Outdoor Wood Furnace Info
All-Purpose OWF Discussions => General Outdoor Furnace Discussion => Topic started by: wreckit87 on January 25, 2018, 11:56:19 AM
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Hey guys. After a couple weeks of dreaming, I think I've decided to take the plunge and install a backup boiler in my basement. Got my fingies on a Lochinvar Knight 94% condensing boiler with a 5:1 turndown. The plan is to pipe a 50 plate into the OWB loop, before my A/W exchanger in the furnace plenum, and yes, I do already have an LP furnace for the house but it's more complicated than that. My shop is 3 feet from my OWB and is heated with the OWB. I do have an electric boiler in there as backup but the stupid thing costs $400/mo to run, thanks to the astronomical electric rates here. So the ultimate plan is to basically use the Knight to heat up the entirety of my OWB water to still be able to heat the shop in the event of a wood outage, vacation, etc. Essentially the return line to my OWB would now be the "supply" from the gas unit when running. Does anyone see an issue with dumping hotter water into the return ports on top and continuing to draw from the bottom ports to serve both buildings? I'm wondering if stratification is going to be an issue from dumping the hot water on top from the LP boiler but still dumping colder return water in the other top port from the shop? I am flowing about 16 GPM total with both loops so I should have a full exchange every 13 minutes or so, in theory.
I've installed a lot of these Knight units and have a pretty good understanding of them from a hydronic standpoint, but given my commercial nature, I don't touch the electronics. Now it seems to me that if I strap an aquastat on the incoming line to the plate from the OWB to activate the pump/boiler on the LP side when the OWB falls behind, it's going to want to run 100% until the aquastat is satisfied. Given the 5:1 turndown in this particular unit, I would like to utilize the turndown when possible to keep efficiency up a little bit if possible. Say I set that aquastat to kick the LP boiler on at 130 degree water temp from the OWB, but want it to maintain maybe 160 degrees while the Knight is running, how does one go about making the controls function? I'd ultimately like it to run 100% until it reaches 160 and then just maintain it at 20% fire or whatever. Seems like the aforementioned aquastat route isn't going to allow that to happen, but I'm electrically challenged so maybe I'm looking at it wrong.
Many questions, sorry. Some insight would be greatly appreciated. Thanks!
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I don’t have a C but a G series, in effect that’s exactly what my waste oil boiler does returns hotter water on the return than the supply when its running. Now I do know the G’s have some internal baffling to help direct water to the hottest spots. I used to do the same with my old wood boiler as well and didn’t have any issues then either as long as your have plenty of flow to aid in mixing.
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With two lines mixing you shouldn't stratify in the owb.
I'm thinking through the controls...
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I would use a Ranco or similar to control it. If you want it to come on when the OWB drops to 130 but maintain 160, set a 30 degree differential on the Ranco and set gas boiler to maintain 160.
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Thats basically what I’m going to do for the waste oil boiler, I turned the cutout on mine to 150 from 140, seems by time mine drops to 140 with ash its about dead cold. Figure to set my strap on aqua stat at 178 or close to it and 145.
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I would use a Ranco or similar to control it. If you want it to come on when the OWB drops to 130 but maintain 160, set a 30 degree differential on the Ranco and set gas boiler to maintain 160.
Call me crazy, but isn't that going to just run wide open to 160 and cut out until it drops back to 130? What's going to make it maintain?
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I would use a Ranco or similar to control it. If you want it to come on when the OWB drops to 130 but maintain 160, set a 30 degree differential on the Ranco and set gas boiler to maintain 160.
Call me crazy, but isn't that going to just run wide open to 160 and cut out until it drops back to 130? What's going to make it maintain?
You would need to adjust the temps a little to make it work. If you set the ranco at 130/160 the gas boiler to 159, it would come on once it dropped to 130 and keep running / cycling at the gas set point as long as it doesn't hit 160. If it does then it will shut off the gas till it drops to 130 again.