Plumbing Forum - Professional & DIY Plumbing Forum > General Plumbing Discussion > Water Heaters and Softeners > Two 50 gal hot water heaters - one or two expansion tanks?



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Old 04-17-2014, 07:32 PM   #1
Patriot
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Default Two 50 gal hot water heaters - one or two expansion tanks?

Hi all - newbie here. I think I got conned by the plumber yesterday. I have two 50 gal hot water heaters set up in parallel with a single 4.75 gal expansion tank in the common supply line, downstream from the shut-off valve. The tank failed at the nipple weld (horizontally mounted with no extra support - ugh). The plumber said that I needed two expansion tanks (one for each heater) - making it sound as if the one 4.75 gal tank that I had was inadequate with repsect to expansion capacity. I made the mistake of trusting him, as I didn't know the required expansion tank capacity per heater off the top of my head. Later, I see that he merely installed two 2.1 gal tanks on the common supply line, clearly at a higher cost to me compared to installing one larger new tank (paying for two tanks vs one plus the extra labor).

Here's where the engineering comes in: the system doesn't "know" how many tanks there are - it only "sees" expansion capacity. That being said, you can get the job done with 1,2 or 4 tanks for that matter, as long as the aggregate expansion capacity is adequate. Clearly one tank is the cheapest way to go, but the plumber said "one tank per heater is what we've always been taught" - translation - "we've always been taught to rip people off." I used to be a marine engineer, so they can't try to BS me and justify why they ripped me off after the fact.

The only semi-plausible explanation, one that I finally got from the plumber's estimator after a round of BS answers from the plumber (along the lines of the tanks "balancing each other out" - what? Utter gibberish!), is I now have a back-up if one tank fails - but, alas, a single 2.1 gal expansion tank is inadequate by roughly 50% in terms of required expansion tank capacity for two 50 gal hot water heaters....at some point, both heaters will run at the same time and I'll likely get to see if the relief valves are working properly - much the same way if I only had one 4.75 gal tank that failed.

Again, I got conned - silly me for trusting someone.....

Thoughts anyone? Differing views?



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Old 04-18-2014, 11:42 AM   #2
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Post some pics showing all the installation. (water heaters , piping , expanison tank)



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Old 04-18-2014, 02:22 PM   #3
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photo attached.....I think
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Old 04-18-2014, 09:53 PM   #4
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While you would've been just fine with a larger single tank, from your picture that appears to be a clean, neat installation. Most service plumbers have seen enough rank hackery to appreciate the care that was put into that. One very minor pick I might have with the installation is that if one of the tanks were to fail and fill with water, the weight of that would have some leverage against the piping due to the offsets in the piping, but I doubt that it would ever cause a real problem. And a single larger tank failing in the same manner would be worse in that respect.

From the perspective of "one water heater, one tank", I have many times seen instances where equipment got removed at a later date. Say you sold your house next year, a year later one of the heaters started leaking, so the new owner cut that heater out and capped it off. If they cut out the single expansion tank at that time, there would have been no expansion protection. Not saying that this is why your plumber did it, but I think of these kinds of scenarios when I do both new installs and repair.

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Old 04-18-2014, 10:43 PM   #5
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Thx Phishfood - sure, it's a clean, neat installation, as would it have been with a single, large tank - and cheaper with a single, large tank to boot - that's my gripe here. Not sure why the plumber put two smaller tanks in on the common inlet line, but I have my suspicions - call me a pessimist!

Re: offsets in the piping for the expansion tank connection creating a moment arm, I hear ya. It's tough to gauge from the head-on picture, but with an elbow connecting directly to the tee fitting in the horizontal straight-run piping, the moment arm is only about 1.5 inches, so I'm not too concerned about that. In looking at the clearances around the tanks though, the plumber could have put the right-hand tank directly over the horizontal piping by simply rotating the tee fitting 90 degrees - not so for the left-hand tank, as the hot water discharge line is in the way.

As for people removing equipment at a later date, why someone would remove an expansion tank is beyond me - that's simply moronic - but I'm sure it has happened before. Speaking of moronic - we've only been in our house for one year, but I'm embarrassed that I didn't catch the fact that there was no support for the old 4.75 gal expansion tank that was mounted horizontally. It failed at the nipple-tank weld in the area directly below the nipple. Half-ass installation job there if you ask me! The home inspector should have noticed that as well! Live and learn.

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Old 04-18-2014, 11:53 PM   #6
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Although one tank will work some manufacturer recommend two tanks on parallel installation. Balancing of the system is a real thing, that is why the piping should match both system. They did a pretty good job and you have good water heaters (Bradford White). I would have tee the tanks directly above the cold water inlet and installed a shut off valve for each water heater.

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Old 04-19-2014, 12:19 AM   #7
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The tank on the right could have been mounted above the inlet line, but not the one on the left due to the proximity of the hot water discharge line that can be seen behind the tank.

Please explain what you mean by "balancing the system". Agreed re: one shut-off valve per heater.

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Old 04-19-2014, 04:25 AM   #8
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When two water heaters are piped parallel the piping to both water heater should mirror each other. The hot & cold water truck lines should tie in at the center point of the system parallel piping = a balance system. If the system is not balance one water heater will have less friction which will caused that water heater to become the lead water heater causing it to fire more than the other water heater.

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Old 04-19-2014, 05:03 AM   #9
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Understood, and you're correct - basically one should aim to design for equal pressure drop (friction) across the piping run both into and out of parallel hot water heaters. If you don't, you're setting yourself up for a lead/lag heater situation. If you look at my pic, the inlet side trunk line is common to both hot water heaters, so having one large expansion tank (versus my brand spanking new two smaller tanks on the horizontal pipe run in my pic) on the vertical inlet trunk line has zero impact with respect to putting the system out of balance. I'll note that the failed, single expansion tank was located on the vertical trunk line - shame is wasn't properly supported!

So, we're back to my original assertion, in that I got conned by the plumber with the more expensive installation of two smaller expansion tanks, as shown in my pic above.

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Old 04-19-2014, 01:15 PM   #10
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No, I don't think you got conned. It is more likely that lack of experience on the plumbers part and the excuses that falsely justified the installation is making you feel that way. While the work is neat, an experienced plumber would have put isolation valves for each supply per tank, hot and cold. The explanation from his manager would then make sense. The manager was just covering his butt and should have been more honest. An experienced plumber would not have said "That's just how we've always been taught to do it." or some similar comment. He would have followed up with a sensible, mechanical explanation.
He may not have the experience to "think outside the box" which comes with many years in the field and thousands of various mechanical circumstances.
We all have been there in our journey as plumbers and you may have traveled the same path when working as a young marine engineer. Somebody may have covered your ass before and you learned from it, also.
It's understandable that the run around would make you a bit bitter and remorseful towards the plumbing company, the plumber and your personal decision to call them in the first place. If you want satisfaction, I would recommend holding the contractor to his word and ask to have the isolation valves on the supply piping installed at no additional cost so that their justification of the design would actually make some sense.
"Good decisions come from experience and experience comes from bad decisions."



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