Hi all
Just a point of interest really. Why is it necessary for a sealed CH system to be initially pressurised? Provided it is flooded, bled etc properly should it not work without the initial pressure charge?
Phil
When pressurised sealed systems were first done it was so you could run at a higher temp and get more heat through the pipes and more heat from the rads (water boils at a higher temp when under pressure).
These days I think its done as a leak detection system, I can't think of any real benefit of it being under pressure other than that and stopping cavitation on the pump. The pump will require a minimum static head to prevent cavitation but its not usually much.
Prevention of boiler kettling is also handy if your boiler is fitted in the loft or similar and has a very low head.
Tim
I don't know the official answer to this, but several points are relevant.
No. In commercial yes. In domestic the temp never went over 80C, so not so.
It is? New to me. It was widely adopted so as to fit in flats with a compact system boiler incorporating the vessel. Installers like it as they were easy to get air out and they were away faster and less pipework to fit.
Many vented boilers and sealed boilers use the same heat exchanger.
They usually have a pressure switch to prove that there is water in the system.
They will work with minimal pressure, but are quieter at high temperatures with the normal 1 bar pressure as the water won't boil until 115deg resulting in lower kettling noise and less chance of cavitation at the normal temperature.
(I've just fixed some minor leaks on mine and am running at normal pressure rather than the minimum I could get away with and the system is noticibly quieter.)
Why are compression fittings not a standard (BSP) thread. I've just removed a Danfoss zone valve, and found that the thread is different to Drayton and Salus zone valves (which aren't the same either!). These are all finer than BSP, as are Kuterlite fittings, but none of the finer threads are interchangeable.
The different threads also mean you can't use an olive puller, but I found about 27p and the old zone valve did the trick.
The system pressure must nearly equal the charge in the pressure vessel. If the system is at 0.01 bar, just over atmosphere and the vessels is at 1.5 bar it will take some pressure to push the vessels air cushion.
In theory this should not make much difference. The pressurising to 1 bar gives the installer some leeway when the pressure drop to say 0.75 bar after air has been ejected from the system. He does not have to return. Many installers charge to 1.25 bar. Many fit a Pressure reducing valve on the system permanently open set to 1 bar. If caught out, they plead ignorance saying they forget to turn off the tap and the PRV is for fast filling only.
You need some pressure to get the water from the height of the pressure guage up to the highest point in the system (1 bar per 10m roughly). You need pressure over and above this in order to actually bleed the system. If the system pressure goes negative, air will be sucked in through small leaks (such as valve stems) much faster than water would ever leak out - some seals are designed to work better when pressurised. System has to remain pressurised right down to the lowest ambient temperature, which might be significantly colder than the temp when you would top it up. Some boilers use a pressure switch to detect loss of pressure/water to shutdown the boiler, and you need enough to operate that, as distinct from the likely pressure of a burst system.
I converted an open vented system to a sealed to stop kettling. It worked. The boiler had a hot spot in the heat exchanger and the water there boiler. The pump filly on and all TVRs open stopped it, but tat was not ideal. A sealed system also reduces corrosion inside a system, as oxygen is not constantly being admitted into the system.
Plant pot replying...
I get it now, you don't have any qualifications in plumbing, you are just a salesman.
Dave
Utter bollocks.
Anyone running CH at 100C plus needs their head examined.
It isn't.
I can't think of
Its a good way to basically make sure that what fluid in there is the fluid you put in., I.e. it cant evaporate, get replaced and generally import more corrosives in than it has already.
A condensing boiler should never get hot enough to kettle, its not going to be condensing if it does.
Oh dear the idiot speaks.
I have seen CH running on pressurised steam so 100C is nothing.
It is one of the reasons, why do boilers stop when zero pressure if it doesn't matter?
That does not require pressure, just a sealed system so you are answering the wrong question again. PS you also have to put inhibitor in a sealed system to prevent corrosion as a sealed system won't be able to get rid of the oxygen in there and will corrode without it.
A condensing boiler can get to 82C. If there is a restriction of flow through the boiler there can be local boiling and kettling.
Thank you plantpot for your reply. The self-confessed plantpot goes on..
I am not a plumber. I don't do drains.
Not in a domestic house in this country you haven't.
That is low water not leak.
Oh and how does it get to low pressure unless there is a leak?
>
Ejecting air through an auto vent.
Only if the plumber doesn't commission it properly and that counts as a leak anyway.
It takes a while to get out.
>HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.