FAQ: General Plumbing

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by Rich Grimes

You have covered Pressure Vacuum Breakers. What is the difference between an Atmospheric Vacuum Breaker and a Pressure Vacuum Breaker?

A pressure vacuum breaker (PVB) is designed to be under constant pressure. An atmospheric vacuum breaker (AVB) is NOT designed to be under constant water pressure. Both devices are designed to relieve backflow but the AVB opens when the pressure drops to atmospheric pressure.. This breaks the vacuum and opens the poppet. This allows air to enter and water in the spout to drain out. A hose bibb vacuum breaker works similarly. Most failures of AVB’s occur because they are exposed to constant pressure on mop/utility sink faucets. Pressure backflow should be handled a PVB, a testable device that is designed for pressure backflow relief.

How are AVB’s exposed to constant pressure on a mop/utility sink faucet?

Many maintenance personnel will attach a hose-end type shut off valve on the faucet spout. They tend to leave the faucet valves (hot & cold) open on the faucet but shut off at the end of the spout. This puts the AVB under continuous pressure and eventually the poppet cartridge fails. There are also chemical stations that utilize the same set up which pressurizes the chemical station but also expose the AVB to constant pressure.

What other issues can arise with mop/utility sink faucets?

Open, Two-handle faucets will put an AVB under continuous pressure. A 3-Handle faucet (One on the spout – mixing hot & cold water) will also apply constant pressure. Many installations of these faucets do not have check valves (internal or external) to prevent bleeding of hot and cold water. If check valves are not employed, a cross connection has been made and it creates hot water supply and recirculation issues. The cold water can mix with the hot water return which causes the recirculation pump and aquastat to remain on. This uses a lot of fuel and puts a lot of stress on the water heater. This can be very expensive to operate a tank-type water heater this way and even worse on a flow-activated tankless heater. It is critical to make sure that check valves are pre-installed on the faucet or installed externally.

Where else are AVB’s used besides on mop/utility faucets?

They can be used in a variety of applications that require gravity drainage and backflow. Sprinkler systems and Lab Faucets use AVB’s for drainage and backflow prevention. Steam-fired heat exchangers and commercial dishwashers use AVB’s as well. AVB’s prevent siphoning and backflow by allowing air to enter and break the vacuum. Note that systems under constant pressure should use a PVB.

I have been seeing newer residential gas heaters that do not have an FVIR (flammable vapor ignition resistant) screen in the combustion box yet they are not required to be mounted on an 18” high stand. Please explain this technology and how this is possible.

Rising efficiency standards inspire new designs. These new designs must meet all of the efficiency standards and also meet the requirements of FVIR design. The design you refer to uses an elevated air intake, or “snorkel”. The air intake is located higher than 18” from the bottom of the heater, above the floor. This negates the need for an FVIR screen and stand. It also provides a more robust air intake for cleaner combustion.

There are other new designs for high efficiency residential gas that employ a draft inducer (fan-assisted exhaust) and electronic controls. The new Federal efficiency standards that come into effect in 2015 will bring about new designs as well as major changes to some products.

I have heard of tankless heaters (up to 199,000 BTU’s) that claim to be able to operate on a ½” gas supply line up to 24 Feet. How is this possible?

Common sense says that you can only flow a given amount of air, water or gas through a given pipe size. In terms of gas piping, higher pressures allow more BTU’s to be carried through a smaller pipe.

This claim is made on low pressure operation (5” to 13.5” WC). The part that is not advertised is that to operate on a reduced gas input, you must reduce burner input and thus reduce hot water GPM output. You simply cannot burn 199,000 BTU’s on a ½” low pressure gas supply. However, a 199,000 BTU heater that has been modulated down to 50% firing is only burning 100,000 BTU per hour. Measured percentage of modulation, GPM flow rate and temperature rise would be indicators of a unit’s true performance.

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