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Should I replace open system with a combi?

Postby dimitripn » Mon Jul 19, 2010 9:04 pm

Hi there,

We just moved into a new house, well an old house to be technically correct. The gas boiler is an old Potterton and it's an open system with a hot water cylinder and a tank in the loft. We are thinking of replacing this with a combi and simply cannot decide whether it's worth doing this. Some gas engineers noticed that we have microball downstairs and 15 mm upstairs so they are saying that 99% it will work but there is that 1% chance that they may need to drop pipes in the corners to replace the microball downstairs.

So provided that it works what are pros and cons of the two systems? I have read the description and it is really good. However it does not provide any decision making information.

The two things I can think of are reclaim the space taken up by the hot water cylinder and the tank in the loft and also all the pipes, the noise that it makes when in use.

Any other thoughts?

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Simply Build It

Postby plumbbob » Thu Jul 22, 2010 9:52 pm

To keep this post to a manageable length, I will just detail the most obvious points (in no particular order).

Advantages of a combi

Clean hot water. Water from storage tanks is not considered "wholesome".
Better pressure at hot taps.
Cheaper to run as heated water is never wasted.


No back up way of heating water if the boiler fails.
Flow dependant on incoming mains supply which can be quite poor in older houses.
Many combis are not as "instant" as you might expect.
Combis are less suitable for homes with more than one bathroom as most cannot supply two outlets simultaneously.
No warm airing cupboard.

From these points you might come to the conclusion that a combi is not a good option. This is not true exactly, as the running cost is such a big factor in most peoples mind it over rides everything else making this option the most popular by far.

Of course, you have two other options to deliberate about.

Simply keeping your existing system and replacing the aged Potterton with a modern efficient condensing boiler, or better, replace the low pressure hot with a nice new unvented cylinder. (No header tank required). This has to be the best choice for a larger family.
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Postby eggers » Fri Jul 23, 2010 5:42 pm

i would be careful of putting a sealed system on old (open vented) pipework. I have had to attend uncontrolled water leaks in the past, from where old joints have failed under the pressure.
I would advise against a combi unless you are really in need of the space as combi's tend to be more complicated, and very susceptible to dirt from the system, especially in the dhw heat exchanger after a few years.
i won't go over the pros and cons because they were discribed perfectly by plumbbob in the last post
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