Personally I would think that replacing a defunct boiler was indeed part of house maintenance as if a house is kept unheated the fabric of that house starts to deteriorate. What you are doing when you use his money to pay for half of house maintenance is keeping the value of the house. As he has a legal interest in the property, then it is to his benefit to maintain this interest and the LA recognises this interest. If the house goes to wrack and ruin, then the value will be lost. Obviously, you need to use common sense - if you installed an indoor heated swimming pool, or paid thousands for the wallpaper, the LA would not accept it, but general maintenance to keep the value of the home is fine.
What I would suggest
@yorkie46 , is that you make sure you keep all receipts for any maintenance done, so that you can justify it if there is any query about sums removed from his account.