combi boilers installation


Combination Boilers (combi boilers), are without question the most practical way of generating your hot water demands.

However, retrofitting one for your existing standard boiler is far from straightforward.

In this post i’ll explain the process, so you can decide if this route is an option for your home.

There are 4 main points to consider when installing combi boilers

A- The existing boilers location

Physically a combi boiler is identical in appearance and size, to a standard one.

The combi basically has a small tank called a plate heat exchanger inside it, which stores the hot water.

The main difference from an installation point of view is basically 2 extra pipes.

1- the cold water in, from the incoming mains.

And 2- the hot water out, that goes to the taps and showers, etc.

In all homes you are guaranteed to find these 2 pipes at the kitchen sink.

The below picture illustrates a perfect location, as We simply just run our 2 pipes from the sink to the boiler, behind the kitchen presses.

If the boiler is unfortunately in another location, then it’s trying to find the easiest most practical route to run these 2 pipes.

This picture is a perfect location for a new combi boiler, close to a kitchen sink. Most modern new built homes have the boiler in this location.

B- Your Hot Water requirements on combi boilers

In the domestic market the smallest combi boiler is a 24 kw, and the largest is around 40 kw.

Physically they are all identical, the heat exchanger is basically larger or smaller depending on the size.

A 24kw is perfect for someone just running a shower, say an apartment or a couple with no children.

If you have a bath then a 30 kw will deliver more hot water where a 24 might struggle.

If you’re planning on running multiple showers at the same time with baths and en-suites etc, then that’s when you will require the larger 40 kw types.

However these are incredibly expensive, upwards of €1000 or more when compared with 24 and 30 kws.

In this case it’s probably better to keep a hot water cylinder as it will guarantee hot water in this amount, and certainly will be far cheaper.

How much hot water do you require? Sinks, showers and Baths. The average 3 bed home will have 3 sinks, a shower and a bath with 10 rads. A 30- 35kw will be perfect in this case.

C- The Gas Supply on combi boilers

The Gas supply, from the installers point of view, is probably the main obstacle.

A combi boiler ,when supplying hot water, will “HIGH FIRE”, basically going into maximum output.

If there’s not enough gas it simply won’t work and if you have other gas appliances, this further complicates things.

A standard gas boiler rarely uses it’s maximum output when heating the rads and your hot press cylinder, that’s why the gas is not really an issue when installing them.

If it’s undersized a new larger gas supply may need to be installed.

Domestically a copper gas supply leaves the gas meter in the larger 3/4″ pipe . However it’s not uncommon for the existing gas boiler to be piped with the smaller 1/2″. Unfortunately this may not suffice for a combi boiler, as the smaller pipe simply doesn’t provide as much gas to the boiler.

D- Water pressure on combi boilers

Ideally a combi boiler should run directly from your incoming mains.

This will eliminate the need for any water storage in your home, and enables the removal of the attic tank and hot press tank.

In most modern homes and newly built estates I have found the pressure, although not fantastic, is greatly improved.

However older homes closer to the city, especially around the Dublin area, have incredibly poor pressures.

In this case We need to take a closer look at your hot water demands.

I’ve noticed most homes have electric showers, like TRITONS or MIRAS, these showers don’t have to be considered as they self heat the water.

Assuming there’s no bath, this will leave just sinks and even with a poor water supply a 24 kw should be fine in this case.

If you need to run a shower or bath then the only way to guarantee a decent water supply is to introduce a booster pump.

The booster will replace the mains, however everything (with the exception of wash machines and dishwashers) will now be pumped toilets, sinks, showers and baths.

The most likely location for the pump will be the hot press, it will guarantee fantastic showers but at a cost, they tend to be loud and are incredibly expensive.


Hopefully the above info helps you understand what’s involved.

A scenario where the boiler is close to the kitchen sink in a newer built home is perfect. In this case the gas line is usually fine also.

However, if you require allot of hot water with poor incoming water pressures and the gas lines size is questionable, then it becomes a costly venture.

In this case a new standard boiler with a good quality hot water cylinder is the most practical and without doubt cheaper option.





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