Hot Tub HX - 9 year failure report

[Nofossil]

I built a hot tub heat exchanger back in 2002 and plumbed my hot tub as an extra zone on my heating system. Some folks wanted pictures of the construction. Unfortunately, I can now provide them.

A couple of weeks ago I heard a hissing noise from the heat exchanger. A quick exploration uncovered a hole (!) on the back side of a copper elbow on the inlet. Since it was under suction, a piece of duct tape fixed it for the short term. I just drained the hot tub and removed the heat exchanger for inspection.

I disassembled the HX (requires some hacksaw work) and took pictures.

The copper was eaten away from the inside, but only in very limited locations. There was no corrosion on the inner tubing that carries boiler water, though there's some kind of funky green patina on them which flakes off.

Corrosion seems to be limited to places where there could be turbulence and eddies, perhaps with trapped bubbles. The hot tub had an ozone generator for years, and it's a bromine-based system.


More posts and pictures to follow.

 

[Nofossil]

I rebuilt the HX with all plastic plumbing to and from the hot tub:

 

[tom in maine]

If I understand this, an outside elbow failed.
What where the actual hx tubes like?

If they were okay, then I wonder if you are seeing erosion from the impingment on the elbow?

Interesting failure. We used to make similar hx for interfacing solar systems with pools and hot tubs.
Only had one failure (inside the hx) with a customer who let his pool chemistry go way off.

 

[maple1]

I had a failure that looked a lot like that a couple of years ago. It was on an elbow on the inlet to my in-furnace domestic coil.

After draining & disassembly, I found that all the elbows in that area (4 in total I think) were all eroded. When I had my system put in, the water tested high in manganese & iron (I was told). I had it tested again, was advised that manganese & iron weren't the problem but that acidic (or basic) water was - it was leaching the copper out. We have green residue in places like under a dripping tap, and bathtub - it is the copper from the water lines. I was supposed to get a water treatment recommendation write up after the water test - it was never delivered like was agreed on, and I have kind of forgotten about/neglected it again. I think it is only a matetr of time before I have another leak.

I must not be understanding the layout in the pics, looks like internal water corrosion when I'm looking at them - but I guess it is the tub water going thru that elbow?

 

[Nofossil]

Yes, it was hot tub water going through that elbow. The 4" plastic pipe is filled with hot tub water (carefully controlled chemistry and PH, but with bromine and ozone to make life interesting). The 1/2" copper pipes carrying boiler water are immersed in hot tub water inside the HX. They have a coating of copper oxide(?) but show no erosion after 9 years service..

Flow rates are way low and the corrosion is not in the classic impingement areas.

I'm posting this for anyone who's built such a device or is thinking of it. My best guess is that hot tub water is at least mildly corrosive to copper fittings. May be that chlorine based tubs would be better... or worse.

 

[jebatty]

Interesting. Did you check the other L's to see if corrosion was present? Just maybe the one L ended up being especially thin.

Less than a year ago I put a new bladder expansion tank on an LP hot water boiler in our rental house, not because it was needed but because I made some minor valve changes in the system and since the system was down, and the tank had been on for about 15 years, i thought it would be wise to change it out. Last week there was a puddle on the floor, and I looked for a long time before I found the leak -- a very small micro-pinhole leak in the expansion tank. At about 15 psi one small drip every minute or two. I depressurized the tank (isolation and drain valves were some of what I added), put some JB Weld for water over the tiny leak hole, and the system is back in operation.

So maybe there was a manufacturing defect in the L.

 

[tom in maine]

I think Chlorine would be even worse.
9 years is not a bad run given the electrochemistry that could be taking place.

I do recall that if the pH goes acidic, all bets are off.

 

[Nofossil]

There were only two L fittings in the hot tub loop. The upper one had perforations on both sides of the L, while the lower one was fine. There were also a couple of other spots with pronounced localized erosion in both the top and bottom fittings, including a very thick section in the threaded portion.

I'm chalking it up to copper being incompatible with hot tub chemistry. I know at least a couple of people have built similar exchangers and wanted to share that there may be an issue here. I'm also interested to see whether this is expected behaviour. I'm used to thinking of copper being forever.

In my case I didn't come home from vacation to find my hot tub water all over the basement floor, but it could have happened.

 

[DaveBP]

When the elbows are formed, the outside of the tubing is stretched and the inside is compressed a bit. I've seen some imported 'economy' priced elbows that even had little accordion folds on the inside radius.

I wonder if the compression of the metal made some crystal structure change that is more susceptible to aggressive chemistry?

Would long radius elbows be less liable to corrode out at this area under your conditions? (quasi-educated guess over the 1st cup of coffee in the morning)

 

[goosegunner]

Not sure about your chemicals but copper and pool water do not mix well. Especially if you have blonde haired women in your house. They will soon have green hair. I have also been told that hot water is much more aggressive than cold water if the chemistry is off.

gg