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Hot Water Heating Systems: Transitioning from Gravity Systems to Forced Circulation Systems

Hot water systems have long been a preferred way of transferring heat from a central point (a boiler) to remote spaces or rooms where heat is needed.  The first hot water heating systems were gravity systems.  When water is heated, it increases in volume; hence it becomes lighter and rises.  Simultaneously cooler, heavier water falls.  This is the principle on which gravity circulating systems work.  Gravity systems have many features to recommend them.  They produce an even heat, are quiet, use low temperature water, are reliable, very efficient, and are virtually maintenance free.  There are many buildings still using gravity hot water heating systems, some over 100 years old!  The disadvantages of gravity systems are:  They require very large diameter piping for supply and return mains.  The low temperature water provided a heat emission rate of only around 150 BTU’s per square foot of radiation per hour.  Consequently, radiators had to be large.

As labor and material costs went up, gravity systems became very expensive to install.  People no longer would tolerate large bulky radiators needed with gravity systems.  To accommodate 6, 8, even 10-inch pipe for mains became prohibitively expensive.  The gravity system’s slow response time to changing demand was also a detriment.

The invention of circulating booster pumps in 1929 overcame all the objections of the gravity systems, while retaining all the advantages of heating with hot water.  The booster pump so greatly speeded up the movement of water that smaller radiation could be used, supplied by much smaller piping.  Forced circulation systems allowed design using higher water temperatures resulting in higher emission rates.  A radiator of 60 square feet with an average water temperature of 170°F will emit heat at a rate of 150 BTU’s per square foot per hour, or 9000 BTU’s per hour.  A radiator of 45 square feet with 197°F water will emit 200 BTU’s per square foot per hour, producing the same 9000 BTU’s per hour.

The use of automatic firing devices and more accurate controls made use of higher water temperatures without sacrificing good design practices.

Power is consumed in moving water through pipes, radiators, boilers, etc.  To utilize the savings of smaller pipes and radiators of the forced circulation hot water systems, water velocities must be higher than in gravity systems in order to carry the BTU capacities necessary.  The booster pump created a pressure head (DP) much greater than in gravity systems, in order to achieve the needed velocities.

DP is the amount of pressure lost between any two points in a system.  The friction created between the inner walls of the pipes, radiators, boiler, and the moving water causes pressure drop.  In a horizontal pipe filled with water, in which there is no flow, the pressure is equal at all points.  The instant flow starts, friction is set up which increases in direct proportion to the velocity of the flow.      A change in DP can be calculated when there is an increase or decrease in flow rate (GPM).  Divide the final GPM by the initial GPM and square the result.  Multiply this result by the initial DP.  The answer is the new DP.


A system with a volume flow rate of 3 GPM and a DP of 5 lbs. needs to be increased to 6 GPM.  What will be the new DP?  (This would need to be known to properly pick a booster pump.)




20 lbs. is the new DP.  (Velocity in feet per second can also be used in this formula.)

Head pressure is used in designating the capacity of a booster pump.  It is a way of describing DP.  Maximum “head” of a pump is really the maximum DP against which the pump can induce a flow of water.  Head pressure is often expressed in “feet of water”.  Only the friction in the system limits the capacity of a pump.  This value is called “head pressure”.

Enough power must be available to overcome the DP of a system and provide the design GPM.  This means the DP of each component part of a system must be known at design GPM.

The booster pump provides the power.  Manufacturers of pumps publish DP and GPM figures or charts for their pumps.  The data may be expressed in pounds per square inch, feet of water, or mil inches.  These figures can easily be interchanged.

1 psig. = 2.31 ft. of water

1 ft. of water = .43 lbs./sq. in.

1 ft. of water = 12,000 mil inches

Static pressure should not be confused with head pressure.  They are totally different pressures and have no relationship to each other.  Static pressure is created by the weight of water in a system.  It has no effect on pump capacity.  To illustrate static pressure, think of a closed hot water system as being an upright loop of water.  See Figure 1.  If gauge 3 is 40 feet above the boiler, and the loop is completely filled with water, but not pressurized, gauge 3 will read 0 psig.  Gauges 1 and 5 are located 10 feet above the boiler, gauges 2 and 4, 20 feet above the boiler.  With the pump off, the pressure in vertical pipe “A” is identical to the pressure in vertical pipe “B”.

Figure 1.

If all the gauges are scaled in psig., gauges 1 and 5 would read 12.9 psig., (30 feet of water is above them and a foot of water equals .43 lbs.), gauges 2 and 4, 8.6 psig.  A gauge on the boiler would read 17.2 psig.

It is good practice to pressurize a closed system, especially if design water temperature is close to or above the boiling point of water at atmospheric pressure.  An additional 4 psig is the recommended minimum additional pressure added to the static pressure necessary to get the water to the high point of the system.  In our illustration, gauge 3 would read 4 psig. and all the other gauges would read 4 pounds more.  Additional static pressure adds equally throughout the system.

It is worth repeating again.  Do not confuse static pressure with head pressure.  These two terms are often misused.  One has nothing to do with the other!

What happens to our Figure 1 system if after filling to the proper static pressure, we turn the pump on?  Maybe nothing; maybe a lot of noise!

Before selecting a pump, we need to know the design flow rate and the design head pressure.  The pump only has to deal with the friction loss, DP, developed at the flow rate, GPM needed.

Let’s assume our system was designed to circulate 10 GPM at 6 feet of head pressure.  By consulting the pump manufacturer’s charts, the correct pump can be selected.  See figures 2 and 3.  These are pump “curves” for some B & G pumps.  Enter the charts on either the “total head in feet” side or “capacity in gallon per minute” side.  Mark the intersection of the GPM and head lines.  Select a pump closest to, but above this intersection.  In our illustration, the pump could be a SLC-30 (Figure 2) or a series 100 (Figure 3).

Figure 2.

Figure 3.

If a pump were needed to supply 80 GPM at a 25-foot head, the correct selection would be a PD38 (Figure 3). 

Note:   Do not greatly oversize a pump.  While under sizing a pump will result in poor or no circulation, over sizing will result in velocity noise and excess cavitation.  Cavitation will soon destroy the pump.  A small increase in flow rate is preferred to reducing the flow rate below design specifications. 

Forced circulation hot water systems are classified as one or two pipe systems.  These classifications are further broken down into direct or reverse return systems.  Figures 4, 5, 6, and 7 illustrate these classes of systems.

Figures 4, 5, 6, and 7

Figure 4 shows a “two pipe direct return” system.  Note that the hot water delivered to the first radiator is also the first to return to the boiler.  This progresses through the circuit so that the last radiator is the last to return its cooler water to the boiler.  There is a tendency for the radiators closest to the boiler to short-circuit the water so the units farther away do not get proper circulation.  This system should be installed using balancing valves and carefully balanced. Figure 5 illustrates the “two pipe reverse return” system.  This system is recommended when two pipe systems are designed.  It is more expensive to install because more piping is required than the two pipe direct return system, but it functions much better.  In this system, the first radiator to be fed hot water has the longest return, and the last radiator to be fed has the shortest return.  This system has a tendency to balance itself as long as supply and return drops are the same size and length.

Figure 6, the “series loop” system, is the cheapest to install.  It simply consists of running a pipe into and out of each radiator, thus making the radiators part of the piping circuit.  The length and size of a series loop is very important.  Due to pressure drop and temperature drop of a series circuit, it is limited in length.

Series loops have to be carefully designed.  As the water passes through each section of radiation, it cools.  As the water progresses through the circuit, each successive radiator is being supplied with cooler water, and consequently, its emission rate is dropping.  If the system’s designer takes all of the factors into account, series loops can be effective.

Figure 7 is a representation of a system using diversion tees, often called a monoflow, or “monoflo” system.  Hot water is diverted into the radiators by using specially designed venturi tees, and the cooler water returned to the same pipe that serves as both supply and return main.  This system combines the efficiency of the two pipe systems with the low installed cost of a series loop system.  Monoflo tees can be obtained in both supply tees and return tees.  See Figure 8.  The supply monoflo tee restricts the flow of water, causing some water to flow up the riser.  The return monoflo causes the main supply water to increase in velocity as the flow goes through the nozzle.  This increase in velocity causes an area of lower pressure around the nozzle and in the return risers “sucking” the water back into the main (Bernoulli’s effect).

Figure 8.

For radiators above the main with normal resistance, only one tee need be used for each radiator, usually used on the return side.

For radiators with high resistance or where the radiators are below the main, both a supply and return monoflo are needed.

Figure 9.

Figure 9 illustrates a radiant panel heating system.  In this system, coils of tube are buried in ceilings, floors, or walls, turning the ceiling, floor, or wall into a radiator that emits radiant heat into the room.  Special care must be taken in design of a radiant panel system.  Because of the small tubing, pressure drop is high and circuit lengths are critical.  Headers with balancing cocks are used.  Radiant panel systems are the most expensive systems of all the hot water systems to install, but are the quietest, cleanest, and most comfortable of all the systems.

In order to function properly, a forced circulation hot water heating system needs certain specialties and accessories.

Beginning with the cold water supply, a “feed valve”, which is actually a pressure-reducing valve, is installed to reduce the incoming system water pressure to a workable pressure.  It is used to fill the system initially and will add water when system pressure falls below the valve setting.  The standard factory setting is usually 12 lbs.  This setting is the correct setting for a static height up to about 18 feet, good for most two-story buildings.  For higher static heads, the valve can be adjusted up to 25 lbs.  Valves are available that can be adjusted up to 60 lbs.  All B & G reducing valves have a built-in strainer and check valve.  Many can be equipped with a fast fill feature, allowing for quickly filling a system initially, or after a system has been drained for repairs.  (While most boiler reducing feed valves supply too slowly to be used on plumbing fixtures, the B & G models 6 and 7, high pressure reducing valves, can be used to protect plumbing fixtures from excessive line pressures.)

The purpose of a compression or expansion tank is to accommodate fluctuations in water volume in a closed system.

Water expands when heated in direct proportion to its change in temperature up to the point of saturation or boiling.  The compression tank acts like a spring on the system, keeping pressure on it at all times.  If the tank is too small, or becomes waterlogged, the relief valve will open when the boiler is heating and discharge water.  When the heating cycle is over, the water will cool, system pressure will drop, and the feed valve will open and feed water until system pressure is back to “normal”.  On the next call for heat, the water will again expand, causing the relief valve to open.  The cycle will repeat over and over until the too small tank is replaced, another expansion tank is added, or the waterlogged tank drained and properly refilled with the correct air and water charge.

System water volume and temperature determine tank size.  If the tank is too large, the system pressure increase may not be enough as the system heats up and approaches boiling, especially at the high point of the system where low static head exists.  Proper compression tank sizing is very important for trouble-free system operation, whether it is a pre-charged tank with a bladder separating the water and air, or a standard expansion tank.

To properly size an expansion tank is a tedious task.  Assuming a compression tank will be properly equipped with an airtrol tank fitting, so that the tank will not parallel a rise in system temperature, the following formula can be used to determine compression tank size:


VT            =    Compression tank size in gallons

VS            =     Volume of system in gallons

EW          =     Unit expansion of water

EW-EP    =     Unit expansion of system

PA            =     Atmospheric pressure in PSI absolute

PF            =     Initial pressure in tank in PSI absolute

PO           =     Final pressure in tank in PSI absolute

.02VS       =     Air released from new system water upon heat-up, 2% of water volume.

Easy!  Simply fill in all the numbers and solve the formula.  Correct tank size!

            There is an easier way.  It’s not as accurate, but will be close enough.

First, the system’s water volume must be known.  This can be estimated using Table A.  Enter Table A in the MBH column closest to the boiler’s input rating.  Then read across, and add up the gallons of water for each condition of the system.  For example:  The system consists of a 150,000 BTU input conventional boiler, copper fin tube baseboard, two pipe piping system.

Boiler =                                  36    gallons

Non-Ferrous baseboard =     5.5 gallons

Two-pipe system =              34    gallons 

Total =                                  75.5 gallons of water in the system

Table A.

Next, determine the “mean design water temperature”.  This is simply the average of the design supply and return temperatures.  If the highest design temperature is 190°F and a 20°F temperature drop was used for design, a very common DT, 180°F is the mean design water temperature.  190 + 170 ÷ 2 = 180.  Enter Table B in the “water vol in gals” column and go down to the closest volume found for the system.  In our example, this is 80.  Go across to the number shown under the mean design temperature column.  In our example, this is an 8.  The 8 is the size, in gallons, of an expansion tank for our example system.  Note our selection was based on a 12 lb. fill pressure and 30 lb. set relief valve, or an 18 lb. allowable system pressure increase.  For other conditions, correction factors need to be applied to the tank selected from Table B.

Table B.

Had our fill pressure been 18 lbs. with a 30 lb. relief valve, we would need to use Table C to correct the tank size.  Enter Table C under the "Initial Pressure . . ." column and go down to the closest setting for the fill valve.  Go across to the factor found under the column representing the relief valve setting, 30 Lbs., minus the fill valve setting, 18 Lbs., or 30-18 = 12.  The factor is 1.94.  Multiply the tank size found in Table B by 1.94 to find corrected tank size 8 x 1.94 = 15.52.  Use the closest commercially available tank.  In this case, a B & G 15 gallon tank.

Many systems are filled with a mixture of antifreeze and water.  The expansion of a glycol and water mix is greater than that of water alone.  Table D shows the correction factor for a glycol water mixture.  If our example system was filled with a 50% mixture of glycol and water, the correction factor multiplier could be 1.6 or 1.5, since our maximum design temperature was 190°F.  Multiplying the 15.52-gallon tank size by 1.5 or 1.6 will result in a tank size of 23.28 or 24.83 gallon, a 24-gallon tank is the commercially available size.

Table D.

All of these figures have been based on using a standard or A.S.M.E. compression tank, that is a tank without a bladder.  Many expansion tanks are available today that are precharged and have a bladder in them separating the air and water.  The basic formula for sizing these tanks is the same, but an “acceptance volume” allowance has to be made.  Other factors enter into the installation and sizing of these kinds of tanks, but since Climatic Control Company does not, as of this date, sell them, this article will not go into the details of sizing one.  Those interested can request B & G bulletin TEH-981 from Hydro-Flo, for a discussion of pressurized tanks.

The expansion tank should be the only air space in the system.  Air is absorbed in water, so some means of preventing the gravity circulation of the cooler air-laden water in the tank into the system is needed, without restricting the passage of free air from the system to the tank.  The B & G ATF is such a device for tanks up to 24 inches in diameter, and the ATFL for larger tanks.  On cold fill, a compression tank should be 2/3 full of water, 1/3 full of air.  The ATF and ATFL vent tubes can be cut to accomplish this, even on tanks equipped with a sight glass.

The ideal place to separate the air from the water in a system is at the point of highest temperature and lowest velocity.  These parameters are met in the boiler.

B & G’s ABF top outlet airtrol fitting, installed in the top of a boiler, does an excellent job of removing the air bubbles from the top of the boiler and passing them on to the expansion tank.  Bubble-free water is then able to circulate through system.  B & G used to make an ABFSO, side outlet boiler Airtrol, but no longer makes them.  Side outlet boiler Airtrols did not work as well as the top outlet did, and demand for them dropped off to the point that continued manufacturing of side outlet Airtrol fittings was no longer feasible.

Air scoops, such as the B & G IAS, are in line air eliminators.  They work on the principle that air, being lighter than water, travels along the upper portion of a horizontal pipe.  As the air enters the air scoop, the air bubbles are scooped up by baffles in the air scoop and rise into the upper chamber.  There, the air can be vented if a bladder-type expansion tank is used, or connected to a standard expansion tank to collect the air.

The elimination of air in a system, except in the expansion tank, is of the utmost importance.  Air must be vented from the system or noisy operation and even complete blockage of circulation can occur.  Air vents must be used at all high points in the system.  This is the only way to completely vent all the air when initially filling a system.  So called “purge and drain” valves do not work good enough to eliminate all the air, and do nothing for accumulated air after a system is in operation.

There are two basic types of air vents, automatic or manual.  Automatic air vents come in two styles.  Float type and fiber disc type.  Float vents have a float attached to a valve, all contained in a shell.  When the shell is full of water, the float keeps the valve closed.  When enough air accumulates in the shell, the float drops, opening the valve and air passes out until water again fills the shell, closing the valve.  As fast as air accumulates, the cycle is repeated.

Float vents work well and last a long time.  Unfortunately, even the smallest float vent may be too big to fit inside fin tube baseboard covers.

Fiber disk type automatic vents are physically very small, the same size as manual “loose key” or “coin” vents.  They use special discs that swell when water touches them.  As air accumulates and replaces the water around the discs, the discs dry out, shrink, and open a small vent port.  Air is vented, water again reaches the discs, and the cycle repeats — for a time.  Fiber disc automatic vents are prone to quick failure such as sticking closed, or dribbling water all of the time.

The best air vents are the manual vents, called loose-key or coin vents.  Coin vents can be opened or closed with a dime or small screwdriver.  Loose-key vents require a small key to open or close them.  Either one is just a small needle valve, metal to metal seat.  Besides being virtually indestructible, they are cheap!  Their only drawback is that they must be manually opened and closed.  If air accumulates, someone has to vent the air.  If a system is equipped with manual vents, it is good practice at least once a year to open each vent to allow any air that has accumulated to escape.

Most air problems can be eliminated by careful design, good maintenance, and proper initial startup of the system.  The most often overlooked part of a forced hot water system is proper startup.

Once a system has been installed, flushed, and filled to the proper static head, the boiler should be fired and slowly heated to at least 225°F water temperature and held there for about one-half hour.  This will liberate the entrained air in the water and send it to the expansion tank.  The hotter the water, the more air it will liberate.  The circulating pump(s) should be off during this initial heating.  Now, allow the boiler to cool to normal operating temperature and start all circulators and open all the zone valves, if used.  Again, run the water temperature back up to at least 225°F and circulate all the water for 15 to 30 minutes.  This will drive most of the air out of the fresh water, and as long as there are no leaks in the system, air problems will be prevented.  Anytime the system is drained, say for some repairs, and re-filled, the startup procedure should be repeated.

Figure 10.

Figure 10 is a representation of a typical boiler installation with a standard expansion tank.  The cold water supply should always enter the system at the compression tank so any entrained air will immediately go to the tank.

Figure 11.

Figure 11 shows a system with a pressurized or bladder equipped expansion tank.  Note the in-line air separator being used with a float vent. Flo-control valves or flochecks, are specially designed valves, similar to a piston check, that stop gravity circulation in a forced hot water system to prevent overheating when the circulating pump(s) is off.  B & G SA flow control valves have a manual opener on them to allow gravity circulation in an emergency, should the pump fail.  Even though the forced circulation hot water system pipes are small, gravity circulation can be quite effective to keep some heat on, if needed.

Every hot water boiler must have a safety relief valve that will keep the pressure at or below the boiler’s working pressure.

The A.S.M.E. (American Society of Mechanical Engineers) code states:  “Every hot water heating boiler shall have at least one officially rated pressure relief valve set to relieve at or below the maximum allowable working pressure of the boiler.  Relief valves shall be connected to the top of boilers with the spindle vertical if possible.  No shutoff of any description shall be placed between the relief valve and the boiler, or on the discharge pipe between such valve and the atmosphere.”

The relief valve has to perform satisfactorily under two conditions.  It has to relieve pressure by discharging water due to thermal expansions and relieving pressure by discharging steam.  Discharging of water will usually be a sign of a waterlogged expansion tank, or defective fill valve.  It is easy to diagnose.  If the static cold fill pressure increases rapidly to the pressure setting of the relief valve when the boiler is fired, the tank is waterlogged.  Drain and re-fill the expansion tank to the proper level of water and air.  Too small an expansion tank for the system may exhibit similar symptoms.  If you suspect the tank is too small, recalculate the tank size and either add another tank or replace the existing tank with a properly sized one.  A hole in an expansion tank will quickly cause it to waterlog.  Again, it will fill with water and leak.  Expansion tanks on hot water systems don’t sweat so any water dripping from an expansion tank is evidence of a leaky tank.  A defective or leaking fill valve will over pressurize the static fill pressure on a cold system.

The discharge of steam through a relief valve is an emergency condition and places a critical demand on the valve.  Whenever the temperature of the water in the boiler is about 212°F or above, and the relief valve discharges, the sudden pressure drop causes the water to flash and to steam.  The capacity of the relief valve must handle this.  There is a vast difference between discharging water and discharging steam.  A pound of water occupies 27.7 cubic inches of space.  A pound of steam, at atmospheric pressure, occupies 26.8 cubic feet!  Over 1600 times more space than water!  Thus, an A.S.M.E. relief valve is tested and rated on steam, even though it is a valve for a hot water boiler.

Properly sized relief valves have to handle the gross output of the boiler. Hot water boiler relief valves are rated in BTU’s per hour at a specific pressure rating.  As long as this rating meets or exceeds the burner input rating, the relief valve will be big enough for the boiler.  To aid in selecting a valve, manufacturers of relief valves print charts showing their valve capacities at various pressure settings.  See Figure 12.

Figure 12.

Dual units, those units that combine a fill valve and a relief valve, do not meet code.

Most boiler manufacturers now recommend putting low water cut-offs on hot water boilers.  Many local codes will require this.  Even though a boiler may be protected from exploding because it has an A.S.M.E. relief valve, dry firing can still ruin it.  Most hot water boiler damage can be traced to low water conditions.

There is a misconception that the pressure reducing fill valve will keep a system full under all circumstances.  This is not true.  To illustrate the problem, a typical system will have a pressure reducing fill valve set around 12 to 18 lbs., and a relief valve set to open at 30 lbs. and close at 26 lbs.  Should the relief valve open to discharge water due to excess pressure, it is obvious the fill valve won’t make up the water lost.  Without make-up water replacing the loss through the relief valve, a low water condition can result. 

There are many other reasons a system can lose water so that a low water condition will result.  Leaks in the boiler, piping, or through the pump seals.  Carelessness, such as draining a boiler for repair and forgetting to refill the system is yet another common reason for a low water condition.  A low water cut-off will save the boiler by not allowing the burner to come on until the low water condition is corrected.

Under certain circumstances, a low water cut-off may not be enough protection.  A fuel valve could stick open; contacts could weld closed due to an overload or short circuit making the low-water cut-off ineffective.  The best recommendation to cover all installations, to provide the most safety, is to use a combination water feeder and low water cut-off.  The feeder portion is usually capable of feeding water into the boiler as fast as it can be discharged through the relief valve.  While the feeder cut-off combination will add to the cost of an installation, when compared to the cost of replacing a boiler, it is “cheap” insurance.  Remember, codes are minimum requirements, the “at least”, that must be done.  It is always good practice to exceed the code requirements, especially where safety is concerned.

       While Climatic Control Company does not usually design forced hot water heating systems, knowing what is required may enable you to help a customer find a problem with a troublesome system he is working on, and sell the proper devices to correct the problem.

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