Iron corrosion in a FHW heating system with indirect hot water tank

55Trucker

An elderly neighbor of mine has a Buderus oil fired forced hot water heating system with an indirect tank for hot water storage. The system was installed around 19 years ago, and looks to be very professionally done.

Last year she called me to say she had water in her basement. I went over and found that a 2" cast iron elbow attached to the back of the Buderus had a water stream coming out of it from a pin hole perforation. I shut the water off, shop vac'd the floor, and had her call her heating guy, who replaced the elbow. He didn't give her an explanation for what happened, I just assumed it was a casting defect or something.

Yesterday she called me again with water in the basement. I went over to find a cast iron flange attaching a circulator pump to 1" copper tubing was spraying water from a pin hole leak in the iron.

She called her heating guy and he replaced the flange, and the circulator, which he said was failing. After he left I went over to check the system over. She does have radiant floor heat, so I checked whether the PEX was oxygen barrier rated. It was. I noticed that she had a 10"x2" water filter near her well tank, and the filter was full of red sediment. I asked her if she ever had her water tested. She said she did in January 2021. She showed me the results. It passed every test. Notables:

pH: 7.27
calcium: 25.5 mg/L
magnesium: 3.85 mg/L
hardness (calc): 79.5 mg CaCO3/L

She mentioned to me that last year she had to have a second circulator replaced because it failed. She also said she kept the elbow from last year's issue. I looked inside of it and there was massive pitting.

In my experience Taco circulators rarely "fail". And I have never seen rust through on iron fittings this heavy/thick.

Can someone please point me in a direction where I can help her permanently address this? This sure seems like a water issue, and the heating guy has no idea. I also called a Culligan office and sent them the water test results. The rep said he has no idea what is happening. He put the water test results into a Langeliers Saturation Index spreadsheet, and said the water is "slightly scale forming on the hot while the cold is corrosive. Not unusual."

Does an indirect tank have a sacrificial anode? Does the entire system have one some where?

Thank you in advance for any help you can provide.

WMno57

Does she have a water softener? Water softeners are bad for boilers.

55Trucker

No, she does not.

Pumpguy

Typically, water has to be on the acid side, below 7 pH, to corrode iron. Could this be a water velocity problem?

Could more corrosion resistant (stainless steel) fittings be a solution here?

I have a customer with a similar corrosion problem with black piping between boiler feed pumps and boiler, and also drain piping from F&T traps that drain the dropped steam headers. These drain directly to the boiler feed tank.

55Trucker

How did you solve your customer's issue? Is there a way for me to verify if the velocity issue you propose is the issue here?

I will be going over to her house on Saturday to take a closer look at the system and inventory the components. My biggest concern now is whether the tank inside the Buderus will be the next thing to fail.

hot_rod

I like to add hydronic conditioners. They buffer ph, scavenge oxygen and give you some film providers 

A small leak somewhere could be adding fill water, more O2. 

Does it have a fill valve?

If so turn it off for a day and see if pressure drops. Assuming it has a working pressure gauge somewhere.

Jamie Hall

I don't see chlorides mentioned on that water test. They're lethal to cast iron...

mattmia2

A leak and fresh water being added is the most common cause of corrosion.

55Trucker

The chloride reading is 16 mg/L. What is the best way to determine if there is a leak? (I would guess that if there is a leak it would be in th)e heat exchanger in the indirect tank?)

mattmia2

The leak would be elsewhere in the system. If the coil in the indirect were leaking the pressure in the system would rise because the higher pressure domestic water would be forced in to the system and usually the relief valve will eventually open.

The way to test for a leak is to shut off the feed and see if the pressure in the system falls.

ScottSecor

I am trying to picture how many homes we've worked in that have 2" forced hot water piping, not too many. I assume this is a very large house if the near boiler piping on a modern boiler is 2". My cheat sheet shows 2" steel pipe will easily pass 50 gallons per minute. With most hot water systems 50 gallons per minute equals about 500,000 btu of boiler output. I have visited about a dozen homes here in NJ that require 500,000 btu output hot water boilers in the last thirty-five years. None of these systems would have circulator pumps so large that they would erode a 2" cast iron fittings. As a matter of fact, I don't think I've ever witnessed a cast iron fitting on a hot water boiler that was leaking from erosion.

That being said, I have seen many cast iron boilers, pumps, fittings and steel pipe that were rotted from poor water quality. However, you had it tested and it appears to be fine.

Is there any chance that there is some sort of electrolysis going on? Could the near boiler piping be acting as some sort of ground rod? I doubt this is the case as you did not mention any issues with the domestic hot water system, that are typically more prone to electrical current issues.

My guess is a leak in the system as others have mentioned. My guess is there is pretty substantial leak and the make up water is being added to the system on a regular basis. What did the circulator pump and the fitting look like inside? How about the piping? Any idea what the boiler water looks like and smells like?

55Trucker

Unfortunately, the only thing I have is that rusted through elbow. I wasn't there when the heating guy replaced the circulators, and he didn't leave them. I did open a shutoff when I went over there for the latest leak to relieve pressure so the latest pinhole would stop spraying. I didn't notice any smell in the water. Her hot water tap in the kitchen also has no smell.

Pumpguy

55Trucker said:

How did you solve your customer's issue? Is there a way for me to verify if the velocity issue you propose is the issue here?

I will be going over to her house on Saturday to take a closer look at the system and inventory the components. My biggest concern now is whether the tank inside the Buderus will be the next thing to fail.

No solution AFIK. The boiler company just replaces the leaking 3/4" schedule 40 black pipe with 3/4" schedule 40 black pipe. Sounds like insanity to me.

55Trucker

I looked at her heating system today. It was a work of art. All of the piping was so clean and well done. There are no leaks anywhere that I could find. I drained some water out of one of the drain valves to look at it and smell it. It was crystal clear with no odor.

So I am stumped. The only thing I can think of, which I mentioned before, is could there be a leak in the indirect water tank heat exchanger, causing the tap water and the heating system water to mix. I couldn't find any pressure gauges anywhere, only temp gauges. The tank is about 20 years old. It came with a 10 year warranty, covering the tank and heat exchanger. The tank itself hasn't failed yet after all this time, but what if there was a leak in the heat exchanger? There would be a constant influx of fresh water (and oxygen) into the heating system, too.

mattmia2

It is easy to find out if it is leaking, shut off the makeup water and see if the cold pressure drops. Finding it can be more challenging.