sizing thermal expansion tank

Mark_39

I am installing an indirect water heater on a new boiler system. I noticed in the installation manual they mention an expansion tank after the mixing valve. Is this really needed, and how do I size it? Thank you, Mark

Uni R

sizing thermal expansion tank

The expansion tank for the domestic side is needed if you have backflow prevention / pressure reducing valve on your incoming water. That tank is sized by the size of your water heater and the heat gain expected. The companies selling the expansion tanks have charts that will show you how to size it. Normally the tanks are mounted on the cold side of the water heater.

hr

Are you on

a city or community water system? If so you many have, or potentially getting a backflow meters or other backflow device. This is the reason that thermal DHW expansion tanks are becoming more prevelent.

If you have a well and well pressure tank, without any check valves between the tank and supply, you should not need one.

If you water supply is a pipe from the street, I'd recommend installing one. Generally the sizing info is on, or in the box. Or visit the supply house, rep, or manufactures webpage for correct sizing info.

hot rod

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Dave Yates (PAH)

be wary of manufacturer's ratings

for thermal expansion tanks as they are based upon non-real-world conditions. The rules are actually pretty straight forward and follow Boyle's Law.

P1 (initial pressure) x V1 (tank volume) must equal P2 (pressure following the expansion) x V2 (volume following acceptance of expansion) P1xV1 = P2xV2

For instance, if your coldest incoming water will be 40F and you'll be heating to 140F, the expansion multiplier is .01501.
A 50-gallon tank will therefore see 50 x .01501 for .75 gallon of thermal expansion.
A 2-gallon TXtank will, in a perfect world, look like this if incoming pressure is forever static at 70 PSI and you increase the bladder's pressure to 70 PSI as well: 70 x 2 = 140. Subtract .75 from 2 and the answer becomes 1.25 x ? = 140 140 divided by 1.25 = 112 PSI. Looks ok, right?

Water heater regs allow for stacking temps to reach 190F and still be in compliance. That multiplier is .03127.

Given that we all know there is no such thing as a constantly static flat-lined incoming water service pressure, you can bet your sweet bippie you'll see spikes. Backflow preventers and PRV's trap those spikes in the house. Take the same formula of 70 x 2 = 140 and change the 70 to a 90 for P2. That represents lost volume before taking thermal expansion into account. Now, utilize the .75-gallon of thermal expansion for V3 x P3.

You'll see that the charts do not reveal the real truth and that the 2-gallon tank will not be the one to utilize if you're hoping to protect your potable water system from excessive pressure.

30-gallons is our cut-off for upgrading to the 4.5-gallon TXtank. Design for design conditions and you'll be adequately protected. Follow pie-in-the-sky optomistic projections and you'll be doing warranty work - for free.



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hr

I suspect most tank manufactures

assume some industry standard pressures. Most codes call for a 45psi pressure reducer, if incoming water exceeds a certain pressure. I believe the UPC used to be 60 or 75? before a PRV was required?

I think some of the Water Conservation acts also stated 45 psi regulators be installed. I used to follow this better when I was a Rural Water Association member.

I imagine this is why the DHW tanks are precharged to 45psi. I suspect the tank manufactures would rather sell the larger sized tank, and don't think they are purposely trying to mislead users with their rule of thumb sizing.

Although the numbers never lie, if you apply them correctly If you have the math and can predict the highest pressure, use the formula Dave provided.

Actually wouldn't the highest possible pressure in the house be the WH relief valve setting? So 150 psi 33- 210° temperature swing?

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Dave Yates (PAH)

air charge

is a function of DOT regs. Hence the 40-PSI factory charge. Our new regs for water PSI list 80 as the break point, but even with a PRV, presure spikes will still affect final delivery pressure, but they do tone them down.

Manufacturers had to settle upon some hard numbers. I just wish they were more realistic. 40 F inlet temp is seen even in places like Florida! Using 50 F and 50 PSI for calculations just doesn't cut it in my school of hard knocks.

The York Water Co tells me that 99% of the failed TXtanks are the 2-gallon variety and that often kills the water heater too. We've seen the same results. For less than 10-bucks, adequate protection can be had using the upgrade.

Besides which, you can make more $ on the higher priced TXtank(G).

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hr

karma-ticously speaking

I error on the safe side with expansion.

I'm switching to #60 tanks on my hydronic systems. When you do the numbers, I use the sizer on the HDS program, you'd be surprised how many times you are on, or over, the line with high volume radiant systems, etc. I've guesstimated too long.

Now if my supplier would only stock the larger sizes

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Barry_2

Sorry to Correct

Dave,

There needs to be an adjustment to your Boyle's law caculations. Boyle's law, as well as the other ideal gas laws, require that absolute temperatures and pressures be used. You need to convert the gauge pressures to psia by adding 14.7 to the gauge reading (30 psig = 44.7 psia). Fahrenheit temperatures can be converted to Rankine by adding 460 to the Fahrenheit temperature. 100 degrees F = 560 R). The volume can remain in cubic feet.

Dave Yates (PAH)

Teach me!

Let's do a V1 x P1 = the constant to V2 x P2 = TC to V3 x P3 = TC exercise using the same numbers I utilized above. IE: 70 PSI, TXrate for 40 to 140 and a 20 PSI pressure spike for P2. How do the numbers compare?

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Dave Yates (PAH)

same results?

I got the same results, but find changing the numbers for practical applications a bit cumbersome. Bear in mind that for those of us out in the field, the first tool of choice for analyzing TX problems will be a standard pressure gauge. Working with what we see is better on site, but I can see where using your numbers would be better for technical papers.

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