PlumberDave

Most towns in the Seattle area require an expansion tank on all water heaters. But all areas require an expansion tank with any type or style of closing a system PRV DC RPBP whatever. To many problems with high pressure here I have seen 215psi on city lines. UPC Code requires we allow for thermal expansion but most localities add there bit to it.

Tankless

Intersesting enough....and I made a comment on the recent threads about this topic. I had a job where I was walking the inspector through everything. Out at the street there is an RPZ on 1" risers. At the house service entrance, there is 1". There is also 1 1/4" for (what I assumed) suppression. Inspector told me the RP was for fire. I showed him the piping and its sizes and he scratched his head. Said "looks like a goof". I just told him there is an XTX on the domestic. I get him to the location of my tankless heaters and he said I didn't need the XTX. Never gave me a good reason for it, just told me with the setup I had it was overkill but he said he likes that I airred on the side of safety. I just told him, I found what I thought was a closed system, therefor without further question it gets an XTX. He said....."yeah...but with the tankless it's just not necessary." I didn't really know what else to say.

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ToUtahNow

I believe it is only hot water storage they are concerned with. In theory, the system is open when a tankless fires.

Mark

robthaplumber

Go ahead and install an expansion tank. It will prevent a call back. I have installed prv's with integral bypasses and still had to install a XTank. Save yourself a trip back and put one in.

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AKdaplumba

is the bypass intergrated within the PRV? I have never noticed one on any that we have installed.

robthaplumber

According to the manufacturer, yes. Check out the Watts website.

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Redwood

Some PRV's have an integral bypass while others don't.

Truth be told the bypass is only effective if the water supply pressure is low enough to absorb the pressure caused by thermal expansion without getting close enough to 150-psi to activate the T&P valve.

One thing you should consider with water heaters is they are pressure vessels and there is a certain amount of expansion and contraction from pressure changes built into the design.

If your installation of an internal bypass PRV without a thermal expansion tank is allowing the pressure to cycle from the set pressure of the PRV @ 75-psi up to 145-psi on thermal expansion where it matches the 145-psi line pressure in the street and it bypasses through the PRV without the T&P activating, even though there are no symptoms that would generate a callback, you aren't doing your customer any favors.

Similar to the nine lives of a cat each pressure cycle from 75-psi up to 145-psi and back down has an effect on the service life of the water heater. The glass lining will crack allowing water to reach the steel and start the corrosion process, and the steel tank itself will start to experience metal fatigue from the flexing during the cycles. Simply stated the tank will start to leak much sooner than a tank which is at a constant pressure.

Even if the water supply pressure is in the lower ranges such as 90-psi and you install a PRV with an internal bypass your installation is only good as long as the water company doesn't come and install a new meter and check valve or, increase the water supply pressure to cover the water supply needs of that new development up on the hill.

Metal fatigue from pressure cycling is real and the illustration I commonly use is Aloha Airlines Flight 243 where an Boeing 737-297 airliner was used it's whole life in service on short hops island to island in Hawaii. In doing so it racked up a huge number of pressure cycles despite having a low number of hours on the airframe. The aircraft had the second highest number of cycles at 89,090 in the entire 737 fleet, the airframe had only 35,496 hours in flight, the Boeing design was expected to be good for 51,000 hours and 75,000 cycles. However due to many of the flights not going high enough to require full pressurization not all the cycles were counted in accordance with the methods used at that time, Flight 243 changed that! The aircraft was at 24,000 feet in flight when the skin of the aircraft failed and they had an explosive decompression. http://www.aloha.net/~icarus/index.htm

Do yourself and your customer a favor and install an expansion tank!
I would consider it good plumbing.

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Protech

Well said, Red

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grandpa

Even with a bypass on the regulator, the pressure on the house side could increase up to the point where it equals street pressure. Remember that at 150 PSI the relief valve should be wide open. In my experience, a relief valve may start to "weep" at pressures as low as 120 or so. The expansion tank is the only way to avoid that.

ToUtahNow

There's no question you need to know the historic highs and lows of a system. I am on a tank so my pressure is constantly 185 psi and would need an expansion tank. Up the hill and across the highway they have 35 psi from the same tank. Now if I go into Westwood they are on a reservoir and depending on the water level at the time they vary between 85 psi and 150 psi so I would install an expansion tank.

Mark