Location For Expansion Tank - Before or After WH Shut-Off

[hdtvkeith]

50 Gallon Gas AO Smith

 

[Twowaxhack]

50 Gallon Gas AO Smith

What’s your incoming water pressure ?

 

[hdtvkeith]

Last time I looked at street side was about 90+ PSI I have PRV set to 62ish and expansion tank is around 64-65. I need to look at street side again it has been a while.

 

[Twowaxhack]

Last time I looked at street side was about 90+ PSI I have PRV set to 62ish and expansion tank is around 64-65. I need to look at street side again it has been a while.

I wouldn’t really worry about street pressure if your PRV is keeping it at 65ish

 

[skeezix]

Yet showers and tubs don’t require service stops.

I could never figger that one out

 

[teamarrows]

I oversized my expansion tank because during winter my tap is around 55F and my water heater is set to 140 and with variation can generate water anywhere from 135-145 depending on how much was used and replaced and how long it sat. On my to gallon gas heater I use a 4.5 gallon expansion tank. I was told that is larger than needed, but a few extra $$ for piece of mind for me is fine. Twice a year I drain the tank, release pressure from water lines and double check pressure in expansion tank to the house pressure. My expansion tank it at least 15+ pipe feet away from heater. There is a shutoff at the heater and one as soon as the main enters the home. I never close the heater shutoff without putting the heater into Vacation/pilot mode.

Do you monitor the pressure? If so, how much of an increase do you see with the tank installed? Any idea what it was before the tank was installed?

 

[hdtvkeith]

You should not see any real increase in pressure with an expansion tank installed if it is balanced to match the water pressure.

 

[teamarrows]

The expansion tank allows the expanding water to compress the air charge that you put into the expansion tank rather than spike the system pressure.

Is a 3.2 gallon tank on a 40 gallon WH enough to absorb a 40 PSI spike? What I've been seeing since the new tank was installed is 45-50 PSI static and 85 - 90 PSI after the WH does a full recovery after a shower, etc. Once the water is off, it takes about 6 minutes climb to the 85 - 90 range then holds that until a faucet is open.

Strange part is that even when I increase the static PSI to 60 via the PVR, I still max out at 85 - 90 after recovery.

I like to exceed my water pressure by a few pounds to insure My air charge is more than my water pressure. This insures my tank is sufficient for my application. It allows for the gauges to be off a little and not affect the tanks performance.

But doesn't that mean the system pressure in the system spikes by the extra PSI that the tank is set at? I'm still missing the physics behind the extra 5 PSI in the expansion tank.

 

[Twowaxhack]

Is a 3.2 gallon tank on a 40 gallon WH enough to absorb a 40 PSI spike? What I've been seeing since the new tank was installed is 45-50 PSI static and 85 - 90 PSI after the WH does a full recovery after a shower, etc. Once the water is off, it takes about 6 minutes climb to the 85 - 90 range then holds that until a faucet is open.

Strange part is that even when I increase the static PSI to 60 via the PVR, I still max out at 85 - 90 after recovery.



But doesn't that mean the system pressure in the system spikes by the extra PSI that the tank is set at? I'm still missing the physics behind the extra 5 PSI in the expansion tank.

Yes the extra 5 psi makes the pressure go up 5 psi before the tank controls it. That’s not a problem.

Your expansion tank should prevent the system pressure to rise above the expansion tanks set air pressure charge if the tank is operating correctly and adequately sized.

What temp are you heating your water to ?

 

[hdtvkeith]

Calculator. The key is knowing temperate rate of rise and the water pressure then the air pressure in the tank. Here in GA the cold water now is 55F, I have water heaters (50 gallon) set to 140F, assuming my water pressure is 65 and the expansion tank is set to 65 then the pressure relief vale is 150psi I only need like a 2 gallon tank, To me I set the calculation to use a relief value of 100psi to that there is no change the pressure never gets too high. I lost a water heater over 7 years ago becasue the expansion tank pressure had dropped a bit (I was not checking it the expansion tank regularly) and the tank sprung a leak due to excess pressure. This is why I oversize my expansion tank to 4.5 gallons. A few extra $$ yet larger margin of protection.

https://www.watts.com/resources/planning/etp

 

[Twowaxhack]

I don’t agree with watts on setting the the starting temp to 90 degrees unless your tank is depleted on each use.

The starting temp for sizing purposes should be no more than the coldest water the heater would ever see.

In my case that temp would be around 50 degrees after a 10 gallon purge.

 

[hdtvkeith]

We take long showers so I just use tap temp of 55F and target of 140F

 

[teamarrows]

Yes the extra 5 psi makes the pressure go up 5 psi before the tank controls it. That’s not a problem.

Your expansion tank should prevent the system pressure to rise above the expansion tanks set air pressure charge if the tank is operating correctly and adequately sized.

What temp are you heating your water to ?

As I mentioned earlier, I'm aiming for 120° - 125°F. Tank was heating to 140°F after install so I'm slowly turning it down. The stat sure isn't linear so it's taking some playing around.

 

[teamarrows]

I don’t agree with watts on setting the the starting temp to 90 degrees unless your tank is depleted on each use.

The starting temp for sizing purposes should be no more than the coldest water the heater would ever see.

In my case that temp would be around 50 degrees after a 10 gallon purge.

I'm confused by your wording. It seems to be the opposite of how I see it.

Unless the tank is depleted on each use, the water temp in the tank will never get down to 50°-ish range. A starting temp of 90° seems reasonable as the lowest temp the water would be in normal, everyday use. Sure, the WH will see 50° water as it "refills" but it's not raising a full e.g. 40 gallons of 50° water to 1XXish, it only has to raise the blended temp (90°-ish).

Doesn't the amount of expansion within the system depend on the delta-T during recovery?

 

[Twowaxhack]

I'm confused by your wording. It seems to be the opposite of how I see it.

Unless the tank is depleted on each use, the water temp in the tank will never get down to 50°-ish range. A starting temp of 90° seems reasonable as the lowest temp the water would be in normal, everyday use. Sure, the WH will see 50° water as it "refills" but it's not raising a full e.g. 40 gallons of 50° water to 1XXish, it only has to raise the blended temp (90°-ish).

Doesn't the amount of expansion within the system depend on the delta-T during recovery?

When a water heater is sized to the typical home it’s not sized thinking the tank will never drop below 90 degrees.

I don’t want my thermal expansion controlled most of the time, I want it controlled all the time.

That’s my thinking.

 

[teamarrows]

When a water heater is sized to the typical home it’s not sized thinking the tank will never drop below 90 degrees.

I don’t want my thermal expansion controlled most of the time, I want it controlled all the time.

That’s my thinking.

OK. Makes sense.

It was impossible to find a 3.2 gallon tank locally, so I went with a 2.1. Even the biggest plumbing supply houses in the area only stock 2.1 and 4.6. When I gave them my specs, they all said 2.1 would be fine.

After I installed the tank, with the PVR set at 55 psi, I haven't see anything higher than 68 psi. That was after a bringing the WH down to about 80°F and letting it recover to 125°.

I'm comfortable with those numbers.

 

[Twowaxhack]

OK. Makes sense.

It was impossible to find a 3.2 gallon tank locally, so I went with a 2.1. Even the biggest plumbing supply houses in the area only stock 2.1 and 4.6. When I gave them my specs, they all said 2.1 would be fine.

After I installed the tank, with the PVR set at 55 psi, I haven't see anything higher than 68 psi. That was after a bringing the WH down to about 80°F and letting it recover to 125°.

I'm comfortable with those numbers.

Look at how they tell you to put the tank in service when you size it based off 90 degrees.

They want you to bleed the extra pressure off manually that caused by heating a completely cold tank upon initial installation.

I size mine based off a cold tank. This covers me no matter what.