil what correction factor they use

[unclebingo]

Ask guys selling propane or fuel oil what correction factor they use

Thanks for laying it all out Carnak, much appreciated!

Pity the correction factor can't be calculated... It sounds like it has something to do with octane and elevation?

[Carnak]

local fuel suppliers, as in those who deliver it, will have a good feel for the region

They need to know when to show up

[WhoIsThat?]

The ASHRAE stuff is very hard to get but this book
http://www.amazon.com/Mechanical-Engineering-Reference-Manual-Exam/dp/1591260493/ref=sr_1_4?ie=UTF8&s=books&qid=1251573598&sr=8-4
is probably available in your local library and has chapters that give a good overview of heat loss/gain.

[unclebingo]

Carnak, you were right re Degree Days. Cedar Rapids has 6924.

Month 1% dbF Days HDD
Jan | 45.7 | 31 | 598.3
Feb | 54.6 | 28 | 291.2
Mar | 70.6 | 31 | -173.6
Apr | 80 | 30 | -450
May | 85.2 | 31 | -626.2
Jun | 91.4 | 30 | -792
Jul | 94.4 | 31 | -911.4
Aug | 93.7 | 31 | -889.7
Sep | 88.7 | 30 | -711
Oct | 79.5 | 31 | -449.5
Nov | 65.8 | 30 | -24
Dec | 56.5 | 31 | 263.5
HDD 1153

But all I get is 1153? Is 1% Dry Bulb data the correct source to use?

[Carnak]

1153 is the therms using the correction factor I pulled out of my ass?

[unclebingo]

I managed to find the ASHRAE 2005 Fundamentals at my local library but the climate data doesn't include HDD / CDD. So I was hoping I could calculate it by subtracting the 1% Dry Bulb data for each month from the base 65F... turns out I was wrong!

[Carnak]

you already have the yearly total of 6924

the degree days does not have much to do with the design dry bulb, it has to do with how many hours out of the year it is colder than 65F

[Carnak]

Carnak, you were right re Degree Days. Cedar Rapids has 6924.

Month 1% dbF Days HDD
Jan | 45.7 | 31 | 598.3
Feb | 54.6 | 28 | 291.2
Mar | 70.6 | 31 | -173.6
Apr | 80 | 30 | -450
May | 85.2 | 31 | -626.2
Jun | 91.4 | 30 | -792
Jul | 94.4 | 31 | -911.4
Aug | 93.7 | 31 | -889.7
Sep | 88.7 | 30 | -711
Oct | 79.5 | 31 | -449.5
Nov | 65.8 | 30 | -24
Dec | 56.5 | 31 | 263.5
HDD 1153

But all I get is 1153? Is 1% Dry Bulb data the correct source to use?

Those are not 1% dry bulbs, its almost like it is an average high temperature during the month. If those were 1% dry bulbs you would be somewhere like Florida. 45.7F in January you would have a citrus crop if that was what you designed a heating system around

Sioux City gets below 0F

[unclebingo]

I'm referencing the ASHRAE 2005 Fundamentals > Climate Data > IP Stations > 725450.PDF

Row:
Monthly Design Dry Bulb and Mean Coincident Wet Bulb Temperatures

Column:
1% Dry Bulb for each month

[Carnak]

For commercial buildings, you usually use the 99% Heating DB, so in that file it is -4.7F to size your furnace. For houses you usually use the 97.5% level, although it is not in that particular file you referenced.

When they say 0.4%,1%, 2% and list a mean coincidental wet bulb along with it, it is cooling data, you are not sizing a heating system in Iowa based on it being 45.7F outside, watch out for GIGO

[unclebingo]

I'm pleased my Garbage In has a Carnak Out filter!