Adding propane boiler to existion wood boiler system

[wtxing]

Existing heating system is a custom made 1050 Gal wood fired boiler. Radiant water heat in concrete basement floor, 1st floor of house, baseboard on second floor of house. Radiant glycol in garge floor by means of exchanger inside water boiler. Wood boiler also has 2 electric elements in it for backup or when on vacation. I'm now 70 and am adding a propane boiler for when I no longer want to cut wood. Manual shutoffs allow me to run in three modes. 1 - wood boiler only as it is today, 2 - propane boiler only with return feeding back to itself or, 3 - propane boiler with return feeding back into wood boiler to allow garge floor heating , Keep in mind I used this design because of my desire to be able to heat my garage floor. I attached a picture of my design and have some questions..... 1) It's my uderstanding I need a weighted flow valve in this design. Not sure I have placed it correctly.
2) When using mode 3 if I choose not to close shutoff on propane return how do I prevent return water from going to both wood boiler and propane boiler ? For a more automated design with no maual shutoffs I looked at a parrell design but that did not take in my desire to heat the garege floor with return water. I realize when using mode 3 return water to the boiler will only be around 100 deg but I only heat the garage floor to about 60 deg anyway. Any other problems you see or suggestions to do it better would be greatly appreciated.

 

[ewdudley]

I believe a "weighted flow valve" would only be needed to prevent convection flow to the upstairs baseboard, and so it would go on the leg going upstairs.

If the garage is reasonably tight you should not need the glycol loop for the garage. It won't take much heat at all to keep the slab from freezing where you live. So just connect the garage loop to system supply and return.

Then run the supply from the wood boiler into the return of the gas boiler and let all flow go through the gas boiler whether it is running or not. In other words, delete the short pipe shown between the supply and return of the gas boiler, and delete the pump on the line into the gas boiler. Now both boilers can be online with an aquastat to activate the gas boiler as backup to the wood boiler.

The line to the left of the gas boiler would then be used to bypass the wood boiler when the wood boiler is offline and isolated.