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Heating boiler backflow preventer valves (BBFP): this article describes check valves installed on the water supply to heating boilers to prevent heating system water from re-entering the building potable water supply system and its piping. Backflow preventers are installed on hot water (hydronic) or steam boilers on the boiler water feed line as required by both good plumbing practice and by building codes in many jurisdictions.
This article describes backflow preventers used on heating boilers including their function, location or placement, and their troubleshooting and repair of leaks or other problems.
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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.
[Click to enlarge any image]
Why do we need a Backflow Prevention Device on Heating Boilers?
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. This back-contamination, or back-flow could occur, for example, if there were a loss of water pressure on the building supply-side of the heating boiler. In that condition, the boiler water, typically at 12-30 psi depending on boiler temperature, could be forced back into the building's potable water piping system, contaminating it with boiler chemicals or debris, even bacteria.
At our hot water heating boiler backflow preventer photo (above left) the automatic water feed valve (red arrow) includes a built-in backflow prevention device, while a separate, code-required backflow preventer valve is shown by the green arrow.
Our photo at left shows a similar backflow preventer valve (green arrow) installed at a steam heating boiler. Again you'll see that the valve is installed between the potable water supply piping and all other boiler controls and components. Click to see this close-up photo of this backflow preventer on a steam boiler.
More generally connection of potable water piping to any device that could potentially send unsanitary water backwards into the potable water system is called a cross connection - a potential drinking water sanitation issue explained in detail at CROSS CONNECTIONS, PLUMBING.
So we install a backflow preventer - a special check valve - at the heating boiler.
Separately from the backflow preventer valve shown in the photos above, at both hot water (hydronic) heating boilers and steam heating systems a backflow preventer may also have been built into the automatic water feed valve itself - a separate control described in our article on WATER FEEDER VALVE, HYDRONIC BOILER in that article's section at BACKFLOW PREVENTER, HEATER WATER FEEDER.
Depending on local plumbing codes, a separate (from the water feeder valve) dedicated backflow preventer valve may also be installed "upstream" of the automatic water feeder - that is, between the heating system water feed valve and the incoming municipal water supply cold water piping (green arrow).
These are two separate devices for heating system water backflow prevention, but often you will see both of them installed.
The blue arrow in our photograph shows the direction of water flow into the water feeder and onwards into the heating boiler.
In sum, backflow preventers found on heating systems are installed to prevent higher-pressure heating system water from being forced backwards (say through a water feeder valve) into the building potable water supply piping.
Backflow Preventer Valve Installation Position on Heating Systems
The backflow preventer valve such as the Watts 9D Dual Check valve can be installed either horizontally or vertically on the building water supply piping, but it should be positioned at the heating boiler before (on the supply side) the pressure regulator and as you will note in our photos below, it is also ahead of the expansion tank.
[Click to enlarge any image.]
Shown above: a Conbraco™ Series 40-400 continuous pressure backflow preventer installed at a residential heating boiler. The company's catalog describes this product as follows:
Reader Question: in what order should the backflow preventer be installed?
(Mar 17, 2013) DW said: I am in New York city where a check valve or back flow preventer (3/4 BBFP) [Boiler Back Flow Preventer] is required for domestic heating systems. I have a indirect hot water tank attached to a gas boiler. My question is - What order should the back flow preventer be installed in A or B. We have had no problems but neighbors with the same set up have had a lot of leaks from the check valve
A. city water>BBFP>expansion tank>hot water tank
B. city water>expansion tank>BBFP>hot water tank
Note: BBFP Dual Check Valves with Intermediate Atmospheric Vent prevent the reverse flow of hot or cold polluted water from entering into the potable water supply. It consists of a brass body with stainless steel internal parts, integral strainer, and two durable, tight-sealing, rubber check valves.
Designed for continuous pressure, non-health hazard applications in smaller supply lines, such as laboratory equipment, processing tanks, sterilizers, dairy equipment, and specifically for boiler feed lines. Minimum Working Pressure: 25psi (172kPa), Maximum Working Pressure: 175psi (12.06 bar)
Above at DW's question we include photographs of Conbraco™ backflow preventer check valve installed at a residential heating boiler. You'll see that the back-flow preventer is installed on the building potable water supply piping ahead of the pressure reducer/regulator valve and ahead of the expansion tank. This is the proper location.
Thanks for the question DW, above I have included an adapted schematic provided by Watts corporation showing the installation position of the 9D Dual Check Valve with an atmospheric vent (above left) . The BBFP you cite probably refers to Watts BBFP, 3/4" CxC Backflow Preventer Watts 0063193 which is a similar device shown at above right.
In the backflow preventer valve photo at left our blue arrow shows the direction of incoming water from the potable water supply and the small yellow arrow at the discharge port shows where water may be vented by the valve in normal operation.
Watts describes that BBFP backflow preventer as follows:
I'm doubtful that I have the whole picture of the building piping and requirements you describe but it seems to me that if you have a back flow preventer valve BETWEEN a heating appliance and its related expansion tank, you've defeated the purpose of the expansion tank - it's as if you eliminated it from the system. The result could be relief valve leaks etc.
The backflow preventer should be installed on the fresh water feed line before any pressure regulators.
Expansion tanks on the system will be on the building side of all of that equipment.
Strainers to Protect the Backflow Preventer Valve From Debris
In some plumbing jurisdictions such as in Australia and New Zealand, Watts' description of the proper installation of the company's double check valve assembly (required by building codes) includes the following advice for installation of a strainer to reduce the chances of debris entering the check valve assembly. Debris can cause the assembly check valve ports to stick open, leading to valve failure to protect against backflow for the Series 719, or debris may cause leaks at the atmospheric vent opening on the Series 9 check valves.
To avoid debris-clogging or interference with the operation of a backflow preventer, the supply piping should be flushed of any debris, solder, flux, pipe chips etc. before the valve is installed. In addition, the valve's strainer should be cleaned "... every six months or as conditions require" - Watts.
References for heating boiler backflow preventer valve installation
Backflow Preventer Valve Troubleshooting: Leaks
Reader Question: diagnosing leaks at the backflow preventer valve
Edward Martin said:
Could a defective automatic water feeder valve cause backflow preventer valve to leak at the Vent side of the "T"
Reply: causes of leaks at inline check valves
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