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HVACR115 – Mechanical for Gas Heat

HVACR115 – Mechanical for Gas Heat

Gas Combustion EfficiencyGas Combustion Efficiency

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BTU InputBTU Input

• BTU Input is measured by clocking the gas meter (time the dial for one minute) and then multiply the number of revolutions by 60. The result is Cubic Feet Per Hour.

• Multiply the input by the efficiency percent to find output.

• BTU Input is measured by clocking the gas meter (time the dial for one minute) and then multiply the number of revolutions by 60. The result is Cubic Feet Per Hour.

• Multiply the input by the efficiency percent to find output.

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Firing Rates to High Or LowFiring Rates to High Or Low

• These means the input is not what it should be.• The wrong firing rate is caused by:

– Incorrect Orifice Sizes– Incorrect gas pressure

• These means the input is not what it should be.• The wrong firing rate is caused by:

– Incorrect Orifice Sizes– Incorrect gas pressure

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Firing Rates to High Or LowFiring Rates to High Or Low

• First, check the gas pressure. – Use either the U-Tube or another manometer.

• Normal gas pressure will be 3.5”wc for natural gas and 11”wc for propane.

• First, check the gas pressure. – Use either the U-Tube or another manometer.

• Normal gas pressure will be 3.5”wc for natural gas and 11”wc for propane.

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Firing Rates to High Or LowFiring Rates to High Or Low

• If gas pressure is correct or it can not be corrected to fix an over-fired situation the orifice sizes must be reduced.

• If gas pressure is correct or it can not be corrected to fix an over-fired situation the orifice sizes must be reduced.

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Firing Rates to High Or LowFiring Rates to High Or Low

• Fixed orifices are sized by:– Specific Gravity of Gas– Manifold Pressures– Altitude– Cubic Feet Per Hour

• Fixed orifices are sized by:– Specific Gravity of Gas– Manifold Pressures– Altitude– Cubic Feet Per Hour

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Sizing OrificesSizing Orifices

• Find the correct orifice for the input. Find the input rate from manufacturer, divide this by the number of orifices to find the btu/h per orifice.

• Use a correction chart for higher altitudes if needed.

• Find the correct orifice for the input. Find the input rate from manufacturer, divide this by the number of orifices to find the btu/h per orifice.

• Use a correction chart for higher altitudes if needed.

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Combustion EfficiencyCombustion Efficiency

• Combustion efficiency is measured by measuring the products of combustion and the flue temperature.– The percentages of oxygen and carbon monoxide that

should be present in the flue gasses is known.

• Combustion efficiency is measured by measuring the products of combustion and the flue temperature.– The percentages of oxygen and carbon monoxide that

should be present in the flue gasses is known.

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Combustion EfficiencyCombustion Efficiency

• Measuring the products of combustion can be done electrically or mechanically.– An analyzer that has a fluid which will trap temporally

O2 and CO– The percentage of these chemicals in the fluid will

change the level of the liquid in the flue pipe.

• Measuring the products of combustion can be done electrically or mechanically.– An analyzer that has a fluid which will trap temporally

O2 and CO– The percentage of these chemicals in the fluid will

change the level of the liquid in the flue pipe.

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Combustion EfficiencyCombustion Efficiency

• Comparing these by products and temperature on a sliding scale will give the efficiency.

• Comparing these by products and temperature on a sliding scale will give the efficiency.

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Temperature RiseTemperature Rise

• The temperature rise over the heat exchanger must be correct for the furnace.– The tag on the furnace will give the correct temperature

across the heat exchanger.

• The temperature rise over the heat exchanger must be correct for the furnace.– The tag on the furnace will give the correct temperature

across the heat exchanger.

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Temperature RiseTemperature Rise

• Incorrect temperature rise is a sign of system problems.– Air flow in duct system may be incorrect– Efficiency problems– Firing rate may be low or high

• Incorrect temperature rise is a sign of system problems.– Air flow in duct system may be incorrect– Efficiency problems– Firing rate may be low or high

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Temperature RiseTemperature Rise

• Measure the supply and return temperatures after 10 minutes of run time.

• Subtract these two to find temperature rise..• Take your supply temperature far enough

away to prevent the radiant heat from the heat exchanger from affecting readings.

• Measure the supply and return temperatures after 10 minutes of run time.

• Subtract these two to find temperature rise..• Take your supply temperature far enough

away to prevent the radiant heat from the heat exchanger from affecting readings.

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Temperature RiseTemperature Rise

• The CFM of the furnace can be found using the the temperature rise and the combustion efficiency.

• The formula is:– CFM = BTU / (1.08 x temp rise)

• The CFM of the furnace can be found using the the temperature rise and the combustion efficiency.

• The formula is:– CFM = BTU / (1.08 x temp rise)

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Temperature RiseTemperature Rise

• Example:– TD = 65 degrees– BTU/Hr = 80,000– CFM = 80,000 / (1.08 * 65)– CFM = 80,000 / 70.2– CFM = 1139.6

• Example:– TD = 65 degrees– BTU/Hr = 80,000– CFM = 80,000 / (1.08 * 65)– CFM = 80,000 / 70.2– CFM = 1139.6

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Temperature RiseTemperature Rise

• If the temperature rise is to high or low make sure the CFM’s meet the manufacturers recommendations or use the rule of thumb number 10 CFM per 1000 BTU.

• If the temperature rise is to high or low make sure the CFM’s meet the manufacturers recommendations or use the rule of thumb number 10 CFM per 1000 BTU.