Cast Iron Boiler Buffer Tank Plumbing ?

I still think that with C.I. boiler purposeful mixing of BT by putting the hot boiler water into bottom of tank is better.

The boiler will fire longer to get entire tank up to target temp.

NRT.rob on diychatroom:
"Buffer tanks are more problematic with cast iron though because of the potential for sustained low return temps, so they must be handled with care there or you will REDUCE your boiler's lifespan."

I think you are looking at two different tasks?

A stacked (stratified) tank is desired to store the most amount of useable energy. It's usually more about the boiler off cycle, or helping with micro zones, that the buffer aids.

At Siggys home he had a solar tank built that is about 20 feet tall and small diameter. Built from the basement to the upper level ceiling. solar and wood feeds into it.

The center, hall wall of his house is a 12" wide tank 10 feet long site built rectangular tank. It also goes from the basement to the upper ceiling , as I recall. Probably the largest steel radiator I have ever seen same concept tall and skinny. It has since been drained after 20 plus years of service for potential leaks and he no longer has a wood boiler.

Keeping a cast, or any non-con boiler happy, and above condensing temperatures is a different task. IF you plan on pulling the tank below the lowest return the boiler can stand, correct you need a protection device. A simple 3 way thermostatic is one solution. You can still send cool water back to a non-con, just blend it at the return. Certainly better efficiency with a boiler designed to directly handle low returns.

Or consider one of the German cast boilers that claim 120°F of cooler is no problem.

Remember the cooler you run ANY boiler, even non-cons, the higher the efficiency.

The stratification goal can be accomplished with any boiler, it is mostly how you pipe the energy in and out to help maintain that layering. really Idronics 17 was intended to show general buffering formulas and the difference between two and four pipe options.

Pros and cons for both methods. the designer/ installer now has the tools within Idronics 17to see which best fits their design.

I think it's more of a thermodynamics answer. I don't know if the boiler manufacturers have any test data.
Hot goes to cold, always. the rate of HX increases as the delta t increases.

Also standby and jacket losses decrease with lower boiler temperature.

Same for solar collectors, the cooler they run, the more efficient they operate. With a collector it's more about losses to the ambient air.

Now if the goal is to load as much energy into the "jug" as possible running the boiler to 180- 200F or whatever, it's a moot point.

In that case the best overall boiler efficiency would be long run cycles to eliminate as many start stop cycles as possible.

Of course the MOST efficient boiler is the one that is not running

Really that is where the whole buffer tank discussion starts is with folks burning sold fueled boilers. You need to run them hot to get then gasifying and at top efficiency.

And all that is obtainable.

It's always a balancing act as to how much is enough when it comes to buffer or storage, knowing the more you store, the more you loose.

We also want to be aware of cycle life of components, in addition to efficiencies. Sure tankless heaters start and stop every time a faucet flows and still maintain fairly high efficiencies, but how many cycles are designed into all those electronics, valves, fans, etc? I don't think you would want to run a cast boiler, any boiler really, like that.

Which is why, on the webinar we also talked about tank size and shape as related to surface area and tank losses. All my tanks have additional wraps around them, either the fiberglass vinyl like you find at home centers, or the foil bubble foil wrap. Slide you hand underneath it in the winter and see how well it actually works. Until we have 4" insulation tanks

We all long for the day of an infinitely adjustable, high efficiency, fossil fueled heating device, starts the first day of the heating season and never stops, just modulated from 10 btu/hr to design load. I think it's doable in our lifetime.

In the meantime we have mass and volume to store energy in, you're on a good path for you specific application.

the key to all these two pipe methods is the short, fat piping connection to the tank. That "becomes' a mini hydro separator with very little pressure drop in that section. Think closely spaced tees, or large chamber hydro separators. By keep flow velocity low in that detail the flow will take the path of least resistance.

Certainly I would have the flow checks installed in the circ pumps also. S&R check protection would be a good idea. Those would limit the potential for thermal migration, when all pumps are off.

Remember thermal migration, like the old gravity powered heating systems is driven by the height of the water column, tall tank and or loads high above the tank = more potential for thermal migration.

Same concept with the thermosiphon solar systems with the tank above the collector.

The graphic just demonstrates the concept, probably a typical system building would have more, possibly higher, piping circuits than shown, tempting ghost flows.

Supply and return from the boiler only a few inches apart, at tank bottom?