Case Studies: Primary Loop Installations

(Note: To view the figures, read Bristol's column in the July digital edition of PHC News!)

In this column, our focus is on Solar Thermal Hydronic Combisystems that typically include multiple sources of heat and multiple heat loads. After designing countless numbers of these combisystems over the years, I have developed a standardized piping configuration that I now use as the skeleton for each new combisystem that we create. I have often referred to this method of design and control as the New Standard hydronic combisystem. 

Figure 84-1 shows a block diagram of how a typical New Standard piping system is put together. As you can see, it is based on a familiar Primary Loop Configuration where Secondary Loops are attached using closely spaced tees to allow heat sources and heat loads to be plugged in with two pipe connections in a consistent and systematic way. 

This block diagram and the other piping schematics that have appeared in this column may seem to impose rigid limits over what the builder or mechanical installer can do with a hydronic heating system. But, in fact, we have found the opposite to be true. The primary loop configuration can be used to connect just about any heat source to any heat load, but in an orderly and systematic way that can then be controlled with pre-existing control methods and software. Here are a few recent examples from the field that demonstrate the design flexibility and how it is adapted from one installation to the next.

A wall-hung mini check-loop

Figure 84-2 shows one installer’s interpretation of the Primary Check-Loop, described in this column a number of times in the past. This is perhaps the most compact version of a Primary Loop in captivity. Using the check-loop approach, the primary pump has been eliminated by placing swing check valves between the ‘closely spaced tees.’ This allows any secondary pump to act as a primary loop pump by forcing the flow from any secondary to proceed all the way around the primary loop, since it cannot pass backwards through the check valve. When more than one circulator pump turns on, the swing checks can open to allow the flow from any or all of the pumps to circulate around the primary loop without interfering with each other. So, the check loop acts as a flow separator or a flow center but with one-way directional flow.

A stainless steel flat-plate heat exchanger is used to complete the right hand side of primary loop, which circulates through the water side of the heat exchanger. The glycol side of the heat exchanger is pumped by a circulator built into a Solar Pump Station, which adds to the compact and modular appeal of this installation.

Although the installation was not completed in this photo, we can see that the temperature-order around the primary loop appears to be correct. The flow around the primary loop is clockwise, and in this installation the flow appears to go from the heat exchanger, to the boiler secondary, next to the DHW Indirect secondary, then to higher temperature baseboards, finally to radiant floors (with a tempering valve) and then back to the heat exchanger.  

The horizontal loop near the ceiling

The photo in Figure 84-3 shows a Solar Hydronic Combisystem that was recently installed in a new house in New Mexico. This is fairly typical of a whole-house solar hydronic heating system in a residence, when following the New Standard design method. The solar heat comes into the system through the stainless steel heat exchanger seen in the photo above the boiler. Just as in the previous system, the heat exchanger is used to complete the primary loop, which flows through the water side similar to the detail seen in the block diagram above.  

The primary loop hangs from the ceiling horizontally, and extends around the mechanical room far enough to easily connect to each heat source and heat load.  The equipment in the room is arranged so that it can be easily connected to the primary loop in the correct temperature order with a pair of secondary pipes that drop down from above. In this configuration, the main air separator can be mounted on the primary loop which serves as the highest point in the plumbing for positive air removal.

If you look closely at the photo, you can see a Solar Pump Station module, used here for rapid installation of the solar plumbing components connected to the solar heat collectors on the roof. The blue tank is an indirect tank for DHW with an internal heat exchanger coil used for both boiler and solar hot water. There is no other heat storage water tank in this system, since most of the solar space heat is accumulated and distributed using the heat storage capacity of the concrete radiant floors. The modular integrated control system can be seen on the wall to the left, the SolarLogic Integrated Control (SLIC), which among other things, controls the heat storage in the radiant floors throughout the house within a carefully maintained range of comfort thereby eliminating the need for a large solar heat storage tank.

A wall-hung extended primary loop

Figure 84-4 shows yet another clever interpretation of the New Standard piping configuration in a recent major remodel in a home in Colorado. The mechanical room is in a basement, and the radiant tubing from the different heating zones throughout the house terminate at various locations across the basement wall. The installer extended the primary loop across the middle of the wall where connections to the multiple zone manifolds could be most easily accomplished. You can see in the photo that the primary loop was reduced to a pair of pipes running across the wall and connected with a 180-degree elbow that makes a U-turn. The solar glycol loop was done the same way, mounted behind the main primary loop.  
If you look closely in the photo, you can see a solar pump station, a primary pump, four heating zone manifolds with pumps and mixing valves, a water expansion tank, a glycol expansion tank, a rectangular heat exchanger and an indirect DHW tank.  The edge of the SLIC control system box is just out of the frame to the right. In other words, many of the same components as found in the other standard systems, but arranged to fit the building, its size and geometry.

This method of extending the primary loop to connect remote components together has been used on other jobs to even greater extremes.  n some jobs where there are multiple buildings, we have extended the primary loop as a pair of pipes insulated and underground from building to building. So, the same basic piping configuration is just as useful in a small building as it is in a small district heating system using the same design procedure and control scheme.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit for more information.

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