Atmospheric Outdoor Wood Boiler- Piping Question

Dave Rothrock

An existing coal stoker is being replaced by an outdoor wood boiler which is open to atmosphere (non-pressure vessel). The original 7 zone circulators push into the existing coal boiler, with the diaphragm expansion tank (Amtrol Fill-trol) and air scoop on the boiler discharge like was commonly done 20-30 years ago. The home is three stories with hot water baseboard. There are manual air purges on each return pipe. This system has worked well, with no history of air binding problems.

The new wood boiler is located such that the water fill line is approximately ten feet lower than the height of the highest sections of baseboard. The wood boiler vendor instructed the homeowner to move the pumps to the supply side of the new boiler connections and there would be adequate pressure for the system to work fine. I don't see how this can work.

I recommended hydraulically separating the 7 zone circulators from the wood boiler pump via 2 closely-spaced tees. I also recommended isolating the wood boiler with a brazed-plate heat exchanger so that a higher fill pressure could be used on the closed loop portion of the system, enabling adequate air control at the top of the system. In this scenario, I felt that the system would work fine leaving the zone circulators where they originally were installed on the return piping. This would eliminate alot of work.

How would you pipe this system?

Thanks for your opinion!
Dave

Steamhead (in transit)

This sounds like

the same thing one would do when adding a hot-water zone off a steam boiler. It will work if done right.

The first thing you must do is eliminate any and all air vents from the baseboard loops. There must be no way air can enter via a vent with a slight leak or some other source.

The second thing is, the water must never get hotter than 180 degrees. You don't want it to boil up there in the zones. You'll want to use some sort of buffer tank to take the extra heat from the boiler as it's generated, and store it for later use.

Once the zones are purged, using the purge stations in the basement while the circs are running, the water will stay up in the loops since air cannot get in to let it fall back down.

Your vendor is right that you must Pump Away from the boiler and toward the zones. This will add the circulator's ∆P to the zone when the circ starts up, making the water in the zones less likely to boil and flushing air away. You'll also want to use a flo-check on the supply and return at each zone, to help hold the water up in the zone when the circ is off.

Don't try to get away with leaving those circs on the returns. They never should have been put there in the first place. Gil Carlson taught us that.

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hot_rod

That's the best way

in my opinion. Isolate the non pressurized OWF from the hydronic loop. It sounds like the current system is pressurized so that should be fine.

That OWF can be maintenance item. water quality needs to be watched on open systems. And they can boil over if overfired. LWC is a nice option, consider an alarm light to notify owners of low water condition.

hr

Steve Ebels_3

Ditto

You can put the plate HX right where the old coal boiler was. Pumping away may help some but itwon't solve the problem, which is, the circs can't make enough pressure to push water to the top of the system. Seeing as that there is now no static fill pressure due to it being an open system, it ain't gonna work. The house side has to be isolated from the open side and pressurized enough to raise the water to the top radiation. Remember that you still need an expansion tank on the closed side even if there's no boiler.