BTU
Before you replace that burned out furnace or boiler with a new one, make sure it is the right size to begin with. Don’t take for granted that the original installer didn’t make a mistake. Especially where forced air furnaces are involved, the sizing of the ductwork is critical to the heating capacity of the entire system. A 150,000 btu furnace feeding ductwork sized for 1000 cfm does not deliver 150,000 btu to the dwelling. Most likely, the burner kicks out on high limit due to over heating. Which is what it should do if there is not enough air flow. In such a case, this furnace has never produced at full capacity, and it’s longevity was shortened by the steady over heated condition. All furnace manufacturers match btu and cfm output; if the ductwork cannot support the cfm requirement, the furnace will not perform to it’s potential. When replacing a burned out furnace, match the btu to the ductwork as best you can.
BREECH
Venting of gas and oil burning sytems The smoke pipe, or connector from the combustion chamber to the chimney must run uphill. Before you purchase a replacement boiler of furnace, make sure the breech on the new unit will be lower than the opening in the chimney. For warm air furnaces, it is also necessary to match the style of furnace with the ductwork. Furnaces are produced in different configurations for different applications. There are counter flow models with a downward flow, low-boy models and high-boy models with an upward flow, and horizontal models with a sideways flow. Some models also come in front or rear flue arrangements which must also be matched. Make sure you equal or improve the venting of any replaced unit.
BLOWER
In response to the increased popularity of central cooling in the last twenty years, many furnace manufacturers raised the air flow capacity of their warm air furnaces by converting from belt driven blowers to variable speed direct drive blowers. This change not only increased the air flow capacity of the system, but the noise factor as well. These blower motors run at higher speeds than the old lumbering belt driven versions, and can therefore produce more air noise. Extra precautions, such as canvas or flexible connecters are needed when installing a direct drive blower in place of a belt drive.