McAvoy’s Patent Model of a Heat Regulator for Hot Water Apparatus - ca 1869

This model was filed with the application to the U.S. Patent Office for Patent Number 93,104 issued to Hugh L. McAvoy of Baltimore, Maryland on July 27, 1869. His patent was for a new and improved mechanism for automatically regulating the fire of a boiler for hot water heating or other purposes . Although such regulators then existed, he claimed that his was the first that was designed to be independent of the actual water being heated, thereby allowing flexibility in design. To monitor the temperature of the water being heated he provided for a compartment separate from the boiler but which would be heated to the same temperature as the water in the boiler. This compartment was partially filled with water with a column of air above it. A pipe extended from near the bottom of this compartment and exited through a hole at the top where it was carried to a shallow reservoir containing a float. The reservoir and pipe were filled with water which held the float at an altitude proportional to the pressure of the sealed air in the regulator chamber. As the water increased in temperature, the air pressure increased thereby forcing more water up the pipe which raised the float. The float was attached via a rod to a lever which rotated vertically around a fixed pin in such a way as to raise one end of the lever while lowering the other. A chain at one end of the lever was connected to the draft door of the boiler. The chain at the other end was attached to the fuel door. As temperature increased the first action was for the chain on the draft door to lower thereby slowing the fire. If temperature continued to rise the float rod would force the other end of the lever to raise the fuel door thus rapidly cooling the fire by introduction of cold air to the fire. As the boiler water temperature subsided the float would descend until equilibrium would be found at the desired temperature. This control point was set by adjusting the lengths of the two chains. Research of available trade literature and other sources has not revealed any commercial product that may have made use of this invention by Mr. McAvoy. He held many other patents in the areas of heating, carburetion, firefighting, lighting, oyster canning, air brakes, and refrigeration.
The model is constructed of unpainted tinplate and in its original form modeled the key elements of McAvoy’s regulator design: the fire pit and fire door, fuel door, boiler chamber, regulator chamber with associated pipes, float, and the fulcrum and chains that operated the fire and fuel doors. In its present condition as shown in the image, the float, fulcrum and chains are not installed. Nor is the fuel door present on the model. Diagrams showing the complete design can be found in the patent document online (
Currently not on view
Object Name
patent model, regulator, heat
patent date
McAvoy, Hugh L.
overall: 12 7/8 in x 7 1/4 in x 8 in; 32.7025 cm x 18.415 cm x 20.32 cm
associated place
United States: Maryland, Baltimore
ID Number
accession number
catalog number
patent number
Engineering, Building, and Architecture
Patent Models
Patent Models
See more items in
Work and Industry: Mechanical and Civil Engineering
Patent Models
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Transfer from Department of the Interior

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