If your boiler and central heating system have not been replaced or. The larger your house, the more powerful boiler you’ll need, since it will need to heat more space and possibly provide more hot water. Which? gave the baxi duo-tec combi 28 he an overall score of 79%, saying: 'this combination boiler is efficient, user-friendly and a great choice for flats and smaller houses.'. Looking for an environmentally friendly product? the combi wall is for you! this energy.