integrating a propane mod/con boiler into a gasser system

[kuribo]

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.

 

[ewdudley]

My fossil fuel boiler feeds top of storage, same as the wood boiler. There is an aquastat at the top of storage that is used to generate a call for heat to the fossil fuel boiler, and then there is a time delay relay that is used to extend the call for heat for ten minutes (or whatever) so storage is used as a buffer to prevent short cycling of the fossil fuel boiler.

The load side of the system works same as with the wood boiler. The only disadvantage I can see is that the storage tanks are always filled with water that is at the average return temperature of the system, in my case that's 115 degF (but would be a lot lower in your case). But the storage is located inside the house, so that doesn't bother me much.

 

[kuribo]

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?

 

[ewdudley]

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?

I happen to have tees on both the hot side and the not-so-hot side of storage, with boilers connected to one side of the tees, and loads connected to the opposite sides. But separate ports and dip tubes would work the same; a layer of hot water is maintained at the top of storage and the remainder of storage is filled with return water. When the layer on top is depleted the fossil fuel boiler gets a call-for-heat and the layer is replenished according to how long the call-for-heat time delay relay runs.

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[kuribo]

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?

 

[ewdudley]

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?

Need someone more knowledgeable of mod-con operation, but I would think that the mod-con would modulate supply temperature according to outdoor reset setpoint and the storage aquastat and time delay relay would just serve to shut the boiler off before it filled all of storage.

 

[kuribo]

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?

 

[arbutus]

I have a mod con boiler hooked up on a secondary loop to the primary/secondary system. The outdoor boiler / storage would be a secondary loop as well. Then just use a snap action thermostat set to 55F or whatever you want if you are going to be away and not burning the wood boiler or heating from storage.

 

[kuribo]

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

 

[heaterman]

It would be simplest to install the M/C in the basement and use closely spaced tee's, located between the storage and your initial load (3-way)? to dump the M/C output directly into your system.
We usually just put an aquastat on the piping coming from storage, or in the storage itself that will call the M/C boiler on at whatever setpoint you would choose.
As long as your storage stays above your chosen temp the M/C is off, if it drops below that (maybe 140*) the M/C will switch on.
The Viessmann will work without an outdoor reset just like a standard boiler. Just connect the aquastat or whatever control to the connections right on the boiler just like the tt terminals on an old style iron boiler.
Personally, I like to see equipment like that Viessmann inside a warm and dry conditioned space.

 

[kuribo]

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.

 

[heaterman]

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.

The main reason I suggested locating the gas boiler in your basement is that is the best place to tie into your system (unless I'm reading your description wrong)

Your storage and the 3-way you mentioned are in the home not the outbuilding?

 

[kuribo]

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.

 

[arbutus]

If I had a pressurized boiler and storage, yes, I would set it up with the wood boiler charging storage and the house would draw from storage on a secondary loop. The back up gas boiler would also be on a secondary loop

BUT I have an unpressurized wood boiler, and no storage. My primary loop runs through a heat exchanger. The wood boiler is connected to the other side of the hx. The back up gas boiler is on a secondary loop. I have only one air vent in the system.

I'm a diyer, not an expert. I sort of followed the schematic posted by Bob Rohr in the primary secondary piping thread stickied at the top of this forum.


You mentioned plumbing the boilers in parallel so you don't have to use a second pump. Do you mean series?

 

[kuribo]

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

 

[maple1]

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.

What all would you have to duplicate? Or what extras would you need for putting the m/c in the house rather than the outbuilding?

 

[kuribo]

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....

 

[mr.fixit]

It would be nice if both were in the outbuilding,that way if you went away for a few days(or a week or 2) in the winter and the gas boiler took over,you wouldn't have to worry about the wood boiler or lines freezing.

 

[kuribo]

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...

 

[arbutus]

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

Got it. My homebuilt system is set up differently. My Weil Mclain Eco came with its own pressure relief. Maybe the Veissmann doesn't? My MC boiler displays the return and supply temperatures digitally. I would be surprised if the Veismann doesn't display these also. Are additional temp wells really needed, especially if this is a back up? I'm not sure why the air vent couldn't still be shared if the boiler was in the basement, but if you wanted two, maybe a good Spirovent and a less expensive air scoop?

Location of the exhaust seems to be a bigger problem than what I had to deal with. I went through the rim joist, not the foundation. Is that an option for you? My boiler also supports fairly long runs of vent, provided they are sloped to drain condensate back. Can you find the manual for the one you are considering, see what length is supported, and maybe exit out the back or side of the house rather than the front?

If it were mine I would try to put it in the basement, but if it's too much headache, the dry and well insulated outbuilding sounds reasonable too.

Regarding freezing while away my plan is to let the circulator run on low and keep the boiler shed 50 degrees with an electric heater.