Expansion tank for hot water tank?

[Mike Garrod]

According to Canadian NPC, provision must be made for thermal expansion when check valves or pressure reducing valves are installed. Is the T&P relief valve sufficient to meet this requirement? Or must an expansion tank be installed. I always install an expansion tank when check valves are installed, but I never considered a pressure reducing valve as creating a closed loop system since many prv’s will indeed allow flow each way. Professional plumbers here, do you always install expansion tanks if a PRV is installed?

 

[Terry]

I used to always install an expansion tank when there was a PRV, until someone pointed out the bypass feature.

Specifications Standard Specifications: A Water Pressure Reducing Valve with intergral strainer shall be installed in the water service pipe near its entrance to the building where supply main pressure exceeds 60psi (413 kPa) to reduce it to 50psi (345 kPa) or lower. The valve shall feature a bronze body suitable for water supply pressures up to 300psi (20.7 bar). Provision shall be made to permit the bypass flow of water back through the valve into the main when pressures, due to thermal expansion on the outlet side of the valve, exceed the pressure in the main supply. Water Pressure Reducing Valve with built-in bypass check valves will be acceptable. Approved valve shall be listed to ASSE 1003 and IAPMO and certified to CSA B356. Valve shall be a Watts Series 25AUB-Z3.* *A water saving test program concluded that reducing the supply pressure from 80-50psi (551-345 kPa) resulted in a water savings of 30%. **The bypass feature will not prevent the pressure relief valve from opening on the hot water supply system with pressure above 150psi (10.3 bar).

I first noticed issues with "check valves" on water meters when the city changed out my meter and within a month my water heater failed. It was old anyway, but the check valve must have done her in. That home didn't have a PRV.

 

[Sponsor]

 

[FullySprinklered]

In some areas of Metro Atlanta, the street pressure can be as high as 140lb.

 

[Jadnashua]

The goal should be consistent, safe pressure on the house side. The T&P valve opens under pressure at 150psi which is not particularly safe. In a closed system, since the pipes don't expand much, that can happen quite quickly when you start heating water up. So, with each significant hot water use, you may get a discharge from the T&P valve. They aren't designed to be used in that manner on a regular basis.

A bypass on the PRV will limit the internal house pressure to slightly more than the supply pressure...but, that could be quite high, and while maybe a backup, doesn't work to maintain that safe internal house pressure as it will rise to just above the incoming pressure before it can then push back into the system. Plus, many utilities also have a check valve to help prevent that from happening to protect their water from potential contamination, so a bypass can't actually do anything.

So, IMHO, put in an expansion tank. They do fail, but are fairly reliable and cheap to replace when they do fail. I don't see why people object to installing one. That pressure fluctuation isn't great for your washing machine hoses, and other supply hoses either.

 

[Mike Garrod]

I used to always install an expansion tank when there was a PRV, until someone pointed out the bypass feature.

Specifications Standard Specifications: A Water Pressure Reducing Valve with intergral strainer shall be installed in the water service pipe near its entrance to the building where supply main pressure exceeds 60psi (413 kPa) to reduce it to 50psi (345 kPa) or lower. The valve shall feature a bronze body suitable for water supply pressures up to 300psi (20.7 bar). Provision shall be made to permit the bypass flow of water back through the valve into the main when pressures, due to thermal expansion on the outlet side of the valve, exceed the pressure in the main supply. Water Pressure Reducing Valve with built-in bypass check valves will be acceptable. Approved valve shall be listed to ASSE 1003 and IAPMO and certified to CSA B356. Valve shall be a Watts Series 25AUB-Z3.* *A water saving test program concluded that reducing the supply pressure from 80-50psi (551-345 kPa) resulted in a water savings of 30%. **The bypass feature will not prevent the pressure relief valve from opening on the hot water supply system with pressure above 150psi (10.3 bar).

I first noticed issues with "check valves" on water meters when the city changed out my meter and within a month my water heater failed. It was old anyway, but the check valve must have done her in. That home didn't have a PRV.

So you no longer install expansion tanks when there is a PRV? To me that requirement sounds pretty specific, meaning you have to have the Watts 25AUB-Z3.

I used to always install an expansion tank when there was a PRV, until someone pointed out the bypass feature.

Specifications Standard Specifications: A Water Pressure Reducing Valve with intergral strainer shall be installed in the water service pipe near its entrance to the building where supply main pressure exceeds 60psi (413 kPa) to reduce it to 50psi (345 kPa) or lower. The valve shall feature a bronze body suitable for water supply pressures up to 300psi (20.7 bar). Provision shall be made to permit the bypass flow of water back through the valve into the main when pressures, due to thermal expansion on the outlet side of the valve, exceed the pressure in the main supply. Water Pressure Reducing Valve with built-in bypass check valves will be acceptable. Approved valve shall be listed to ASSE 1003 and IAPMO and certified to CSA B356. Valve shall be a Watts Series 25AUB-Z3.* *A water saving test program concluded that reducing the supply pressure from 80-50psi (551-345 kPa) resulted in a water savings of 30%. **The bypass feature will not prevent the pressure relief valve from opening on the hot water supply system with pressure above 150psi (10.3 bar).

I first noticed issues with "check valves" on water meters when the city changed out my meter and within a month my water heater failed. It was old anyway, but the check valve must have done her in. That home didn't have a PRV.

So you no longer install expansion tanks on hot water tanks if no PRV is installed?

 

[Mike Garrod]

The goal should be consistent, safe pressure on the house side. The T&P valve opens under pressure at 150psi which is not particularly safe. In a closed system, since the pipes don't expand much, that can happen quite quickly when you start heating water up. So, with each significant hot water use, you may get a discharge from the T&P valve. They aren't designed to be used in that manner on a regular basis.

A bypass on the PRV will limit the internal house pressure to slightly more than the supply pressure...but, that could be quite high, and while maybe a backup, doesn't work to maintain that safe internal house pressure as it will rise to just above the incoming pressure before it can then push back into the system. Plus, many utilities also have a check valve to help prevent that from happening to protect their water from potential contamination, so a bypass can't actually do anything.

So, IMHO, put in an expansion tank. They do fail, but are fairly reliable and cheap to replace when they do fail. I don't see why people object to installing one. That pressure fluctuation isn't great for your washing machine hoses, and other supply hoses either.

Thanks. I think I would prefer to err on the side of caution and install one like you said. It’s a small price to pay to for added protection!

 

[Reach4]

So you no longer install expansion tanks when there is a PRV? To me that requirement sounds pretty specific, meaning you have to have the Watts 25AUB-Z3.

Terry was quoting the specification from Watts that they hope the specifying engineer copies into the plan specs verbatim. So it describes the valve. It was not code or law.

If there is a check valve before the PRV, you would still need a thermal expansion tank.