are you saying that was consumption was reduced by 4.5 cords by just switching from series to parallel?
The oil boiler was the one pictured on the left. The boiler had a 6" flue connection. There was an asbestos lined fire box on top which sat three cast iron sections of water tubes which made its heat exchanger. These sections weighed just under 300 lbs and held a total of 3 1/2 gallons of water. There was also a tankless coil for domestic hot water. The boiler's primary function was to produce DHW with house heat being secondary so it maintained a water temperature of 170 F to 185 F for domestic supply with the circulator running continuously to ensure a constant flow of hot water through the coil.
Years later with storage and the Jetstream in operation and the oil boiler now just a back up, all valves for domestic supply and house heat were closed to the oil boiler and an hour meter was connected to record the operating time of the oil burner. Over a three day period with no load, the burner ran for 13 hours. This 13 hours represented the stand-by loss, wasted heat going up the chimney. The boiler would run for more than 3 hours day to maintain standby temperature for DHW, so over a week this stand-by loss represented more than 21 hours of operation.
I'm a person who likes to tinker so I connected the oil boiler to the storage. Average operating time of the oil boiler dropped to between 12 to15 hours per week when heating storage.
Connecting your oil boiler to storage is not something that I would recommend for everyone without doing your homework. Our storage is well insulated with a very low heat loss of around 1,200 BTUs per hour for 1,000 gallon tank , plus the oil boiler in this set up was lower than storage. No gravity flow when the boiler was not operating.
The wood boiler pictured is the same as was being used at the time. The instructions in the boiler manual were for a series connection which required the oil boiler circulator to operate continuously to moving the water between oil and wood boilers to ensure a constant flow of hot water through the tankless coil in the oil boiler. The aquastat on the wood boiler was set to maintain a water temperature of 170 F / 185 F. The wood boiler in series with the oil boiler burned 22 cords the first year!
This little wood boiler was pretty much burnt out at the end of the second year. The constant cycling of a hot fire, metal around the flue and draft glowing red hot, then having the draft suddenly close, the steel around the draft control and flue had cracked and had been welded several times.
The second year of operation the boilers were plumbed to operate in parallel, with the smallest tankless coil like the one pictured being used for DHW.
With the wood boiler operating without heating the oil boiler's 300lbs of cast iron, wood consumption dropped to between 17 /18 cords (4'x4'x8') in the second year.
These are some observations that may or may not be of some use to you
Allan
