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A Mid(co) Summer Night's Dream
It's been a few months since we converted to gas and installed the Midco LNB-500 modulating burner, and I wanted to provide an update. When the Steam Odyssey 2 thread ended in April, the system was performing well except for high stack temperatures. Over 40 or so spring heating cycles, the modulating controls did a good job of controlling the burner to maintain a steam pressure setpoint at the boiler, even though the controlling voltage was at the low end of the range (max 2.8 volts on a full scale of 2-10 volts). The burner has seen light duty over the summer producing DHW.
In an attempt to mitigate the stack temperatures, the folks at Midco developed a new LNB-250 burner head that's rated for 50-250 MBH. The development rep and an engineer from Midco came out in mid-August to install and test the new head, which uses the same blower, valve train, and controls as the LNB-500. If nothing else the new head is better matched to the boiler's rating of 245 MBH. The new head is shorter (14" vs 28"), with the burner mesh centered in the firebox and has a smaller orifice plate at the inlet than the LNB-500.
We ran tests with the firebox insulation installed, and another set of tests with it removed. While we unfortunately did not see a large drop in stack temperatures, removing the insulation did yield a 30 to 50 degF reduction in stack temps. At high fire we're seeing 800-850 degF at the boiler and 500-550 degF a foot from the chimney, so I'll be replacing the galvanized vent pipe between the boiler and chimney with something that's rated for 1000 degF continuous before the heating season sets in.
Over the summer I added a few new features to the EcoSteam modulating controls. Each of the DHW, Preheat, and Heat On cycles now has an individual set of configurable parameters that define how the boiler is controlled during that cycle. The parameters include the initial output voltage, control strategy (fixed output voltage, pressure control, stack temp control), a control delay time, and setpoints for the steam pressure and stack temperature controllers. I've also added a "setpoint control" option in the outdoor reset model so I can bypass the math and simply run until the indoor temp hits the setpoint-minus-anticipator target.
The website describing the EcoSteam boiler control project in detail has been updated with the recent changes. Here's the link: EcoSteam project
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