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Because combi boiler systems are pressurised are they more prone to leak than gravity fed systems? What pressure do combi systems work at and how is the pressurisation achieved?
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I have two combis, one in Ifithelps Towers and the other in my static caravan in leafy North Yorkshire.
Both are pressurised from the mains via a tap and a short flexible hose.
There's a gauge with a green zone indicating the acceptable pressure range.
I think the gauge is marked in bars and the range is about 1 to 3.
I've always run mine at the lower end without any problems.
Both systems have been reliable, although neither is worked very hard.
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My combi runs at between 1.75 and 2.5 bar. It looses a little pressure over time, but its a 2 min job to re-pressurise it, maybe once every 6 months so far.
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We've never had a combi-boiler. But might soon buy a house with one. So do you pressurise it by turning the tap?
When we swapped boiler at the old house we didn't go for a combi, partly because someone said due to the pressure some of the radiators might leak. Because we had a 100 year old house with old radiators we didn't go for it and got a normal (huge) boiler for around £1200.
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It's not just Combis - most boilers these days are 'sealed system' whereby there is no feed/expansion tank - expansion of the water as it heats up is taken care of by an air bladder expansion vessel in the boiler. Mine has a low pressure cut out at 0.5 bar, and the relief valve is set at 3 bar (gauge, for any of you engineering pedants out there :-) )*. A filling loop which consists of a double non-return valve, isolation valve and flexible hose is fitted to allow the system to be filled and pressurised. They are perhaps slightly more likely to leak when first filled due to slightly higher pressure, but that is easily solved with a tweak of the spanner. Of course if you get a 'proper' leak then you will only lose the water in the system (100l or so) rather than having an open ended pipe pouring water out of the main!
This of course is the heating side of the system - the hot water side is under pressure directly from the main - although if the main pressure is above 3 barg then you should fit a pressure reducing valve. This might make the HW system leak somewhere if you change from gravity to combi, but again usually solved by nipping up the offending compression fitting.
* although, to confuse matters I have a thermal store system, and due to a lack of head above the boiler have a sealed system boiler with a coil in the thermal store, whilst the store itself (which the CH system runs direct off) is open vented with a FE tank! That will confuse any future purchasers of the house....!
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In our new house, I've had to "top up" our combi boiler (Worcester 28i junior) once in 5 years. Runs at around 1.25 bar.
HTH
Terry
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...So do you pressurise it by turning the tap?...
Yes - you watch the gauge rise.
You are introducing plain water into the system this way, but for a top-up the tap will only be open for a second or two.
As others have said, you shouldn't need to do that more than once every few years, so the impact on the corrosion inhibitor is negligible.
I have a Worcester 28i Junior in the caravan, it runs three hot taps, a shower, and six smallish radiators no bother at all.
Very neat little unit, which I imagine is why it was specced for a caravan.
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My combi boiler keeps the bar pressure at pretty much the required level for up to a year - resetting is merely a matter of connecting the short hose to two pipes and briefly opening and closing one or both valves' screwdriver slots as necessary. The two pipes, in this instance, are close together just under the boiler itself.
In most cases a higher than recommended pressure won't cause any problems with modern boilers as there is normally a pressure safety valve contained within the design.
However, if the pressure falls consistently than either a leak or a boiler problem is a probability.
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Just a bit of info which might help somebody.
If you have a pressurised combi or a system boiler and you are constantly having to re pressure it, but you cannot find a leak, there are two things to check first.
The expansion tank inside the boiler is itself pumped up to usually 1 bar but sometimes this pressure can leak away through the schrader valve ( just like the one on your car tyre). If it does, then expansion of the water in your heating system will rise too much and the pressure relief valve will open and water will dribble out.
As this is/should be piped outside, you may miss it and then when the system cools down, you have lost water so the cold system pressure indicated on your gauge is too low, hence you need to keep filling it up.
The thing to do is find the expansion tank and put a tyre pressure gauge on it. If it less than 1 bar, you can pump it up with a bicycle pump.
The other problem is that sometimes in a system with lots of radiators and pipework, the integral expansion tank is not big enough to cope with the volume of water contained in the system and the scenario above happens.
In this case, an extra external expansion vessel can be teed into a heating return pipe.
A 12 litre tank cost about £30 and could solve your problem.
A usual system pressure at cold should be about 1 bar and fully hot should not excees 2.5 bar
(usually). You should check the manufacturers installation instructions for the figures for your boiler.
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If I buy the house I am looking at with a combi boiler I have some learning to do. The boiler is in the cellar so not sure how it's all plumbed in etc.
Some informative stuff today. But I think we've diverged from the OP's question though.
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>> But I think we've diverged from the OP's question though.
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No probs. It's all good information.
Last edited by: L'escargot on Thu 25 Mar 10 at 06:49
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My other thought with a modern combi boiler is that it needs to be 'looked after' in use. They have a 3-way valve inside that diverts the flow from CH to HW when a HW demand is made, and if the boiler is not already running, the pump and fan starts. This is OK if you let the tap run till it warms up, however, if you just 'bump' the boiler by turning the hot tap on momentarily, eg to rinse you hands, then you are either: cycling the 3-way valve back and forwards rapidly (boiler already running); or: starting the pump for a short period, starting the boiler fan, and possibly cycling the gas valve open and closed rapidly (boiler not running - not sure what state the 3-way valve sits at if there is no CH demand on the boiler, at the HW position probably). Doing this is not likely to do much for the longevity of these parts. So, unless you want, and wait for, hot water, just use the cold tap!
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Combi boiler question - Iffy
