I have on order a Leisure Line 220K boiler for my home. I've been doing lots of reading on this site for how other people hook up their boilers. I am certainly not a plumbing expert by any means, but I have some thougts and observations.
I understand there are various multi-boiler designs. For the most part, people are going with Series, Parallel, and Primary Secondary loop designs (people are mistakenly calling this parallel) when integrating dual boilers in their homes. Series makes sense for keeping both boilers warm, but it may have some inefficiencies as far as a second boiler being warm all the time. It also provides a single point of failure. Parallel seems to be the most efficient, in my opinion, however you could potentially introduce leaks or corrosion as you have a cold boiler. Primary Secondary is a neat, high tech design, however I think it is overkill and inefficient for a home design. Especially if you have multiple 80 Watt circulators running when you only need one in a residential heating system. I'm also thinking in terms of being able to run your system on UPS or generator when you have no power. If you have a call for heat on 4 zones, 5 circulators running, that's 400 watts of power. It also has a cold boiler. These are my observations, and I welcome anyone to counter my thoughts.
I currently have 8 zones in my house. One zone is for a Boiler Mate, one zone (although it is deactive) is for the basement, three zones for the first floor, two zones for the second, and one zone in the attic. The house is 3300 SQ Feet. There is already a recent Burham oil burner in the house and I want to add a Lesuire Line 220K as a second boiler.
My plan is to go with a Hybrid Series design that I came up with. It is a series design which will use the LL 220K to warm the Oil Boiler, however it will utilize 4 3-way valves to allow me to isolate each boiler. It will have a single circulator that sits on the return side. Expansion tank and air purge will sit on the supply side. Please see the below drafts I came up with on my current and dual boiler design.
Obviously these designs are missing some valves and I missed the expansion tank.. I would consider a parallel system, but is it a real concern about a cold boiler leaking (mine doesn't today when it is cold) and is there any damage that could occur to the boiler in terms of corrosion from it sitting for years full of water but cold? Does anyone have any thoughts or comments?