Primary/Secondary Piping

Variations of Primary/Secondary Boiler Piping Ideas

Note :All drawings are conceptual drawings for illustration purposes and may be incomplete. More specific drawings may be required for your application. Refer to manufacturer's installation manuals

The way a high efficiency boiler is piped is determined by the application. Do you have different water temperature zones or are they all the same temperature? Do you have an indirect water heater? Many things need to be considered when considering the laying out the boiler piping. While the cast iron boilers are very forgiving the new high efficiencies are not. I have seen many homeowners install cast iron boilers and they did a good job, and many were very bad jobs. High Efficiency boilers should be left up to the professionals.

The drawings below are all piped differently. Why? It is dependent on flow rates, how your zoning, or indirect hot water heater or not for domestic hot water.

Drawings #1 and #2 - Is a system using circulators or zone valves# for zoning. All zones should be the same water temperature. 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- Zoning with circulators

Fig. 2 - Zoning with Zone Valves

Drawing #3 - Is named a boiler primary loop with system circulators. This means a pipe leaves the boiler supply taping, goes around a loop, and enters the boiler return taping. All the zones come off the closely spaced tees from the side of the tees. The closely spaced tees separate the flows between the boiler flow, the system flow, which is known as hydraulic separation. The temperature of the primary loop is affected by the running zones. The first zone gets hottest water. When the additional zones are running along with the first zone they will get cooler water. This is OK if the zones require lower water temperatures as we go around the loop. In this drawing the second baseboard zone will receive water that may be too cool to heat the zone properly. The rule is the shortest hottest zone first and the coolest longest zone last. Remember a mixing valve may still be required on zones that may receive the cooler water such as an in-floor radiant system in a concrete slab.

Fig. 3 - Boiler Primary Loop All zones drop in temperature

Drawing #4 and #5 - Is the same drawing as #3 except multiple zones on the first set of close spaced tees. This is a way to get the same temperature to the zones if zoned by pumps or zone valves, piped on this small manifold. The reason for this is the zones 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 or all the zones off this manifold are running.

Fig. - 4 - Same temperature zone manifold, zoned with pumps

Fig. 5 - Same temperature zone manifold, zoned with zone valves

Drawing #6 & #7 - This are similar to Fig. #1 and Fig. #2 but we added an Indirect Water Heater (IWH) shown with circulators and zone valves. Note, the drawing with zone valves still shows the IWH with a circulator. I believe that is the best way to maximize the hot water recovery in the tank. These drawings were popular when high efficiency boilers were first introduced into the USA. Since than IWH piping has changed with many manufacturers.
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 flow. The problem I have with these drawings is the flow requirement for the IHW tank and the boiler may var yto much.
If the flow in the IWH is less than required the recovery will be really slow. If the boiler flow is too low the boiler water could flash into steam or cause damage to the boiler. The boiler will attempt to modulate down but may not be able to modulate down fast enough.

Fig. 6 - IWH connected to boiler piping, zoned w/pumps

Fig. 7 - IWH connected to boiler piping, zoned w/pump, heating zones w/zone valves

Drawing #8 and #9 - Is another variation of drawing Fig. #6 and Fig. #7. The difference here is if the known flow through the boiler and indirect is very different. With the close spaced tees, you setup hydraulic separation and the flow of the indirect will not affect the flow of the boiler. Each appliance will receive their proper flow.

Fig. 8 - IWH piped on System Piping, all zoned w-pumps

Fig. 9 - IWH piped on system piping, Heat zones w-zone valves, IWH w-pumps

Drawing #10 and #11 - These drawings are showing a boiler primary piping again. It is like drawings Fig. #4 and Fig. #5 except with an IWH. The hydraulic separation allows the flow to be different between the boiler and indirect water heater. I am also showing a mix of circulators and zone valves. The IWH is first to get the hottest water of not doing DHW priority which I would suggest with this type of piping.
The zone valves are on a couple of high temperature zones which both have the same temperature. The second set of closely spaced tees are going to be a cooler hydro-air zone. The last set of closely spaced tees is going to be the coolest zone, a radiant heat floor zone. It is not uncommon to add a mixing device for a radiant zone. It will probably have a mixing valve or injection to control the temperature of the water below a maximum level. All the heat zones will shut down on a call for domestic hot water if wired to do so. Just because the boiler control shows DHW priority is on it may not shut down the heat zones if heat zones pump control is not controlled by the boiler.

Fig. 10 - Boiler primary loop, IWH first zone, heat zones pumps

Fig. 11 - Boiler primary loop, IWH first zone, heat zone valves

Drawings #12 and #13 - Drawing #12 is my go to drawing for retrofitting a high efficiency boiler into an existing system and adding an IDW. This drawing you may or may not use DHW priority especially if the boiler is oversized. The drawing can be used with circulators and zone valves.
As a side note all these drawings on this page can also be applied to a cast iron boiler installation if you like.

Fig. 12 - IWH piped P/S on boiler supply, zoned w-pumps

Fig. 13 - Boiler primary loop, IWH first zone, heat zoned w-valves

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 differences 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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