When you operate a fleet of 32 delivery trucks year-round in upstate New York, you have some decisions to make regarding how to start those diesel engines when the outside temperature is -15

Issue: 7/04

When you operate a fleet of 32 delivery trucks year-round in upstate New York, you have some decisions to make. One is how you would expect to start all those diesel engines when the outside temperature is -15

Figure 1. The new 23,000-sq.-ft. garage at McCraith Beverages.
These costs certainly played a role in their decision to build a new 23,000-sq.-ft. vehicle storage and maintenance garage in late 2003 (Figure 1). They wanted to maintain the trucks at 50-55

Equation 1

Small Tubes Delivering Big Performance

The underlying concept in a minitube distribution system is transporting relatively high temperature water from the mechanical room to the location of the manifold stations, and then mix as necessary to supply the floor circuits.

This approach takes advantage of the large temperature difference between the water sent to the manifold station(s) and the water returning to the mechanical room.

At design load conditions, a typical minitube distribution system supplies water at 170-200

Example using Equation 1
For example: Assume the requirement is to move 250,000 Btu/hr to a large manifold station under design load conditions. Hot water is available from the primary loop at 180

Second example
If, on the other hand, the mixing were done in the mechanical room, the required flow rate to the manifold station would be based on the temperature drop of the floor circuits, as shown at the right.

This flow would require a 2-inch tube set between the mechanical room and manifold station. The difference in installed cost of between these tube sets over a total distance of 1,000 feet or more would be substantial.

Figure 2. A single-zone minitube system.
Minitube systems use injection mixing for water temperature control. In the McCraith garage, the injection mixing is regulated by a small circulator operated by a variable speed controller. The faster this injection pump runs, the faster the rate of hot water input to the mixing point, and the warmer the supply temperature to the floor circuits. The basic concept of a single-zone minitube system is shown in Figure 2.

Notice that the minitube piping connects to the parallel primary loop as well as the manifold station piping using a pair of closely spaced tees. This detail eliminates interference between the pressure differential created by the injection pump and those created by the primary loop circulator and manifold station circulator.

The minitube system at the McCraith garage has four individually controlled manifold stations, each serving a different zone of the building. The basic system schematic is shown in Figure 3 (Editor's Note: See print issue for Fig. 3).

Figure 4. The four injection mixing controllers each operate an associated injection pump. The larger blue controller operates the multiple boiler system.
Each injection pump draws hot water (at the same temperature) from the parallel primary loop. Each injection pump is independently operated by its own variable speed controller (Figure 4). This allows the possibility of operating each manifold station at different temperatures if necessary. Although this was not needed at this time in the McCraith garage, it could be very desirable in other buildings due to changes in loads or floor coverings. Each injection controller also monitors the inlet temperature to the boiler system and adjusts injection flow as necessary to prevent the boilers from operating with sustained flue gas condensation.

Figure 5. Each waste oil boiler operates as a stage. Total capacity is 700,000 Btu/hr.

Black Gold

The heat source for the system is a pair of waste oil boilers, each having a rated output of 350,000 Btu/hr (Figure 5). A microprocessor-based staging control operates both the burners and circulators on each boiler. Each boiler circulator operates for a short time before the burner is fired, and continues to run for a few minutes after the burner is turned off to purge residual heat from its associated boiler.

Figure 6. Small circulators operate at variable speed to regulate injection flow to each of four manifold stations. Parallel crossover pipes supply same-water temperature to each injection system.
The boilers supply a hybrid primary loop. This loop has upstream takeoffs for a high capacity indirect DHW heater, which operates as a priority load. The primary loop then splits into four parallel crossovers, each with its own balancing valve (Figure 6). Within each crossover is a pair of closely spaced tees for the supply and return connections to the minitube circuits. The upstream tee on each crossover leads to the associated injection pump. After the pump, the 3/4-inch or 1-inch copper minitubes head out to the manifold stations via an overhead strut/rail hanger system.

The return minitube from each manifold station returns to the mechanical room and passes through a flow metering balancing valve before connecting back to the crossover pipe. The balancing valve allows the resistance of the minitube circuit to be adjusted so the injection pump runs at full speed under design load conditions. This maximizes the rangeability of the injection system for precise temperature control.

The manifold stations use standard cast brass components. The two large manifold stations have design loads of approximately 260,000 Btu/hr each, and are supplied by 1-inch copper minitubes. Two smaller manifold stations serve a wash bay and maintenance bay at about 90,000 Btu/hr each, and are supplied by 3/4-inch minitubes. All minitubes are insulated between the mechanical room and manifold stations.

Each manifold station has its own distribution circulator. The circulators are automatically turned on when the outdoor temperature drops below 60

The staff at Arcuri Plumbing & Air Conditioning: (from left) Frank Scharf, Ron Roth, and Rick Arcuri. Not pictured: Harvey Youker.

The Benefits Continue

Besides saving on material and installation cost, the small minitubes reduce system volume. This, in turn, may reduce the size of the system's expansion tank. It also reduces the volume of antifreeze in systems that require it.

Because the distribution circulators only handle the flow and head loss of the manifold stations, and not that of the piping between the manifolds and mechanical room, it may be possible to use smaller circulators.

The small minitubes also reduce piping heat loss between the mechanical room and manifold stations relative to that of larger piping operating at lower temperature drops.

The McCraith garage system has been in operation since February 2004. It was brought online during one of the coldest winters in recent history in upstate New York, and has already proven it can deliver the heat precisely when and where it's needed.

If you design floor heating systems for larger buildings where the manifold stations may be a considerable distance from the mechanical room, a minitube distribution system may be just what you need to reduce cost, improve control and speed installation.

Modern Hydronics Seminars Announced

John Siegenthaler will discuss the design details for minitube systems as well as other contemporary topics in hydronic heating during a full-day seminar entitled, "Modern Engineering Concepts for Hydronic Heating Design." These seminars will be offered in six locations during July. For more information and to register for the seminars, visit www.bnpmedia.com/events.htm#mec.