You have the two boilers in series and have the coal boiler running essentially at a manual feed rate so that it feeds preheated water to the gas boiler and then the to the radiation. I don't think I would have done it that way. Without some form of modulation based on demand for the coal boiler you will either:
1. Have the coal boiler set so low as to not deliver as much heat as needed in really cold weather. You will lessen the risk of over temp doing this.
2. Have the coal boiler set so as to deliver a good portion of the heat during the cold weather, but have to then worry about over temps when the heat demand drops.
Q. Why not use the aquastat to control the coal boiler like it should and set it to a temperature higher then the gas boiler, or just set the aquastat on the gas boiler to a lower setpoint then the coal boiler.A. I have the temp on the gas boiler set to 140 and I have the Aquastat set at max of 180 on my stoker.. It sometimes gets over that but then the Johnson Controls will shut it down until it gets below 175.. then power will turn back on to the feeder and blower returning them to regular cycle.
I think if you hit a really warm day without much of a demand for heat, your coal boiler will hit that high temp quite easily, and this with PEX tubing is especially bad.
On a warm day it will get to the high temp quickly and stay there but the Aquastat and Johnson controls will keep it at the max temp. On a warm day I do back the feeder down or adjust the on / off time on the stoker control.
Q. What kind of house are you trying to heat? What was the capacity of the gas boiler? What kind of radiation do you have?A. I have a split level house with 3000 square feet... Hot water base board and I think the gas boiler is about 110000 BTU
The Harman can deliver about 80 to 90K BTU, comfortably, much more then that and I think it would run out of steam.
I too am a Sussex Co resident, have been for the past 27 years. Not far from you in Sparta, maybe 20 to 30 minutes.
I have been in Sparta for 33 years and will most likely be here another 10 or so...