tankless coil leaking around the mounting bolt

ilya980

Hi,
I have a Burnham boiler with a tankless hot water coil. About 5 years ago a plumber was replacing the rubber gasket on it and could not take one of the bolts out due to corrosion. He drilled it out and threaded the hole with a slightly smaller diameter. The new bolt is a bit smaller than the rest. It started to leak very slowly around the bolt shortly after the repair. About 2 years ago my tankless coil started to leak inside the boiler causing the pressure to build up and emergency relief valve to open. I replaced the coil and the gasket myself, but did not address the leak around the bolt because it was small. Now the leak is worse. How can I fix it? Will it help if I put an o-ring on the bolt? I have green o-rings used in a car AC system. Is there a better fix? I believe, the pressure inside is up to 30 PSI, but the temperature could probably go all the way to 212F or maybe even a bit more. Is there a good thread sealer for this? Or should I put a flat rubber washer?
Thanks.

EBEBRATT-Ed

The thing with tankless coils is eventually the gaskets dry out and start to leak, the bolts get rusted in then you have a problem. HB Smith was the only boiler MFG (that I know of) that USED to have a cast iron flange on the boiler so the tankless was held in by nuts and bolts, easy removal

The problem is it was expensive and was dropped. People neglect the leaks and things get rusted up

Some tips

Use studs cut from threaded rod and use nuts on the outside so when you remove the coil you have two chances to get a thread to come out.

Coat the nuts, bolts, studs with anti seize compound a lot of it

If you can get the bolt out you could try an O ring, rope wicking, teflon tape twisted up etc it's whatever works at that stage of the game.

I have drilled them out and used smaller bolts I have drilled them out and used larger bolts if their is enough meat on the boiler.

Most tankless bolts are probably 3/8, so 5/16 would be the next size down and 7/16 the next size up.

I never checked to see if there was a metric bolt that would be only slightly larger but you could look


problem is if it leaks for a long time the boiler and the coil can get rusted and pitted so the gasket won't seal.

An angle grinder with a wire brush and a flap disk can clean things up.....use carefully

mattmia2

I'd try rtv to seal up the leaking part, on the gasket or the threads, whichever is leaking. I'd use brass or stainless bolts so they won't seize along with the antiseize. You probably will have to use rtv without the antiseize on the leaking bolt. Stainless studs and brass nuts would be even better, if it is a nut, you can remove it with a nut splitter but brass and stainless won't seize except under much more extreme conditions than a domestic boiler. You can buy pre made studs or stainless threaded rod from mcmaster-carr.

HVACNUT

The hole can't be tapped to the correct size?

EBEBRATT-Ed

@HVACNUT

There is a broken bolt in it that was then drilled and tapped to a smaller size. Anything is risky now. If the smaller bolt threads in and can be tightened I wouldn't mess with it

Grallert

Left hand twist drill of the correct size might turn the stud out. At least it will drill. retap

ilya980

Thanks for the suggestions. With a lot of patience I loosened all bolts without breaking them. The old bolt is thin near the head. If I waited longer, I would probably snapped the head off. Sadly, I still don't understand how to fix this. I did other things successfully today (replaced main pressure relief valve and the vent above the expansion tank). I have small kids and need working boiler, so I put a band-aid fix over this bolt. But it is still leaking.

The problematic bolt is 3/8-16 with 1/2 head. The head size (1/2'') is smaller than the hole size in the face plate. The head of the bolt does not cover the entire hole in the face plate. I need a washer with this bolt to cover the face place hole. How would I seal this with rtv? How to make it watertight between the plate, washer, and bolt head? Should I try to find a suitable flange bolt and try to eliminate the washer? It looks like I would need 3/8-16 1'' length with 1'' diameter flange. Where do I find such a bolt, especially in brass?

Replacing the main gasket would probably help, but it is very difficult, and I am not sure it will last long. A plumber first replace the leaking gasket in 2013. Then in 2017 the tankless coil leaked and I replaced the gasket together with the coil. Now it is leaking again, but only on the "inner" seam, i.e. through the bolt holes, not on the perimeter. Looks like 4 years of a lifespan is too short.

Thanks.

EBEBRATT-Ed

Take the bolt out and get a 3/8" thread X 2" stud and thread it in the hole. Clean the hole and any rust around the plate as well as you can before you put the stud in. Make sure everything is dry Then fill the recess between the stud and the tankless plate with RTV. Leave it a little bit proud of the tankless plate. Let it cure overnight

Try and find some washers that fit the stud tightly maybe a 5/16 washer as washer holes are oversized. Put washers and nut on it should compress the rtv enough to get you into the summer

ilya980

Thanks a lot for all the suggestions. Very helpful to understand the issue!

I ended up getting a new yellow zink plated bolt. The size is 5/16'', not 3/8'' (I misstated the size above). Also I got a large diameter (covering the coil face plate almost to the edge) washer with 9/32'' center hole and drilled it to 5/16'' so it would be a tight fit. First, I drained the system and the boiler to the water level slightly below the problematic bolt. Then filled the gap around the bolt with Permatex Right Stuff Black 90 minute. Before tightening the washer I put Permatex behind the washer to form the gasket and also on the bolt threads near the head to seal the washer to the bolt. Hand tightened everything and then while sealant was still liquid rapidly dropped the pressure in the system by opening the boiler drain valve and creating a vacuum. The idea was to suck a bit of the permatex inside the tankless coil compartment. Then waited an hour and tightened the bolt with 1/2'' wrench to compress the formed gasket. Let it sit for another 30 minutes. Filled up the system and everything seems tight. Spent 2 hours and several gallons of spilled water to remove air from the bathroom radiator, but that is another story.

EBEBRATT-Ed

@ilya980

Good job. Hopefully it holds up for a while

HVACNUT

MacGyver would be proud.