Quote from Yanche - "On the existing boiler side of the run, I don't have enough of an understanding of what you want to do. Boilers in series, parallel, manual water flow switch over, automatic switch over, parallel-secondary loop, etc?
I also don't know your heat distribution piping, 3/4" basboard, radiators, zone circulators, zone valves, single returns, common returns, any radiant manifold, injection mixing pumps, on and on?"
Thank you so much for the reply and help. I hope this post isn't to long. This project is driving me crazy but I think I'm almost there. I'm planing on puting the boilers in series (they are about 90' apart). Please take a look at the pictures I posted in the beginning post. I'm going to move the pump from the return on the oil boiler to after the exp tank because of what I've learned here(pumping away) I think for now, I will run that pump (the existing pump, a taco 007) constantly to keep things simple. After water leaves that pump it will go to a flo check (I'll reuse the one in the picture) about 10X pipe diam downstream. Then it will loop up and back to meet up with the Pex return to the coal boiler. Suppy from the coal boiler will enter at the return for the oil boiler, a big series loop, also my primary loop. I think I should tap into this loop after the flow chek and before it gets to the return pex with a secondary loop for the house radiators. 2 closely spaced T's with a taco 007 IFC(with flo check)on the supply side and a swing check before it meets back up with the primary. I'm thinking I'll have the circ pump run power from both boilers go to a relay and then either boilers thermostat can turn on the house circ pump (I'll set the oil t-stat lower than the coal t-stat for backup). My whole house has only 1 zone with 1 pump at the boiler inlet now going to 12 large old radiators fed by various sizes of iron pipes, some are up to 2". It branches out to 2 pipes on the supply side and returns back in 4 pipes on the return (you can see them in the picture), very simple control but it works just fine. I'm going to try and use the DHW coil in the oil boiler as long as I can keep the oil boiler from kicking on. If it draws too much heat two quickly and even with a large aquastat differential between the 2 boilers the oil kicks on, I'll probably route the output of the DHW coil to the inlet of the electric heater to help it.
I'm glad you said 1" copper would be good at the coal boiler.
Based on my description of the system:
1. Whould it also be acceptable for the rest of the primary loop at the oil boiler to be 1" copper? and
2. Would it be acceptable to make the beginning and end of the secondary loop 1" copper till it joins the house iron pipes?
3. Does my relay idea make sense for the t-stat control of both boilers?
The price jump to 1 1/4" copper is substantial when you add up all the isolation valves, pipes, fitings etc. Hundreds of dollars more. If I don't need it, I don't want to waste the money.
I think I'm almost there, finally. Your expertise would be very much appreciated for these last few concerns