Monday, September 19, 2011

How to calculate cost in heating per degree raised on thermostat? ball park calculation?

How do i calculate how much more I have to pay per degree I want to raise my room? I tried using specific heat for air and have trouble finishing calculations.



anyone has this done? thanks. (i know it depends on the volume of my space, so let's leave that at say V).



say a general 2 bedroom apartment , wooden structure. I set thermostat at 59 degrees F. vs. 63 degrees F?How to calculate cost in heating per degree raised on thermostat? ball park calculation?You have to plug in you own values;



Lets say each room is 12' x 8'.

You need the insulation values of the walls, ceiling, floor.

Let's say you have r12 in walls, r30 in ceiling, r20 in floor.



The 2 rooms are connected, so they have a common wall. Disregard this wall and assume a 12' x 16' room.



Heat loss is calculated by;

Q = A dT / rThe formula you are looking for is Q = A dT / r

where;

Q is the heat loss in BTU

A is the area

dT is the temperature difference

r is the r-value of the walls, ceiling, and floor.



Now let's say it's zero degree outside and 63 inside.





ceiling - 12 x 16 = 192 sq ft.

Q = (192 x (63 - 0)) / 30 = 403 BTUH



floor - 12 x 16 = 192 sq ft

Q = (192 x (63 - 0)) / 20 = 605 BTUH



walls - 12 x 8 + 12 x 8 + 16 x 8 + 16 x 8 = 448 sq ft

Q = (448 x (63 - 0)) / 12 = 2352 BTUH



Qtotal = 403 + 605 + 2352 = 3360 BTUH



Now account for air leaks and lets say 3 air changes per hour. That will just about double the size of the furnace, so 3360 x 2 = 6720 BTUH to heat this area to 63 degrees for 1 hour, or 161280 BTU per day.



Now look at your gas bill and see the cost of gas, it's probable in therms - like $0.50 per therm.

A therm is 200,000 BTU, so that's $0.40 a day.



At 59 degrees;



ceiling - 12 x 16 = 192 sq ft.

Q = (192 x (59 - 0)) / 30 = 378 BTUH



floor - 12 x 16 = 192 sq ft

Q = (192 x (59 - 0)) / 20 = 566 BTUH



walls - 12 x 8 + 12 x 8 + 16 x 8 + 16 x 8 = 448 sq ft

Q = (448 x (59 - 0)) / 12 = 2203 BTUH



Qtotal = 378 + 566 + 2203 = 3147 BTUH



Now account for air leaks and lets say 3 air changes per hour. That will just about double the size of the furnace, so 3147 x 2 = 6294 BTUH to heat this area to 63 degrees for 1 hour, or 151056 BTU per day.



151056 x 0.50 / 200000 = $0.38 a day



So you save 2 cents a day.



If you want to include windows, the r-value for a window is around 1.1. For an outside dooe, it's about 5. Make sure you subtract their area from the area of the walls. For example, there are 448 sq ft of walls. If there is a 3'x7' door and a 3'x3' window, then there are 448 - 21 - 9 = 418 sq ft of wall, 21 sq ft of door, and 9 sq ft of window. The heat loss for the window at 63 degrees is;

Q = 9 x (63 - 0) / 1.1 = 515 BTU per hour

The door is;

Q = 21 (63 - 0) / 5 = 265 BTU per hourHow to calculate cost in heating per degree raised on thermostat? ball park calculation?PK, Blue, you have the right approach and good R values. You deserve the points because you went first and did all the work. But I would use a couple different values.

The infiltration rate seems quite high, especiall since you have R11 in the walls and R30 in the ceiling. There would also be a vapor barrier with the insulation so the infiltration thru these surfaces would be insignificant.

Also you have assumed a btu rate for infiltration, but it can be calculated if you do know the flow rate. BTUH = 1.08 CFM dT

For the 63 F room at 3 changes/ hr the heating load would actually be 4838 BTUH. Again I would discount most of this.

The dT through the floor would only be about 20 Deg. because the ground temp will be higher than the air temp. If the rooms are on a second floor, there might be a small heat gain thru the floor from the apartment below.

I cannot comment on the heating fuel .

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