Viessmann mixing valve threads I.D. too large

Bill_B · January 24

We bought a 3-Way mixing valve for a job. My installer said the threaded connections keep leaking. I had him bring the valve back to the shop. I tested a 1-1/4" black plug in the ports. I fits way too loose. I can tightening the plug until bottoming out WITH MY FINGERS! Has anyone else ran into this? My vendor says he's been selling these for 20 years and never heard of any issues. Any sealant you can think of that would work? Maybe Expando? Or some automotive sealant like RTV or Hylomar?

https://www.youtube.com/shorts/YbtvKiDn2Vc?feature=share

hot_rod · January 24

Sounds like a BSP straight thread version made it through.

If it isn't too sloppy and you can get it clean, with acetone, we use Loctite on all our BSP connections.

#545 or 272 high strength.

For BSP nipple to BSP female fitting hemp or Loctite 55 Sealing Cord will work.

BSP and NPT is always a work around.

LRCCBJ · January 24

This usually occurs when you attempt to put an NPT fitting into a straight fitting that seals with an O-ring…………as Viessman is likely to have done. Some people attempt to do it with tape and dope but it usually doesn't go well for them in the end.

@hot_rod will know the suitable adapter from Caleffi.

hot_rod · January 24

I installed a Viessmann indirect/ solar tank recently it came with BSP to NPT adapters in the box, up to 1-1/2” size. So somewhere in their catalog must be adapter fittings?

Some connections depend on a fiber washer on the face to seal. Piping for boilers and fuel piping is generally done with hemp.

Caleffi machines hemp specific brass fittings. The threads get knurled so the hemp doesn’t roll off so easily.

When German boilers first started arriving in mass, they came with a fist full of hemp.

You know what that means to a young thrill seeking installer🫣

GW · January 25

I remember the hemp days, almost 30 years ago or so. I was baffled. Paul Ross the rep had to come make up the joints.

Doesn’t the Loctite string do a good job?

hot_rod · January 25

https://forum.heatinghelp.com/discussion/comment/1837712#Comment_1837712

It is much easier to use, you don't need to seperate it and hold it in place.

It looks like a siliconized dental floss.

Screenshot 2025-01-25 at 2.01.55 PM.png

Screenshot 2025-01-25 at 2.02.03 PM.png

GW · January 25

yes the dental floss stuff. I have some tucked away

Bill_B · January 27

Hello everyone and thanks for your replies. Hotrod great to see you are still on here sharing wisdom.

To address the comments about a sealing washer, I thought the same thing but if you look inside the F.M. fitting there is not a nice flat surface in there. It is a welded bead so not uniform at all.

This is what I have on hand currently for sealants-

Bill_B · January 27

I just ordered the Loctite 55 incase that is the way to go. I will tell you what. We learned one very valuable lesson. DO NOT install one of these valves without UNIONS piped in on all ports!

hot_rod · January 27

get the threads good and clean, maybe sacrifice a fitting brush.

Are you heading back to the future soon?

GGross · January 27

Why won't your supplier just warranty the thing? I had a couple of these leaky "out of the box" a year or two ago and it was no trouble to just warranty them with Viessmann

Bill_B · January 27

https://forum.heatinghelp.com/discussion/comment/1838204#Comment_1838204

Heading back to the 80s! The vendor is sending a new valve. I'll test fit the plug in the new one. If it is loose then we will use Loctite then.

Bill_B · January 27

They are saying they will warranty it "As long as something is wrong with it" (the valve we currently have).

They ordered another valve, and it is supposed to be here any day now. Actually they said that they direct shipped it and it was supposed to be here last Friday the 24th.

I was just asking for advice in advance in case the second valve has the same issue and the vendor didn't have any answers.

Viessmann mixing valve threads I.D. too large

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