Currently:
Large 4 bed house 2 baths, 2 showers
Tanked hot water with Y plan heating (and now a thermostat !)
Ideal Mexico RS125 65% efficiency 37KW depending on burner setting which is either 15.5 or 10.5 mbar 1000000 or 125000 Btu/h
If we replace it with a Vaillant can we downsize
e.g.
ecoTEC plus 630 - 10.0 - 30.0kW
34000-102000 Bthu/hecoTEC plus 637 - 12.0 - 37.0kW
40000-126000 Btu/h