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A smaller radiator will offer you less heat. Is the kitchen too hot? …then reduce the size of the radiator.

If your kitchen is just the right temperature when the heating system is operating, then installing a smaller radiator with less heating output, will result in your kitchen will be colder.

If the system is set up and adjusted properly, the boiler should only run to heat domestic water when there’s a call from the indirect through the 2107 Logamatic control on top of the boiler.

The Logamatic control operates from a heating curve which means it doesn’t call the boiler on for SPACE heating from a thermostat demand, but instead, it uses an outdoor sensor and water temperature sensor to regulate the boiler’s water temperature. This is known as outdoor reset (ODR). It also has a feature known as warm weather shut down (WWSD). This turns off SPACE heating at around 68* outside. The control probably has a radiant heating card installed which operates a mixing valve for a lower heating curve for the radiant floor since the boiler must operate at a warmer temperature to prevent flue gas condensation.

I suspect that something (maybe a bad outdoor sensor) is keeping the boiler on all the time at a high temperature resulting in excessive fuel consumption.

Unfortunately, the Logamatic control is no longer available from Buderus unless there’s one on a shelf somewhere. There are other aftermarket controls that can be incorporated by a competent tech should the Logamatic be faulty.

The boiler is the best cast iron made and was the most efficient system around when it was installed. I believe the AFUE rating was 84%. If properly maintained, it could easily last 40+ years. The indirect, depending upon water conditions, 25-30 years on average.

I would recommend that you get someone like @Alan (California Radiant) Forbes to evaluate the system and see what needs to be done to correct any issues. Again, it could be something as simple as an outdoor sensor gone bad.

If the time comes, or has come, for a system replacement, a 95%+ mod/con boiler with fire tube heater exchanger + a new indirect would be the best way to go.

@amaiale , take a pic from the end of the radiator so we can see what type and capacity are the sections it uses. Also how tall is it?

perfect application of a Thermostatic radiator valve. Turn up or down as you need or as your cat prefers. Just swap the valve, much easier than a new radiator.

Edit: Except that’s not the problem you are trying to solve! Less radiator not just less heat.
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It's hard to follow, it doesn't matter if its a triangle or has some 45's, the shape doesn't matter. If you have zones on the secondary loop, and all the zones have their own pumps, and those zones are NOT hydraulically separated from the secondary loop (which would give you tertiary loops?) then no you do not need a pump to circulate water around the secondary loop. Each zone pump will pump its own zone, and will circulate through the low loss header, and from what you described its not really an issue to have the expansion tank piped into the LLH like that, though personally I like to have it on the return of the boiler loop on the suction side of the pump, the location you described is OK as well just makes techs scratch their heads when they see it. I would draw this out if I were you, and not draw it like a triangle or anything, just make straight lines representing each zone so you can read it easier, it will more than likely answer the question for you.

Photo might help