Ongoing Issue with Bosch Combi Boiler

Bobichael

I've been having an ongoing issue with a Greenstar Combi 151p boiler since it was installed over three years ago. At this point, I can no longer get plumbers to work on the unit, and Bosch tech support is refusing to respond to any additional calls. Essentially, the domestic water is malfunctioning where the system misreads the water temp and deactivates the burner after the thermostat runs to 199. The thermostat gets pegged at 199 and the taps run cold, as the circulator pump won't kick on while the thermostat is maxed out.

The issue appears to be related to the radiant side of the system, and replacing the heat exchanger fixes the problem. At this point, I believe something on our water is reacting with the metals on the radiant side. That is creating sediment that collects on the radiant side, which eventually breaks the domestic water. The radiant side continues to work.

After several letters, I was able to get Bosch to send a technician out, and they promptly blamed the problem on the onix tubing the plumber used for the heating. I had magnetic dirt seperator installed on the radiant return in case that was the issue, but I'm not seeing any sludge or other major sediment issues. What I do see in the heat exchanger is a lot of bluish green sediment that smells metallic and is the size of ground pepper. It breaks down if you rub it between your fingers. We do have high iron content in our water, but the water for the unit is all being chemically treated, first through magnesium and calcium, followed by chlorine injection, the through cerapure and catalytic carbon. Iron, hardness, and pH are all within Bosch's recommended levels when going through the system.

Two questions:

1. Does anyone have any idea about what could be causing the buildup on the radiant side of the brazer plate?
2. Is there anything I can run through the radiant system to breakdown and flush the sediment out of the brazer plate?

The attached image shows the sediment in the unit.

1. 20240720_170805.jpg

Big Ed_4

Since the Pex used is the problem , add a tube type heat exchanger for the radiant to isolate the Pex from the boiler …. Rather than chase your tail….

hot_rod

bluish green is sometimes an indication of the copper in the system corroding,

Any hydronic treatment you use needs to be aluminum friendly for that boiler

You could send a water sample to a hydronic lab like Rhomar. You need a professional opinion on the fluid quality.

Teemok

Someone is claiming the Onix tubing is the problem. What's the logic /evidence for that claim? Age of the Onix tubing? hot_rods advice is, as usua, correct. A lab would likely be conclusive. Has anything (chemicals) been put in the hydronic water? Are you 100% sure the hydronic side holds it's water (no leaks or water from reliefs). Is pressure maintained over weeks with the hydronics system fill connection turned off? Is Onix and copper the only types of piping? Flushing the hydronics side then running an Onix and aluminum safe cleaner and then thoroughly flushing it all out and refilling the system with de-ionized water might be a good next step. PH and TDS should be verified after the new water has run for a few days. Some sort of filter/ screening on the return piping might help protect if material is coming from the tubing. Removing the plate HX and treating it with cleaners on a rigged up cleaning system is possible. Results are hard to predict not knowing what you're trying to clean out. Once a plate HX is very well plugged up it's probably not worth a cleaning attempt. Preventing the new HX from plugging is a better focus.

kcopp

I had this very issue w/ a Greenstar combi last Autumn. The brazed plate was plugging up w/ debris from a 40 yr old hw bbd system.

Tubing out to the system was polybutylene.

The solution I have used in the past is a bit time consuming and certainly not inexpensive.

Swap out EVERY component in the system that has steel or cast iron. Use bronze, brass or stainless steel. This includes the circulator.

Add a good cleaner to the system. Power flush it out w/ a flush cart. Typically after the cleaner has been in a week and circulated around.

Then add the inhibitor that is friendly to Aluminum. I have used Fernox in the past but have found Rohmar to be a better fit…less aggressive.

That may be the reason you are getting the bluegreen coloring. Also you do need to measure out the appropriate dosage. Some companies think that you cant overdose.

Fill water is also important. If your water is aggressive you may want to use a deionized mixture. Axiom makes a nice filter.

I did need to swap out the BPHX.