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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)?

  • When new homes are receiving their inspections, it's fairly standard for the plumbing systems to be capped and the lines to be pressurized above the max expected pressure with a gauge to monitor for leaks. Not sure how feasible this is for you in an existing system and if it can be done on a heating system, but might be worth asking your plumber if he can test your system first. – BMitch Feb 16 '16 at 16:22
  • Have you gotten an answer to your question? If not I may be able to help. – d.george Dec 20 '16 at 11:21
  • @d.george: I never found a proper answer, but I went ahead with the sealed system in February. I haven't spotted any internal leaks, but there's an overflow pipe from the loft tanks that is, pretty much, running constantly outside the house for the last weeks. – Ratler Dec 22 '16 at 1:44
  • If the contractor installed a sealed system there should be no overflow pipe coming from anywhere.. What are the loft tanks for? If they are for water expansion they should have been capped or plugged with the new installation. Usually a new tank is supplied with a new heating system unless cost is a factor. As for the system pump they are usually extremely quiet. You mentioned an airing cupboard, that is a term I am not familiar with. You were concerned with the system pressure and water leaks I have seen hot water systems 75+ years old that don't leak. – d.george Dec 22 '16 at 12:04
  • Did you find where the water was coming from that was running outside the house? – d.george Jan 13 '17 at 11:52
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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.

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