Convert from unvented heat only to system boiler?

forum_james123

It looks like system boilers are more advanced (heat geek also suggest this), most notably supporting lower modulation levels and the pump being included in the warranty.

What is involved in switching from an unvented heat only boiler to system and what are the pro/con with each approach?

molerat

What is involved mainly depends on the location of the hot water tank, both current and proposed.  If close to the boiler then it is pretty straightforward, the further away then the more pipework changes are needed.  The pros and cons IMO are based on size of house and how many people live there, a big house with loads of people would be the ideal situation for a system boiler and tank as it can provide loads of hot water and have an alternative to using the boiler for that water, small house with a couple ideal for a combi which could struggle in the former situation.  What I like about my system boiler is if away during a colder spell I can get home and put both the boiler and immersion on which gives both loads of hot water and heat very quickly.

doodling

Hi,

I think we need to understand what you mean first.

If you mean switching from a boiler without an integrated pump to a boiler with an integrated pump (the usual definition of a system boiler) then the only work required (assuming your property isn't so huge that the integrated pump can't cope) is to replace the boiler and remove the existing pump.

If you want to move from a system with a vented central heating circulating loop to a sealed one then that involves (a) confirming that the boiler is suitable for that configuration (pretty much all new boilers are), (b) removal of the central heating header tank, (c) installation of a "pressure vessel", (d) installation of suitable filling loop, (e) installation of suitable overpressure valves and vent pipework.  Some system boilers include some of (c), (d) and (e).  There is a risk that moving to a sealed system reveals poor quality plumbing (a sealed system is generally under more pressure than one with a header tank) which might need rectification.

In theory, a vented central heating circuit should be slightly more reliable as there are fewer components to fail, they can however suffer from other problems - e.g. pump-over which can lead to high levels of oxygen in the central heating water and consequent corrosion of radiators.  A vented system also generally has a tank in the loft which takes up space and can be subject to freezing in cold weather whereas this is not required for a sealed system.  With a vented system, if you get a leak it can run continuously until it is fixed whilst with a sealed system there is a fixed volume of water so the size of any leak is limited (but still potentially quite a lot of water!).  In general, sealed central heating circuits have completely replaced vented ones for new installations.

Your use of the phrase "heat only" is ambiguous but I'll take it mean that it is not a combi boiler but it does heat hot water in a cylinder.  Separately, you may want to consider replacing a vented hot water cylinder with a mains pressure hot water cylinder which would eliminate the large cold water header tank in the loft.  This is however completely independent of the type of boiler you have fitted and whether the central heating circuit is vented (although it is unusual to see a mains pressure hot water cylinder with a vented central heating circuit).