BY JOHN SIEGENTHALER WHAT S WRONG WITH THIS SCHEMATIC? In the September/October 2005 issue of HPAC Magazine, methods to draw hydronic piping schematics were discussed. These drawings are an efficient way of communicating the intricacies of modern hydronic systems from designer to installer to troubleshooter. Learning to spot design errors in the schematic phase of a project and making the necessary corrections is much easier and far less costly than correcting as built errors. Three of the schematics shown in this article contain serious design errors. All are based on actual installations that have no doubt disappointed the owners of these systems. If you are well versed in hydronic heating and have been following HPAC articles on hydronic heating you may spot the design error(s) in the schematics shown in Figures 1, 3 and 5. Most involve the violation of the basic design principles summarized at the end of the article. If the errors are not as apparent, carefully study the differences between Figures 1 and 2, 3 and 4, and 5 and 6. ACROSS THE HEADERS Take a look at the schematic in Figure 1. It represents part of a $200,000 mechanical system installed in a $6.5-million house. The owners were convinced they were buying top shelf comfort in the form of radiant floor heating. What they got was a heating system with a mind of its own in regards to when and where comfort would exist in this trophy house. After looking at the installation it appeared that the installer s intent was to create a primary/secondary (P/S) system. That is the way the system started out in the mechanical room. However, once the was outside the mechanical room, the secondary circuits were attached across the mains rather than by closely-spaced tees. Perhaps the installer was so used to connecting branch loads across headers in previous installations that he lost faith in the concept of P/S at that decisive moment when tees meet tubing. When you design-as-you-solder sudden impulses to change what you are about to do occur frequently. The piping system in Figure 1 creates a significant pressure differential on all zone circuits whenever the primary circulator is running. In this system that circulator operated whenever the outdoor temperature was below 65F. The flowcheck valves present in each branch circuit could only hold back about 0.3 psi. The pressure differential created across the and return of the branch circuits by the primary circulator was many times higher than this. The result was overheating due to unanticipated flow in nearly all zones. The fact that the heating curve (e.g. reset ratio) on the boiler reset control was set higher than necessary only exacerbated the situation. One way to correct the situation is shown in Figure 2. This approach creates true secondary circuits that are hydraulically isolated from each other as well as the primary circulator. This eliminated the previously described heat migration. Several crossover bridges now connect between the and return mains. The total flow divides up across these bridges based on their individual flow resistance, the same as it would in a twopipe distribution system. Each crossover is supplied with water at about the same temperature. When a given secondary circulator operates, flow is routed from the first of the closely-spaced tees, through the circuit, and back to the downstream tee. No flow develops in secondary circuits in which the circulator is not operating. This approach also allowed the existing installation to be corrected with minimal invasive surgery. Case solved. CONTINUED ON PAGE 26 FIGURE 1 FIGURE 2 zone manifold station parallel isolation valves closely-spaced tees (primary / secondary connection) "primary circulator" "secondary circulator" flow-check higher pressure here lower pressure here partially open ball valve heat input primary circulator zone manifold station heat input NOTE: the zone circuits are connected across the mains flow balancing valve 24 HPAC NOVEMBER/DECEMBER 2005
Circle #18
CONTINUED FROM PAGE 24 REALLY MIXED UP The piping schematic in Figure 3 represents the original installation of a radiant floor heating system in a new 7,000 square foot municipal office building. During its first winter this building could not be maintained above the mid-50s. Rather than a typical 15 to 20F, the temperature difference between the and return ends of some radiant floor circuits was in the range of 60F. Can you spot the cause of this? FIGURE 4 outdoor sensor injection mixing controller zone circulators FIGURE 3 flow-check valve return temp. sensor 3-way thermostatic mixing valve purging valves temp. sensor piping offset between injection tee and sensor to ensure good mixing prior to flow past sensor T T zone circulators One major problem was that the zone circulators were not installed between the mixing valves and the manifold stations. This is why the circuit flow rate was so low and the temperature drop along the circuits was so high. Another design flaw is that the oil-fired boiler has no protection against flue gas condensation. Assuming the circulators were properly relocated upon recognizing the error described above, another problem would instantly show up sustained flue gas condensation within the boiler. Still another issue is that the circulators are all pumping toward rather than away from the location where the expansion tank tees into the system. Hence, the circulators are attempting to suck rather than push water around the system. This often makes it hard to purge the system of air and, given the right circumstance, can lead to pump cavitation. The fix for this system was to remove the individual mixing valve and install a variable speed injection mixing system. The latter can provide fully reset water temperature control to the zone circuits while at the same time protecting the boiler from sustained flue gas condensation. The circulators pump into their respective zone circuits and away from the injection riser location, which is the pressure reference back to the primary loop. Purging valves on all circuits ensures efficient air removal. Interestingly the corrected piping/control design would have been far less expensive than the original installation. Live and learn. WHAT TO WATCH FOR Here is a summary of things to watch out for as you design and document your hydronic systems, as well as when you look over schematics from other sources. Many of them have been discussed in previous articles. Once you have taken your system schematics and the associated calculations through this list the schematic is ready for take off. Always protect conventional boilers from sustained flue gas condensation. Do not create flow bottlenecks. Do not create heat transfer bottlenecks. Use closely spaced tees for primary/secondary piping. Provide purging for all secondary circuits. Provide proper thermal traps for variable speed injection mixing. Control differential pressure when zoning with valves. Provide hydraulic separation of high flow resistance devices. Do not circulate heated water through unfired heat sources. Do not oversize primary circulators. Pump away from expansion tank connection point. Protect against thermal migration due to buoyancy of hot water. Be sure there is flow past temperature sensors when their associated control is active. Be sure to sense the final blended temperature for mixing systems. Do not use gas- or oil-derived heat to maintain a solar storage tank or buffer tank for a wood-fired boiler at an elevated temperature. 26 HPAC NOVEMBER/DECEMBER 2005
This works fine in modest applications, but when the shower stall comes equipped with a seat belt you had better get serious about producing lots of DHW. ALL BOTTLED UP The schematic in Figure 5 might be found in a number of highend custom homes. A multiple boiler system is being used to provide plenty of power for space heating, snowmelting and, FIGURE 5 anti-scald rated tempering valve cold water sensor for boiler staging controller closely spaced tees primary circulator 1/25 hp circulator 3/4" copper presumably, high capacity domestic water heating. Assume the total heat output of the three boilers is 450,000 Btuh. When there is a call for domestic water heating the designer s intent is to focus the full output of the multiple boiler system on the DHW tank, treating it as a priority load over space heating. The designer may have all the necessary control actions for this to take place, but take a look at the piping details secondary circuits (and sizes) used for the DHW subsystem and make note of what you think is a problem. If you can t find anything that appears to be wrong let me ask another question: Do you think that 450,000 Btuh will make it from the boiler plant to the DHW tank through 3/4 copper tubing with the assistance of a 1/25 horsepower (hp) zone CONTINUED ON PAGE 28 Circle #19 NOVEMBER/DECEMBER 2005 HPAC 27
CONTINUED FROM PAGE 27 FIGURE 6 FIGURE 7 circulator? If you do you are going to be very disappointed. This situation represents a severe bottleneck to flow. Even if the tank s internal heat exchanger had a very large surface area, the flow developed through the piping connecting the heat exchanger with the rest of the system would be woefully incapable of ing the necessary flow rate. For example, if we assume the temperature drop across the heat exchanger at design rate of heat transfer was a typical 20F, the necessary flow rate would be: FLOWRATE = BTUH = 450,000 = 45 GPM 500 X T 500 X 20 This flow would be pushing the velocity limits of a two-inch copper tube. It is also well beyond the flow capability of a 1/25 hp zone circulator, even if the piping connecting the tank to the system was two-inch or larger. So why does this happen? The answer is that many installers are accustomed to piping smaller indirect water heaters that handle the load of a typical three-bedroom house with 3/4-inch tubing and a zone circulator. This works fine in modest applications, but when the shower stall comes equipped with a seat belt you had better get serious about 28 HPAC NOVEMBER/DECEMBER 2005 plenty! of pump plenty of pump closely-spaced tees sensor downstream of P/S interface w/ boilers closely-spaced tees sensor downstream of P/S interface w/ boilers check to prevent heat migration plenty of heat exchanger strainer check to prevent! heat migration plenty! of heat exchanger bronze or stainless steel circulator!! anti-scald rated! tempering valve! cold! water! secondary circuits anti-scald rated tempering valve cold water secondary circuits producing lots of DHW. The wet-side corrections for this situation are shown in Figure 6. It includes an upgrade in both pipe size and circulator capacity, as well as a calculation to verify that the DHW heat exchanger can handle the full heat output rate of the boiler plant while working at an acceptable temperature drop. If the latter calculation shows the indirect tank cannot keep up with the boiler plant you will need to go to a properly-sized external heat exchanger as shown in Figure 7. Another error in Figure 5 is the sensor placement for the boiler staging control. If installed as shown, there will be times when the boiler staging control is satisfied that the water temperature leaving the boilers is adequate for the load. When this occurs the boilers and the boiler circulators will be turned off. This stops flow past the sensor. The controller thinks everything is fine based on the temperature of the sensor and does not fire the boilers or operate the boiler circulators. Meanwhile the loads may be screaming for heat. The correction is to place the sensor downstream of the closely-spaced tees that interface the boiler piping to the load piping as shown in Figures 6 and 7. Do these details right and you will have one mean hot water making machine. AN INVITATION If you have run across a bad system design (and few who have been in this industry for any length of time have not), send it to me via e-mail. Perhaps there will be another round of these challenged hydronic system designs in the future. No names or identifiable specifics will be used, just fundamental design information that can help others avoid a similar problem down the road. John Siegenthaler, P.E. is the author of Modern Hydronic Heating. Visit his website at www.hydronicpros.com for reference information and software to assist in hydronic system design. He can be reached at john@hydronicpros.com. RATE THE ARTICLE! Will this information be useful? Please circle the appropriate number on the Reader Postcard. Thank you. VERY USEFUL.................... 106 USEFUL....................... 107 NOT USEFUL.................... 108