Boiler Breeching Draft Pressures

[BronYrAur]

I have a situation with three (3) Lochinvar PB1501 boilers venting into a common breeching. Each boiler has a 13' long 8" riser that connects to an 18" horizontal breeching. Overall horizontal length is 30'. Breeching then enters a 38' tall vertical stack. There is a barometric damper at the far end on the horizontal breeching that allows air to enter.

Attached is a sketch showing draft pressure measurements that were taken with all boilers at high fire. At the far end near the barometric, pressure was -0.03" WC. Between the boilers and the stack, the pressure was -0.15" WC. Down near each boiler, the pressures were -0.30" , -0.25" and -0.23" WC. The boilers have power burners, so I was not expecting pressures near the boilers to have larger negative values than the breeching.

The boilers all fire correctly and are within combustion parameters. Do these pressures make sense?

Thank for your help

 

[EnergyProfessional]

It is category I (negative pressure) venting. At the top of the stack you have 0 in. In order for exhaust gas to flow, the pressure at the boiler has to be the most negative. In between boiler and top of the chimney, it will be in between.

The reason it is nearly 0 (-0.03) near the damper is that it is likely slightly open and/or leaking.

The barometric damper provides somewhat stable pressure to make up for wind etc. that will impact pressure. Basically to prevent too much stack effect. Read here.

With 3 boilers you also see the boiler closest to the barometric damper has the least negative pressure, so it will have the least air flow (it also has the longest vent pipe length). Ideally you don't combine multiple devices to one venting system.

To me the measurements make sense