Apologies in advance for the long post.
I currently have a gravity-fed, fully-vented central-heating (CH) and hot-water (HW) system (large cold-water tank and expansion tank in the loft, hot-water tank and pump in airing cupboard, gas boiler in the kitchen), and I need to replace the boiler.
I'm trying to decide between a boiler that would keep the system exactly as it is (open-vented) or switch to a sealed-system (e.g., see http://www.gasapplianceguide.co.uk/central_heating_systems.htm), which means the expansion tank and pump will be cut out as the boiler itself provides the necessary functionality.
The sealed-system boiler was recommended by the plumber, but I'm trying to understand a bit more in order to choose correctly. A specific advantage of the sealed system that the plumber mentioned was that the pump, which can be a bit noisy, would be removed from the airing cupboard, which is closer to bedrooms. However, he also mentioned that a sealed system operates at higher pressure, so pipework that is not in great shape can start leaking.
This sounds like it should be a major concern, especially if you start getting leaks in walls , floors etc., so I'm not sure why he's saying it so casually. The information I've seen on the Web is not much more helpful: there are various pros and cons (more important than noise) for both options, and the high-pressure/leakage issue is also mentioned, but different experts/sites disagree which one is preferable overall.
To make this more specific, given everything I've read, I would probably go with the plumber's recommendation for a sealed system, if it weren't for the major concern of water leaks. My house is not very old (about 20 years), but I've repeatedly seen that the developers have consistently done bad work, either out of greed or pure incompetence in some cases, so I generally assume the worst for things I don't know.
So the question is: how likely would this scenario of leakage problems be with a sealed system? I try to investigate things properly before making a decision, but there's also some urgency to this because my house is freezing at the moment.
Side-question or clarification: the page I linked to above mentions cases of exploding cylinders. If I understand correctly, is this only related to cases where the hot water is also at high pressure, hence the risk of hot-water tank explosion (I understand that in my case, only the central heating would be a sealed system and the hot water would still be open-vented)? Would there be a risk of anything else "exploding" (vs just leaking)?
Best Answer
While it's difficult to quantify the risk, based on my experience, I believe it's quite real.
I went ahead with the sealed system, and I've experienced since then a couple of leaking radiators (e.g., at the venting valve). I think the system might also be losing water from somewhere else that I haven't realised because the system pressure went too low on a few occasions, and I had to top it up with water manually.
So, I'm not saying it will happen, but if you're facing a similar choice, I think you should weigh in this risk as well.