Navian Combi Tankless NCB-240 knocking

[encoad]

Hi Guys,

Just got the radiant floor heating going today and I've got some knocking. I'm guessing this is some air? Should I just the let the system work itself out and continue to knock? Or maybe I should stop using it and try to purge the air manually (how though?). I have an automatic air vent. (that I've never caught doing anything)

Here's a video:

Thanks in advance!

 

[Jadnashua]

A couple of questions:
- is there an expansion tank in the system?
- what pressure are you running in the system?

 

[Sponsor]

 

[encoad]

I'm running at 12psi when cold. I do have an expansion tank which is mounted to my air vent. There is also an air vent inside the NCB-240 (which I have also never witnessed do anything)

 

[Dana]

It looks like spikes in pressure, possibly due to flash-boil on the heat exchanger. Flash boil can be due to insufficient pressure at the heat exchanger, or insufficient flow.

It doesn't appear that Navien's hydraulic separator manifold was used in your installation, but it's a bit hard to tell without a wider angle view of the near-boiler plumbing. Is there a different hydraulic separator that isn't making a cameo appearance in the video? While it's often possible to run an NCB-240 pumped direct rather than primary/secondary, with many systems that might require swapping in a different pump to get sufficient flow under all operating conditions.

The easiest thing to try as a quick test is to raise the system pressure to something like 18-20psi to see if it affects the symptoms. If it's being pumped direct it may make sense to follow the manufacturer's recommendation and set it up primary/secondary with an adequate hydraulic separator to guarantee that the boiler always has sufficient flow.