Expansion Tank fails repeatedly

I have a setup for radiant floor and DHW running on a Munchkin boiler. It all works great, with one exception. In five years I've gone through 3 expansion tanks! Each time, the tank fails by rusting through from the inside. I cannot believe I would get 3 defective tanks (they were bought at different times, from different vendors, but all same manufacturer).

I've tried to include some photos to show the setup.

Here is my theory: the tank that fails is at a low spot in the system (not the lowest spot, but close). The boiler and DHW are connected with black pipe, and the radiant zone pumps with copper. There are proper dielectric unions between them. Perhaps the iron pipe is corroding slightly, and the sediment is falling into the expansion tank, somehow causing excessive corrosion inside the tank? Does this make sense?

So, do I replace all the black pipe with copper? I am afraid there may be damage to the boiler that I cannot see, because that is truly the low point in the system. Would a trap or filter solve the problem?

By the way, there are 2 expansion tanks on the system, one up high near the inlet and one near the radiant manifold. It is only the one near the radiant that fails.

Comments

For example, Uponor make HePEX Plus, and AquaPEX. HePEX is OK with mild steel components, but AquaPEX is not...

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

I do not know who made the tubing, but it was described as "POLY PXC" tubing (7/8"). I can tell you that it is NOT an oxygen barrier tubing. That may have been a mistake, but it is too late now. I would welcome any suggestions to minimize the deterioration.

Fortunately, any components that would fail are accessible, i.e. the expansion tank and any steel piping in the near boiler piping configuration.

You could maintain the pH above 8.0 and it would slow any oxygen degradation. As for the tank replace it with a potable water expansion tank, which is rated for oxygen. Adjust the diaphragm pressure to around 12 to 15 PSI, assuming no more than a 2 story building.

The next components to start showing sign would be any steel pipe, then cast iron pump.

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

replace the expansion tank with a rubber lined tank designed for domestic water. the air pressure should be adjusted down before installing, but this will eliminate corrosion in that part of the system.

1. Replace expansion tanks with potable models, ask HTP how to make a munchkin handle o2 permeated water such as in an open system (there are parts that need replacing, I believe), replace all of your pumps either with non ferrous models or with a non ferrous system pump or two with zone valves (I'd go with the zone valves, cheaper up front and cheaper to run as well judging by your current massively overpumped sitaution), and replace any iron with copper.

2. Find a good corrosion inhibitor water treatment and use it. We don't do this so I am not familiar with the options there. But with this many pumps, this is probably your least expensive route.

Curious Rob, why don't you offer water treatment as a part and parcel of your "system" designs?

For guidance, I'd suggest Rhomar Water, speak with Dwight Hedgepeth.

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

we only do closed systems, and in something like 1000 active projects water treatment might have helped only 1 avoid a problem that I'm aware of (that one has a gunked up PBV and flow screen, cleaning might have helped). so I haven't seen the urgency or utility of making water treatment standard.

The amount of black pipe & possible corrosion is making me nervous. I am not worried about a hole in a pipe -- I know that would take a very long time to develop.

What worries me is the gunk & debris that might be flowing around inside the system. I can't change the tubing, and I will look into the chemical treatment option. In the meantime, would it be smart to consider replacing all the black pipe with copper? I know that's very expensive, but am I looking at bigger problems down the road?

but that would include the pumps. I'm thinking that ongoing chemical treatment may be your best option, but that is just my opinion.

As has been stated, if you go that route, there are a few components in the boiler that are ferrous (the elbow going to the pressure relief valve, for example, is an black iron fitting).

Even if you do change all components, you will still want to maintain a proper pH on the system. It just won't degrade as fast due to eliminating the oxidizables in the system.

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

I hope you are covering your butt in paper making sure that SOMEONE addresses WQ issues, otherwise you could be co-named in a legal action if it were to become an issue.

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

Well, the fortunate thing is that you have now identified the problem. The unfortunate thing is that you are going to have to do some re-piping to provide a "fix". You are going to have to seperate the boiler side of the system from the system side of the system by means of a heat exchanger. My choice would be a brazed plate exchanger. Go to a website such as Triangle Tube, and look at the sizing charts for your system. You should do this as soon as posible. The system will need to be flushed, and cleaned before putting it back into service. You should also use some kind of boiler water treatment, and check the water periodically for maintainence. The water, if left as is, will have the look, and consistence of brown gravy. That wont be good for air seperators, flow checks, hy vents, and of course degraded heat transfer in the system. Once the heat exchanger is in place, the system is flushed and cleaned, and a conditioner used, you should have an improved situation. And of course, I would have an experienced heating company do the upgrade, NOT the person who installed the system initially.

I vote braze plate heat x, bronze or ss circ. Zone valves and ecm pump would be nice except I have yet to be impressed with any certain zv mfr. Don't know yet on Calleffi's reliability but do know on many others.

While I am a fan of new technology, I have also seen enough come and go to be skeptical. I do agree with the bronze or stainless pumps, but be careful installing out of the ordinary components. Im not saying that ecm pumps are no good, but its no fun to get a call in the middle of the night to find that you are looking at a pump or controller that no one stocks. Make sure your supplier has the items in stock, and will continue to support the line. I had a job recently that the contractor had been sold on a Wilo pump for a very large apartment building, one pump, multiple zone valves. I advised him to use Taco pump instead, as everyone stocks the pump that would be on the job, so finding a replacement on a no heat call would be a snap. As far as Caleffi zone valves, we have been selling them for a while, and are having no problems with them.