Getting It Right

This review of mechanical piping and radiant loop layouts and their differences, as well as control strategies, will help you on the road to designing top-notch systems in terms of performance, efficiency and cost.

Piping of Mixing Devices

When working on a single water temperature boiler system, the mechanical piping arrangement is quite simple and the load can be piped right off the boiler. However, many boiler systems today consist of a high temperature load such as DHW and fan coils, as well as a low temperature load such as radiant. In that case, most commonly, some form of mixing device is installed. This allows for simultaneous operation of loads with different water temperature requirements and improves efficiencies. 

Mixing device options include: two-way injection valves*; three-way or four-way mixing valves; or, variable speed pump injection systems. Three- and four-way mixing valves are more common than the two-way and should be modulated with either a floating (24 or 110Vac) or modulating (0-10Vdc) signal to provide an outdoor reset water temperature to the radiant system. When the piping configuration of those mixing devices is implemented improperly the system performance can be compromised.

Figures 1 and  2 show two piping configurations with flow in the boiler loop, but the primary pump and heat source, as well as any high temperature loads, are omitted for clarity. It is assumed that all high temperature loads are taken off and a primary pump is installed prior to the mixing device. The same is true for the remaining Figures.

Figure 1 shows how 3- or 4-way mixing valves are frequently installed. In these two examples, there is no hydraulic separation between the primary pump and the radiant pump and once the mixing valve is fully open, two pumps work in series effectively, changing the system flow considerably. This can lead to velocity noises on the radiant side, but also to increased strain on the system components, particularly the motorized mixing valves. 

Figure 2 shows the proper installation of a mixing device off the primary loop. Two closely-spaced Tees allow for hydraulic isolation of the primary loop and the secondary/radiant loop, effectively isolating the two pumps from working in series. To attain effective isolation a few basic guidelines must be followed. Those are shown in Figure 3, where:

A = MIN 6 x diameter of primary loop pipe size; and

B = MAX 4 x diameter of primary loop pipe size.

Pay close attention to A and B, as references are sometimes in inches for each that do not align with the actual requirement.

Note: Mixing Valves need to be sized based on flow requirements, not to the pipe size of the main loops, i.e., a100 000 Btuh radiant system operating off a 10F ΔT requires 20 GPM flow (Flow = Btuh/(ΔT x 500)). The system piping, if in copper, would be 1-1/2″ according to Table 1, where the mixing valve would only need to be at 1-1/4″, according to Table 2 (see CV Value).

Another common mixing device is the variable speed pump injection system. Here, the speed of a pump is controlled by controllers, which change the frequency and the voltage supplied to the pump. This modulates the amount of heat transfer between the boiler/primary loop and the secondary/radiant loop to attain water temperature control. The piping requirements are similar to those of the mixing valves. A and B remain the same for variable speed injection systems as shown in Figure 4.

Additional requirements include:

C = MIN of 12″ drop (creates a thermal trap, which prevents convective heat transfer)

D = MIN of one pipe diameter smaller than the secondary/radiant loop

E = Balancing or Globe valve for flow balancing

Since most pumps on the market today vary with output based on head pressure and a balancing valve is installed on the injection loop’s return, fine-tuning of the output is easily attainable while the controls output is at 100 per cent.

In my experience, the simple Flow = Btuh/(ΔT x 500) formula gets me into the ballpark, with the exception that in this example, the ΔT is the temperature difference of primary loop supply and secondary/radiant loop return.

For example: Injection Flow = 100 000 Btuh / ((180F-110F) x 500) = 2.85 GPM

Sizing an injection pump exactly requires you to have the following, along with the Injection Flow Ratio Chart in Figure 5:

• Primary/Boiler Loop Supply Temp. (Tb)

• Secondary/Radiant Supply Temp. (Ts)

• Secondary/Radiant System ΔT (ΔTs)

• Seconday/Radiant System Flow Rate (SF)

Using the chart in Figure 5, identify the Flow Ratio (FR) in GPM using 180F (Tb) – 120F (Ts) and 10F (ΔTs). In this example, that would be roughly 0.14 GPM. Now, take your 20 GPM SF x 0.14 GPM flow ratio (FR) = 2.8 GPM design injection flow rate. Pick the best wet rotor circulator that can give you that flow and use the balancing valve on the injection loop return to adjust the heat pressure to suit.

CONTROL STRATEGIES

Water temperature control is essential when working with radiant floor heating systems. Without it, the system efficiency and performance would be greatly compromised when the heat loss of a building is reduced due to a milder outdoor climate.

Figure 6 pictures the differences within space temperature control using a standard thermostat with a 1F on/off differential when compared to no outdoor reset during reduced building heat loss on the top, with outdoor reset in the middle and finally, outdoor reset with indoor temperature feedback on the bottom.

No outdoor reset water temperature control – even though the thermostats turns ON 0.5F below room setpoint, the floor does not immediately put out heat into a space until the heated water is circulated for a while and the mass of the slab warms up. That timed delay can cause the space temperature to continue to fall until then. At the same time, when the thermostat turns OFF 0.5F above room setpoint, the floor now continues to put out heat into the space even though the heated water circulation has stopped while the mass of the slab cools down. This will often result in temperature swings of greater than three to four degrees Fahrenheit during milder outdoor temperatures, since the water temperature, traditionally set for the worst case scenario (coldest day of the year), is fixed.

With outdoor reset water temperature control – having the water temperature now regulated based on outdoor temperature and in coordination to address changing heat losses of the building, that same thermostat now seemingly performs better. That is mainly due to the fact that its ON and OFF times are now greater and the heat input versus output to this space is more dynamically controlled with water temperature. Outdoor reset water temperature control can get very close to the actual requirements of a building, but cannot consider internal heat gains and additional losses. Nevertheless, outdoor reset alone should be the absolute bare minimum on any radiant system.

Outdoor reset with indoor temperature feedback – Adding indoor temperature feedback to outdoor reset provides even greater accuracy to radiant space temperature control. In this case, a traditional thermostat would be replaced with a sensor or communicating thermostat where it provides internal heat gains and additional losses to a reset water temperature controller, resulting in constant delivery of just the right water temperature to the space. Here, the comfort level is typically maintained with an accuracy of 0.5F.

The user interface and configuration of outdoor reset controllers/devices has become straightforward and manageable for virtually anyone, providing the knowledge of the input requirement is there. Most modern digital controllers requir
e you to input the following three pieces of information in order to setup a heating curve:

• Design Outdoor Temperature (Dsgn Outdoor) which is the coldest outdoor temperature the building’s heat loss is being designed to (e.g., 10F)

• Design Indoor Temperature (Dsgn Indoor) which is the desired Indoor Temperature the building has been designed for on the coldest day of the year (e.g., 70F)

• Design Supply Water Temperature (Dsgn Sup) which is the water temperature required on the coldest day of the year to heat the building to the desired indoor temperature (e.g., 120F)

For non-display controllers/devices, a heating curve/ratio needs to be calculated, as shown in Formula 1.

Contractors who are committed to achieving maximum efficiency and comfort for their clients must understand the ins and outs of radiant system piping strategies, system components and controls. A healthy dose of the keep it simple principle does not hurt either. 

*I very rarely see 2-way injection valves, which is why they are excluded from this article.