Primary/secondary options with wood boiler
Last Post 10 Oct 2014 02:42 PM by Dana1. 8 Replies.
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jacoblastUser is Offline
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09 Apr 2014 10:48 AM
Hi all-

This is a question about system design with a GARN 2000 wood boiler.

Suppose in our system we will have radiant floor zones needing two different water temperatures (there are both slab areas and suspended joist areas). Suppose we circulate full temperature water in the primary loop (75' underground insulated pipe run) and mix to the required temperatures for secondary circuits inside the building. I understand this is a typical configuration when the boiler is inside the house (any heat loss from the primary loop stays in the house). But in the case of an outdoor underground primary loop, with a system that doesn't require any high-temp water (no DHW), it seems that circulating 180 degree (or whatever the GARN has at the time) water underground at a fixed design flow rate every time there is any call for heat (no matter how small) would create unnecessarily high heat loss. Comments?

Another option that occurred to me is mixing the primary loop water (at the boiler shed) to the suspended joist area design water temperature (with outdoor reset control). This lower temperature might have the advantage of less heat loss in the ground, but the flow rate would be higher, requiring larger pipe and primary circulator, and thus higher electricity use. There is an optimization problem here and I can't quite make full sense of it. Thoughts?

Could another option be to circulate full boiler temperature water in the primary loop, but use a variable-speed modulating pump? Then the flow rate in the primary loop could be dependent on the actual current heat load. But then again, with a decreased flow rate, there would also be increased heat loss to the ground, right?

Or might one use an outdoor reset control for the primary boiler loop temperature, in addition to the two water temperatures already required by the radiant floors? But this seems to be getting into excessively complicated controls. It seems to me that in feeding the GARN, we're already going to be doing a sort of "manual reset" anyway, getting to know what our GARN tank target temperature needs to be throughout the heating season. (We're in central Virginia.)

One other basic control system question: how does the primary loop circulator get turned on when a zone calls for heat? I assume it doesn't just run continuously. Do you need a system controller to coordinate this? Or do the zone thermostats also signal the primary loop pump as well? Much appreciation in advance for any feedback.

Jacob
Dana1User is Offline
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09 Apr 2014 02:33 PM
Methinks you're mis-using the terms primary & secondary relative to how hydronic designers use them. In hydronic design the primary & secondary loops are driven by separate pumps to account for different pumping requirements. What you ar describing is a thermostatically controlled recirculation loop, which can be driven by a single pump. And yes, there are ODR controlled mixing valves that could be put to good use here. With dual-temp requirements in different parts of the system the flow from the boiler needs to be at the higher temp, but the lower temp loops can be mixed down.

With a big ol' self-buffering high mass boiler like the Garn there is no minimum pumping requirement for the boiler- it just heats up until it trips the high-limit, throttles back the fire and it's fine.

To know the cost crossover points on pumping power requirements you'd need to actually design the system. In general fatter pipe takes less pumping power than skinnier pipe at a given gpm and length, but fatter pipe isn't really any more difficult or expsive to insulate in a ditch than skinny pipe. With either you'd want to fix the pipe to 1.5-2" EPS and spray over it with 1.5" of closed cell spray polyurethane. You'd have to mix the water temps down to 150F or less for the polyurethane to tolerate it over the long term, but that's probably higher than what you were looking for anyway(?). With only one pump driving the mixer loop from the Garn, depending on flow requirements you might be able to use zone valves for the rest, but I strongly recommend taking a Hydronic Heating 101 course before committing to any of it. Doing the math and getting it right the first time is really worth it.
sailawayrbUser is Offline
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09 Apr 2014 03:26 PM
What Dana advised plus think about spending some time reading Modern Hydronic Heating by John Sigenthaler, PE. You should also first accomplish a building/room heat analysis too.
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jonrUser is Offline
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13 Apr 2014 08:57 AM
Keeping the underground run as cool as possible will save energy. But it's a small and presumably well insulated pipe - so I'd look at the actual numbers before worrying about it.
jacoblastUser is Offline
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15 Apr 2014 10:30 AM
Actually I think I was using the terms primary/secondary in the proper hydronics sense, i.e. the primary (boiler) loop having its own circulator and each secondary being hydraulically isolated from the primary loop. My explanation was unclear, though. At any rate, after Dana's helpful reply (and also finally biting the bullet and starting to read Siegenthaler's MH book) I realize that I may be able to simplify like he suggested and use a direct circult (one pump & O.R. mixing valve) to feed the joist areas at the appropriate temperature, then have the slab manifold(s) as a hydraulically isolated secondary branch off the return leg of the joist manifolds' circuit, with it's own circulator.

As far as I understand, I might (depending on requirements) be able to accomplish this by carefully designing the flow rate & temperature drop so that the slab manifold's supply temperature is proportionally correct to the outdoor resetted joist temperature...this would be a very elegant and minimal solution if it worked out. Or if not, maybe using a "dumb" 3-way mixing valve to do proportional reset as described in MH. Or a thermostatic 3-way mixer, or a second O.R. controlled 3-way? Any insight here?

This raises the question: how necessary is O.R. for slabs? How about slab-on-grade (above ground) vs. basement slabs? It seems that a basement slab's heat load will have much less seasonal change than an above-ground s.o.g.

Also, how hot is an acceptable supply temp for a slab? Are there disadvantages to using a relatively hot supply (say, 120deg) and a floor sensing thermostat to keep it in check?

I am ready to start doing the real calculations, but it seems to me that proportional reset is an elegant solution, keeping controls simple (the areas that would be proportionally reset don't need to be setback regularly, etc.) Any comments or suggestions would be very much appreciated!

Thanks,
Jacob
BadgerBoilerMNUser is Offline
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09 Oct 2014 07:25 AM
The Garn boiler is a batch-feed built here in the Minneapolis area for decades.

The is no reason to use P/S in the system design of a Garn save in a mixing strategy and then only to mix the return with supply should the storage water built into the Garn run below the design supply water temperature for a given day.

Remembering that "design" represents but a few days per year and the typical design temperature of a sub-floor system in a well-insulated home is perhaps 20 F higher than the slab, in most weather one temperature will do for both systems.

The choices are injection with circulators, zone valves or three-way mixing valves.

We use a plate heat exchanger to isolate an open system like the Garn, and most solid fuel outdoor boilers, from the more delicate and expensive radiant hydronic components. We also use outdoor reset to conserve stored energy in our boiler and lower the fuel usage. Of course it is all guesswork unless you have a proper heat load using radiant dedicated software, which will give you the DWT
for each area and all you to choose to-mix or not-to-mix.

Oh, yeh. Also, what Dana said...
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Dana1User is Offline
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09 Oct 2014 06:19 PM
Hmmm...


What I'm wondering now is why it took a week shy of 6 months for Morgan to respond to this thread!?! :-)
BadgerBoilerMNUser is Offline
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09 Oct 2014 08:29 PM
Always with the math.

I work for a living...and a little ADHD...

heheheheeee
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Dana1User is Offline
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10 Oct 2014 02:42 PM
 
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