Some hydronic systems use a separate reset-controlled mixing device for each water temperature required. It works, but it's not always necessary.

We've discussed the theory and benefits of outdoor reset control in several past columns. You'll recall the basic idea is to reduce water temperature to the heat emitters as the outdoor temperature increases. This helps balance system heat output with building heat loss so interior temperatures remain stable. It minimizes on/off cycling of heat delivery. In effect, the heat is always "on," but at just the right delivery rate to match the existing load.

Outdoor reset control is usually implemented one of two ways:

    1.A control can automatically adjust the operating temperature limits of the heat source based on outdoor temperature.

    2.A mixing device can be used between the heat source and the distribution system. It blends cooler return water from the distribution system with hotter water from the heat source to maintain the desired supply temperature to the heat emitters.

The method of choice depends upon the heat source used and the "depth" of reset allowed.

For example, most conventional gas- and oil-fired boilers that aren't designed to operate with sustained flue gas condensation should not be reset down to operating (return) temperatures below about 130 degrees F, even on mild days. To do so allows extended periods of condensing operation and sets the stage for fire-side corrosion or scaling. If "deep" reset of the distribution system is desired, a mixing device must be installed between such boilers and distribution system to reduce water temperatures in the latter while maintaining suitable return temperatures to the former.

If the boiler temperature is not lowered below the condensation threshold, (often referred to as partial reset), a mixing device is not necessary. Reset is achieved by varying the operating temperature limits of the boiler in response to outdoor temperature.

If One Isn't Enough

When designing hydronic heating systems it's not uncommon to encounter jobs where some parts of the distribution system need to operate at higher water temperatures than others. Case in point: A house will uses a tube and plate radiant system for wood-framed floors, embedded tubing in slab-on-grade areas, and some zones of baseboard heat. There are a number of ways to accommodate such requirements. Most use a portion of the hottest water from the heat source for the high temperature loads, feeding the remaining hot water to mixing devices that reduce its temperature to that needed by the other loads. Some systems even use a separate reset-controlled mixing device for each water temperature required. It works, but it's not always necessary.

Look at the piping schematic in Figure 1. It couples a tube and plate radiant floor heating sub-system into a primary loop that also supplies a secondary circuit of baseboard. Let's assume the primary loop temperature will be reset by a control that adjusts the operating temperature limits of the boiler. What do you think happens to the water temperature in the floor heating circuits as the primary loop temperature changes?

If you said "it changes to" you're right. But how much does it change? The best way to answer this is work the numbers through an example.

Let's say the system designer intends the primary loop temperature to be 180 degrees F at an outdoor design temperature of 0 degrees F. At warmer outdoor temperatures the primary loop temperature tracks down along the reset line shown in Figure 2.

The radiant sub-system is designed to provide 10,000 Btu/hr. at design conditions using 130 degrees F supply water and a circuit temperature drop of 10 degrees F.

The injection flow rate that must be "pulled of" the primary loop at design conditions is:

The total flow rate through the radiant distribution system at design conditions is:

Assume valves (V1) and (V2) are manually adjusted to provide these flow rates. Once these valves are set, they establish a fixed relationship between the injection flow rate and the radiant distribution flow rate. These flow rates do not change as the water temperature in the system changes. Furthermore, since the radiant sub-system is connected as a secondary loop to the primary loop (using closely spaced tees) these flow rates won't change even if the primary loop flow rate does. The latter situation won't happen with a true primary/secondary system, but could in a hybrid system where some other loads are connected in parallel.

If you apply the hydronic mixing formula (see my May 1997 column) at point A, and write a formula that states that heat output from the radiant floor is proportional to the temperature difference between supply water and room air (a very reasonable approximation) and factor in the flow rates we calculated above, you'll get the following formula for the water temperature supplied to the radiant circuits (in this example):

Now the interesting part. Use the reset line in Figure 1 to find what the primary loop temperature would be at 100, 75, 50 and 25 percent of design load and plug these numbers into Formula 4. Plot these temperatures on the same graph with the reset line of the primary loop, connect the dots and see what happens. Anticipating that you probably don't feel like plotting graphs as you enjoy this month's issue of PM, I've done it for you in Figure 3.

Mind Reader

As if the system could read your mind and grant your wish, the water temperature in the radiant loop tracks down its own reset line. In effect a second reset function has been delivered without using another mixing device. Is that cool (or warm) or what! Two resets for the price of one.

This piping configuration always produces a proportional relationship between the primary loop reset line and the secondary loop reset line. This means the left end of the two reset lines always start from the same point, but the right end of the lower reset line can move up or down depending on the flow proportions set by the manual valves in the lower temperature sub-system.

Although sufficient for most hy-dronic systems, it's still doesn't provide the total flexibility that two independently adjustable reset mixing devices could. For example, it doesn't provide return water temperature protection for the boiler under cold-start conditions. Still it's an underutilized design tool available for the savvy Wet Head to deploy.

If the primary loop is only partially reset down to some minimum supply temperature, the secondary loop will also bottom out its reset line at the same outdoor temperature where the minimum temperature setting of the primary loop takes effect. This is shown using dashed lines in Figure 3. In such situations preventing overheating during mild outside weather requires on/off flow control in the secondary circuit.

Figure 4 shows another piping design using this concept. In this case a variable-speed injection system is used to provide reset control to an intermediate loop. Two independently controlled radiant floor manifold stations are supplied from this loop. A third manifold station that needs to operate at a lower temperature is tied to the intermediate loop using primary/secondary connections and a manually adjusted three-way valve. This manifold station will be proportionally reset as the intermediate loop tracks up and down its reset line. In this system the boiler is also protected against low return water temperature by the variable speed injection control. Many variations of this piping concept are possible.

Perhaps you can use the proportional reset strategy on the next multiple temperature hydronic system you design. In most applications it should reduce cost over that of installing separate reset-controlled mixing devices. Who says Mother Nature doesn't give out "freebies" once in a while?