Setting ET Pressure

[Sam M]

Hi All. I have read as many online threads as I can to determine the proper ET pressure on my cold water line. Below is what has been replaced:

New 40 gallon water heater
New 2 gallon ET
New Watts PRV set at 50psi

During the day, water pressure stays mostly consistent at 50psi, this was measured with my Watts gauge and with my plumbers gauge when he replaced the PRV. Overnight, the pressure peaks at 75PSI. Prior to getting my PRV replaced, the pressure at the house was around 75psi which is where I set the ET but I knew the PRV was going to get replaced so I wasn't concerned about being correct for the time being. Also with the ET at 75psi and that is where my pressure peaks, I guess the ET is doing its job. Now with the pressure being lower, I want to adjust it correctly. From what I can conclude, my water pressure mostly stays at 50psi during the day but I've seen it at 55psi. So should I set my ET to 55psi or 60psi?

Also, what is the considered a normal expansion of water over night? I assume if my ET was not installed or set at 75psi, the pressure would be higher? The water heater is set to 120F.

Thank you in advance. Hopefully this is not a repeating thread.

 

[Jadnashua]

If your PRV is set to 50-psi, precharge your ET to the same value. When it is set higher than the normal pressure, it cannot accept any expansion until the pressure rises to the ET's precharge level. If set to 50 where your PRV is, then any expansion can push in on the ET's air bladder. If done this way, your home pressure will stay as close to the PRV's setting as possible. A larger tank would peak slightly lower than a smaller one, but as long as the tank has at least the recommended minimum, it will work fine.

To check and adjust the precharge, you must shut the water off first, then open up a water valve to relieve system pressure pushing on the ET. Then, while the valve is still open, you can adjust the thing. This puts the bladder at the nominal position, ready to accept its maximum amount of water. with minimum effects on the static pressure.

You should not expect overnight pressure to rise. While the utility's pressure might go up, with a properly functioning PRV, that should have no effect on the home's pressure. You only get expansion when you've first used hot water, adding cold to the tank, then stop using water. While it is being heated back up to temp, that cold water expands. Once it has expanded to temp, that stops. I guess if you use a setback thermostat for your home, the pressure could drop slightly as the water in the pipes (not within the WH) cooled off a little, but the delta T won't be much, and the volume in there is small, so while you could probably measure it, you would have to look really close!

 

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[Sam M]

If your PRV is set to 50-psi, precharge your ET to the same value. When it is set higher than the normal pressure, it cannot accept any expansion until the pressure rises to the ET's precharge level. If set to 50 where your PRV is, then any expansion can push in on the ET's air bladder. If done this way, your home pressure will stay as close to the PRV's setting as possible. A larger tank would peak slightly lower than a smaller one, but as long as the tank has at least the recommended minimum, it will work fine.

To check and adjust the precharge, you must shut the water off first, then open up a water valve to relieve system pressure pushing on the ET. Then, while the valve is still open, you can adjust the thing. This puts the bladder at the nominal position, ready to accept its maximum amount of water. with minimum effects on the static pressure.

You should not expect overnight pressure to rise. While the utility's pressure might go up, with a properly functioning PRV, that should have no effect on the home's pressure. You only get expansion when you've first used hot water, adding cold to the tank, then stop using water. While it is being heated back up to temp, that cold water expands. Once it has expanded to temp, that stops. I guess if you use a setback thermostat for your home, the pressure could drop slightly as the water in the pipes (not within the WH) cooled off a little, but the delta T won't be much, and the volume in there is small, so while you could probably measure it, you would have to look really close!

The specs on the tank are in line with what my system is doing so I should be ok on the sizing.

The reason the pressure rose overnight is because most of the hot water gets used at night from running the dishwasher and people showering. During the day hot water is only used occasionally which is why the pressure remains fairly constant.

Thanks for the advise, I will monitor the pressure one more night to be certain prior to adjusting the tank pressure.

Edit: I currently have the tank mounted vertically. From what I have read, when mounted this way, it doesn't really need any support. Is this typically correct?

 

[Reach4]

Thanks for the advise, I will monitor the pressure one more night to be certain prior to adjusting the tank pressure.

Since you have the PRV, there is no need or advantage in waiting. 50 or 51.

 

[Jadnashua]

When the tank eventually fails, it will fill with water. Two gallons will make the thing probably just under 20#. You want to make sure that extra weight isn't going to create undue stress on things. Functionally for the tank when vertical like that, it doesn't really matter, it's the connection that may be at issue.

Any use of water after the WH has reached its set temperature and the heat turns off should relieve the system pressure to the prv setting, regardless of when that happens, but yes, lots of hot water use then everyone goes to bed would likely be your maximum volume of expansion. But, if the precharge is higher than your 'normal' pressure, any expansion will only occur once the pressure rises to that of the ET's precharge. The pressure will spike to that point quickly, then gradually rise as the air in the ET gets compressed and the bladder expands.

 

[Sam M]

Thank you all for the help. I will get a support strap for the tank just in case for when it does fail. I dropped the pressure down to 52psi, landed there after letting air out and decided that should be good enough. I'll monitor the pressure overnight and see if I get another spike.

 

[Sam M]

Highest pressure overnight after pressure adjustment(tank at 52psi) is just under 60psi. Is this acceptable or do I need a bigger ET?

 

[Reach4]

Highest pressure overnight after pressure adjustment(tank at 52psi) is just under 60psi. Is this acceptable or do I need a bigger ET?

60 is amazingly low. A pressure of 120 after expansion would not be a problem IMO.

 

[Sam M]

60 is amazingly low. A pressure of 120 after expansion would not be a problem IMO.

So in a perfect world the ET would absorb all the expansion pressure but realistically with my incoming water pressure of 50psi, ET at 52psi, and overnight spike after using hot water of a hair under 60psi isn't bad at all? I am sure I don't have things set 100% with gauge's potentially being slightly off as well.

 

[Reach4]

I am sure I don't have things set 100% with gauge's potentially being slightly off as well.

I suspect it is the same gauge you used for setting the PRV to 50 psi.

What could possibly be happening is that your gauge's lazy (tattletale) hand works its way lower overnight due to tiny vibrations. I have seen the lazy hand jump up due to flicking the gauge.

 

[Jadnashua]

Code guidelines call for pressure to not exceed 80psi. If yours peaks out at 60, your systems are safe.

When you compress the air bladder in the tank, the pressure there goes up as well as the water pressure. The relative volume of air in the tank will determine how much it raises the pressure...a larger volume of air, the lower the percentage of change. You're forcing the pressure higher by having the precharge higher than your nominal water pressure setting, but it's only 2 psi. Not a big deal. The bladder won't start to compress until the water pressure matches and then exceeds the starting air pressure. Lowering the ETs precharge below the nominal water pressure means that it's already been compressed some, and the percentage change will be higher.

 

[Sam M]

I suspect it is the same gauge you used for setting the PRV to 50 psi.

What could possibly be happening is that your gauge's lazy (tattletale) hand works its way lower overnight due to tiny vibrations. I have seen the lazy hand jump up due to flicking the gauge.

Funny you mention that because when I tap the gauge the lazy red needle does creep up.

Code guidelines call for pressure to not exceed 80psi. If yours peaks out at 60, your systems are safe.

When you compress the air bladder in the tank, the pressure there goes up as well as the water pressure. The relative volume of air in the tank will determine how much it raises the pressure...a larger volume of air, the lower the percentage of change. You're forcing the pressure higher by having the precharge higher than your nominal water pressure setting, but it's only 2 psi. Not a big deal. The bladder won't start to compress until the water pressure matches and then exceeds the starting air pressure. Lowering the ETs precharge below the nominal water pressure means that it's already been compressed some, and the percentage change will be higher.

Thanks for the explanation, that makes sense. I will leave my system the way it is and do routine checks to make sure everything stays in check.

 

[Reach4]

Code guidelines call for pressure to not exceed 80psi. If yours peaks out at 60, your systems are safe.

I doubt that you will find a code entry that forbids the pressure at the expansion tank from rising to more than 80.. I am confident that you will find 80 psi, but not in that context.

Your interpretation would prohibit using a 100 psi relief valve to deal with thermal expansion.

 

[Jadnashua]

Pressure in a residential distribution system by code, for whatever reasons, is not supposed to exceed 80psi. Valves, seals, WH, hoses, etc., are built with that in mind. So, if yours did get above 80psi, for whatever reason, you'd want to figure out why and fix it. All of those devices are tested to a higher pressure to allow for some margin for error, but, things will last longer if you abide by that number.

Things like your washing machine hoses will balloon some. Anything that you blow up and then deflate adds wear, and eventually, could cause it to fail. The goal is to minimize the variations. The WH will expand and contract with temperature changes as you use water, and, pressure can change through expansion. The glass lining, like anything else, can stress crack eventually. That lets water get to the steel, which starts it rusting. The sacrificial anode helps, but most people don't change them, and they do get eaten up over time. Better them than the WH itself!

 

[Reach4]

Pressure in a residential distribution system by code, for whatever reasons, is not supposed to exceed 80psi.

You will not find a citation to that effect. The static pressure is not the same as the pressure after thermal expansion.

 

[Jadnashua]

FWIW, the static pressure could easily be on the system for days as the result of expansion if no other water is used and there are no leaks. In a closed system with a working check valve, it could easily be at 150psi, limited by the T&P valve. Everyone showers, then they go on vacation...could be weeks that it stays high. One needs to take into account the intent of the regulations.

You do not want your pressure to be above 80psi...if it gets there, fix it.

Code does allow the use of a relief valve at 100psi with a prv set to 80psi in a properly engineered system to account for expansion, so it could peak to 100. A lousy situation, IMHO. Throwing that extra pressure on the system just isn't a great idea.

 

[Reach4]

Code does allow the use of a relief valve at 100psi with a prv set to 80psi in a properly engineered system to account for expansion, so it could peak to 100. A lousy situation, IMHO. Throwing that extra pressure on the system just isn't a great idea.

With a thermal expansion tank, the pressure can properly peak even more than that.