Buff it out

did you already buy the stuff? a better way to go would be to size the boiler for the dh load and use a bigger indirect to get the dhw capacity you need unless you have some unusual condition where you have a continuous dhw load.

the indirect stores energy, a bigger tank stores more energy. unless you need continuous hot water you can size the indirect to store enough hot water for your use then recover while you're not using hot water.

i thought the boiler was a modulating boiler but it is a 3 pass oil boiler. you probably can't find an oil boiler small enough to match your load so the buffer tank would be the better way to go although you could probably do some things with controls to keep it from short cycling. the buffer is the cadillac installation. a bigger indirect and smallest boiler you can find along with controls with some anti short cycling features would be the more cost effective option. might be able to downfire the burner some too.

there are also some schemes to make the indirect the buffer.

with a buffer tank you are buying 2 storage tanks, one for domestic heating water and one for domestic hot water.

what type of heat emitters, what temperature do they need? Will you run outdoor reset control?

How often would the smallest zone run by itself?

How much dhw do you want?

a reverse indirect may work depending on dhw load,

This is a two pipe buffer, reverse indirect.

The injection option if you want ODR control

Here is what a 11,000 load looks like.

Rule of thumb you want a minimum 10 minute run time every time a boiler like that fires.

So a 50 gallon buffer gets you close at 7 minutes. 44 minute off cycle is great.

With ODR if you pulled down to 145F you get a 13 minute run.

If you want to do more number crunching, determine how much fin tube in each zone. Compare it to design load. Then determine how often and how low you could run the fin tube on less than design days.

Probably 80% of the heating season you are below design. That data is available also.

The tank will run 160- 180, so the boiler is protected from cold return. The key is to pull off the buffer via ODR, also have enough boiler capacity for DHW.

With a control logic, you could drop heating load with DHW call. So a 177K boiler coiuld supply 2.5 gpm or so. Check with the TurboMax output chart to confirm DHW output at various boiler input and incoming water temperatures.

An example of BIN temperature data

I think SlantFin output charts went down to 120 SWT.

This Idronics has some ideas and formulas for lowering SWT.

https://www.caleffi.com/en-us/magazine/25-lowering-water-temperature-existing-hydronic-systems

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control logic is just referring to the control logic, could be tekmar or others.

to do odr you would need either to do injection mixing with a variable speed circulator or a modulating 3 way valve, there are pros and cons to each, either will work. others know much more about this than i do.

on the dhw supply you want a fixed thermostatic mixing valve to bring the dhw temp in the coil or tank to safe temps and to keep the temp consistent as the tank temp varies.

What is the coldest outdoor temperature you have experienced? Does the home maintain a comfortable temperature, able to stay at 70 indoor?

If so that could indicate that you have plenty of heat emitters, fin tube.

Then determine the boiler operating temperature at those high load conditions. Does it operate at a common - 180 temperature?

It is possible with sufficient or oversized radiation that the boiler could run lower temperatures even at design conditions. That would make a better case for ODR.

Next I would look at the reverse Indirects performance output at lower temperatures. If it gives you adequate DHW at a 160 or lower tank temperatures that too helps the boilers and systems efficiency. Dig into the performance data at Turbomax I know they have a software that you can input all you specific data and see what tank can cover the loads. Either call tech support or a local Thermo 2000 rep for sizing help.

Assuming you have multiple goals, reduce micro zone short cycling, maximize efficiency, and take advantage of the near constant circulation that ODR can provide.

So, how good is good enough? If you want the very best bang for the bucks I’d suggest some more data collecting and number crunching.

Spending big $$ to upgrade should give you big improvements.

Or just buy the smallest boiler and let it short cycle as I suspect it has been?

You could also build your own buffer/ DHW system. A plain insulated tank with external plate HX A FPHX is much more efficient than a tube and shell HX, which is basically all an indirect is.

the only caveat to the bphx is that it usually requires descaling once a year or so.

Fuel oil at 4 bucks a gallon, electricity (resistance element) needs to be around .13, delivered cost, to compare.

A HPWH changes that if you can run 2, 3 4 COP or better.

The Idronics that @hot_rod made some screenshots of is probably great although I don't know which issue it is.

Turbomax probably has some training materials for professionals on their web site too. Look for presentations and whitepapers and design guides, don't just look at the manual for a specific product. I'm not sure if there is a good way to do DHW priority.

It can be this simple. Boiler pipes to top of TurboMax, supply circ and zones connects there also. Zone valves on supply to prevent ghost flow.

Large piping, 1-1/4" at the tank where the boiler and zone piping connects. That serves as the hydraulic separation function between boiler and zone circs.

Air sep and exp tank in that pipes somewhere. TurboMax comes with high point float vent also.

A delta P circ modulates with ZV opening and closing, Grundfos Alpha 15-58 for example.

Isolatiopn under system pump, ZV are top isolation.

Isolation valves at boiler pump.

Purge valve, like a Webstone at the return from the zones

Thermostatic mix valve at dhw connection from tank.

You will just need to use trial and error as far as tank operating temperature, if you don't have dhw requirement numbers. Basically start at 180, crank it down until dhw doesn't provide what you need. Lets say it can run as low as 150- 170. Now you have the points to set the boiler ODR. The "blue" 1.8 curve may be a good start:. You tune the curve to what works for you and the family, there is no "right" number.

The perfect system would have the zones open on first heat call and never shut down. The boiler water modulates to the load condition, pump always runs for constant circulation. ODR can get you close to this condition if you play with settings for a heating season.

The bottom graph shows fin tube at 194°F on design day. That may be too high. Unless they skimped on fin tube 180 or lower should get er done.

The goal is to lower the operating system as low as possible and cover DHW and heat loads. Basically the 3-1 rule, see below.

As mentioned there is an Idronics and Coffee with Caleffi for most every portion of hydronic system installation, wiring, troubleshooting, down to the individual components.