integrating a propane mod/con boiler into a gasser system

Hello-

I am in the process of figuring out how best to integrate a mod/con back up boiler into my gassifier system and was hoping for some input from those with much more experience...

Currently, my plan is to send the gassifier output by variable speed pump from an outbuilding to my basement to (2) 500 gal propane tanks, then out of storage to a 3 way mixing valve, variable speed delta p pump to zone valves. I will have 4 zones (2 basement, 2 in floating slab on first floor). Design load is about 75,000 BTU/hr. I figure I will need roughly 15 gpm at design conditions. I have about 6000 feet of pex in total in the slabs.

I would like to put the back up boiler in the outbuilding with the wood boiler and plumb them in parallel, so I can then use one set of circulators, air vent, etc., as well as the pex underground lines. My questions surround how best to handle the basement end- I am assuming it would be best not to use the mod/con boiler to heat the 1000 gallons of storage, but to run directly to the zones. The mod/con boiler I am looking at (Viessmann 100-35) which has about 97,000BTU/hr maxo output, puts out only 6.5 gpm. So, would I basically be using it to "inject" to my zones, after the storage tanks, before the 3 way mixing valve? I would then need some sort of relay activated valves to route the mod/con output to the zones, bypassing the storage? Would it be good to use some sort of small, say 50 ~100 gallon buffer tank between the mod/con and the zones?

Any input appreciated....Thanks.

Do you have your storage tanks plumbed with separate dip tubes for boiler supply/return and load supply/return, or do you have storage plumbed in with a "tee" so that boiler supply(return) feeds both storage and your loads at the same time?

In this scenario, the mod/con boiler would not modulate, it would just run full out, floating a layer across the top of the storage tanks, running until the heat loads where satisfied, plus an extra 10 minutes or so? It would be seeing say, 85 degree return water, and putting out its 6.5 gpm at 175F or so, which would then be mixed down by the 3 way mix valve to 105F or so for the floors, correct?

I guess that is one way but with the three way mixing valve after storage serving the 4 zones, I am thinking I would just use an outdoor reset (if used at all) to control the mix temp out of the mix valve....Hmmmm....anyone with any experience with these mod/con boilers?

So your storage is on the secondary loop as well as the mod/con boiler?

I was hoping to avoid duplicating all the accessories but I understand the sentiment about placing it somewhere warm and dry. The boiler building I build is concrete block with spray foam inside of steel studs covered with cement board. The roof is poured concrete. No doubt it will be warm and dry but not quite the same as inside the house envelope.

Thanks.

Yes, the storage and three way, etc., are all in the basement of the home. The wood boiler is in the outbuilding. I thought it would be easiest and most economical to put the mod/con in the outbuilding as well, just plumb it in parallel with the wood boiler. Then I wouldn't have to cut a hole in my concrete foundation or add extra air separator, etc.

No, I mean in parallel like in the nofossil "simplest pressurized system" sticky....

air separator, temp wells, pressure relief, off the top of my head....plus I would have to cut a hole through my 10" thick poured concrete foundation for the flue which would then put a flue on the front of the house where I would rather not have it. Never mind the hassle and costs of renting the equipment to cut the hole....

I think if I plumbed them in parallel, I might still have to worry about that wood boiler freezing unless I have its circulator run a bit off and on.....if I plumbed them in series, that wouldn't be a concern....I could always put a small heater on a thermostat in the room as well...