I have a 4 zone hot water baseboard heating system. When the thermostat setting is satisfied, the zone valves close with an extremely loud banging. The Valves are 3/4 inch copper, Honeywell #40004850-001. The heating system is 4 years old and this problem started about 2 years ago and seems to get worse each season. I have bled the system several times with no success to solve this problem. I have been told that some Honeywell valves exhibit this problem by closing too quickly. Is there another zone valve manufacturer that closes slower, that someone can recommend? Thanks for your Help!

What water pressure/altitude is shown on the boiler gauge when the unit is operating? What temperature is your boiler operating at? The temperature will also be located on the same gauge as the pressure.

What type of expansion tank do you have? Is it a bladder tank or a standard expansion tank? If bladder style what pressure is it pumped up to?

Water pressure ranges between 15 and 30 PSI.
Water temp. ranges between 130 and 190 degrees. Temp. and pressure fluctuates because the boiler is wood fired with a oil fired backup system. Expansion tank is a Watts pressurized tank with diaphragm No. ET-30.

30 is to high and I suspect the bladder/diaphragm might be reptured/leaking internally. The test for this is to remove protective cover on bottom of tank pump valve and depress the shrader fitting for just a second or so. If water comes out the tank is defective.

It should not hit anywhere near 30 PSI at 190 degrees. Systems at 190 will usually run about 15/16 max. Now there are systems and then there are systems if you know what I mean but the system water pressure has no business being near that high.

Usually a higher pressure means that the exp tank is not absorbing the pressure by making space available for the expanded water. Since the water has no where to go in the exp tank the pressure rises.

((Technically speaking, hydrostatic head is a result of a combination of volume and column height. It is defined as follows:

"...the pressure at a given point in a liquid measured in terms of the vertical height of a column of the liquid needed to produce the same pressure..."

((Expansion tanks, themselves, primarily serve as a receptacle for the expanding water. Fluid in these tanks are a byproduct of the fact that heated water expands. When the sealed units fail, and the air bladder no longer provides a sealed cushion, they become water-logged and require replacement.))

I checked the tank as you suggested and it does leak water from the bottom. How difficult a job is it to replace this tank? Where can I buy a replacement tank? Do you think this will solve the banging noise problem?
Thanks for your help!

Tanks are available locally at a supply house.

Not difficult BUT you have to usually drain part of the system so water does not run out of the opening where the tank is threaded into.

If properly done your banging should go away.

If you do not feel comfortable doing this job it might be better to let a on site professional fix it.