What would you think of a fellow engineer who suggested the use of 3/4-in. tubing to carry 250,000 Btuh from a boiler to a large floor heating manifold station located 200 feet from the mechanical room? You might do some quick math using an old rule of thumb that equates 10,000 Btuh of heat transfer to each gpm of water flow. Based on this you'd probably conclude there's no way a 3/4-in. tube could carry the necessary 25 gal/min flow. Perhaps you'd then tell your colleague to think in terms of 2-in. rather than 3/4-in. piping.

Rules of thumb are good for conceptualizing a design, provided all the assumptions they are based on are appropriate to the situation. In this case, equating 10,000 Btuh with each gpm of flow is based on an assumed temperature drop of 20 degrees F between the supply and return piping. However, who says that all hydronic distribution systems must operate with a 20 degrees temperature drop?

A large Delta T can be useful

Formula 1 can be used to find the flow rate necessary to transport a given rate of heat flow for various hydronic system fluids and "target" temperature drops between the supply and return piping:

Formula 1

f = Q

k(Delta T)

Where:

f = required water flow rate (gpm)

Q = heat transport rate (Btuh)

Delta T = temperature drop between supply and return piping ( degreesF)

k = 490 for water, 479 for 30% glycol, 450 for 50% glycol

Let's apply this formula to a distribution circuit supplying 250,000 Btuh to a manifold station in a large floor heating system. Assume the supply water temperature to the floor circuits at design load is 110 degrees, and the design temperature drop across the manifold station is 20 degrees. The supply water temperature is created by blending hot boiler water with cooler return water at a mixing device in the mechanical room. The flow needed to transport 250,000 Btuh to the manifold station is:

f = Q = 250,000 = 25.5 gpm

k(Delta T) 490 (20)

This flow requires 2-in. piping between the mechanical room and manifold station if the flow velocity is not to exceed 4 feet per second (fps).

Now suppose that the mixing took place at the manifold station rather than the mechanical room. Assume 180 degrees water is sent to the manifold station to be mixed with the 90 degrees return water. The temperature drop between the piping leaving and returning to the mechanical room would now be 180-90 = 90 degrees, or 4.5 times greater than in the previous calculation. Formula 1 can again be used to determine the flow rate necessary under these operating conditions.

f = Q = 250,000 = 5.7 gpm

k(Delta T) 490 (20)

The flow rate has dropped to about 22% of what was required in the first calculation; 5.7 gpm could be handled by a 3/4-in. pipe with the flow velocity remaining under 4 fps. This tradeoff between temperature drop and flow rate has profound implications that can lower cost and improve control if recognized and "exploited."

Minitube distribution systems

Figure 1 shows a piping schematic for a minitube system designed to take advantage of the large temperature difference available between hot boiler water and the relatively cool water returning from a heated slab-on-grade floor.

Hot water from the primary loop is pulled into the supply minitube at a flow rate dependent on the speed of the injection pump. The speed of this wet-rotor PSC-motor pump is regulated by a variable-speed (VS) injection mixing controller. The hot water is carried to the manifold station through the insulated supply minitube and injected into the tee marked (A). Here it mixes with some of the cooler water returning from the floor circuits. The mixed stream flows past the supply temperature sensor for the mixing controller, and on to the floor circuits.

The faster the injection pump runs, the greater the proportion of hot boiler water entering the mix point, and the higher the temperature supplied to the floor circuits. Heat input to the manifold station can be regulated from zero, (when the injection pump is off), to full design load (when the injection pump is running at full speed).

Notice that both the supply and return minitubes are coupled to a primary loop as well as the manifold station piping using pairs of closely spaced tees. These create a primary/secondary interface that prevents the pressure distribution in the minitube subsystem from being influenced by the pressure distribution is in either the primary loop or the manifold station.

To select an appropriate injection pump, first determine the required flow rate through the minitube circuit at design load. Next, determine the head loss of that circuit. If smooth tubing such as copper, or PEX-AL-PEX is used for the minitubes, the head loss can be estimated using the formula and data in Figure 2. Finally, select a circulator with a pump curve passing through or just slightly above the required operating point.

MultiZone minitube systems

When planning a floor heating system for a large commercial building, it's often necessary to locate manifold stations at considerable distances from the mechanical room. Such situations are perfect for a multiple zone minitube system. Each manifold station is paired with its own VS injection mixing system and minitube piping. The mixing subassemblies share a common primary loop as their source of hot water.

One piping approach for a three-zone system using a multiple boiler array is shown in Figure 3. The primary loop has been split into three parallel crossovers. The closely spaced tees in each crossover are the beginning and ending points for each minitube circuit. This allows the same water temperature to be available to each injection pump. It also eliminates any flow interference between these pumps. Finally, it allows crossover flows to be balanced in proportion to the loads served.

Another possibility is the primary/secondary low loss header assembly shown in Figure 4. This arrangement provides the necessary isolation between the primary loop and the minitube circuits while ensuring the same supply water temperature to each injection pump. If the secondary header piping is generously sized and relatively short, there will be insignificant interaction between the injection pumps. However, because the injection pumps are in parallel, it is necessary to install a swing check valve in each minitube circuit.

Some necessary details

The manifold circulators in a minitube system should be operated continuously during the heating season. This allows for constant flow past the supply sensor for the injection mixing controls. With the injection mixing control(s) configured for outdoor reset control, the water temperature supplied to each manifold will be just warm enough for the prevailing heating load.

If internal heat gains are present in a given zone, the injection control may totally stop the injection pump for a time. During this time, it's important to prevent hot water from slowly migrating through the supply minitube and potentially entering the mixing point in the continuous flow manifold system.

One approach is to use an injection pump with an integral spring-loaded check valve. Several models are currently available from at least two major pump manufacturers. Flow-check valves or external spring-loaded check valves are not recommended for this duty because they can create chattering sounds. Another approach is to ensure that the closely spaced tees connecting the minitube to the manifold piping are at least 18 in. lower in elevation than those connecting the minitubes to the primary loop. This drop in elevation creates a thermal trap that discourages heat migration.

When a conventional boiler is used as the heat source, it's important to ensure it operates at a temperature high enough to prevent sustained flue gas condensation. This is especially true when the load is a large, concrete floor slab with very high thermal mass. Most VS injection controllers currently available have the logic necessary to monitor the boiler inlet temperature and slow the injection pump when necessary to prevent the load from extracting heat faster than the boiler is generating it.

Minitube benefits

The minitube approach offers several benefits relative to other methods of supplying remotely located manifold stations:

It uses much smaller piping between the mechanical room and manifold station. Besides saving on installation cost, smaller tubing is also easier to route and support. It reduces system volume, which in turn may reduce expansion tank size, and the volume of any antifreeze used.

If desired, each manifold station can operate at a different supply water temperature or outdoor reset schedule. This flexibility is very useful if different areas of the building have different floor coverings or widely varying load characteristics.

Minitube systems allow the distribution circulator at each manifold station to operate continuously during the heating season. Besides helping to provide stable supply water temperature, constant flow through floor circuits has many benefits in larger buildings. It transports stored heat from internal areas of the slab to localized "heat sinks" such as areas just inside large overhead doors, or the wet floor under a vehicle shedding snow. Should the boiler be inoperable, continuous circulation can help delay freezing in these high loss areas.

Minitube systems improve the rangeability of VS injection mixing systems. When VS injection mixing is done using short injection risers, a throttling valve must be installed in the return riser to remove a significant portion of the pump head. This is necessary to force the circulator to operate at full speed under design load conditions, thus using the full range of speed control.

In a minitube system, the vast majority of the head loss occurs as water flows through the minitube piping. When the head loss of the minitube circuit at design flow rate equals the head of the injection pump at full speed and the same flow rate, it is not necessary to install a balancing valve in the minitube circuit.

In some minitube systems it may be possible to use a smaller manifold circulator. Since it isn't responsible for flow between the mechanical room and manifold station, the size of the circulator may be reduced. This saves in both initial cost, and more importantly, in lifecycle operating cost.

The small minitube piping loses less heat to the surrounding air than would larger diameter distribution piping of equivalent heat transport capacity. Still, prudence dictates that the supply minitube must be insulated to minimize heat loss. The return minitube can also be insulated, although energy savings will be minimal given the small surface area and low operating temperature. Obviously, any tubing passing through unheated space should be insulated.

The minitube concept is highly scalable. A small system using 1/2-in. or even 3/8-in. minitube piping may be ideal for larger residential space heating and/or snowmelting applications. It is also well suited for retrofit applications given the ease of routing small tubing from the mechanical room to distant manifolds. Larger systems using 1.5- to 2-in. minitube piping are capable of transporting several million Btuh from a boiler plant to a low temperature load.

Such systems are well suited to large agricultural floor heating installations such as multiple hog or poultry barns supplied from a central boiler plant. Large snowmelting systems using conventional boilers are also good candidates for minitube distribution systems. The cost saving relative to traditional distribution piping increases with the size of the system.

Field experience

The author and others have successfully deployed minitube systems in many buildings such as municipal highway garages, churches, and large industrial buildings.

The largest known application to date is a 170,000-sq-ft manufacturing facility in upstate New York with 13 independently controlled minitube-supplied manifold stations. An example of one of the large manifold stations used in this facility is shown (partially completed) in Figure 5. Notice the 1-in. copper supply and return minitubes at the top of the manifold, which at design load delivers 500,000 Btuh to the floor circuits.

In one 70,000-sq-ft facility, the decision to use the minitube system saved an estimated $11,000 in piping cost over a more traditional (mix-in-the-mechanical-room) system. It also provided the ability to operate each manifold at a different water temperature if ever necessary.

Summary

Minitube distribution systems demonstrate the basic thermodynamics of how flow rate can be traded off against temperature drop. The resulting system is less expensive to install and more versatile in terms of control. The approach lends itself to systems where low temperature manifold stations are to be located at substantial distances from the mechanical room. Keep it in mind when your next big radiant floor heating or snowmelting system comes along. ES