Many of you reading this column have gained a healthy respect for the ability of a cool concrete slab to suck heat out of warm water passing through embedded tubing. Under the right conditions, a cold slab can pull heat from that water three to four times faster than when the system is operating under design load conditions. It’s amazing to watch this happen. A cool concrete slab is like a black hole for Btus. It gobbles up any heat that dares to get close.

Over the last 25 years, our industry has learned to deal with this in systems where the heat source is a gas-fired or oil-fired boiler. The solution is to install a mixing assembly operated by a controller that measures the boiler’s inlet temperature, as well as the water temperature supplied to the distribution system. This concept, and the placement of the associated temperature sensor, is shown in Figure 1.

The hardware that makes up the mixing assembly can be a three-way or four-way motorized mixing valve, a two-way motorized injection valve, or a variable-speed injection pump. All these devices rely on the same thermodynamic principles when it comes to boiler protection.