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All About Hydronic Multiple Boiler Systems

While Climatic Control Company does not, at this time, actually design hydronic systems; we do deal with the control systems and the people that service and design them.  Having knowledge of good hydronic design principles can come in very handy when repairing or upgrading a hydronic system.  You can speak intelligently about a system, enabling you to solve a problem or problems. 

Let’s consider a typical hydronic multiple boiler, primary/secondary system, as found in many small to medium size commercial buildings such as hospitals, churches, nursing homes, office buildings, even large residences.  These systems consist of three major parts:


  1. The boilers; heat generators
  2. Primary loop; a heat transport system
  3. Radiators; heat distributors


Boilers are sized for worst-case conditions.  If the heat loss calculations are correct, a boiler will run continuously at design day conditions.  “Design day” conditions will probably be reached on only two, maybe three days a year.  If the boiler runs continuously on more than just the “design days”, it will be very inefficient.  It makes no sense to have one big boiler generate its maximum output on warmer than design days.

To overcome this problem, gas-fired boilers are available with modulated firing rates, even small residential boilers as small as 45,000 BTU/Hr input.  They are very expensive, and should the boiler fail and need repair, no heat is available until the boiler is fixed.  This could be a disastrous situation if the repair takes “too long.”  The building could “freeze up,” resulting in broken plumbing, loss of income, etc.

By splitting the load between two or three boilers, piped in primary/secondary systems, we have built in a stand-by feature, and still generate just the amount of heat needed to match the building’s heat loss at any given moment.  The chances of all the boilers needing repair on the same day are extremely remote.   Comfort, economy, and peace of mind are attained.

By splitting the load, we recognize that not every day is the coldest day of the year.  On an “average” winter day, probably one boiler can heat the building.  Chances are it will run longer than a single, large boiler.  By splitting the load, we recognize that not every day is the coldest day of the year.  On an “average” winter day, probably one boiler can heat the building.  Chances are it will run longer than a single large boiler, improving overall operating efficiency, thereby reducing fuel usage.  As the weather gets colder, the second boiler will come on, but only on the really frigid days.  In addition, by piping the boilers in a primary/secondary system, no water will flow through an “off” boiler, reducing heat losses through the stack and boiler jacket of the off boiler.  It’s as if the off boiler were valved off from the rest of the system, even though it isn’t.

Small commercial buildings that can utilize these systems abound:  churches, schools, convenience stores, etc., even large residences, will benefit from these systems.

The load can be split to use more than two boilers.  However, in buildings where the design load is 1,000,000 BTU/Hr or less, the economic return using three or more boilers is so small it doesn’t justify the extra installed expense.  (Over a million BTU’s three boilers may return the extra expense, but seldom would four boilers.  Careful calculations to figure the payback on systems of four or more boilers should be made.  Since, in this Info-Tec, we are dealing with buildings in the 400,000 to 1,000,000 BTU/Hr range, we will concentrate on two boiler systems, the most economical to install and operate.)

Multiple boiler primary/secondary systems are comparatively small in size.  They can be easily installed in buildings during remodeling or in new construction.  They are easy to pipe.  Usually two (even three) boilers will fit in the same space an old cast-iron or steel tube boiler occupied.  Labor costs will be decreased in handling smaller, lower weight boilers.  Small boilers will fit through most doors, making them perfect for retrofit jobs.

The boilers in a primary/secondary system are the “heat generators.”  They inject heat into the primary flow system, but the boilers themselves are on a secondary loop.  Consequently, one need only to size the boilers circulator and piping to meet each boilers needs only.  By using the primary/secondary system, the circulator will usually be an off-the-shelf in-line pump, and the boiler piping will be much smaller than needed for one large boiler.

Figure 1 is a “rule of thumb” chart for a typical boiler.  The 25°F is based on using 25°F as the system temperature drop, or put another way, 25°F is the temperature rise through the boiler.  It is always best to check the actual boiler manufacturer’s specifications, but for illustrative purposes, Figure 1 is typical.

Figure 1.

Figure 2 shows the basic primary/secondary system. 

Note:   Always keep the boiler supply and return lines about six inches apart where they connect to the primary loop.  Never more than a foot apart!  (See Info-Tec No. 36).  Piped that way, no water will flow through the “off” boiler when its pump is not running.   


Note:   Always pump down into the boiler with its secondary pump, away from the common primary piping. 

Figure 2.

Regardless of how many boilers are used, use only one connection to the primary loop for the compression tank.  If the system is big enough for multiple compression tanks, manifold the tanks together, but still only connect at one single point in the primary loop.

The compression tank is the “point of no pressure change” in a closed hydronic system.  It’s the one place a circulator’s differential pressure can’t affect.  If you pump away from the compression tank, the pump will add its pressure differential to the systems fill pressure.  If you pump toward the tank, the pump will remove its pressure differential from the fill pressure.  Air is always in the systems water, and if the pump drops the system pressure, the air comes out of solution and forms bubbles (think of a bottle of soda, when you open the cap the drop in pressure releases the dissolved Carbon Dioxide). 

Note: To avoid air problems — always pump away from the compression tank!

That’s another reason to always have the secondary boiler circulating pumps pump away from the primary loop.  The secondary pumps use the primary loop as their compression tank.  Also, always bring feed water into the point the compression tank connects to the system.  It’s the only place in the system where the pressure can’t change due to the circulators.  Therefore, the feed valve will get a true reading of what’s going on in the system.

Primary Loop

Now, let’s look at that “primary loop.”  The primary loop is the transportation system for the heat.  It carries the heat from the boilers to the radiators.

When zone circulators draw heat out of the primary loop, the boilers turn on and put heat back into the primary loop.  In this way, the primary loop acts as an extension of the boilers.

The primary loop circulator is run continuously during the heating season.  The circulator needs to be sized only for the flow and head-loss for this loop.  You usually end up with an off-the-shelf, in-line pump.  There is usually very little resistance to flow in a primary loop, since there are no boilers or radiators in the loop.

With commercial single boilers, single pump systems, you almost always need a single large base-mounted pump.  These types of pumps are expensive to buy and install.  They must be mounted on heavy concrete bases, grouted in, and take up valuable floor space.  In primary/secondary systems, you work with small, inexpensive, in-line circulators.

To size the primary loop circulator a “rule of thumb” can be used.  It is:  “One gallon per minute of primary flow will transport 12,500 BTU/Hr to the system.”  (This is based on 25°F temperature drop.)

Let’s begin an example using a building with a calculated heating load of 500,000 BTU/Hr. We’ll split the load using two 250,000 BTU/Hr output rated boilers.

To get the flow rate for the primary circulator, divide 12,500 BTU/Hr into the total load of 500,000 BTU/Hr:


To get the proper copper pipe size for the 40 GPM flow rate; Figure 3 can be used.  Figure 3 is based on industry accepted flow rates for the sizes shown.

Figure 3.

Now we need to know the head-loss.  Another “rule of thumb:”

“For every 100 feet of primary-loop piping, allow six feet of pump head.”

In our example, let’s say our primary loop measures 300 feet.  Based on the flow rates in Figure 3, we find we will need a circulator that can pump 40 GPM at an 18-foot head.

Once you know the flow rate and head loss, it’s a simple matter to select a pump from manufacturer’s catalogs.


The radiators, and their secondary piping loop become the final part of our system.  Look at Figure 2 again.  Note the two closely installed tees (about six inches apart) and the circulator pumping out of the primary loop.  The secondary radiation piping should be sized to match the flow rate needed for each zone.

For sizing radiation zones, we have another “rule of thumb.”

Figure 4 is based on the same 25°F DT we’ve used throughout our example.  If a zone was sized to use a baseboard to put 15,000 BTU/Hr into the zone, you select 1/2" nom (5/8 OD) copper tube, tee off the primary, keeping the tees about six inches apart, and install a secondary circulator pumping away from the tee.  When a zone thermostat calls for heat, the circulator is turned on.  The zone circulators will almost always be small ones, such as the B & G SLC, since this pump sees only the flow rate and DP through the secondary loop.

Figure 4.

Figure 5 shows how to handle a radiant panel heat zone, mixed with baseboard zones that require lower temperature water than the baseboard zones.  A three-way valve is piped on the primary loop side of the circulator to keep the flow stable through the radiant panel.  The three-way valve need only be a manual valve, adjusted to maintain the desired radiant loop water temperature.  It’s the simplest, least expensive way to handle this loop.  (Once properly adjusted, it is a good idea to remove the handle of the three-way valve to prevent unauthorized personnel from changing the adjustment.)  Once again, the circulator goes on and off in response to a room thermostat.

Figure 5.

The multiple boiler primary/secondary system is beautifully simple:

Boilers inject heat into the primary loop.  This heat is orbited in the loop and extracted as needed into the zones where people are.

The small zone circulators used are as inexpensive as zone valves, and using the primary/secondary system results in a relatively small, inexpensive, in-line primary pump too.

One need not employ an expensive hydronic engineer to design a system.  The “rules of thumb” work well.  Overall, these systems are less expensive to design, install, and operate than a single large boiler system with zone valves.  These systems result in customer comfort and the peace of mind that comes with multiple boilers. An example will best illustrate how all this comes together.  Our example is even going to include a Climatic Control Company designed control system.

Our example building is a commercial building with nine baseboard radiators and zones.  Heat loss calculations are:

                             Three zones at 18,000 BTU/Hr each =      54,000 BTU/Hr

                             Four zones at 48,000 BTU/Hr each   =     92,000 BTU/Hr

                             One zone at 70,000 BTU/Hr each      =     70,000 BTU/Hr

                             One zone at 80,000 BTU/Hr               =     80,000 BTU/Hr

                             Total load                                            =   396,000 BTU/Hr

Boiler Selection: 

The total load will be split between two boilers, each rated at 200,000 BTU/Hr output.  25 °F is to be used for figuring the system DT.  From the boiler manufacturers catalog, we find that a 250,000 BTU/Hr input boiler is rated 200,000 BTU/Hr output, requires 16 GPM, and is equipped with an SLC B & G circulating pump.  The supply and return lines from the boilers to where they tee into the primary loop could be 1-1/4" or 1-1/2" copper pipe, it’s a close call.  If those lines are short (and they should be) 1-1/4" is okay.  If, for some reason, the piping from the primary loop to the boilers begins to approach a total of 80 feet or more, 1-1/2" pipe should be used.  (Supply and return lengths are added together to get a total length.)

Now, let’s deal with the primary loop: 

Using Figure 3, we find the primary loop will be 2" copper pipe.  Let’s say our primary loop measures 360 feet.  Using our rule of thumb that for every 100 feet of primary loop we allow 6 feet of pump head, we find we’ll need a pump that can pump 32 GPM at a 22 ft. head.  (6 x 3.6 = 21.6 round off to 22).  Looking at the B & G catalog, we find that the in line 60 - 13 will fit our need.  A PD37 will also work, but is more expensive.

The piping for the 9 zones is sized using Figure 4.

   Three 18,000 BTU zones — 1/2" copper pipe

   Four 48,000 BTU zones — 3/4" copper pipe

   One zone at 70,000 BTU, and one zone at 80,000 BTU — 1" copper pipe

(Note:  Those of you familiar with friction loss and flow rates through residential 3/4" baseboard will see that the large zones would require commercial baseboard with 1-1/4" pipe.  But, we are not dealing with baseboard sizing in this Info-Tec.)

The “hydronics” of our hydronic system have now been done.  But the hydronics are only half the system.  The other half is a control system.

For maximum comfort and economy, the control system should utilize all the features of the system, and yet be affordable.  Climatic Control Company is an expert at designing and supplying these control systems.

As you will see, enhancements can be added to the basic control system.

The boilers used in these systems are usually sold as “packaged boilers.”  That is; they come complete with limit controls, circulator, gas train, etc.  One needs only to supply power to the boiler and a contact closure to make the boiler operable.  One thing to watch for on these packaged boilers is how the circulator is wired to operate.  Some manufacturers will wire the circulator to run all the time.  Re-wire these boilers so the circulator will run only when the boiler fires.  A relay may be required.

Each zone has a thermostat that simply turns on and off the zone circulating pump.  Since it is much easier and cheaper to run low voltage wiring instead of line voltage wires, a pump relay will be needed. This relay can be of many different configurations, but the Honeywell RA89A is a popular pump relay that incorporates all the necessary features.  It has a built-in transformer, for our low voltage circuit, comes in a NEMA 1 case, and is UL approved.  The 10.2 amps at 120VAC contact rating are more than enough to handle the small zone circulators being used.  Installed cost is low.  Always consider “installed” cost, not just the item cost.  See Figure 6.

Figure 6.

We need to have hot water available at all times in the primary loop so when a zone calls for heat, response is immediate.  There should be no lag to bring the supply water to temperature.  But — it is not necessary to maintain the supply water temperature at design temperature at all times.  Remember, the design water temperature is only needed on the few coldest days.  It would be “fuelish” to maintain, say 180°F supply water all winter.

The A350R reset controller is the solution.  It is designed to raise or lower the temperature of the supply water based on outside temperature.  Because of the A350R’s many adjustment features, the supply water temperature can be matched to the heat-loss characteristics of the building.  Add-on stage modules can be plugged into the A350R, as can a power module.

In our example building by using the A350RN-1, S350AA-1, and Y350R-1, we would have a very inexpensive, but completely adequate efficient control system.  The total control materials list would be:

  • Nine: Low Voltage Zone Thermostats
  • Nine: RA89A Pump Relays
  • One: A350RN-1 Reset Controller
  • One: S350AA-1 Stage Module
  • One: Y350R-1 Power Module
  • One: WEL11A-601R Well

The A350R includes both the supply water sensor and outdoor sensor.  The outdoor air sensor comes with an outdoor enclosure, even wire nuts and a conduit connector!  Only a well for the supply water sensor needs to be added.

Figure 7 would be the wiring diagram for boilers with their own power supply.

Figure 7.

System Enhancements

As stated, this control system will work, is efficient, and is certainly low cost, but by adding some enhancements, the system can be made more efficient and even easier to install.  Most often, these options are very useful. 

The first add-on to our basic system should be lead/lag.  As it stands now, boiler one will always be the first boiler to come on line on a call for heat.  Boiler one will probably get 80 to 90 percent more run time than boiler two.

This unequal wear and tear results in more maintenance and a shortened life span for boiler one.  The lead/lag add-on will even out the “on” times of the boiler, just like rotating the tires on your car, resulting in longer life and consequently less cost.  Equalizing the boiler’s run times will save money.

Another useful add-on is the D350 digital temperature display.  It can be used as a tool for setting up the A350R at installation time.  When the D350 display is plugged into the left side of an A350R it will continuously display the outside sensor temperature.  When the button on the front of the D350 is pressed, the supply water sensor temperature will be displayed.  The D350 plugged into the left side of the A350R is the most used position.  (The D350 can be plugged into the right side of the A350R.  It will then continuously display the supply sensor temperature, and pressing the button will display the supply setpoint.)

We’ve now added to our materials list a D350AA-1, an ARA-24-ACA Diversified duplexer, and a PF083A-E base for the ARA. 

Climatic Control Company can custom-build a panel.  All the controls will be mounted, wired, tested, and placed in one good-looking, convenient enclosure.  The installer only has to mount the enclosure and bring a few wires to it to complete an installation.  While the cost of this panel will be more than the cost of just the parts, the contractor’s installed cost will be less than if he were to field mount and wire the system.  Climatic Control Company even includes computer generated wiring diagrams!

Extras, such as pilot lights to show which boilers are “on,” add nice touches that customers come to appreciate, and can be helpful in trouble shooting if something should fail in the future.

Figure 8 shows the completed Climatic Control Company diagram for just such a panel.

Figure 8.

Adding another staging module and changing the duplexer into a triplexer will allow one to control a three-boiler system.

On many of these systems, a stand-by primary pump will be installed, such as in hospitals, nursing homes, schools, anywhere it is crucial that heat be maintained at all times.  The stand-by pump is to automatically come on line if the primary pump fails.

This feature can easily be incorporated into our panel.  First, remember that the primary loop pump runs all the time during the heating season.  Therefore, there is no need for automatic lead/lag.  That leaves two ways to configure a stand-by pump arrangement, as far as the controls are concerned.

One way is to have the stand-by pump (pump 2) automatically come on when the lead pump (pump 1) fails, but pump 1 will always be the lead pump.  This is illustrated in Figure 9.

Figure 9.

We’ll call this “stand-by pump, auto on, no lead change.”

The devices needed to construct this type of circuit are shown in Figure 9.

An explanation of how the circuit works will help us understand it.  The on/off switch gives manual control to turn pump 1 on for the heating season, off for summer.  When the switch is turned on, current flows through the closed contacts of 1R3 and 2R3, energizing pump 1.  At the same time, the one-minute time delay is energized.  This delay is to give pump 1 time to build pressure, moving contacts of the P74FA-5 differential pressure control to break R to B.  After a one-minute delay (delay timing is adjustable to be able to match any system’s response timing), relay R1 is energized making contact 1R1.  Nothing more happens.

Should pump 1 fail, the P74FA-5 will sense the loss of differential pressure and make R to B, energizing relay R2.  Contacts 1R2 will close, energizing R3.  Contacts 1R3 and 2R3 will switch, energizing pump 2, and opening the circuits to pump 1.  Contact 3R3 is also closed, latching the circuit to R3 to keep it energized.  Pump 2 rebuilds pressure and the P74’s R to B contact is once again broken.

Relay R2 is de-energized, opening contacts 1R2, but R3 remains “latched” through its contact 3R3, keeping the circuits to pump 2 closed and pump 2 on.  As long as the on/off switch remains closed, pump 2 will run.  Pump 1 can now be repaired or replaced.  The circuit will be only reset when the on/off switch is opened, and, of course, if there is a total loss of power.  Note that when powered, pump 1 will always be the lead pump.  Pump 2 will only run when pump 1 fails to maintain the necessary differential pressure.

The circuitry can be improved, for very little extra cost, to be able to select which pump will be the primary pump.

Figure 10 shows the addition of a 3-position selector switch in lieu of the on/off switch.  The rest of the circuits are the same as Figure 9, as is the sequence of operation, except now the lead pump can be manually selected.  This manual selection of the lead pump can be done once each season, once a month, whatever an operator decides.  This way, run time on each pump can be equalized, resulting in prolonged life of the pumps.

Figure 10.

Indicating pilot lights can be easily added to show whether pump 1 / pump 2 is on or off or both.

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