Boiler check valve or flow control valve (C) Daniel FriedmanGuide to Heating System Boiler Check Valves, Flow Control Valves, Backflow Preventers

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Check valves & flow control valves on hot water heating systems:

This article explains types of check valves used on hot water heating systems.

Here we explain heating boiler check valves, also referred to as flow control valves, flo-control valves: controls that prevent hot water from circulating in the heating system when it is not supposed-to. We also discuss IFC circulators - internal flow control circulators, and heating system backflow preventer valves.

We discuss: What are flow-control valves & how do they work? What are IFC circulators - internal flow control valves? How do automatic water feed valves also function as a check valve on heating systems? Cleaning & maintenance guide for heating system of well prime

This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Check Valves on Hot Water Hydronic Heating Systems - Flow Control, Flo-Control Valves, Inspection, Settings, Problems

Boiler check valve or flow control valve (C) Daniel FriedmanThe photo above shows a modern cad cell relay on an oil fired heating boiler - one of the safety controls which we discuss in this article.

Because on some hot water heating systems the check valve or flow-control valve is integrated into the circulator pump assembly, also see Circulator Pumps & Relays.

[Click to enlarge any image]

For an example of how the check valve or flo-control valve functions on hot water heating systems, and what goes wrong, also see Heat Won't Turn Off - Stop Unwanted Heat. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Check valves on heating boilers prevent hot water from circulating through the building when it is not supposed to be doing so - such as when the boiler and its water are still hot but the thermostat has stopped calling for heat.

Why Are the Baseboards or Radiators Hot Even Though the Thermostat is Set Lower than Room Temperature? Un-wanted Gravity or Convection Circulation of Hot Water

If electrical power switch to a hot water heating boiler is in the on position (see ELECTRICAL POWER SWITCH FOR HEAT) and the room thermostat is set below the current room temperature, the heating system (hot water or steam) boiler or (warm air) furnace should not run.[1]

But for hot water heating systems (baseboards, radiators), other defects could cause or permit hot water to circulate through the heating system by "gravity" (convection, warm water rising on its own through the heating piping) even though the thermostat is not calling for heat.

[Warm water is less dense than cold water and will rise up through cooler water, displacing it and sending cooler water down to the boiler.]

The problem, if this is occurring, is usually that a check valve (photo above-left) (found internal to some circulator pumps, or external as a physical device) intended to prevent hot water from circulating on its own - when the circulator pump is off - is either set to a "forced open" position, or it has become defective (less likely).

If that's the problem (diagnosed by a heating and service technician) then the valve or circulator needs to be replaced. While waiting for that repair to be made, you can still turn off the heat when it is not needed, by turning off electrical power to the boiler. With the heating boiler switched off it will cool down and water will stop heating the radiators by gravity or convection circulation.

Heating control installation sketch BandG (C)ITT B&G modified by DJF

The red Bell & Gossett B&G flo-control valves shown in our photo above and in the sketch at left are designed to prevent hot water from circulating in a heating system unless the circulator pump is also running.

The valve provides a vertical-lift check valve function: until the circulator pump begins to run, the valve will prevent hot water from circulating through the piping system.

The B&G installation sketch (left, edited) shows a simplified but typical location of a flo-control valve - the red device at the right-most side of the sketch.


Flow-Control Valve Installation Position

It's important that B&G flo-controls and similar flow control valves on hot water heating boilers are properly installed: the stem with the little lever needs to be upright. This flo-control valve is installed only in a vertical position, either at the top of a vertical pipe (where heat piping changes direction from vertical to horizontal) or in a horizontal heating pipe, but with the valve in an upright position.

The valve has to be installed with its cast-in flow-arrow pointing in the correct direction - in the direction off flow of heating water.

Flow control valves such as the Bell & Gossett Flo-Control Valves V51843E series (Xylem brand) must be installed upright - with the operating stem or handle "up" and in the proper direction for hot water flow from the boiler into the heating zone piping as indicated by the arrow on the valve body.

Watch out: The stem or control handle is always in the top or vertical position when the valve is installed. Otherwise the automatic flow control or check valve feature of the valve will not work properly.

Some flow control valves are manufactured with a side or bottom inlet from which water is fed from the boiler but in any case the valve body itself is upright and flow is in the direction of the arrow cast into the valve body. The un-used opening in the valve (side if the valve is fed from the bottom, bottom if the flo-control valve is fed from the side) is plugged with a pipe plug.

Which way do we turn the lever on a flo-control valve?

B&G Flo Control Valve lever setting (C) Daniel FriedmanThe lever on the top of the B&B flow control valves shown here allows the user or service technician to force the valve to an "open" position, overriding the valve's automatic internal operation.

Manually Open Flo-Control Valve: the valve is turned fully counter-clockwise to put the valve in a manually-opened position. This position may also be used to manually open the valve during filling of the heating system with water or during air-purging operations.

Watch out: with the valve in manually-open position some hot water is likely to circulate from the boiler by convection or "gravity" . When that circulation is not needed or wanted be sure to return the flow control valve to automatic operation.

Automatic Flo-Control Valve Operation: depending on the valve brand and model this position may be marked as Winter or Automatic. Or the valve lever is turned fully clockwise. As we illustrate later in this article, other flow control valves will have position markings cast into the valve body and marked Open, Winter, or Closed.

Clockwise, or Winter is the normal setting for this control.

When would we turn the lever counter-clockwise or to the Open position? We can think of several instances:

Flow control valve showing which direction to turn the lever for gravity heat (C) Daniel FriedmanManually Closed Flo-Control Valve: The valve handle is turned fully clockwise to prevent hot water circulation [for some models the instructions also include for this position: other than when the circulator is running. ]

[Click to enlarge any image]

The label on the B&G Flo Control valves at left includes instructions on which way to turn the lever.

  1. Emergency heat: the circulator pump is broken and we want hot water for heat to be able to circulate in the home by gravity until the circulator can be repaired or replaced - this is an "emergency heat" function that could be very useful. See Circulator Pumps & Relays.

    Emergency heat means the building has lost electrical power and we want hot water for heat to be able to circulate in t he building by gravity.

    Of course the length of time you'll have hot water for heat in the first place may be limited since the heating boiler's burner or safety controls will usually also be "off" if there is no electricity. But there are exceptions such as a combination wood-oil heating boiler system. See Wood Burning Heaters Fireplaces Stoves.
  2. Filling the system with water or removing air: The heating system piping is air-bound and we are trying to force water through the system to flush out un-wanted air. See Air-bound radiators or piping. This feature is also used when the heating system piping is being filled with water after system drain-down or servicing.
  3. Indirect fired water heaters: to permit hot water circulation to an indirect fired domestic hot water heating system even when no building heat is called for the valve may be set to OPEN in summer weather.

Watch out: B&G warns users to remember to return the lever to the "closed" (fully clockwise) position after the emergency or service. Otherwise you'll get un-wanted hot water heat circulation by gravity even if the room circulator is not calling for heat. Also, these valves sometimes leak around the packing nut below the flo-control valve lever. The valve instruction and service manual describes how to remove, clean, and repair the valve without disassembling system piping.

Reader Question: Definition of Open Winter Closed positions on a flow control valve

Flow control valve lever positions (C) InspectAPedia MFI have a home hot water heating system with two loops, one for the bedrooms and another for the rest of the house.

[Click to enlarge any image]

The gas fired boiler is in the basement. Above the boiler where the piping turns horizontal on both loops are two of what I assume are check valves. They have arrows for flow direction and a lever that can be turned to point at; 'Open', 'Winter' or 'Closed'.

From your helpful articles, I think I understand that 'Winter' might be used to prevent unwanted gravity circulation and that 'Open' could be used for emergency heat and to help purge air from the system, but what about 'Closed'?
Thanks, - M.F. Dec 7 2014


IF the valve is in WINTER and the circulator is OFF, and if you then feel along the pipes on the house side of this valve, once you get a few feet away from the boiler the pipes should be cool (unless heat was just recently running). If the pipes are always hot even when you are not calling for heat (say after 4-8 hours), then the valve is probably no longer working as it should, and it'll need to be replaced.

You might decide to leave it alone until warmer weather just so you don't have to shut down and drain the system during very cold weather - or if the heat waste is bugging you go ahead now.

Reader follow-up

The valves do have the words "Flow Control Valve" on them but no branding info.

1. I now understand the Winter and Open position but is the Summer lever position the same as what you describe as Closed?

2. If I might digress. The reason I've been getting so involved with the furnace again is that when I first turned it on this year it smelled when warming up. It had never done that before in its' 24 years. I thought it might be a dead mouse that was drying out but then I started to hear gurgling in the piping and radiators. Now I believe that the boiler has a leak and I'm smelling the water on a rusty boiler. I have no proof of this.

I know from the schematics on the web that the boiler is made of three pieces of cast iron and bolted together. I have not opened the housing. There is no evidence of a leak around the furnace, floor or anywhere around the copper pipes in the two zones. It's now 24 years old. Is a leak something that I might expect at this stage?

Does one normally fix or replace a boiler of this type? It's a Weil-Mclain Series 3 Model HE-4


If a cast iron boiler is not cracked and the problems are in its controls or piping it usually makes sense to fix those. For an old rusted-out steel boiler or a badly cracked cast iron boiler typically the boiler gets replaced.

B&G Flo-Control Valve Maintenance & Repair

Reader Question: 12/20/2014 Ryan said:

I want to know how to clean a flow control valve. Have heat, but no hot water.


Ryan the control valves shown at the top of this page are not intended for field cleaning but can be replaced if necessary.

Without understanding how your hot water is made I can but guess that you are using an indirect-fired water heater whose heat exchanger operates as an additional heating zone provided by your boiler. A control, relay, thermostatic sensor, or circulator pump for that zone could certainly explain having no hot water. Start by determining that the circulator runs and feeling the pipes for hot water flow to the heater.

Reader follow-up:

We have been looking at that. We have a wood furnace that by passes the oil furnace in the winter and heats the house and the hot water. We have noticed that the circulating pump for the hot water tank is working. The pipes are hot up to the red flow valve to the hot water tank, then go cool to cold. hot water is going into the valve, and then when I reversed the water it came through cold from the holding tank. All the water in the holding tank is cold. The light is on and is calling for heat.


So if your valve has a manual OPEN lever you can try putting it into that position so that the valve is manually kept open - that may give hot water until you can replace the valve. "B&G Flo-Control Valves are used for preventing gravity flow in forced water systems and to permit summer-winter operation of indirect water heaters. " - B&G

On the B&G Flo-Control valves such as the B&G V5182E series, turn the handle fully COUNTER-CLOCKWISE to put the valve into a manual OPEN position.

The valve body CAN be removed without removing the whole assembly from the system, but only if there are isolating valves on either side of the Flo-Control valve.

You'd have to cool the heating system to less than 100F, remove pressure by opening a drain valve, put the valve in manual open (fully counterclockwise), remove the retainer clip-ring at the valve top, remove the valve handle and packing nut, clean the assembly, install new B&G valve packing, and re-assemble. When reassembled the handle is turned fully clockwise to return the flo-control valve to automatic operation.

Watch out: B&G warns: "System fluids under pressure or temperature can be very hazardous. Be sure that pressure has been reduced to zero, the system temperature is below 100F, and the drain is left open during service. Failure to follow these instructions could result in serious personal injury or death and property damage." - REFERENCES

Watch Out: don't try this unless you have the required re-packing/repair parts on hand and don't try it if the system is hot (scalding burn risks) and don't try it in weather so cold that if you're forced to keep heat off for a time there could be building freeze-damage risks.

B&G Flo- Control Valve References & Manuals

B&G Hydrotrol™ Check Valves

Hydrotrol flow control valve from BandG

Bell & Gossett (and other manufacturers) also provide a separate Hydrotrol™ valve to prevent hot water from circulating in a building by gravity when the room thermostat is not calling for heat.

This brass valve includes a knob that permits bypassing this check valve with a simple half-turn, achieving the same emergency heat or service functions we described above.

The Hydrotrol HT flow control valve - a specialty check valve e is used to prevent hot water from circulating through a building where the boiler is kept on to provide domestic hot water via a tankless coil or indirect water heater, during non-heating-season months.

It is also installed to prevent overheating of individual heating zones by gravity. But unlike the red B&G Flo-Control valves discussed above, the Hydrotrol flo-control valve can be installed both vertically and horizontally in heating piping.

Taco Flo-Chek Check Valves

Taco Flo-Chek heating zone flow control valve, manual (C) Taco InspectApedia

Here we illustrate Taco's Flo-Chek heating zone control valve. The flow adjustment on this valve is made by the valve-top mounted knob.

Quoting from the company's product literature, the properties of this valve combine a check valve (don't circulate hot water when the system is not calling for heat) a the thumb screw that can be opened manually for gravity feed applications. This is a check valve, not a zone balancing valve.

Internal Flow Control Valves - IFC Valves on Heating Circulator Pumps

IFC circulators (C) Daniel Friedman

Check valves may be present on a hot water heating system (hydronic heating) but may be less easy to spot.

IFC valves or "internal flow control" check valves are built right into the circulator pump of some models, including some circulators made by Taco® such as the Taco nnn-IFC® (nnn=model number and IFC = internal flow control) circulator pumps.

Taco's product literature comments on the benefits of an IFC type circulator pump:

By locating the patented IFC® inside the pump casing, a separate in-line flow check is eliminated, reducing installation costs.

The reduced pressure drop of the IFC®, increases the 005 flow performance up to 240% over in-line check valves.

Reports are that failures of the check valve function in IFC type circulator pumps are rare.

Manually-Operated Heating Zone Flow Control Valves

Manual heating zone balancing valve adjustment (C) Daniel Friedman

Our photograph shows three Taco hot water heating zone circulator pumps and three lever-type blue-handled heating zone flow control valves that are used to balance the flow of hot water among the three heating zones.

The two bottom valves are in the fully-open position (handle parallel to the heating water pipe) while the top valve is about half-way closed.

When all three heating zones are calling for hot water at the same time, that partly-closed valve reduces the heat flow into that zone.

Why would we do this? Well in most buildings heating zones are not all of the same length. Or zone areas may vary in their rate of heat loss - some needing more heat than others.

By using a manual heating zone balancing valve on each hot water heat zone piping we can tune the flow of heat to prevent some building areas from being too cold while others are too hot - when all of the zones are calling for heat at once.

In the installation shown at above left, the three lever-type ball valves, one on each zone, also make it easier to swap out a circulator pump assembly - if we can close off the heat piping on either side of the circulator pump we can change it out without having to drain down the system and without having to then force air out of the heating piping.

Below we illustrate Bell & Gossett's current heating zone circulator flow control and service valves, the company's Check_Trol valve. Quoting from the product literature:

Can be used with most circulators, including NRF/NBF/SSF wet rotors, Series PLs, 100, PR and LR, and other manufacturers’ pumps. Available sizes: 3/4”, 1”, 1-1/4”, 1-1/2” NPT and Sweat. - B&G Check-Trol Isolation Flow Control Flange, Xylem, Inc. retrieved 1/25/2014, original on file

B&G Check-Trol heating zone flow control valve - InspectApedia & Bell & Gossett

Manually-Set Heating Zone Pipe-Mounted Heating Zone Flow Control Balancing Valves

Heating zone valves with flow balancers (C) Daniel Friedman

A smaller and harder-to-spot manually-set hot water heating zone flow balancing valve is installed on some hydronic heating systems, usually where heat is provided by a single circulator forcing hot water through multiple zone valves.

Because heating zone piping and zone size or zone area heat loss may vary widely within an individual building, if we cannot regulate the hot water flow in individual zones we may find that when more than one of the thermostats are calling for heat at once, some building areas may be too cold while others may be too warm.

The heating system installer or service technician addresses this heat imbalance by fine-tuning the volume or the rate at which heating water is delivered to different building areas.

Notice our blue arrows pointing to those bronze & brass fittings just below each of the red-handled zone drains in our photograph? By loosening the lock-nut and using a flat-bladed screwdriver to open (CCW) or close (turn clockwise) these little valves, the service technician can balance the heating flow among multiple hot water heating zones.


Radiant Heat Flow Control Valves

Radiant floor heating systems provide water flow controls for individual heating zones or even tubing loops within zones. For example see the controls provided by

Variable Speed Circulator Motors Offer Heating Zone Flow Balancing Alternative

Below we show an UpStart hot water heating zone circulator pump produced by B&G (Bell & Gossett). This circulator pump includes a variable-speed circulator motor (see that gold knob on the electrical box atop the motor?). This was a four-speed motor whose output rate could be set by the consumer or heating service technician.

By slowing or speeding the motor we could slow or speed the flow rate of hot water through the zone managed by this circulator. The result: this control provided another way to balance heat flow among multiple zones (and circulators) in a building.

This circulator, the Bell & Gossett SLC-25 Upstart Circulator was sold during the 1980's and is not currently available. Control are also obsolete, but B&G offers similar replacements (such as the NRF-22). At below right is a newer but also obsolete product, the B&G NRF-VS variable speed circulator is shown.

B&G UpStart circulator with variable speed motor (C) Daniel Friedman B&G NRF-VS variable speed control with circuator pump - InspectApedia Bell&Gossett Xylem

Contemporary as well as obsolete variable speed circulator pumps from B&G, Taco & Grundfoss are detailed at CIRCULATOR PUMP RELAYS & OTHER CONTROLS.

Backflow Preventer Valves on Heating System Piping - A Different Check Valve

Automatic water feeder for hot water boiler

Heating System Backflow Preventer Valves Protect Public Water Mains / Private Water Piping from Heating System Water Contaminants

Because a building potable water supply piping cold water line is used to deliver water to hydronic or steam heating boilers, we need to prevent heating system water from back-contaminating the building water supply piping.

At a hot water heating system a heating system backflow preventer check valve is used to keep hot, high pressure water in the hydronic heating system from flowing backwards through a boiler water feed line into the building water supply - a sanitation concern.

Details about heating boiler back flow preventer valves BBFP (green arrow in the photo above) are at BACKFLOW PREVENTER VALVE, HEATING SYS. There we discuss BBFP installation position, leaks, troubleshooting, and product sources.

Information about automatic water feeders (red arrow in the photo above) (that may also contain a built-in backflow preventer) are at WATER FEEDER Valves, Hydronic and WATER FEEDER VALVE, STEAM.

Water Feeder Valves on Hot Water and Steam Heating Systems - as check valves

Automatic water feeder valve

On older hydronic or steam heating boilers the expansion tank and water feeder valve are separate physical units (photo at left). This automatic pressure-reducing water feeder combines a check valve and boiler water feeder.

Like the separate backflow preventer described just above, the check-valve function, internal to the automatic water feeder device, prevents back-flow of heating system water into the building water supply piping. This bell-shaped device opens and sends makeup water into the heating boiler and its piping whenever the heating system's internal water pressure falls below a normal level. The feed-pressure i s typically factor-set to 12 psi when the boiler is cold, but adjustable where higher starting water pressure is needed for taller buildings.

Some older heating systems may not have an automatic water feeder and may only provide a manually operated valve to add water to the boiler. Systems without an automatic water feeder are less safe and risk serious boiler damage should boiler water be lost and should there be no low water cutoff installed on the system.

See WATER FEEDER VALVES for details about water feed valves for hydronic boilers. Steam boiler automatic water feed valves are discussed at WATER FEEDER VALVE, STEAM

While going through the detailed sequence in the operation of the heating boiler, watch for and inspect the condition of the heating boiler controls and safety devices (as required by ASHI 9.1.A.3 automatic safety controls).


Continue reading at WATER FEEDER VALVE, HYDRONIC BOILER or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see BOILER CONTROLS & SWITCHES - master list





If you are looking for information on water supply piping & well system check valves see WATER SUPPLY SYSTEM CHECK VALVES.

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