Outdoor wood boiler with condensing gas boiler as back up

bob eck

Contractor told me this is what the home owner wants.
Outdoor wood boiler with high efficiency condensing LP gas boiler as back up.
What is the best way to install and pipe in this type of system.
House has radiant zones plus will have cast iron radiators in the basement.
I was thinking
Outdoor wood boiler
50/80/119 gallon buffer tank with coil in the tank.
Pipe outdoor wood boiler into the coil in the tank.
Pipe condensing LP gas boiler into one side of the buffer tank
Pipe system supply and return out the other side of the buffer tank.
Have a aquastat in the tank so if outdoor boiler not keeping up the condensing LP gas boiler would fire and take over heating the house.
Or should the outdoor wood boiler be piped into a brazed plate heat exchanger and then into the buffer tank.
Will this type of set up work.
Anyone have drawings on how to pipe this in? Or is there a web site to look at for piping diagrams?

Thanks for your help.

GroundUp

What's the heat load of the smallest zone and is it smaller than the lowest fire of the mod/con you have in mind? Reason I ask is the majority of these I do have no use for a buffer, as the OWB circ runs 24/7 anyway. Typically I'd pipe the mod/con as normal in the house, but cut in a plate exchanger in the return line to the mod/con just before the unit. Aquastat on the incoming OWB supply line just before the plate, interrupting the ignition for the mod/con until SWT from the OWB drops below 140 or whatever that magic number happens to be. You may want a second aquastat on the same line and a 3 way valve to bypass the plate with the constant circ OWB water to avoid backheating the OWB with LP. I do a lot of these but I'll admit I'm real dumb when it comes to electrical and I have someone else do that part, but I do know what goes where and does what

Ironman

What type of emitters in the rest of the house and what SWT are they designed for?

hot_rod

There are a number of way to pipe a system like that. If the OWF is an open system, either a HX or the indirect as a HX for the OWF. Upper coil could be for mod con input, tank is the heating loop.

What about DHW?
A solar differential controller works nicely to flip between the various sources, better control compared to a one sensor aqua stat.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf

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bob eck

Hot Rod would a buffer tank 50/80 gallon with coil work?
I can get a Vaughn buffer tank with a coil in it. The coil will be for the OWB because that is an open system.
Then he can pipe the Lochinvar Noble Combi boiler into one side of the buffer tank and system supply and return out the other side of the buffer tank.
Would it make sense to have a second coil in the buffer tank to preheat the domestic water before going into the Lochinvar Noble Combi boiler?

Ironman

@bob eck,
I'd be careful about using an indirect for the heat exchanger. The coil surface MAY be too small. They are sized based on a delta of 120* (50* domestic in, 170* boiler in). Depending upon the required SWT for your emitters, you may have 170* AWT from the ODWB and need that same temp for your emitters if they are high temp. Depending upon the load, an indirect may not be able to do that. I've been down this road.

We really need accurate load calc and radiation numbers to determine what type of HX is needed.

hot_rod

A plate style HX works best as you have two moving flows to better transfer heat energy. With an indirect tank you are limited by coil capacity and water may not always be flowing thru the tank for good heat transfer.

I'd rather a generously sized plate HX, and a buffer tank to act as a separator and possibly DHW production. The buffer tank would also cover micro loads if any on the mod con.

A reverse indirect may be a good option, piped as a 2-pipe buffer. The coil would pre-heat to the Nobel Combi.

Tank buffers and is hydraulic separator.

Mod con directly to load or tank, or both together depending on loads.