# What is Btu: Definition and 29 Discussions

The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. Heat is now known to be equivalent to energy. The modern SI unit for heat and energy is the joule (J); one BTU equals about 1,055 J (varying within the range 1,054–1,060 J depending on the specific definition; see below).
Another legacy unit for energy in the metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius. While units of heat are often supplanted by energy units in scientific work, they are still used in some fields. For example, in the United States the price of natural gas is quoted in dollars per million BTUs.

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1. ### I How does BTU convert to heat in degrees in a greenhouse?

Information online claims sun is 200 btu per sq ft in winter only when sun is perpendicular to plastic or glass of a green house. When sun hits glass or plastic at an angle there are losses. The steeper the angle the more losses will be. Is it possible to calculate how warm a green house will...
2. ### BTU Calculation for a fireplace-based heater

An online seller claimed with an 80 Cubic Feet per Minute blower his output temperature was 350 degrees Fahrenheit from a fireplace heat scavenger. If you assumed his measurements were correct and the room temperature (input) was 70 degrees F. how would you calculate his gain in British Thermal...
3. ### How Do I Calculate BTU Needs for Cooling an Enclosed Space?

Hi all. I'm working on a small project and could use help. I have an 3/8" thick aluminum box that is going to be placed outside. The dimensions of the box are 33"x33"x33" and inside the box there is a 3/4 air gap on all sides that is covered with 1/8" hdpe platic walls. There will also be a...
4. ### Question about 120,000 BTU Furnace Wiring.

I am wiring a new 120,000 BTU furnace. The old furnace I wired with 14 gauge wire going from fused switch to the inside of the furnace wired to the furnace panel. This new furnace has 16 gauge wire going from the inside furnace panel to an electric box on the side on the inside of the furnace...
5. ### Find BTU Required for Water Cooling ?

Homework Statement [/B] Calculate the BTU/hr required for water to lower its temperature to find the compressor capacity required - Cool 30 Liters of water from 50 degree Celsius(T1) to 25 Degree Celsius(T2) – ( 122 to 77 Fahrenheit ) -Water in a closed vessel, Static, not flowing. -Max...
6. ### AC vs evaporative cooler: ratio of (BTUs/hr)/electric watts

Hi Air conditioner vs evaporative cooler: ratio of (BTUs/hr) / electrical watts I know evaporative coolers do not state BTUs/hr. I do not know why. Did anyone try them and record time taken to cool. I found this example, but I do not know if it is practically relevant...
7. ### Power vs BTU: Is There a Direct Relationship?

Hi Air conditioners power should vary with thermostat and fan/Airflow rate levels. Is there data on Power vs BTU? Is it directly proportional? Thanks
8. ### Otto Cycle - calculate the net work per cycle in Btu

Homework Statement A four cylinder, four stroke internal combustion engine has a bore of 3.7 in and a stroke of 3.4 in. The clearance volume is 16% of the cylinder volume at bottom dead center and the crankshaft rotates at 2400 RPM. The processes within each cylinder are modeled as an air...
9. ### Calculating BTU for Heating a Garage from 40 to 60°F in 4 Hours

I have a 40x60 by 14 foot garage... 3 walls are R19... one wall maybe about a R10 (garage doors, somewhat well sealed) ceiling is R30... (how would I calculate in BTU for raising the temperature from 40 degrees F to 60 degrees F in say four hours with an outdoor temperature of 20 degrees with...
10. ### Answer 1 BTU for 1°F Change in 1 lb of Water

I know that 1 BTU is energy needed to raise 1lb of water 1 fahrenheit. Does that also mean that if I were to lower the temp of 1lb of water by 1 fahrenheit that I have used 1 BTU of energy? Does it apply both ways?
11. ### Heat transfer in a large steel box

If I have a large steel box that is 27ft x 8ft x 8ft and has 7 in thick walls with 4 non-insulated windows each 1ft by 1ft, where does heat escape from inside the box, is it the windows and the walls? So if the inside temperature needs to be 72 F and the outside air is 0 F how much heat would...
12. ### What units are BTU compatible with?

Homework Statement Im given a problem where I am supposed to utilize the concept of conservation of energy to find an object initial velocity. There is a force that works on an object. They give me the amount of work done in BTU and the other units are in lb, ft/sec, ft/sec^2...Then I...
13. ### BTU transfer of stainless steel pipe

I know this is a complicated question, and depends on a lot of factors. So, I will do my best in explaining the situations. I am trying to regulate the temperature of a pool of circulating water. I have 8' of stainless 304, 1" OD, 14 awg, pipe, positioned against the flow of water (to provide...
14. ### Determine the amount of energy transfer by work in btu

Homework Statement A closed system consisting of 14.5 lb of air undergoes a polytropic process from p1 = 80 lbf/in2, v1 = 4 ft3/lb to a final state where p2= 20 lbf/in2, v2 = 11 ft3/lb. Determine the amount of energy transfer by work, in Btu, for the process Homework Equations W = ∫v1v2p...
15. ### BTU rating of Earth below frost line

Does anyone know how many BTU's the Earth is capable of transferring per minute or hour per square foot of surface area below the frost line? thanks -Steve
16. ### How do you calculate BTU of expanding compressed air

I would like to know if there is a measure, in BTU's, of the cooling effect from the release of compressed air through a nozzle and how it relates to PSI
17. ### How do different types of elements affect the BTU output of a 24V DC heater?

I had a 24V DC tractor cab heater that decided to let the magic smoke out shortly after hooking it up for this winter season.So,I checked the company website-DC Thermal-and they have the new improved models which claim to output almost 3 times the heat in BTU's.These are quartz type heaters.My...
18. ### Find Approximate BTU of Air in 10 Cubic ft of 95F Water

I've got 10 Cubic ft of water at 95F. I'm agitating it with air at 15psi, with 65F temperature. How can I find the BTU of air? Approximately.
19. ### BTU Calculator for Solar Thermal Water Heater

I am trying to develop a btu calculator for my solar thermal water heater. I would eventually like to use labview to do the acquisition. I have an internal heat exchanger and know the temperature before and after it enters the exchanger. I have a flow meter w/ a pulse output. What would the...
20. ### Calculate BTU Output of Gas Water Heater: Formula & Tips | 475°F Exhaust

I have a gas water heater that has an exhaust of 475 degrees Fahrenheit. I would like to know, what formula can I use to determine the amount of BTU's that the water heater produces (or wastes) per hour of use. I only have the temperature of the exhaust (475 Deg. F) and the diameter of the...
21. ### Need a formula to calculate BTU loss in a pond.

I am looking at building an ortemental pond. The pond will be 15' x 3' with an average depth of 2'. I caculated it out to 673 gallons and 45 square feet of surface area. I plan to put tropical fish in it and will have to maintain a temperature of 72F year round. Worst case scenario would be an...
22. ### Converting Btu to J: Troubleshooting Heat Capacity of Water

Homework Statement I am just trying to convert Btu to J based on the heat capacity of water and it is not giving me the right answer. Homework Equations The Attempt at a Solution Btu = C(water) * m(1 lb) * dt(1 F) 1 lb = 0.454 kg 1 F = 9/5 K 1 Btu = 4.186 kJ/(kg*K) * .454...
23. ### Ice vs. Dry Ice total BTU cooling to 25 degrees C

I wanted to ask which would provide the largest total BTU cooling regular ice from water or dry ice to reach a temperature of 25 degrees C? 1 lb of Ice would start at -3 degrees C to 25 degrees C at sea level = total BTU 1 lb of Dry ice would start at -80 degrees C to 25 degrees C at sea...
24. ### Calculating BTU & Water Consumption for 50 kW/h Steam Turbine

Hi All, For a small steam turbine to generate power at a rate of 50 kW/h. How many BTU's of heat required per hour, also water consumption per hour, how do you work this out?
25. ### BTU Q's about burning water vs. burning gassoline

I would like to know how Hydrogen compares to gasoline or diesel fuel for BTU which is the basic comparison for energy sources. If water is relatively easy to get, then the cost of hydrogen production is irrelevant. but gasoline on the other hand is expensive and controlled by a few individuals...
26. ### Powering 5K BTU 115V Air Conditioner in Auto

Folks This is my first post to Physics Forum. I've started a project to put air conditioning into my '86 Westfalia van. The stock AC never worked and was ill-designed with huge long runs of tubing back and forth in the van, so I've removed it. I located the smallest possible 115 V AC window...
27. ### Question regarding energy, BTU

"Assume that the population of the U.S. increases by 1%/yr. How many BTU of energy will have to be added to the national annual energy budget this year to maintain the same per capita expenditure?" The current populations is listed as 291 million. And the current per capita energy...
28. ### How do I calculate BTU? (for a water heater project)

The last time I dealt with any physics was 6 years ago in college, so forgive my apparent ignorance: The Situation. I have a swimming pool pump outside near the house's air conditioner unit (heat exchanger I think it's called). Well, the freon is coming out of the house in copper pipes to...
29. ### BTU Q's about burning water vs. burning gassoline

In another thread (under Dr. Kaku's EXPLORATIONS program page, the thread about the idea of a "Space Elevator"), the conversation turned to the prospect of using something like a "Space Elevator" as the basis for shifting from a petroleum based economic model to essentially a water based economy...