Well to be fair combis were rubbish when they first came out werent they? lol. No seriously if they are installed in the correct place ie. not in a kitchen in open plan living (had loads of complaints as they are quite noisy) , but manufacturers instructions do recomend to be fitted in garage, utility or a plastic shed like you put the ground source pumps in. They have to be installed well ie. flushed and then flushed again and add loads of fernox, fit spiravents and make sure theyre cleaned out annually, the waterways are very small. An annual service is expensive as theres alot to do at different temperatures (time consuming) plus you need to carry nitrogen to top it up annually and you need a laptop to get it to do what you want on a service. I personally think they need to be fitted in old houses with alot of heat loss as they work better if they just chug along slowly allday (you get more KW put back into the grid or free electric)