Heating Help

Expansion Tank #7 in 4 years

OK, so we are going to replace the expansion tank on a cast iron boiler for the 7th time in 4 years.  Here's what we've done:  Flushed out all antifreeze until clear, added Rhomar Hydro Solv; 12 days later flushed hydro solv until clear, added Rhomar tratment and 15 gallons of non-toxic antifreeze.  That didn't do it, we changed feed line to boiler to pex, instead of a dialectric union. That didn't do it.
The two other possibilities are stray current; we've searched it out, but maybe not completely; oxygen in the system, there is a combination of pex and onix radiant, so thought that wasn't the issue.
HELP HEATING PROS!!!

What type of tank is it?

Is it sized correctly? 

If it's a diaphragm-type tank, is it properly pre-charged? How many stories in the house?

diaphragm tank

It's properly charged, pinhole forms each time. i'm thinking of putting in a potable tank this time, two floors in the house.

How about

Have you closed the feed, and see if you have a loss in pressure? Basically, do you have an unseen leak, that could be causing you to have make up water coming into the system?

haven't closed feed

that's a good idea, then we'll find a leak if there is one, we'll try it with a potable tank.

First what's wrong

Is the tank developing water leaks or it is failing and allowing the system to go over pressure?

Leo

Oxy barrier?

I agree with EddieG to check for leaks first.

Are you sure that the pex has oxygen barrier. When that is the problem you can isolate the boiler water from the radiation water with a heat exchanger.

failing exp. tanks

if tanks keep failing due to pin holing (rot) I would start with a simple PH test. Hydronic antifreeze can become very acidic in a short period of time.

acidic glycol

I have not seen that in my experience, so it must be in combination with your local water.In 35 yrs pipefitting I have only seen 2 boilers with this problem.The 1st, the guy filled the boiler with glycol with his fuel oil transfer pump.The 2d, the house had non glycol barrier quest tubing.
None the less, the acid test is a good idea. I have a supply in my car.

Heat

Is the tank close to the flue pipe of the boiler?    Every ruptured tank I come across where there is close proximity to the flue pipe, the life of the tank was not what I would consider within its expected life span.     I feel the heat affects the adhesives of the rubber diaphram and causes premature failure.

Replace with RX30

I have a few customers that have Onix radiant and each have the same problem with expansion tanks leaking. I have replaced them with an RX-30 expansion tank, it is a plastic lined tank, and they have not had issues again in years. I think that the RX-30 means radiant extrol. It is made by Amtrol.

I have one system that has onix, Rehau radiant manifolds, and the sight glasses on the manifold have rusted up. This system had a Taco 007 that had the volute filled with a slimy rusty type of paste and it took out the pump in 2.5 years.

oxygen

Well, if that is the case that the radiant system is causing the corrosion, then a plastic expansion tank is not a cure for protecting the system.

You will need to separate the radiant from the boiler with a heat exchanger.

If

If tanks are failing, boiler will be failing. Just going to take longer and cost more!

right

Right Eddie, that is what I was trying to say :)