Diverter Valves

Stokehold

I am adding a small solid fuel boiler for supplemental heat. It will supply one zone which will be a fan coil in an existing propane furnace.
I was thinking about using a 3-way diverter/mixing valve similar to a Dan Foss ESBE series on the return to prevent thermal shock.
Since a portion of the supply water is diverted back to the return, how do these valves affect overall flow? In other words, if the coil requires 10 GPM and there is going to be supply diverted back to the boiler (water going in two directions), how do I size the pump?
In your opinion, is it better to use a diverter valve for simplicity or would it be better to add an extra pump and build a primary-secondary loop? It seems to me a primary-secondary may be over kill for a small one zone system.
All comments are appreciated!

STEVEusaPA

Usually people do it the other way around. The propane is the supplemental heat for when the primary can't keep up, or it's a shoulder season. Let both units charge a storage tank, and work your zones off of that.
Great resources:
Specifically:
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf
Generally:
https://www.caleffi.com/usa/en-us/technical-magazine

hot_rod

Remember primary secondary piping alone will not assure return protection.

Here is a couple drawing showing the best way to use V/S pumping in my experience.

Regardless of a valve or pumped decision, you basically split flow, whatever diverts still equals the total flow.

Idronics 19 was put together mainly to address piping gitches that are most common, return protection is one that comes up in the non condensing boiler installs, especially with high mass either thermal buffer or large slabs.

Stokehold

Thanks for the info! Excellent site!
Actually, the 90,000 BTU high efficiency propane furnace is the backup. It is currently running with a 4 ton geo thermal unit.
When the temps get really cold, the geo just doesn't impress me. I like it warm. I usually start a wood stove to make things a little more comfortable rather than firing propane.
I figure that if I am going to burn solid fuel anyway (wood/hard coal) I'll burn it in a central location.
I already have a small 90,000 BTU solid fuel boiler. It's nothing fancy and isn't particularly high in efficiency but I already have it.
My plan was to install a fan coil in the propane furnace and was just wanting to know the best way to control the return temps to the boiler.
Coal/wood boilers are a little tricky to control with high/low limits and ideally you can maintain close to 180°, but they can easily idle close to 200° depending on conditions. A dump zone is present for anything over 210°.
My concern is when there is a call for heat (independent thermostat), and the circulator brings that first round of cold water into the boiler. Obviously, I need to get that return temperature up and am wanting to know the best way. I am assuming that if a 3-way diverter is used, I will only require the one circulator.

hot_rod

Depending how the coil output matches the actual boiler output., Usually low mass heat emitters like a fan coil or fin tube do not need return protection, they heat up quickly.

Some really simple basic methods are a return temperature sensor tied in series with the circulator. If return drops below 130F, pump stops. Fairly primitive bang/ bang control, even Viessmann used that control on basic entry level cast boilers.

Your slow start up will be the boiler and it's content I suspect, how many gallons does the boiler hold? Most wood fired boilers can run without circulation at all, so the temperature control on the pump would help with cold starts also.

A step up would be a thermostatic mixer, chose one with a high Cv and it will not cost much pumping power. Most of the return protection thermostatic valves will be a 12-14 Cv. They give you absolute protection and allow the pump to run from cold start. Your best heat exchange from the boiler will be with water flowing across all the surface.

Stokehold

Hot Rod, you bring up some interesting points.
This boiler only holds 17.9 gallon.
As far as the emitter itself, the manufacturer wants the finned area of the heat exchanger to take up as much of the plenum as possible. In my situation, a 3.5 in. x 20 x 22 exchanger is rated at 176,000 BTU (180° at 17 GPM and 2750 CFM).
Even if I move 8 GPM through this and the fact the propane furnace fan is set at 1805 CFM, I have a feeling this may cool that small boiler down very quickly. I do have an 8124 aquastat that will be regulating low limit and circulator, (burner circuit controlling forced draft fan) however, each time the boiler recovers and the circulator starts, I believe the return temp is going to be low enough to cause a problem.
Are the thermostatic valves you speak of the ones that have actual elements that correspond to various source and return water temperatures?

hot_rod

What is the heat load of the space that you are supplying with that coil? That is the number everything should design around. The air handler, ducting, burner or coil supplying the hat to the "box"

Is 90K the actual output of the wood burner? What size is the furnace that the coil is going into? Is it sized to the load accurately?

The air guys here probably have an idea how much heat energy you move with 1800 CFM.

EBEBRATT-Ed

the air side will depend on the hot air supply temp. Assuming 115 deg supply and 68 return air (47 deg rise) CFM x 1.08 x temp rise=btu/hr 1800 x 1.08 x 47=91,368 btuh

Stokehold

Yes, the actual output of the wood/coal boiler is 90K. The propane furnace that will hold the coil is also 90K.
As far as the load, I never really did a heat loss calculation. Since the geo unit and propane were already in place, that would be like closing the barn door after the horses get out. The home is well insulated with good windows/doors and is not really difficult to heat. This add on boiler will only be taking the place of the wood/coal stove I now use in very cold weather. If it can't keep up, something else (geo and then propane) is going to fire.
I would rather have a larger wood boiler, say 130,000K that would facilitate some of my radiant zones as well (those zones, garage/basement, are on there own heating source) however, my flue is not large enough to accommodate anything larger as far as hand fired boilers go.
This wood boiler will have it's own thermostat set at approx 5° over the geo/propane unit. If the temperature keeps falling, the geo thermal will attempt to take over. If the geo can maintain room temperature, it will run very efficiently for however long it takes to reach set point. If not, then the propane bill goes up!
Again, since I already use a wood stove, I just thought a small boiler would be more apt to evenly distribute heat with more or less the same fuel consumption as the stove. Getting back to the original post, I just want to protect the boiler from too cold return.

hot_rod

I'm thinking that if the furnace is/ was oversized, as most older systems are, then the wood boiler should be able to keep up with the load and you should not have major return temperature issues. But it is just a guess, without an actual load calc on the home as it stands now, it's hard to predict an actual operating condition.

Usually the opposite is the problem with wood boilers, keeping them happy with low load conditions, constant fire stoking may be required to adjust the burn to the load. A lot of wood burners add buffer tanks to smooth out those load swings and the need to adjust the burn. Wood burners much prefer long hot burns to maximize efficiency.

A single pump system with a thermostatic valve would give you the piece of mind. Really no downside other than cost of the valve and piping.

Or run a season and collect some data of actual operating conditions, adjust components as needed.

Stokehold

Thanks much for all the advice!
It's kind of where I was going.
This is is a great site and keep up the good work!