Unvented System - Should I go Combi or System Boiler

[topgazza]

Having gone unvented back in 2004 ish I am debating whether when replacing the boiler its "sensible" or even the right thing to do to just go for a combi boiler rather than another system boiler...Baxi or Worcester.... as its looks logical to keep the unvented which is a nice system and replace boiler with a system boiler ? Would seem a lot of work for little gain to fit a combi even though its a smallish 4 bed house it has a bathroom and ensuite shower and 2-3 people living ?

I live in a hard water area so when my 27 year old reliable and safe Potterton finally passes away the choice of boiler is important but maybe also the type ?

Any thoughts much appreciated please ?

 

[stem]

As I see it, the advantages of sticking with a heat only boiler would be.

1. You won't have to get rid of your existing hot water cylinder and feed tanks, and possibly controls, which will simplify installation, and require less 'making good'.

2. Heat only boilers have a lot of components externally mounted. Diverter valves, hot water thermostat, programmer etc., these can be easily replaced by anyone with parts obtainable from plumbers merchants or DIY sheds. On the other hand, combi's generally have their equivalent components located inside, the components are often specific to the manufacturer and may require the attention of a gas safe registered engineer to replace them.

3 Combi’s can be slow to get hot water to the taps if the boiler is starting from cold. This is because the boiler has to heat itself first before it can heat any water.

4. To have the capacity to heat cold mains water instantaneously from scratch, Combi boilers often require the installation of a larger gas supply pipe from the meter. (This can often be seen routed around the outside of houses-Yuk!)

5. Retaining your hot water cylinder means, you can have an immersion heater to provide hot water for when the boiler fails, and a cupboard you can use to 'air' clothes, or make beer or yogurt.

6. Generally the cold main supplying a combi comes from a single 15mm pipe. That same pipe also feeds the cold taps, WC’s, washing machines, dishwashers, etc., If any of these are used at the same time as the hot water, the hot water flow / pressure will be reduced accordingly. With your existing hot water cylinder the tanks provide a stored supply of water, and are unaffected by the use of the cold water elsewhere.

...and the advantages of a combi:

1. Can provide mains pressure hot water with a good flowrate, provided no other demand is present (see 6 above)

2. Frees up space where your cylinder and tanks used to be.

3. There can be a small energy saving in the summer because of lost heat escaping from a hot water cylinder. In the winter any escaping heat is not wasted as it helps heat the property. However, this benefit can be lost if the combi has a facility that keeps it warm permanently in case you need any hot water.

4. Simpler electrical installation. But only where wiring doesn't already exist.

 

[oldbuffer]

1. You can use a combi with the unvented system, basically run the cylinder off the central heating flow / return. Depending on where the boiler is sited, you could use the "on demand" hot water from the combi for the kitchen supply. However, given that you've already got an unvented hot water system, its a bit pointless.

2. If you've already got the pump and valves in place, why not replace with a heat only boiler. I'd suggest you change pump and valves at the same time, but future replacement would be from standard parts, not boiler manufacturer's specific and expensive bits.

Best bet is to talk to two or three registered installers. Explain what you want to achieve, and ask them to quote accordingly. Don't forget its very likely that you'll need an upgraded gas line between meter and boiler for a modern boiler.

 

[topgazza]

Blimey... good old plain English. Not surprised as that is what "you" get here. But excellent replies.... thank you. I fully understood and made my mind up to go with the system boiler and not rip out , in effect, the unvented system which we like.

Thanks again