Why does a combi boiler have to be pressurised?

[Stevekane]

Hi, just wondering, why is it nessesary to presureise an unvented or combi boiler, I know there are presure switches that need to be triggered etc but why is it all under presure to begin with? My thoghts ranged from the boiling point of water being higher under presure,,but you would get a blast of steam when you bled a rad! To the water reacting differently at both lower temps or at a higher temp under presure,,to prevent kettleing? But do open vented boilers then run at lower temps?
Anyone have any idea?
Steve.

 

[Fitzroy]

That’s an interesting question and my thoughts, based on being a process engineer who deals with industrial heating systems, so educated guesses at best!

It’s not to do with operating temperature. Although the 0.3bar needed to allow the boiler to operate increases the boiling point of water to 107degC the system will never be operated the hot. Most boilers have safety systems that limit the temperatures to below 80C so that radiators can’t burn you. And most combo boilers should be operated much cooler that this so that they are in condensing mode for max efficiency.

My only thought is that a minimum operating pressure ensures that the system is primed and prevents it being run dry. In a traditional vented system with a feed and expansion tank it would be more difficult for the system to be unprimed or have leaked dry. Running a boiler dry will circumvent many of the safeguards and cause damage or a fire quite quickly.

Fitz.

 

[Jacob]

It's the whole system not just the boiler. All I know is that you can't bleed them if the pressure is too low. Can't quite see how they lose pressure in the first place, without water squirting out somewhere

 

[ChaiLatte]

In a traditional vented system with a feed and expansion tank...

If we were to take a worst case of a bungalow with the feed tank in the loft and the boiler at ground level, what would be the pressure head from the tank? I think it would be roughly the same 0.3 bar that you mention above. So in that sense, a combi boiler is not so much different to a conventional one.

 

[Spectric]

It does not have to be a combi boiler, system boilers also run unvented pressurised systems. One of the biggest benefits is not having the header and expansion tanks in the loft and @ChaiLatte has the concept. You need pressure in the system to help heat transfer, any air and as we have all experienced you get cold rads so where a tank provided " THE HEAD" it is now pressurised and instead of the expansion tank you have expansion vessels. Another good thing is you cannot get pull over from the expansion tank pipe, the point at which the header tank feed and expansion pipe were connected to the system was very important. Downside is expansion vessels and over pressure valve which can seep and cause pressure loss.

 

[Terry - Somerset]

An ill informed thought - in a conventional boiler and hot water cylinder the water is heated indirectly via a flow and return pipe. Any temperature variations in boiler output are moderated by the volume of heated water already in the hot water tank.

With a combi boiler the hot water is heated and flows directly to the taps.

Does a pressurised system ensure that water flow through a combi boiler provides more control to ensure constant flow and temperature output and avoid the risk of scalding.

 

[Spectric]

Does a pressurised system ensure that water flow through a combi boiler provides more control to ensure constant flow and temperature output and avoid the risk of scalding.

In a combi boiler it is the mains water that is used for the hot water output, the cold feed going first through a pressure regulator. It is the heating system and rads that are pressurised using the mains water supply and what you monitor on the gauge.

In the older systems with a header tank, this provided a supply of cold water to the hot water cylinder at a pressure determined by the height of the tank above the cylinder and is why some people had low hot water pressure and filling a bath could take some time.

 

[Agent_zed]

my understanding (which could well be wrong) apart from the advantages mentioned already, it also means less corrosion on the system as you can add corrosion inhibitors and no additional oxygen is entering the system. On a vented system you will get additional oxygen ingress and the inhibitors will wear off quicker.

 

[Jacob]

....

With a combi boiler the hot water is heated and flows directly to the taps.

Hot water goes through at mains pressure but the CH is separate closed circuit.

Does a pressurised system ensure that water flow through a combi boiler provides more control to ensure constant flow and temperature output and avoid the risk of scalding.

No.
Sounds like @ChaiLatte might have hit it - there simply has to be enough pressure to keep the CH closed circuit topped up?

 

[hunter27]

Most boilers now are designed for sealed systems (not called pressurised in the trade) as all systems need some pressure to work and to get water up to the highest radiators in the system. The advantages of sealed systems are are, there's no water in the loft to freeze or leak, easier to install with less pipework running through the house, generally the system stays cleaner (most F&E tanks eventually get gunge in them and occasionally a jelly like growth, If you have a slight leak on your system you soon know about it whereas tank fed systems can leak for years without anyone noticing it dependant where the water is going. Some of the old Worcester combi boilers I have fitted could be tank fed if required but that was probably 25-30 years ago.
Draw backs are expansion vessels all seem to be of poor quality and often cause trouble as do pressure relief valves.
If you are concerned about the pressure of boiling water your boiler stat and over heat stat must both be totally shot

 

[Bingy man]

Some of the worst systems I ever worked on were in flats . There was never enough head of water above the pump so they were doomed to be problematic from day one - if the pump speed was set at 2 or 3 they would pump over back into the f/e tank . Set on no.1 and the last rads on the circuit would be luke warm or cold . The system would eventually get fouled up with sludge. Even draining and especially filling them up was difficult and time consuming and then came the inevitable air locks that took forever to clear .

 

[Agent_zed]

The other thing I don't think has been mentioned yet is a pressurised system is more efficient. With a vented system you will lose heat through the vent pipes.

 

[Stevekane]

Many thanks to you all for your help, and I suspect your all right in as much as every single thing thats been suggested is important to the systems safe and efficient operation. My guess is that an open vented system (with a tank in the loft) is automatically topped up, whereas in a sealed system which is a simpler installation with various other benefits too, its not so easy to creat a way of automatically keeping the system topped up, so introducing a pressure sensor/switch ensure that the system cannot run until you come along and top it up?
My reason for asking this is that we presently have an ancient (40plus years old) open vented system in which the Potterton Kingfisher floor standing boiler now really does need to be replaced. The system is run in 8mm microbore and overall works very well. I would idealy like to replace the boiler with a combi.
My system at present is operating at approx 0.5bar (16ft head of water) and maybe a combi will function at around 1.0bar, and increase in presure as the water heats up,,so our old system would have to cope with 3 to 4 times the present pressure, and of course Im worried about springing leaks. I suspect its a questain of just going for it and dealing with any issues if they come up, and the microbore might have the advantage of less joints as lots of the pipe was run off a roll and cabled through,,I hope!
Steve.

 

[Spectric]

Don't just think of a combi when it comes to a boiler, they are simple and easy to fit but not always the best choice and if you have the room use a system boiler with an unvented hot water cylinder. With these systems you can use off the shelf zone valves from Honeywell and smart pumps from Grunfoss and are not tied to the boiler OEM for components like divertor valves etc.

 

[HamsterJam]

I thought (at least for modern combi systems) the burner only heats the pressurised primary circuit and a diverter valve directs the output to either the radiators or a secondary heat exchanger to heat the domestic hot water (DHW).
When the DHW is shut off, there is a short period while the flame extinguishes however in pursuit of efficiency, modern heat exchangers are lightweight and hold a smaller volume of water meaning there is a real risk of boiling. To prevent this, the pump overruns for several seconds and the diverter valve sends the output to the radiators. This why the first radiator in the heating circuit can feel slightly warm even when the heating is off.
I think the pressure is there to ensure integrity of the primary system and to stop air getting into the system when radiators are bled etc.

There is a good diagram here, although the explanation is a bit simplistic…
https://idealheating.com/products/boilers/ideal-combi-boilers/what-is-a-combi-boiler

 

[Fitzroy]

Many thanks to you all for your help, and I suspect your all right in as much as every single thing thats been suggested is important to the systems safe and efficient operation. My guess is that an open vented system (with a tank in the loft) is automatically topped up, whereas in a sealed system which is a simpler installation with various other benefits too, its not so easy to creat a way of automatically keeping the system topped up, so introducing a pressure sensor/switch ensure that the system cannot run until you come along and top it up?
My reason for asking this is that we presently have an ancient (40plus years old) open vented system in which the Potterton Kingfisher floor standing boiler now really does need to be replaced. The system is run in 8mm microbore and overall works very well. I would idealy like to replace the boiler with a combi.
My system at present is operating at approx 0.5bar (16ft head of water) and maybe a combi will function at around 1.0bar, and increase in presure as the water heats up,,so our old system would have to cope with 3 to 4 times the present pressure, and of course Im worried about springing leaks. I suspect its a questain of just going for it and dealing with any issues if they come up, and the microbore might have the advantage of less joints as lots of the pipe was run off a roll and cabled through,,I hope!
Steve.

I concur with your first paragraph as to why a certain minimum pressure is required for the system to run.

A combi boiler will have an expansion vessel fitted, this purpose of this vessel is to prevent the system pressure rising when it heats up. These vessels contain an air filled bladder that is inflated to a certain pressure, normally 1.0-1.5bar. When the system heats up the expanding water compresses the air in the bladder and the system pressure rises less, mine rises from about 0.7bar cold to 1.1bar hot. In theory adding a bigger expansion vessel at a lower pre-charge pressure will limit the pressure rise, so you could run close to the boiler min operating pressure to reduce the strain on the old pipework.

However, when they install the new combi they should pressure test the system up to max operating pressure, normally 3.0barg set pressure of the relief valve, which will be stress test and reveal any leaks.

Fitz.

 

[HOJ]

I agree with @Spectric , Combi boilers have a place, but in the right environment, flats, single occupancy etc. We are are reliant on oil, stuffed when you run out, neighbour bought into one, with a new baby, in the middle of winter, no hot water, let alone CH and has also had no end of call outs on it to keep it running, locks out when pressure is down below 0.3 bar (leaking heat exhanger was one issue) along with no back up, not having a cylinder with an immersion heater anymore, and no airing cupboard. + more options to integrate other heat sources as well with secondary coils fitted, I would fit a system boiler and an unvented cylinder every time over a combi.

 

[johnnyb]

without pressure it's impossible to eliminate the air present in the system.

 

[Stevekane]

Im waiting for the plumber to get back to me,,but again thanks for the replies, I have to say I dont fancy the idea of “stress testing” the old system to 3 bar (30mtrs head) in fact Im rather hoping that we can even avoid using harsh chemicals to clean it through, maybe get away with a non acidic cleaner and good mains flushing. Dont want to start it leaking if we can avoid it. The comment above about bleeding is bang on as they say, fill a closed system and shut off the filling loop, and without a reserve of pressure you wouldn't get any water/air out,,

 

[hunter27]

Im waiting for the plumber to get back to me,,but again thanks for the replies, I have to say I dont fancy the idea of “stress testing” the old system to 3 bar (30mtrs head) in fact Im rather hoping that we can even avoid using harsh chemicals to clean it through, maybe get away with a non acidic cleaner and good mains flushing. Dont want to start it leaking if we can avoid it. The comment above about bleeding is bang on as they say, fill a closed system and shut off the filling loop, and without a reserve of pressure you wouldn't get any water/air out,,

There are probably still boilers that can be tank fed but I suspect there won't be any combi boilers, The Worcester RI range has been going for quite a while now and could be fitted on sealed or open vented systems, I fitted lots of them before I retired 12 years ago and liked them but they are not a combi boiler so need a hot water cylinder. There are probably more makes that don't have to be on a sealed system but this one springs to mind.
If you do stay with your old open vented system get it flushed as well as possible and make sure you have a magnetic filter fitted on the return before the boiler.
PS I believe if you fit a Worcester filter fitted with a new Worcester boiler it can extend the boilers guarantee but you would have to check that.

https://www.worcester-bosch.co.uk/p...aGEQBJtyr0JUhmcfL05tduZWsGhzeZjxoC-HcQAvD_BwE