Have to agree Vaillant are at the top of my list, the design of the EcoTec Pro, Plus and GreenIQ range I think are well thought out for servicing and repairs down the line as well as installing them. The cheaper EcoFit pures are still decent too, however not sold 100% on the HEX in those and more plastic inside so only time will tell. Useful when you need to get in a tight space or need a rear flue though. After service is good too usually next day and reliability is top notch so far on our installs.
Hate Ideal, only fitted 7 Logic+s about 6/7 years ago, think nearly all have needed a new HEX due to the poor quality they've made them from. Ideal engineer in our area is rubbish too.
Fitted a few Ferroli Modenas as a budget/cheaper option after Logic disasters. Not a bad little boiler, only 4 moving parts, round burner, no plate HEX, 7 year warranty and for the price point what can you expect. After service is great in our area, we have two fantastic engineers. However they do seem to have the Ferroli curse and start leaking at some point.
Worcester, our company fitted lots about 5 to 10 years ago, mostly when the Greenstar range came in. Very disappointed, nearly all of them have leaked from plastic parts. Got one customer who's basically had a new CDI from all the parts we've replaced over years. Fans seem to go, gas valves pass, expansions failing (Mainly the ones in the back) and you're forever putting new electrodes in the dam things. Also it really annoys me that they carry on running when there's no pressure in the system! PRVs, who was thinking at Worcester when they put it where it is?! The most reliable out the ones we've fitted seem to be the Si. As others have said Worcester, was good and now bad.
Viessman, now only ever installed one. A heat only. Serviced a few combis. Don't like the upside down expansion, fills with water then you can't pump it out. Not sold on the designs from a point of servicing and replacement of parts but they seem like they're built to last and are pretty reliable. Not keen on the push fit flues either.
Baxi/Potterton, again not keen. They seem to all get to 6 years old round us and then start to fail. Diverter being the favourite, leaking then blowing the fuse as the diverter motor is full of water. Along with the HW sensor failing, HW temp sensor failing, pumps going and expansions again. PRVs seem to go on these too as if there's any airlock in the system boiler keeps plowing along till it reaches 95 then pressure is bouncing up and down.
Interesting fact though, family member at the time needed a new boiler but didn't want to spend the cash so had a Biasi. At the same time parents had a new Worcester top of range at time. Biasi out lasted it by 5 years.
The issue is there's problems/design flaws with all of them. I think if manufactures asked the installers more before designing they could come up with better designs.
