NP Series Flow Centers

The NP Series is a family of non-pressurized flow centers used for closed-loop geothermal (ground source) heat pump systems.  The NP Series flow centers use a water column to provide the necessary suction head for the circulator (pump), and to ensure a flooded pump volute.  Each NP Series flow center consists of a fluid reservoir (tank), flush and service valves, and one or more pumps housed in a foam-insulated cabinet.  The flow center includes a sealing cap to ensure a closed system while providing integrated pressure and vacuum relief to prevent the reservoir from being over-pressurized or dropping below atmospheric pressure.

The maximum system size for which the NP Series flow center can be used depends on the required flow rate and system pressure drop.  However, in general the NP Series can be used on systems up to 10 tons.  Geo-Flo provides free on-line calculators to assist in determining the system pressure drop and required pump size.

Installation instructions are available on our website.

The NP Series flow center assembly includes flush valves that allow the ground loop to be flushed without filling, overflowing, or pressurizing the tank.  Simply connect the flush cart to the flush valves, and rotate the flush valves to bypass the tank.  NOTE: The loop should NOT be flushed with a flush cart through the reservoir.

The cap is hand-tightened until you hear an audible “click” similar to your automobile’s gas cap.

Yes. Although the NP Series is intended to be part of a non-pressurized flow center, it can be pressurized.  The sealing cap has an integrated pressure relief valve rated at 13-15 psig.  If the system is pressurized to a level above the 13-15 psig, the valve will open and air and/or fluid will leak through the cap from the reservoir.

Yes.  The sealing cap will hold pressure to 13-15 psig.  Therefore, it can be mounted up to 30 feet below the loop field or heat pump (13 psig X 2.31 ft-hd/psig= 30 ft).

Yes, but in general you should not need to remove the cap and it should only be removed when absolutely necessary.  The cap is intended to seal the ground loop system from the outside atmosphere thereby ensuring a closed system.  Caution should be exercised when removing the cap.  The reservoir may be slightly pressurized which could cause the fluid in the tank to leak out when the cap’s seal is broken.  In addition, we recommend that the cap only be removed when the circulator pump is running.  When the circulator starts, the fluid level in the tank may drop slightly allowing you to remove the cap without overflowing the tank.  The cap should be removed slowly so that if fluid does begin to exit the reservoir, the cap can be quickly retightened.

Under specific conditions, it is possible.  However, using a high-head high-flow pump available on most flush carts is the fastest and easiest way to ensure that all the air and debris is removed from the loop.  If one does decide to attempt to flush the system with the circulator pump, extreme care should be exercised to prevent damage to the circulator.  More information is provided in the NP Series Installation, Operation, and Maintenance guide available on our website.  Also, see Q10 below.

In general, no.  It is extremely common for construction debris, polyethylene pipe shavings, dirt, sand, rocks, etc. to enter the ground loop piping during installation.  The wet rotor circulator pumps used during system operation require clean, debris-free fluid to function properly.  A small amount of debris in the ground loop could become lodged between the pump’s rotor and stator housing causing pump failure a few days to a few years after initial installation.  This preventable issue is a common mode of failure for circulators.  Conversely, flush carts are equipped with high-flow/high-head pressure pumps, and are designed to flush both air and debris from the loop.  In addition, some flush carts, including the one manufactured by Geo-Flo, have power flushing capabilities which make even difficult loops easy to flush.   A quality flush cart is the fastest and easiest way to ensure that all the air and debris is removed from the ground loop.  If one does decide to attempt to flush the system with the circulator pump, extreme care should be exercised to prevent damage to the circulator.  More information is provided in the NP Series Installation, Operation, and Maintenance guide available on our website.

Connect an acceptable flush cart to the NP Series flow center’s integrated flush/purge valves and follow the industry’s recommended flushing procedures.  The NP Series flow centers have integrated bypass valves that allow the ground loop to be flushed without filling, overflowing, or pressurizing the reservoir.  More detailed directions are provided in the NP Series Installation, Operation, and Maintenance guide available on our website.

First, the installation or service technician can pressurize the reservoir during installation or service of the system.  Second, if the tank is located below the ground loop or unit piping, the water column can create head pressure on the reservoir under certain circumstances.  Third, the reservoir can become pressurized due to the temperature variations of the ground loop.  Polyethylene pipe expands and contracts during seasonal temperature changes.  If the reservoir is filled while the ground loop is warm (i.e. during the summer) and then sealed with the cap, the contraction of the pipe when the loop cools (i.e. during the winter) may slightly pressurize the reservoir.

Fluid can either be removed from the reservoir by removing the cap or through one of the 3-way valves on the flow center.    This is useful for checking the antifreeze concentration in the system.  A specific gravity-to-freeze protection level calculator is available on our website to assist in determining the system’s freeze protection

Heat pump manufacturers suggest measuring the pressure drop across the heat pump’s heat exchanger via PT ports located at the water connections of the unit.  The flow is then determined using the manufacturer’s published tables for pressure drop versus flow.  Also, flow can be measured using a flow meter.  Geo-Flo manufactures a removable flow meter tool called the GEO-METER for this purpose.  More detailed directions are provided in the NP Series Installation, Operation, and Maintenance guide available on our website.

Although there is little static pressure on the ground loop system, the fluid flow through the system will still result in a pressure differential (drop) across the heat pump’s heat exchanger due to the fluid flow and frictional losses.  This pressure drop will be the same whether the system is under high or low static pressure.  Therefore, assuming you have an accurate gage capable of reading the relatively low pressure drop across the heat exchanger, you will not need a new gage.   Be aware that some gages are more accurate near their mid-range value than the upper or lower ranges.

The standard NP and NPP flow centers hold approximately 3.5 gallons of fluid.

No, a check valve on the suction side of the pump is not useful or necessary for the NP Series flow center.  Pump manufacturers (such as Grundfos, Wilo, Armstrong, Taco, and Bell & Gossett) recommend that check valves only be placed on the discharge side of wet rotor circulators which are the type of pumps used in flow centers.  In addition, check valves can cause a maintenance issue (i.e. they can stick open or closed) so they should be located in a position that allows for repair or replacement.

The fluid level in the tank can vary with the seasonal temperature swings in the ground loop.  However, if you find that you are adding fluid on a regular basis you may have a leak somewhere in your system, or there may still in air in the ground loop piping system (also, see Q24). In this case you should contact your loop installation contractor.

You should be cautious whenever you decide to remove the tank’s cap.  Many ground loop systems contain water and anti-freeze such as ethanol, methanol, or glycol in a concentration high enough to reach the system’s required freeze protection.  Adding a small amount of water may not seriously affect the freeze protection level but adding too much water will affect the freeze protection.  In addition, many residential water supplies are high in minerals, contain bacteria, or have an inappropriate Ph level to be used in the ground loop system.  Finally, mixing the wrong antifreeze in your ground loop system can cause considerable harm.  One method of ensuring you are using the appropriate loop fluid is for the installer to provide the system owner with a pre-mixed solution of loop fluid in a clearly marked, sealed, approved container that is stored in a safe, convenient location.

The tank should be filled to about 1” below the bottom of the tank’s neck with the circulator pump turned off.  You should be able to touch the fluid with the tip of your index finger.

The gurgling sound is likely water rushing into the tank from the discharge of the return pipe.  This indicates that fluid needs to be added to the tank.

The whistling sound is air being drawn through the cap into the top of the tank.  The NP Series flow center includes a cap with integrated vacuum relief (older models using a smaller 2-1/4″ cap have integrated pressure and vacuum relief; on current versions the pressure relief is separate from the cap).  This vacuum relief allows the fluid level to drop in the tank without creating negative pressure inside the tank.  The fluid level in the tank will rise to its highest level when the NP flow center’s cap is removed and the circulator is off.  When the circulator starts the fluid level inside the tank drops slightly as fluid is forced into the loop (the loop slightly expands under the pressure of the pump).  If the cap is secured while the pump is off and the pump is subsequently started, the fluid level will drop and air will rush through the cap causing a slight whistle.  This is completely normal and will only occur during the first couple of pump starts unless air remains in the loop.    If air is in the loop it will be compressed when the pump starts causing a larger fluid level drop in the tank and therefore a more sustained whistle.  Running the pump(s) on full-speed for one or two days may dislodge the air in the loop and return it to the tank resolving the issue.  The fluid level in the tank must be monitored during this process.  NOTE: The NP Series allows the pump to be “dead-headed” (see Q24) in order to check for air in the system.

Most likely, there is air in the loop and the system needs to be flushed.  If air is present in the loop and the circulator starts the air is compressed due to pressure created by the pump.  When the pump stops the air expands pushing the fluid back through both the supply and return pipes into the tank causing the tank to overflow.

The flow center’s return valve can be closed while the pump is running “dead-heading” the pump against the valve while the fluid level in the tank is monitored.  If the fluid level drops more than one to two inches during dead-heading, air is present in the system.  Also, see Q23.

No.  An open system is one in which the fluid is continuously open to the atmosphere.  The cap seals the reservoir and prevents interaction between the fluid and the atmosphere.

The NP Series differs from other non-pressured flow centers with the following advantages:

• Allows system flushing/purging using an industry standard flush cart without additional on-site plumbing.
• Allows choice of plumbing connections into the top or the sides of the flow center.
• Single pump NP flow centers (NP1 and NPV) can easily be field upgraded to two-pump flow centers (NP2 and NPV2).
• Allows for pressurized or non-pressurized system operation.
• Provides a sealing cap that ensures a closed system and that cannot be blown off the tank.
• Cap provides pressure and vacuum relief.
• 3-way flush/purge/isolation valves can be field serviced (replacement seals available), if necessary.
• Safety: small reservoir opening will not allow hands or foreign objects to be placed in the reservoir.
• Utilizes Flo-Link Double O-ring connections for fast, easy, and leak-free installation.
• Allows choice between 3-speed circulator pumps and high efficiency variable speed pump.

It’s your choice.  However, putting the header inside requires more labor, additional wall penetrations, additional valves, and does not look as professional as burying the header outside

The terms “pressurized” and “non-pressurized” when referring to a geothermal ground loop system indicates whether there is static pressure on the ground loop and heat pump system.  The circulator pumps used in flow centers require a pressure at their inlet (suction side) that exceeds a minimum level that is established by the pump manufacturer.  In technical terms, the net positive suction head available (NPSHa ) of the system must be greater than the net positive suction head required (NPSHr) by the pump.  In pressurized systems, this positive pressure is induced and captured in the system during start-up/installation by an external pressure source (usually a high-head pressure pump on a flush cart).  In a non-pressurized system positive static pressure is not applied to the ground loop system.  Instead, a standing column of water is used to meet the minimum suction pressure requirement of the pump.

You will find installation contractors who use exclusively pressurized or non-pressurized flow center systems who claim they never have problems with their installations.  The fact is that even the very best system will perform poorly if it is not applied and installed properly.  This is true of both pressurized and non-pressurized flow centers. Note that many of the items listed below for a particular system’s advantages are also listed as disadvantages and vice versa.  Ultimately, the installation contractor, dealer, or homeowner will decide which system is best for them. The following is a list of advantages and disadvantages of each system:
Pressurized flow centers’ advantages

• Do not require or encourage monitoring or maintenance by the homeowner after initial system start-up.
• Generally much smaller in size thereby requiring less space for installation.
• Ensure a closed system which prevents air and or debris from entering system after installation.
• Include 3-way 4-position dual purpose isolation/flush (purge) valves.
• Sold and supported by all major geothermal heat pump manufacturers (not just flow center manufacturers or distributors) since the inception of the industry.
• Fewer components reduce possibility of component failure or system leaks.
• Fewer limitations on installation locations and orientations.  For example, pressurized flow centers have no practical limitation on unit placement below the heat pump or loop field.  Also, they can be installed in relatively confined spaces such as a crawlspace, closet, or very small mechanical room.  Finally, they can be installed vertically or horizontally (12-, 3-, 6-, and 9-o’clock positions) providing greater installation flexibility.
• Flow centers can be plumbed in series or parallel as needed.
• Single point isolation location between the ground loop and heat pump (via the 3-way valves).
• No limitation on the type of pumps used in the system.  Many pumps require (NPSHr) much more than the 1-2 ft of head (NPSHa) that most standing column (non-pressurized) flow centers provide.
• Plumbers and hydronics technicians are very familiar with pressurized systems since most hydronic (i.e. hot water heating, radiant floor hearing, solar hot water, etc.) systems are pressurized.  Non-pressurized hydronic systems are unique to the geothermal industry.
• Require the minimum number of field plumbing connections to reduce field labor and the risk of leaks.

Pressurized flow centers’ disadvantages

• No built-in provision for air elimination.  Therefore, if the installation technician does a poor job of flushing all air from the ground loop it will remain in the system.  This could lead to issues with noise and/or air-lock of the pump under certain circumstances.  NOTE: Some contractors choose to install air eliminators to pressurized systems to alleviate this concern.
• Thermal expansion (during cooling season, i.e. summer) and contraction (during heating season, i.e. winter) of the ground loop piping causes pressure fluctuations in the system.  The loop pipe will slightly stretch and relax over time which causes a decrease in system pressure.  If the initial system start-up pressure is not high enough to overcome this relaxation of the pipe the system pressure can drop below the required suction pressure of the pump (NPSHr) causing pump cavitation which could lead to pump failure.  NOTE: Some installers choose to install an expansion tank in pressurized systems to alleviate this concern. Geo-Flo offers the Pressure Battery polyethylene expansion tank and the Geo-Prime tank to alleviate pressure concerns for pressurized systems.
• Poor installation practices (i.e. small leaks in the ground loop or inside plumbing) result in decreases in system pressure over time which can lead to pump cavitation and failure.
• Generally do not allow homeowner maintenance or interaction with system.  Fluid/pressure can be added to the system via the PT ports but this is typically performed by a qualified service technician.

Non-pressurized flow centers’ advantages

• Standing column of water provides necessary suction head for pump so that the static pressure on the ground loop is not a concern.
• Column of water on suction side of pump prevents air from being drawn into pump (as long as the fluid level remains at an appropriate level) eliminating the chance of air-locking the pump.
• Provides air elimination function.
• Allows homeowner to add make-up fluid to tank if the level gets low.
• Poor installation practices (incomplete flushing, small leaks in piping, etc.) can be tolerated due to the ability for the homeowner to add make-up fluid.

Non-pressurized flow centers’ disadvantages

• Can be used as a bandage to cover up poor installation practices (incomplete flushing, small leaks in piping, etc.).  This is bad both for the homeowner and the industry.
• May require continuous fluid level monitoring from the homeowner.
• Allow homeowners to access ground loop fluid.  This could cause issues with the antifreeze being diluted or the wrong antifreeze being added to the system.  Both of these issues could cause serious harm to the heat pump.  In addition, contact with the antifreeze solution may be harmful to humans and pets.
• Water level decrease in the flow center tank (standing column) reduces the positive suction head pressure on the pump increasing the chance of pump cavitation and pump failure.
• Larger size requires more space for installation.
• Flush/purge valves not incorporated (in some manufacture’s systems) so they must be field fabricated and/or installed.  Note: Geo-Flo’s NP Series does include flush/purge valves.
• Limited installation locations (distance below the loop field or heat pump) and orientation (flow center tank must remain vertical).
• Requires additional ball valves to be field installed to isolate the heat pump from the ground loop (on some manufacture’s systems).
• In general, cannot be installed in series or parallel.
• Requires more field plumbing connections than pressurized flow centers increasing field labor and the chance of leaks.