Incorrect Manifold Pressure on Mod Con / Fog Horning - Cracked Burner?

drescheg

I have a Rheem RCBH199DVLP propane combi boiler that has a couple of issues that I am working through that may be related.

First it is fog-horning bad (sounds like it is going to fall off the wall) on high fire.

Second, the manifold pressure goes from -0.29 in H20 static to 0.00 at both low and high fire. The manual gives a spec of -0.28 and 0.04 respectively. As soon as the fan starts up, the pressure rises to 0.00 and holds without fluctuation when changing the dip switches for low/high fire.

The supply pressure is reading at 12.5 in H20 static, 12.4 low fire and 11.9 high fire (taken at the gas valve). Which all land within spec of 8-14.

Combustion analysis looks to be good though this is my first mod con boiler install.



To address the fog horning, I added a couple of 45s and a 90 to jog first the intake and then the exhaust vent to increase the equivalent length to 30 and 34 ft respectively. There were at 14 and 18 ft which was close to the min (14 -50 spec). No charge to the nature or quality of the foghorn noise.

This unit uses the same single orifice valve as discussed in this thread, which I found while searching how to adjust the manifold pressure. From what I understand, is that the offset valve only adjusts the manifold pressure at low fire and that high fire is set by orifice in fan assembly.

https://forum.heatinghelp.com/discussion/185627/how-to-interpret-combustion-analysis-results-noritz-cb199-dv

At this point I am wondering if both the fog horning and the incorrect manifold pressure could be caused by a leaky burner unit? Other ideas on where I should look next?

Thanks for any guidance you can give.

kcopp

What do you have for gas piping to the boiler? Is it CSST piping or black steel?

EBEBRATT-Ed

Sometimes fof horning is caused by using CSST as @kcopp mentioned

pedmec

Your getting too much gas on high fire or have a leak between the blower and gas valve. You should be in a negative reading at high fire. down stream gas pressure is determined by blower speed. An increase in the blower should result in an increase in a vacuum reading downstream. You can't be at zero at low speed and zero at high speed.

Not exactly familiar with your model but make sure you have installed the venturi for propane. if its correct make sure its not broken. the baffle can break affecting the air flow