HPAC Magazine

What Are Your Options?

Knowledge is power when it comes to distribution piping design.

August 1, 2015   By Mike Miller

Several options are available when approaching residential and commercial distribution piping design. The options are generally: direct return, reverse return or one pipe primary/secondary. Understanding each of them will help you determine which one will provide the best overall results in specific circumstances.

Direct Return
A direct return, two pipe system is the most commonly deployed piping strategy. It has direct piping for supply and return lines to and from the heating terminal unit or zones as shown in Figure 1. The supply and return lines are usually of equal distance. Each branch or terminal unit or zone uses a two-way control or zone valve as a means of enabling or disabling flow to that heating or cooling load.
A proper means of flow balancing should be installed per load in order to achieve adequate flow to each. Fluid will otherwise flow through the shortest distance or the zone offering the least resistance. Flow balancing can be achieved with balancing or globe valves, or better yet, flow setters. If two-way control valves are used, the primary pump is sized for the full building requirement.
A proper means of pressure bypass must be installed to allow for flow to be bypassed when any terminal unit is not calling. If zone pumps are used, each one provides the necessary flow to each load along with the associated pressure drop, therefore balancing may not be needed. The primary piping size is dependant on the loads downstream, so the further down the primary loop you get, the smaller the remaining pipe sizing will be.
Benefits:
• Same supply fluid temperature across all loads
• Easy to follow
Compromises:
• Improper balancing can cause compromised efficiency
• System needs to be balanced (if using two-way control valves)
• Close attention needs to be given during the installation on reduced piping sizes

Reverse Return
A reverse return, two (or three) pipe system is one that has separate piping for supply and return lines to and from the heating terminal unit or zones as shown in Figure 2. Depending on the building structure, this may require a longer return line than that of the supply, as the ultimate requirement being that the first load’s supply is the last return in the system (see Example A, three pipe). Unless, of course, the primary supply and return come in at opposite sides of the building (see Example B, two pipe).
This piping strategy, however, is inherently self-balancing if all loads are equal, or the piping is sized adequately throughout based on the loads. The additional piping can result in savings on mechanical hardware such as balancing and globe valves. The primary piping size is dependant on the loads downstream, meaning that the further down the primary loop you get, the smaller the remaining pipe sizing will be. Note that the inverse is true on the return piping.
Benefits:
• Same supply fluid temperature across all loads
• Self-balancing
• Less mechanical components needed due to self-balancing
Compromises:
• Typically requires more primary loop piping
• Close attention needs to be given during the installation on reduced piping sizes

One Pipe Primary/Secondary
A one pipe primary/secondary distribution system consists of just a single pipe primary loop as shown in Figure 3. All heating terminal units or zones come off through a means of hydraulic separation. Hydraulic separation can be achieved via closely spaced Tees or specially designed fittings. Every load would have a zone pump on it, and its return fluid mixed with the primary loop, causing a cascading of supply fluid temperature throughout the primary loop. The cascading of the primary supply temperature can and should be calculated in order to properly size the zone pumps and terminal units downstream. Note: Upsizing the terminal units downstream is not necessarily required, as the deltaT across the terminal unit can be accommodated for in order to keep the terminal unit size the same (GPM = Btuh/deltaT x 500). This system is also inherently self-balancing as the primary loop is hydraulically separated from the secondary or load loop.
Benefits:
• Self-Balancing;
• One pipe distribution (less pipe)
• Same pipe size throughout primary loop
• Cascading of fluid temperature can improve high efficient heating source by creating larger differential (i.e. condensing boilers)
• Labour/installation savings
• Reduction on primary pump horsepower
Compromise:
• May not be ideal in all applications
Note: Closely spaced Tees require the centre-to-centre measurements to be a maximum of four times the primary pipe diameter.

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CASE IN POINT
The goal in a project involving a five-storey multi-family dwelling with four units per floor was to eliminate any surface piping with the terminal units located inside the closet spaces. Unit layouts from floor to floor were identical, therefore the closet spaces in each unit provided the most ideal location for the installation of the riser distribution piping.
Figure 4 shows this design using the direct return distribution piping method. This particular design would have the supply and return lines brought to the top of the building where they would split into the four risers, one per wing, picking up all apartments in that part of the building in parallel. The primary pump is sized to accommodate the flow for the entire building. Not shown in this drawing is a pressure bypass valve that would be required if the primary pump was constant volume. DeltaT or deltaP pumps, as well as self sensing pumps, could be used instead to regulate flow when not all zones are calling. Please refer to Figure 4‘s table for the make-up of distribution piping needed based on line sizes. Also, note the need for individual balancing valves per zone in order to balance each load.
Figure 5 shows this design using the reverse return distribution piping method. This particular design would only have to supply the top of the building where it would then split into the four risers, one per wing, picking up apartments in that part of the building in parallel. The returns are fed down back into the basement or mechanical room level in order to eliminate the need for a third pipe and to still achieve self balancing of the system by essentially making the fluid travel distance equal for all loads (first supply – last return). The primary pump is sized to accommodate the flow for the entire building.
A pressure bypass valve (not shown) would be required if the primary pump was constant volume. DeltaT or deltaP pumps, as well as self sensing pumps could be used instead to regulate flow when not all zones are calling. Refer to Figure 5 for the make-up of distribution piping needed based on line sizes.  Offsetting the supply and return on opposite sides of the building allowed for less two-inch piping as shown in Figure 5. Note the need for individual balancing valves per zone in order to balance each load.
Figure 6 shows this design using the primary/secondary distribution piping method. This particular design is similar to Figure 5 but picks up all apartments in that part of the building with a form of hydraulic separation. The returns are fed down back into the basement or mechanical room level also. The primary pump is sized to accommodate the flow only with very little resistance in the primary loop. Each load, being hydraulically separated from the primary loop, now has its transformers and zone valves replaced by a small wet rotor circulator. As a result, the primary pump can be significantly smaller than those needed in Figures 4 or 5. DeltaT pumps would be ideal for the
primary pump as it would modulate its flow based on the overall load in the building. In that scenario, a two-way balancing valve is used per riser in order to set the required flow based on full design conditions at the commissioning stage. Constant volume pumps can also be used.
Taking this a step further, a two-way modulating valve operated by a deltaT controller could modulate the flow through each riser based on riser deltaT instead of fixed balanced flow. Self sensing pumps could increase system operating efficiency by reducing the primary pump’s power consumption when the riser modulates down due to reduced building load. Figure 6 shows the piping needed based on line sizes.
Through the slightly larger deltaT (30F) in the distribution piping, additional savings could be picked up by using smaller pipe sizes  for the majority of the building. Even if the deltaT per riser were to be kept at 20F, each riser size would go from the current one- to 1¼ inch.
No single option will be the best fit for all applications but in either of those two scenarios, the primary/secondary distribution piping option could provide not only material but also labour savings.
Determine the best piping option for each project based on the desired overall system design, installation and efficiency outcome. You may find a scenario where it is advantageous to combine a couple of those options within one system. <>

Mike Miller is director of sales, engineered products and systems with Taco Canada Ltd., and current chair of the Canadian Hydronics Council (CHC). He can be reached at hydronicsmike@tacocomfort.com. See Mike at Modern Hydronics-Summit 2015 where he and Steve Goldie will present on this topic. For more information, see www.modernhydronicssummit.com.

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