Failing HEX

waterboy88

Have a Navien NFB200 installed in a large freezer building heating the ground under the slab. Running at 85 degrees supply temp. Boiler is installed in a loadout area kept at 50 degrees. It is plumbed primary /secondary. It was installed 3 years ago and has a fire tube hex. The hex failed (glycol coming out the condensate drain) this spring. After talking to tech support we swapped out the 0015 boiler pump for a 0013 and they sent us a new hex, which also failed 5 weeks later. This time they sent us a new burner with the hex (original burner had some small cracks in the burner gasket). This hex again failed (glycol coming out condensate drain) within 10 days of installation. We now had the attention of every lead tech at Navien and they sent us a new hex and burner head again as well as sending a rep out to check things over. Did some water testing which found ph at 6.8 on one test and 7.5 on another. Propylene glycol content at 36%. Venting was done in accordance with install manual with exhaust extending 1’ out from intake, 1’ above, and 1’ to the side. Our combustion analysis didn’t show any signs of recirculation. Also we extended the intake down another 4’. We have not started the boiler again since, but are circulating the system pump and monitoring the return water temp to make sure we don’t get close to freezing. The pictures are of the second and third heat exchangers after running for 10 days and 5 weeks. What else should we be checking?

pecmsg

What temperature are you keeping the floor?

What temperature is the boiler?

33% is about 5 - 0°F Freeze point, that's rather low.

GroundUp

That is crappy combustion, 100%. Are there any potential chemicals or dust in the air?

PC7060

No insulation? just dumping energy into the soils?

What typing of tubing is under the slab?

HomerJSmith

How about pics of your whole setup.

Looks like iron flakes on the HX. Where is there iron in the sys? Pumps? I would check for electrical currents in the fluid. Bond everything.

HVACNUT

You didn't list the combustion numbers. Natural or LP gas? Any numbers on pressure drop when the valve opens? It doesn't look like coffee grounds so I'd put recirculation on the back burner. That looks like poor combustion. I'm not too familiar with Navien, but I believe you need to go into settings and configure gas type, elevation, max flue temperature, etc. Since a rep was on site I assume this was all done? Aside from what we're looking at, everything else is 100% correct? Combustion.

Is elevation an issue? Where is this freezer building. And what are you freezing that you need a whole building?

hot_rod

glycol should be more in the 8-10 ph. If you look at the spec sheet for the exact brand you have you get exact numbers. Dow suggests dumping glycol if it falls below 7, clean the system with a cleaner, Fernox or others, and use pre blended glycol to avoid bad blend water.

That is for the water side.

On the combustion side, we need to see combustion printout. The rust? Is it vented with ferrous metal?

They are what they breathe, where is combustion air coming from?

The fire tubes look plugged in these pics, that is bad combustion, or aggressive intake air.

At those low temperatures it should be condensing like crazy and flushing those fire tubes.

jesmed1

Maybe far-fetched, but I'm wondering if a slow refrigerant leak in your freezer building is contaminating the combustion air. Refrigerants like Freon can severely damage gas boiler combustion chambers.

https://forthepro.bradfordwhite.com/combustion-air-contaminants/

Quote: "Combustion air for gas fired equipment can also be contaminated by any of the common chloro-fluoro-hydrocarbon refrigerants known under the trade names of Freon, Genetron, etc., and as from refrigeration systems. However, the greatest source of air contamination from these compounds is from their use as pressurizing agents for spray cans of insecticides, paints, hair treatment, and “beauty” solutions. Because of the latter two items, the air in beauty salons is generally contaminated to some extent. Restaurants frequently install refrigeration equipment in rooms or areas with gas fired water heaters resulting in the same condition."

Dave Carpentier

My far-fetched idea was a delivery/shipping truck parking area. Do trucks with refers park nearby ?

waterboy88

https://forum.heatinghelp.com/discussion/comment/1904327#Comment_1904327

I will get combustion specifics. vented with pvc. combustion vented out behind the loadout close to the condensers.(freon leak may be a concern?) although there was no knowledge of a leak at the time of failing hex.

What is "aggressive intake air" ?

waterboy88

https://forum.heatinghelp.com/discussion/comment/1904133#Comment_1904133

Natural Gas. What valve opening are you talking about? What pressure drop should I check?

NW Iowa, 1200 ft elevation. Butter Braid.

hot_rod

did the failed HX get sent back to Navian? A forensic lab could analyze the deposits and the attack mechanism.
after multiple failures I think time to bring in the metallurgy pros. It it failing from the inside out. Or burning through from the fireside?

Maybe disconnect the outside air and pull air from inside, if the space allows. That eliminates recirculated exhaust gas as a cause. On a cold day you can sometimes see the exhaust being pulled into the intake air, certain wind conditions can cause recycling also. So it could be an intermittent condition.

Combustion analysis on the numbers?