The way a Mod/Con (modulating/Condensing 90%+ Efficiency) boiler is piped is determined by the application. Do you have different temperature zones or are they all the same temperature, an indirect water heater or not? Many things come into the thought process of boiler piping. While the cast iron boilers are very forgiving the new mod/cons are not. I have seen many home owners install cast iron boilers and they did a good job, and many very bad jobs. Mod/cons should be left up to the professionals.

I have 9 drawings on this page which are all piped differently. Why? It is dependent on flow rates, zones domestic hot water demands etc. The drawings below are simple drawings not showing all the valves etc required.

Drawings  #1 & #2 - Is a system using circulators (#1) or zone valves (#2) for zoning. All zones are the same water temperature and they have an indirect water heater. The indirect could be removed if not included. This is just removing the old boiler, connecting the supply and return manifolds together and installing a set of closely spaced tees for the boiler piping.

Fig 1             Click to enlarge               Fig 2

Drawing # 3 - Is a boiler primary loop with system circulators for zoning and an indirect. The closely spaced tees separate the flows between the boiler, the system and indirect circulators, which is known as hydraulic separation. The temperature of the primary loop is affected by the running zones. If all zones are on except the indirect water heater every zone after the first gets cooler water. This is OK,  if the zones require different water temperatures. The rule is hottest zone first coolest zone last. Otherwise see drawing # 4. The indirect should be prioritized or first off the loop. I show the indirect last due to being a priority zone. When the indirect is a priority zone all other heating zones shut down when the indirect calls. A better way of piping the indirect would be Picture # 8 where everything is the same except indirect piping.

Fig 3       Click to Enlarge        Fig 4          

Drawing # 4 - Is the same drawing as # 3 except I am showing system zone valves on a manifold connecting to the primary loop with closely spaced tees. The reason for this is the zones (could be made with circulators also) (See # 7) are all the same water temperature. If they are both baseboard they want the same water temperature. Anything beyond the first set of close spaced tees will get cooler water if any of the zones off this manifold are running.

Fig 3        Click to Enlarge        Fig 4                    Fig 7

Drawings # 5 & # 6 - These are similar except one is with circulators and the other has zone valves. These drawings which are very similar to drawing # 1 & # 2 except the indirect water heater is piped off the primary pipe, not the boiler piping. Why would the indirect water heater be piped off the primary instead of the boiler piping. One word "flow". Two very important things in the hydronics industry is Flow and Resistance to flow (pressure drop). Everything has a pressure drop, piping, fittings, valves, boilers, coils, you name it. If the water passes through it, it creates a pressure drop or resistance to slow down water. In drawing # 5 let's assume the boiler requires a minimum flow of 10 gpm. The indirect water heater requires a maximum of 6 gpm. The circulator for the boiler is sized for a 10 gpm flow, the indirect water heater has a circulator sized for a 6 gpm flow. With 4 gpm difference in flow you cannot pipe the indirect tank to the boiler piping. If you send too much water through the indirect coil you transfer less heat to the water in the tank. This slows down recovery of domestic hot water. The only time the indirect water heater may be piped off the boiler piping is when the flow through each are similar. Although the wiring is easier when the indirect is piped to the boiler piping it may not give good recovery in the indirect. Most boiler replacements are going to be piped as drawing # 5 & # 6 dependent if there is circulators or zone valves.

Fig 1      Click to enlarge           Fig 2                                Fig 5                       Fig 6

Drawing # 7 - Is another variation of drawing # 4. The difference here is if the flow through the boiler and indirect is similar you will not have to operate the primary circulator. This will save a little electricity and money.

Fig 4    Click to enlarge    Fig 7

Drawing # 8 - This is the same drawing as # 7 except the indirect is moved closer to the boiler and I show more zones connected. Remember the shortest hottest to the longest coolest.

Fig 7    Click to Enlarge     Fig 8

Drawing # 9 - This drawing is also showing primary/secondary piping.  It is like # 7 and # 8 with the exception the boiler secondary loop is tied in with closely spaced tee's. The hydraulic separation allows the flow to be different between the boiler and indirect water heater. I am also showing a mix of zone valves and circulators. The zone valves are on a hot temperature zone which both have the same temperature. The second set of closely spaced tees are going to be a cooler zone, maybe a hydro-air zone. The last set of closely spaced tees is going to be the coolest zone. It will probably have a mixing valve or injection to control the temperature of the water below a maximum level. These will all shut down on a call for domestic hot water.

Fig 7    Click to Enlarge     Fig 8                              Fig 9

Mod/con boiler piping is not limited to these drawings. There are more variations. As I said above flow and pressure drop is important. Water is best moving at a velocity of about 2 ft per second. Too slow or too fast you will reduce heat transfer and may create piping noise.

One of the biggest difference with the drawings is what circulators should be running. Is the boiler circulator supposed to be on or off, is the primary circulator running or not? Hmmmmmmmm!!!!! Maybe best left to the professional.

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