Need Expert Advice for UFH System - Consolidating 2 Boilers

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Two story standalone home. Each floor has a separate gas boiler providing UFH to a manifold that controls the zones in that floor via thermostats. Total home is 250 sqm. Boilers are Immergas relatively new (last 5 years, inverter units) and installed side by side outside the bottom floor. At any given time, the amount of total square footage requesting heat in the house is less than 100 sqm (e.g. at night its 2 bedrooms on one floor and a single on another). For efficiency/cost sake, it seems correct to me to combine the heating of both floors to a single boiler (which can more than handle that load) so that I'm not cycling two boilers all winter.

A guy I spoke with quoted me ~2000 GBP to do so. He said I need an 80 L balancing tank and two pumps. The boiler would just cycle the water into and back from the tank all day and the pumps would cycle to each floor separately. I'm wondering if this is overkill and I'm not sure in the end I end up more efficient (since then I'd be heating 80 L of water that's sitting in the cold all day).

I was wondering if a viable alternative was simply to connect the cold return from both floors with a T into the boiler input and split the hot word out of the boiler with a T and send it to both floors. As the height differential in the manifolds/floors would probably cause uneven flow, I could use the thermostats (they are connected to my smart home) to ensure that only loops on one floor are open at any given time (e.g. at night open the two bedrooms on the lower floor for 20 minutes, then close them and open the single on the upper floor for 20 minutes and keep alternating all night).

Happy to provide more details if it helps.
 
Personally, I’d stick with what you’ve got.

I don’t think you make any noticeable savings, in fact you’ll probably find it costs you more to run as a big boiler is much less efficient than a small one when the big one is running with low loads.
 

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