As Paul motors on at National Trust Plas Newydd with the marine source heat pump. The other critical part he is managing is to continue to reduce the overall heat load at the site and to deal with the risk zone. Those are zones which are at risk of not reaching comfort heat levels which will result from the lower heat the heat pump produces (from 85c from the oil system down to around 50c from the heat pumps). We can attack these zones in two ways to get them warmer 1) make the historic radiators bigger to emit more heat or 2) reduce the heat loss from the target rooms. The National Trust went for option two which is first reduce the heat load. It makes financial sense not to use more heat but also means we leave the historic feature radiators as they are.
Why not secondary glaze the whole building? Cost, effectiveness and impact of the buildings historic fabric. We have decided to deal with the risk zone which are rooms which are heated to comfort (20c) levels. These consist of offices and accommodation flats in the house. (…or sitting targets as we call them. Moving targets such as corridors need less heat!) The rest of the mansion consists of empty rooms or conservation / museum areas which are heated to control relative humidity to look after the contents (lower heat needed which means a financial payback on Secondary glazing which runs into decades. The carbon saving is low as well) But we still use shutters and heavy curtains in these areas when not in use to keep the heat in and the damaging UV out