Pressurised vs loft tank CH systems?

Apart from not having a header tank in the loft which is prone to freezing, can anybody remind me what the advantages are of the modern pressurised central heating systems, versus the old non pressurised ones? Is it a requirement of new building regs, and if so, what are the reasons for it?

I ask because these days, I seem to spend much more time having to attend to pressurised systems than I ever did to loft header tank systems. Pressurised systems also run at higher pressures - putting more force on seals and joints, and have expensive pressure valves, a sealed expansion vessel, and a filling loop with twin double acting non-return valves. All extra expense and maintenance.

Adding cleaner or inhibitor to a pressurised system is much more difficult than being able to tip the stuff into the header tank.

I asked this very question before my son's house was fitted last year. No mention of building regs, but pressurised boilers can be installed anywhere in the house because there's no need for a header tank high enough to provide hot water pressure, and hot water runs quickly because it's pretty close to mains pressure.

Couple of painful incidents in my parents house put me off loft tanks : first a dead pigeon, and then a major leak about ten years later which started while they were asleep!

Dave

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

UG! Tanks are plastic and have lids now for both those reasons. My father had Bees block the overflow on the hot tank back in 1960, then the washer went.

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

John,

I had a loft tank but now have a pressurised system. The newer system is a pain. I'd go back to a tank tomorrow.

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

All types of boiler are condensing nowadays and need a drain, our 3 yr old conventional one is. Didn't want a combi as fed up of regularly seeing plumbers at different houses in the street fixing faults or hearing neighbours moaning about having to keep pressurising the system.

Edited By Bezzer on 20/03/2022 15:46:24

+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.

i suspect the real reason is on a new build a combi is cheaper to instal or am i just a cynical old ???

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.

Quite a few of the newer boilers simply won't work with a gravity system so at the very least the closed loop needs to be under pressure. Hot water can still be gravity feed.

Pumps will also run more efficiently with less chance of cavitation due to low head pressure

If you also run the hot water as a pressurised system Megaflo etc you do away with the need for shower and booster pumps. This is not the same as a combi type boiler that heats the water as you need it, there is a pressurised hot water cylinder that stores the hot under pressure.

You can stuill run a new boiler that needs high pressure with gravity heating and hot water but the heat exchanges needed for this are not cheap, one of my clients has this system as it was felt the old pipework may not be upto high pressure. They do still need six shower pumps though!

Edited By JasonB on 20/03/2022 16:41:24

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

I live in a fairly rambling Victorian house where the topography results in a lot of unavoidable "uphill/down dale" pipework so I modified the central heating system here from a loft mounted header tank to a filler loop and expansion vessel. The main reason for doing so was that after draining down the system it would take hours to get rid of the numerous air locks due to the low pressure from gravity feed from the loft tank.

Using the mains pressure from the filler loop has meant that there have been no issues with air locks etc. on refilling since changing over. The hot water still uses a conventional vented cylinder fed from a cold water tank in the loft. Obviously there is a potential maintenance liability from the expansion tank bladder and the filler loop non return valves, but I don't consider these to be significantly different to the risk of a header tank plumbing leak and/or ball cock failure

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

Instantaneous water heating? You must be joking. I used to have hot water tank upstairs before I had the new boiler fitted. It used to take a while before the hot water came through, but the new boiler is even worse. I must run off almost of a gallon of water before the hot water comes through and that is in the bathroom where the the boiler is about a metre away on the other side of the wall. The kitchen is even worse. Of course, even though we have been up to are backsides recently with flood water, in a few weeks time we will be told by the water "organisations" that we are short of water and need to use it sparingly. Not much chance of that when I have to run off a gallon before it gets hot.

Edited By Anthony Knights on 20/03/2022 17:45:21

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

I have a new heat only system with the boiler in the loft, two cold storage tanks for potable cold water, a header tank for the central heating system and a hot water cylinder in there as well. It is the latest Baxi boiler with 7 years warranty and it is a condensing non pressurised  boiler. We had a back boiler in the lounge before the boiler was installed in the loft. If the water is off we have a supply to drink and the toilet flush still works and we have no problem with hot water in the shower either going cold when someone turns a tap on elsware in the house or running out during a shower. Plus we have a backup immersion heater.

David

Edited By David George 1 on 20/03/2022 18:54:37

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

Thanks for replies so far.

Some are assuming a hot-water-on-demand boiler. Not my case. I have a normal boiler, and a normal hot water cylinder. Normal, except the domestic water and the CH are both pressurised to about 2 bar and have the filling loop and pressure gauges etc, instead of having open header tanks.

I had a combi boiler in the previous house, and it was absolutely useless. 

Recently in this house, the expansion vessel fell off the wall ! The herbert who fitted it, (years before we bought the house), did a terrible job. I was thinking why do I need an expansion vessel and the whole CH system under pressure, when I could replace the expansion vessel with a small header tank in the airing cupboard, leaving only the domestic water fed from high pressure mains.

In the airing cupboard, there is a mains water supply and a drain that I could use for the overflow, so easy to change over, and much easier to maintain.

Edited By John Doe 2 on 20/03/2022 19:09:16