to cool the firebox crown safely. Stays, or ties, physically link the firebox and boiler casing, preventing them from warping. Since any corrosion is hidden, the stays may have longitudinal holes, called tell-tales, drilled in them which leak before they become unsafe. The fire-tube type boiler that was used in the Stanley Steamer automobile had several hundred tubes which were weaker than the outer shell of the boiler, making an explosion virtually impossible as the tubes would fail and leak long before the boiler exploded. In nearly 100 years since the Stanley's were first produced, no Stanley boiler has ever exploded. Economics and efficiencies Excessive cycling Each time a boiler cycles off and on, it can lose efficiency. When the fire starts combustion efficiency is usually lower until steady state conditions prevail. When the fire stops the warm chimney continues to draw additional air from the interior space until it cools. Excessive cycling can be minimized Modulating boilers can run longer (at firing rates that match the loads) than the non-modulating boilers (which operate at the full firing rate). By using condensing modulating boilers. By using non-condensing modulating boiler. By setting the controls (thermostats or controller with temperature sensors) with greater temperature differentials between STOP and START. At Non-Condensing Boilers make provisions so that minimum return water temperature of to to the boiler to avoid fireside corrosion. By setting the MINIMUM OFF times at 8 to 15 minutes. For comfort heating loads, short time intervals do not usually trigger occupant complaints. Common provisions are to provide a primary piping loop with pump(s) and a secondary piping loop with pump(s); and either a variable speed controlled pump to transfer water from the primary loop to the secondary loop, or a 3-way valve to divert water from the secondary loop to the primary loop. Fireside corrosion in non-condensing boilers A minimum return water temperature of to to the boiler, depending on the specific design, is used to avoid condensing water vapor from the flue gas and dissolving and from the flue gasses forming carbonic and sulfuric acid, a corrosive fluid that damages the heat exchanger. Condensing boilers Condensing boilers can be 2% or more efficient at lower firing rates by extracting the heat of vaporization from the water vapor in the flue gases. The efficiency increase depends on the fuel and the available energy to be recovered as a fraction of the