I Have Read ? About Series Vs Parallel.

Personally, after many years in the HVAC industry and through personal experience, I would recommend parallel boilers each with a pump and isolation valves. This way, if anything goes wrong with one unit, it can be taken off line and serviced while the other maintains heat. Failures usually occur during the coldest weather when the equipment is worked the hardest. I would also recommend primary/secondary pumping with the boiler pumps being the primary system and heating zone pump(s) as the secondary.

Many piping and controls schemes can be used. Personally, I like the articles written by John Siegenthaler , P.E. I have read in PM Engineer, a industry magazine. He has several piping diagrams in his articles that show right and wrong.

I am personally preparing to modify my heating systems for a future installation of a new boiler. My system will be primary/secondary through a hydraulic separator. I plan to build by own separator this winter.

Tekmar, which was previously mentioned, produces a line of controls with all of their documentation on the web. I have not heard anything bad about these controls.

No doubt series boilers will be cheaper and simpler to install.

Good reading, and free! All the tech talk: http://www.bellgossett.com/BG-TechTalk.asp

If you decide to buy a book this is the only one you will need:

http://www.hydronicpros.com/publications/index.php?id=24

It's expensive but worth it. Comes with design software that can design simple heating systems.

I like a series installation with the coal stoker before the oil boiler.
I installed a by pass so you take the coal out of service for the low load conditions.
Series with a circulator between the boiler with the temp switch installed on the water return side of the coal.
So if the coal boiler is at setpoint 180 and the supply back to the coal boiler is above 160 run the circulator until temp switch is satisifed.
I also installed a 3/4"sv nc that opens when the circulator is running wich cause water to run from coal stoker through the oil boiler out the discharge header and back in the return header.And for you siphon guy I have a 1/4 ball valve in parell with the nc sv.
Coal boiler aquastat run the whole thing setpoint is 25 degree higher than the oil boiler.

series bypass
on the return line to the boiler install a ball valve in series and two ball valves in a parell one on each side of the series valve.
The parell valve closest to the return side feeds the coal boiler.
The other parell valve return hot water to the oil boiler.

BDD

Thanks guys, between family and work, I don't have much time to be on the net to search, what ever I did search made me more confused, as far as plumbing goes I know how to (plumbed my house) and his I like things right I take my time and make it look good and work well. I just need some ideas maybe some pics and reassurance that what I want to do will work. The ? in my last post are what I am hung up on at the minute Thanks

Mine is a pretty simple system:

The entire set up looks like an oil-only design, as if there were just one boiler (the high efficiency oil unit). The hot water exits the boiler, goes to 5 zone valves, then returns to the system. The controls are wired to the oil unit.

My coal modification was super simple: where the cooler water returns from the house, I cut the pipe and added 2 tees with valve between them. By closing the valve, water is directed through the 1st tee to the coal boiler, gets hot, and flows back into the system through the second tee. (There are also valves in each line so I can completely isolate the coal boiler.)

The system works and was cheap to assemble. The entire system has only one pump (a Grunfos 3 speed, $79, a great little unit). What's good about it: if the coal unit can't keep up or is slow to respond, the oil kicks in until the coal catches up. If the coal goes out, the oil handles the load. And, all summer, coal is off, valves isolate the unit. It also keeps the boiler warm, preventing condensation. What's not so good: well, I do burn some oil. But not alot. In Central New York (ie: cold windy winters), I used about 300 gal in a calendar year to heat almost 3000sq ft. Probably 120gal was for domestic hot water for a family of 6. What's bad: if the coal fire dies, the system still pumps hot water through the coal unit. That causes oil-heated water to maintain chimney draft (with the thermostatic damper open wide) and sucking heat up the chimney.

Lesson: don't let the fire stay dead. Hopefully I won't have this with the new stoker unit.