You get more choice of mixer taps that work.

You only heat what water you need

You, um, er, give me time now ……..

You need a bigger gas feed right back to the meter

You get enough hot to feed one tap only.

You get a condensate drain that freezes if the outside temp is lower than -8C for more than a day and stops the boiler. Particular fun when the boiler is in the roof space so that freezes as well

As far as I recall using a pressurised CH system allows the boiler to be installed anywhere and operating between 1-2 bar increases the boiling point of the water which prevents heat exchanger hot spots and noise.

Refilling the system from pressurised mains water can introduce dissolved Oxygen so inhibitor should be added every time you refill. In my loft I have a vent valve to release any gas in the system but I also use it to suck in inhibitor when I do a partial drain down before a refill.

Ideally any top up water should be boiled to get rid of dissolved gasses and then pumped into the system along with inhibitor but this not a trivial exercise.

Clive S

The main 'advantage' is that you don't need a loft – not uncommon in modern houses/flats.

+1 for keeping tanks if you have them already. If there is a hot water tank with immersion heater then you still have hot water if the boiler fails. The airing cupboard has other uses such as setting adhesive, hardening paint and storing joinery biscuits.

There are two different answers beind tendered to one question.

Pressurised heating system implies no header tank for the heating circuit. This can be done with any boiler. Advantages are:-
No need for small header tank.
Much reduced corrosion in radiators etc.
Boiler can be sited higher without risk of boiling!!!

Disadvantages are:_
Need an expansion vessel.
Normally need a hot water cylinder which can take up to 3 bars of pressure in the heat exchanger. (Mine couldn't when I fitted the new system ).

Instantaneous water heating.
Advantages are:-
No need for hot water cylinder or cold water header tank.
Less loss from constantly held hot water.
Mains pressure hot water without needing a booster pump.

Disadvantages are:-
Lower potential maximum flow.
Much higher boiler capacity required for water heating than for house heating.

Having had a water tank leak I will not have any water in my loft.

Just a condensing boiler, and sealed pressurised central heating, in the kitchen. I also have central heating insurance which was used, yearly, on the old French made boiler (replaced 6 years ago).

JA

Can the user easily descale a combi boiler in a hard water area? With a separate hot water and header tank it is a simple job to remove and replace or descale the hot water tank and doesn't require a gas fitter.

Brian G

Anyone else traumatised by an old-fashioned gas geyser?

My mother's eyebrows were removed by one as a girl and on another occasion she collapsed due to the fumes. I was terrified by another as a little boy – two settings: cold, then a loud bang, followed by spitting boiling water and roaring clouds of steam.

Dave

Dave Your last 2 points may possibly be true in a very few cases. I have had a 42Kw combi boiler in my loft for the last 10 or so years and have never had a shortage of water. It easily feeds 2 taps or shower & tap or 2 showers at the same time. All that happens is you get a very slightly lower flow rate which is barely noticable. We dont bother about people turning on a tap or flushing the toilet when in the shower, unlike when we had a non pressured system and had to tell everyone "dont turn a tap on" I am going in the shower. Never had the condensate which is on a north facing wall freeze to a point where it stops the boiler. I live in the East Midlands so we get our fair shair of freezing days & nights.

I would go for a Combi over a non pressured system every time.

regards

There is always a little confusion over what is pressurised in a heating system. I don't disagree with any of the above comments. Just thought I would add my personal comments about my system and why I choose it to be as it is. House built in 1989 with standard regular boiler and two headers one for primary heating loop and one for immersion hot water feed.

When I upgraded I choose to fit a seperate pressurised hot water cylinder. Advantages no pumps for shower and mains cold water pressure delivered hot water. In the event of boiler failure I can turn on the immersion heater and still have hot water. disadvantage the pressure vessel will fail at some point between 1-3 years and that is a routine change out. (look at the number of pressure vessels screwfix sell to see this is a good turn around. I am prepared for this minor disadvantage as all other benefits outweigh it.

The boiler is a regular boiler it's sole job is to heat water with no added complication within. Separate standard pump to circulate, standard zone valves to divert water to where required. I split upstairs and downstairs and added workshop loop and underfloor heating. All these things are within my control and need no specialist intervention. I kept the open vent header feed for the boiler and not pressurised because I don't want to find micro leaks that may be no issue currently (although confident there are none) don't want the hassle of topping up when pressure lost. don't want the hassle of an internal special pressure vessel within a "system boiler" needing specialist replacement. Possible negative header tank washer every ten years but system will still work with overflow fault indication, simple. I don't regard freezing of tank or leaks in loft as a probability (although always a risk) and are covered with overflow tray to outside.

Combi boiler I would never consider as have had experience of them and a failure of any aspect means both heating and hot water are likely to be lost, and there are more specialist parts blenders, sensors etc. I found there is always an inconsistency in the hot water delivery and can be affected by draw off elsewhere in the property.

That said combis do have advantages they are quick to fit and make the installer more money, regular boiler and cylinder will cost more to fit. If space is an issue then clearly it can have it's advantages. It's the individuals choice but understanding fully why the choice is made is key to a happy outcome which will always be a compromise of circumstances.

Alan

By the way. Thanks for this thread. I've just done some checking on the screwfix website and realized that things have moved on in the 28 years since I rebuilt the heating system.

I had been thinking that I needed to get a combi boiler or booster pump in order to satisfy SWMBO's desire for a high pressure shower. I see that the world now includes high pressure indirect hot water cylinders. So I can keep the existing boiler and HW pipework, replace the cylinder and get mains pressure hot water.

Luckily, our council tip still accepts asbestos cement, so I can dispose of the cold water tank with no issues.