Best Of

A heat load would add some more info to the question. 1800 sq ft X 25 btu/ ft guesstimate= 45,000BTU/hr required. That is on the coldest day.

80% of the heating season the load could be in the 30,000 range. So the boiler 152,000 you have could be 3-4 maybe 5 times what you need.

While` not hydronically ideal for that boiler, the circs are not causing efficiency issues on the return, unless the system has noisy air issues?

Here is a simple buffer piping, direct to load method. Indirect on priority with its own circ.

The piping size is adequate for the load you have connected. 100' of fin tube could supply about 55,000 BTU/hr. Seems like plenty for 1800 square feet? No sense in having a boiler sized larger that the fin tube can put into the space. So a 60- 75,000 BTU/hr boiler would be adequate.

If your current boiler is twice that size, 120,000 or larger it probably short cycles and runs 70% efficiency range.

Three zones with 50'/zone = 150' x ~500 BTU/ft. @ 180° = 75,000 BTU. One inch pipe is fine for that load.

If you really want to do something, pipe it primary-secondary to keep the large boiler happy.

or a buffer tank. a buffer tank is probably the only thing you could do that might come close to paying for itself