A 100,000 or a 50,000 btu furnace?
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A 100,000 or a 50,000 btu furnace?
BTU's generated is a direct result of the type of fuel you burn, and the more BTU's you generate then the more fuel you consume. How the energy is transferred in terms of efficiency would be another factor. For example, the more fins or surface area you have on a radiator would increase the effectivness of transferring the heat into the room.
If you are trying to decide on a new heating unit like LP vs Oil heater, then you could multiply the cost of each per gallon per BTU to compare which is more cost effective.
For example, gallon per gallon, No 2 oil generates more BTU's than Kerosene, and kerosene generates more BTU's per gallon than Propane.
One caveat, if it is -10 F outside and you want your home at 60F would mean the heater would need to be able to have enough heating energy to maintain the temperature.
Table 1: Average Heat Content of Various Fuels*
Fuel Type BTU/Unit Kilocalories/Unit
Kerosene (No. 1 Fuel oil) 134,000/gallon 8,921/liter
Burner fuel oil (No. 2 oil) 140,000/gallon 9,320/liter
Electricity 3,413/kWh 860/kWh
Natural gas 1,000,000/thosand cu.ft. 7,139/cubic meter
Propane 91,600/gallon 6,098/liter
Anthracite coal 27,800,000/ton 6,354,286/tonne
Hardwood (20% moisture ) 24,000,000/cord 1,687,500/cubic meter
Pine (20% moisture) 18,000,000/cord 1,265,625/cubic meter
Wood pellets (pellet stoves 36,000,000 8,228,572/tonne
* Source: U.S. Department of Energy
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Egon [img]/forums/images/icons/grin.gif[/img]
lbrown, This isn't an answer to your exact question but one you might want to consider. <font color="red"> It depends. </font color> You need to know the efficiency of the heater, i.e. both the input Btu and output Btu ratings. If you compare an oil fired heater to a propane unit then first you have to know how many gallons (or pounds) of oil per hour are consumed while making xxx number of Btu (output.) Then using the data already provided in this thread you can figure the cost per KBtu. Do likewise for propane and then compare costs per KBtu. This shows which gives the most Btu/$ an important concept.
To address, if not answer, your actual question: There is no way for anyone to answer the question as the efficiency of each unit must be found first. Whichever is the LEAST efficient will use the most gas. The smaller unit will run longer and have a higher duty cycle than the larger unit but that is not neccessarily a bad thing and could be a good thing provided that the small unit can provide enough Btu to make up your conductive and radiative Btu losses through walls, floors, windows, and floors as well as make up for infiltratiion and exfiltration losses during worst case conditions of outside temperature and wind and highest reasonable thermostat setting inside.
There are certain startup losses when you fire up a furnace. You burn fuel for a while priior to getting useable heat. Most furnaces have thermostats that prevent the air handler from supplying cold air while you wait for the furnace's heat exchanger to heat up. This will happen more often with a higher Btu furnace since it will run in shorter bursts than the smaller unit. Efficiency data may consider a steady state condition, i.e. really long run time and not take into consideration startup and shut down losses.
Be aware that HVAC sales and installation outfits are often poorly trained in energy efficiency and engineering. They typically sell units that are oversized NOT RIGHT SIZED because they often don't really know how to do a good thermal analysis and get accurate results. If they oversize the unit (heating or cooling) they typically make more profit and the building is adequately heated and cooled (adequately but not efficiently.) Hence no callbacks and typically good customer satisfaction because most customers don't have a clue that they are paying too much to buy, install, and operate their HVAC systems. There are HVAC folks out there who are way above the average in knowlege. Don't think because someone has been selling oversized units for 30 years that his extensive experience is a plus for you.
If you can get some efficiency figures or both input and output Btu ratings then we can help you select the most economical unit, i.e. answer your actual questioin.
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