Febco805Y

If you have questions about equipment size, water heaters or expansion tanks, you should talk to your manufacturing representative to ensure that you’re selecting the best equipment sized correctly for your application.

Can I close the upstream valve, leave the downstream valve open, then use the downstream test cock (red arrow) to feed compressed air in? Looking at the parts diagram, it seems like this would work. I just don't know if this would damage the backflow preventer.

Febco 850manual

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FebcoLF850

Febco 850Repair Kit

As a rule of thumb, a thermal expansion tank should be about 10% of the total water heater volume. Let’s take some examples:

Remember, this is just a rule of thumb. In addition to water heater volume, an engineer will consider other factors when sizing and selecting a thermal expansion tank, such as entering and leaving water temperatures and minimum and maximum pressures. The expansion volume may then be calculated and a tank selected based on acceptance volume and overall volume. In most cases, the engineered expansion tank size will be smaller than the 10% of the water heater volume, but, for budgeting/planning purposes, this estimate will get you in the right ballpark.

FEBCO 850Double Check Valve

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Winter is coming, and I'm planning to winterize my irrigation system. I have a 3/4" Febco 850 backflow preventer. I read the winterization instructions, but they rely on there being a main shutoff valve, an inlet drain valve, and an outlet drain valve. My installation doesn't have any of those valves. I just have the two shutoff valves that are built into the 850.

This is a place for professionals and non-professionals alike to discuss irrigation. Ask a question, ask for help, show off your work (no advertising), or show us what some idiot did. Just keep it cordial. Check out the user and post flair, and think about adding it to your post. If you receive help from this sub, please let us know how we helped you.

To protect the water heater, pipes, and other system components from excessive pressure when heating water, an expansion tank is added. A simple device, it is nothing more than a tank with a bladder (thick balloon) inside. The space between the bladder and the inner wall of the tank is pre-charged with air to closely match the neutral pressure of the system BEFORE the water is heated. Then, when heat is applied and the pressure goes up, the expanded water volume flows inside the bladder. The air outside the bladder compresses against the tank walls, providing the cushion needed to protect the rest of the system.

When you look a water heater in a large building or even a home, you may notice a much smaller tank close by. That small tank is a thermal expansion tank. An expansion tank in this application is a vessel design to accept additional system volume resulting from heating water. Water is incompressible and, when it is heated, it expands. If this happens in a system with a fixed volume of pipes and equipment, you will quickly discover the weakest point in the system as water begins to spray through it. (Hopefully, it is the pressure relief valve releasing water!)