Adding alternative High Eff propane to existing 1050 gal wood gasifier.

[wtxing]

current wood boiler design 1050 gal water heated to 200 deg. Based on wood boiler location 10 degree loss to zone inputs. Raidant zones mixed down to 140 deg baseboard zone (upstairs hardly ever turned on) get 190 deg . Return from radiant zones is 20 deg less (120) looses 10 going back to wood boiler (now 110).

Option 1 - shut off to select wood or propane. Install in line with wood boiler hot output (setpoint = 180 deg) . Returning temp 110 deg. New 123 BTU high eff boiler would have to heat water 70 deg delta when next call for heat.
Option 2 - install so the propane boiler heats the 1050 gal of water in wood boiler (setpoint = 200 deg / heat call from wood boiler temp sensor set to 180 deg. Wood boiler is heavily insulated and typicall drops 20 deg a day when heating. Now propane boiler only has to heat 20 deg delta but has to maintain 1050 gal of water. So theoretically propane boiler would only kick on once a day.

Question: which install option would use less propane ?

 

[Fitter30]

Any brand of a condensing boiler is only 95%+ efficient with return water under 140° above 140° 87%. Confused about your temperatures wood boiler leaving water is 200.° and return temperature 190°. Fin tube after mixing valve 140° and 110° return. Fin tube doesn't need a mixing valve just a zone valve and thermostat. Lp boiler with 70° delta how can that be? Just punched in some numbers 110° return fin tube, 180° return main system, Boiler pump 8 gpm need 6.8 gpm 110°, 1.1 gpm 180°.
If the house is calling for heat the lp boiler will cycle all the time. 1050 gallons x 8.34 lbs per gallon = 8757 btu' s per minute x 60° (180-120) = 525,420 btu's to bring the water temp up back to 180° with no load. To raise water temp by 1° x 1 lb water = 1 btu.

 

[Sponsor]

 

[Fitter30]

delete