By Carol Fey
Hot water is incredibly versatile. It provides domestic hot water, baseboard heating, and radiant panel (floor, ceiling, wall or snow melt) heating. These operate at different temperatures. But with the right heat source, piping layout, and controls, various requirements can be satisfied by one hydronic system.
Wide Temperature Requirements
Not all heating loads want the same temperature. Domestic hot water in the tank needs to be about 140F. A typical range for baseboard is 160F to 200F. Radiant floor heating varies widely, depending upon where the tubing is, and the covering materials. Tubing embedded in a concrete slab covered with tile may want only 100F, but underfloor tubing with carpet might require 160F.
The Heat Source
Looking at a hydronic system, let’s first consider the heat source. There is a full range of equipment that provides both heating and domestic hot water, including boilers, water heaters, solar collectors, and geothermal heat pumps. We must consider whether or not the heat source is capable of producing water hot enough for the high temperature requirements, and additionally contains enough Btus to be mixed down to the lower temperature requirements.
Solar and geothermal may not be capable of serving the needs of the higher temperature demands. Water heaters may have trouble providing enough water fast enough to satisfy all the loads. That leaves us with boilers, which isn’t a bad place to be.
Now we have determine if we want to use a conventional boiler, or a condensing boiler. It’s all about cost, energy conservation, and perhaps surprisingly, the temperature of the water returning to the boiler.
While a condensing boiler costs more, it has a payback in energy efficiency. More complex is the fact that the energy efficiency happens only if the return water temperature is consistently low. That’s because the boiler depends upon the scientific principle of condensation to get extra heat out of the fuel. When condensation happens, heat is given off. The low temperature return water cools the flue gases inside the boiler. They condense there instead of going up the flue. The resultant heat stays in the boiler rather than going up the chimney.
The flip side of the boiler decision is that a conventional boiler needs consistently high return water temperatures. A conventional boiler is not designed to withstand the caustic nature of condensed flue gases. Hot return water keeps the hot flue gases vaporized and moving up the flue.
Mixing it Up After we have determined the heat source, the rest is about mixing cold water with the hot to get various temperatures. This is done with piping and mixing controls.
Think of each temperature requirement as a load which requires its own piping loop. Let’s say we have some areas with baseboard and some with radiant floors. Each has its own loop of piping from the boiler. The baseboard has a higher temperature requirement. Depending upon the system design, possibly it can use the water directly from the boiler.
Let’s say that the in-floor radiant is covered with tile, and needs only 100F. That’s because a hotter temperature will be uncomfortable to people’s feet coming in contact with it. The piping loop from the boiler to the radiant floor will contain a mixing control to mix a certain amount of cold water with the boiler water.
Smart or Not
Mixing controls can be valves or injection pumps. And they can be “intelligent” or “dumb.” Intelligent controls are motorized mixing valves or variable speed injection pumps. The device measures water temperatures and repositions accordingly to provide the set water temperature. When the incoming water temperature changes, the control adjusts so that the mixed temperature is constant.
“Dumb” mixing controls are less expensive, but can’t adjust themselves once they are manually set. The result is that the mixed water is not always a consistent temperature.
There are two important considerations. One is to determine how critical it is to have precise and consistent water temperatures to the loads. The other is to provide hot return water to a conventional boiler and cool return water to a condensing boiler.
Domestic Hot Water
Controlling the temperature of domestic hot water is critical to prevent both scalding and disease transmission. This is complicated by the fact that two different temperatures are required.
To prevent Legionella, domestic hot water must be heated to 140F. That is too hot for safe household use because of the danger of scalding. The U.S. Department of Energy suggests that 120F is adequate at the tap. The solution is again a mixing control, either under the sink, or at the hot water tank. The water is heated to at least 140F and then reduced down to a safe 120F.
With careful design it’s possible to have several different water temperatures within one system. The key is design — being aware of the requirements of the various loads, and what it takes to deliver those temperatures.