I thought I had it figured out, but now I'm not so sure. I'm leaning towards the second option.
Background: Pellet boiler, oil backup, 108 gal. buffer tank, parallel piped, pump for each boiler, zone circs. Also own a couple of AC powered Ranco temperature controllers, so would like to stick with that.
Summary: Use buffer tank temperature or a delay with pellet boiler temperature?
My original thought was to use a 2 stage Ranco to start the pellet boiler when the tank dropped to 160F and start the oil boiler when the tank dropped to 145. As an aside, the Ranco controller has max differential of 30 degrees. I'm second-guessing that now:
-Maybe 15 degrees isn't enough for the pellet boiler to start and get up to temp-maybe I should put a delay timer on it
-There could be a scenario where both boilers were on, as in the previous example, or if the pellet boiler doesn't keep up with the load; in the second case, that might be a good thing, although could having both boiler circs on be problem (piping is 1.25").
-Looking at my notes, it looks like I have 57,000 btu/hr possible load on the baseboards and 51,000 on the pellet boiler, so I don't think multi unit operation could really come into play anyway.
Another thought I had was similar to DZL_Damon's approach. A 1 stage Ranco could monitor the tank temp: if less than 160, send start signal to pellet boiler via pump swtich relay (taco 501sr) at same time tripping time delay relay, RIBD2421C. The relay on the other side of the coil would be connected to a 2 stage Ranco, already serving on stage 1 to delay the start of the boiler pump until boiler is warm. 2nd stage would be connected to TT of the oil boiler controller (either existing or new) which would start the oil boiler. The logic being: tank temp low, pellet boiler fires, pellet boiler temperature less than 130 for 10 minutes, oil boiler fires.
I can't see much downside for this approach, except:
-Both can't be operating at the same time which might be a good thing.
-I guess it might be possible the pellet boiler got to 130 from ghost flows; there's a check valve built into the boiler pump isolation valve, but it's on the return side of the boiler. Perhaps another spring check valve in the supply anyway, closer to the supply manifold?
What do you think? If nothing else, writing it down helped me.
Background: Pellet boiler, oil backup, 108 gal. buffer tank, parallel piped, pump for each boiler, zone circs. Also own a couple of AC powered Ranco temperature controllers, so would like to stick with that.
Summary: Use buffer tank temperature or a delay with pellet boiler temperature?
My original thought was to use a 2 stage Ranco to start the pellet boiler when the tank dropped to 160F and start the oil boiler when the tank dropped to 145. As an aside, the Ranco controller has max differential of 30 degrees. I'm second-guessing that now:
-Maybe 15 degrees isn't enough for the pellet boiler to start and get up to temp-maybe I should put a delay timer on it
-There could be a scenario where both boilers were on, as in the previous example, or if the pellet boiler doesn't keep up with the load; in the second case, that might be a good thing, although could having both boiler circs on be problem (piping is 1.25").
-Looking at my notes, it looks like I have 57,000 btu/hr possible load on the baseboards and 51,000 on the pellet boiler, so I don't think multi unit operation could really come into play anyway.
Another thought I had was similar to DZL_Damon's approach. A 1 stage Ranco could monitor the tank temp: if less than 160, send start signal to pellet boiler via pump swtich relay (taco 501sr) at same time tripping time delay relay, RIBD2421C. The relay on the other side of the coil would be connected to a 2 stage Ranco, already serving on stage 1 to delay the start of the boiler pump until boiler is warm. 2nd stage would be connected to TT of the oil boiler controller (either existing or new) which would start the oil boiler. The logic being: tank temp low, pellet boiler fires, pellet boiler temperature less than 130 for 10 minutes, oil boiler fires.
I can't see much downside for this approach, except:
-Both can't be operating at the same time which might be a good thing.
-I guess it might be possible the pellet boiler got to 130 from ghost flows; there's a check valve built into the boiler pump isolation valve, but it's on the return side of the boiler. Perhaps another spring check valve in the supply anyway, closer to the supply manifold?
What do you think? If nothing else, writing it down helped me.
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