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Central heating feed and vent pipe

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    #32
    John.G
    Registered Users Posts: 3,474 ✭✭✭John.G


    Tom_DCU wrote: »
    If mine is a semi-sealed system then does it mean it is always 'pressurised' to the same pressure defined by the head of water in the F&E cistern in the attick? And obviously there is never a need to re-fill the system because the F&E tank does it (in fact the F&E cistern now becomes a feed cistern only once expansion happens in the expansion vessel in side the boiler).
    What is the advantage of a sealed system? I can see several disadvantages like the need to re-fill the system if there is any loss in the system (say after bleeding the radiators), and also the higher pressure might cause leaking of not well made joints. I understand that a pressurised system is filled form the mains so it fills faster and also there is less chance of trapped air. Any other advantages?

    Assuming a semi sealed system (obviously with NR valve installed): if the head is 6 meters then the boiler pressure will be 0.6 bar after filling the system. The pressure will then start rising as soon as the water starts to get hot because it now can't expand into the "F&E" tank because the NR valve will close, the expansion will then go to the expansion vessel and depending on system contents, system water temperature, expansion vessel capacity and pre pressure, will rise. The final boiler pressure with system contents of 100 litres, system temperature of 70C, expansion vessel of 8 litres and pre pressure of 0.6 bar should be ~~ 1.2 to 1.5 bar. The pressure will then return to 0.6 bar when the boiler cools down to its original temperature and any leaks will be made up by the "F&E" tank now operating as a feed tank only.

    Advantages/disadvantages in other above post.


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    #33
    Conservatory
    Closed Accounts Posts: 1,194 ✭✭✭Conservatory


    John.G wrote: »
    Its the OP that is changing his cylinder, I have a oil fired system with a combined cold feed and expansion and I have a number of my original rads installed over 45 years ago, there seems to be greater corrosion problems associated with fully pressurized systems probably caused in part by these auto filling valves and little or no expansion vessel testing/servicing.
    If it were my house, I would retain the gravity fed system as long as the F&E tank doesn't have to be relocated with any associated plumbing changes.

    The expansion vessel is in the boiler casing. If there was no expansion vessel the safety valve would be spewing out the wall more than likely. The pump would quickly corrode with the constant fresh water rad valves and boiler would rot eventually I reckon.

    Are you sure you understand these new systems?


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    #34
    John.G
    Registered Users Posts: 3,474 ✭✭✭John.G


    Conservatory wrote: »
    The expansion vessel is in the boiler casing. If there was no expansion vessel the safety valve would be spewing out the wall more than likely. The pump would quickly corrode with the constant fresh water rad valves and boiler would rot eventually I reckon.

    Are you sure you understand these new systems?

    Yes, I understand them perfectly well and certainly do not/did not suggest running without a expansion vessel. See my post no32.


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    #35
    Tom_DCU
    Registered Users Posts: 11 ✭Tom_DCU


    So what would happen if I removed the non-return valve? Even though there is an expansion vessel in the boiler, it would be easier for the expanding water to rise through the combined feed/expansion/vent pipe into the F&E cistern than to deform the vessel (which is pre-pressurised). The main problem I can think of is that if the pump is too fast it could overpump water through the vent portion of the f/e/v pipe that overhangs the cistern (and potentially draw in air).


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    #36
    Conservatory
    Closed Accounts Posts: 1,194 ✭✭✭Conservatory


    Tom_DCU wrote: »
    So what would happen if I removed the non-return valve? Even though there is an expansion vessel in the boiler, it would be easier for the expanding water to rise through the combined feed/expansion/vent pipe into the F&E cistern than to deform the vessel (which is pre-pressurised). The main problem I can think of is that if the pump is too fast it could overpump water through the vent portion of the f/e/v pipe that overhangs the cistern (and potentially draw in air).

    Leave it alone young grasshopper


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    #37
    John.G
    Registered Users Posts: 3,474 ✭✭✭John.G


    Tom_DCU wrote: »
    So what would happen if I removed the non-return valve? Even though there is an expansion vessel in the boiler, it would be easier for the expanding water to rise through the combined feed/expansion/vent pipe into the F&E cistern than to deform the vessel (which is pre-pressurised). The main problem I can think of is that if the pump is too fast it could overpump water through the vent portion of the f/e/v pipe that overhangs the cistern (and potentially draw in air).

    If you removed the NR valve then the system will operate exactly as you described above and the boiler pressure should remain practically constant hot or cold. Overpumping/pitching should not happen, as combining the feed and vent pipes has often been used successfully to stop this very condition on systems that formerly had separate cold feed and vent systems and has been suggested in "heating & plumbing" on boards.ie several times.
    If you do remove it, replace it with a spool piece so that you can reinstall it at any time.
    Have you a boiler pressure gauge anywhere on your system.?

    Overall I would prefer to leave the NR valve in place as a bit of system pressure when hot helps circulation (as has been pointed out above) and prevents air being drawn in in parts of the system that may run at negative pressure otherwise.


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    #38
    Tom_DCU
    Registered Users Posts: 11 ✭Tom_DCU


    Yes, there is a pressure gauge near the boiler (and there has to be one inside the boiler as well because it has a function of displaying it) and an automatic vent as well. Since the boiler is on the ground floor this automatic vent seems to protect the boiler from air rather than the rest of the system and does not seem to help much when filling the system (hence the air lock near the coil of the cylinder).
    At the moment in cold stage of the system the pressure gauge measures around 0.5-0.6 bar which is pretty much corresponds to the head pressure above it to the attick cistern. The boiler seems to be able to operate at this low pressure (as was mentioned by K.Flyer).


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    #39
    John.G
    Registered Users Posts: 3,474 ✭✭✭John.G


    Tom_DCU wrote: »
    Yes, there is a pressure gauge near the boiler (and there has to be one inside the boiler as well because it has a function of displaying it) and an automatic vent as well. Since the boiler is on the ground floor this automatic vent seems to protect the boiler from air rather than the rest of the system and does not seem to help much when filling the system (hence the air lock near the coil of the cylinder).
    At the moment in cold stage of the system the pressure gauge measures around 0.5-0.6 bar which is pretty much corresponds to the head pressure above it to the attick cistern. The boiler seems to be able to operate at this low pressure (as was mentioned by K.Flyer).

    Can you recall this (these) pressures when the boiler was hot as it will give a good indication of whether the NR valve is functional or leaking and also a indication of the expansion pre pressure


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    #40
    Tom_DCU
    Registered Users Posts: 11 ✭Tom_DCU


    Unfortunately I do not remember the hot pressure but will check it once the heating is back (at the moment it is too warm in Dublin to switch on the heating). I do not remember the water level ever to have risen much in the F&E cistern, but I might not have checked it when the system was fully hot.

    Now I can see it is a non-return valve as there is an arrow at the back which I can see with a mirror (it is not visible from the front, hence this whole confusion). But I am glad I did not see it at the beginning as I have learnt much from this forum. Thanks to all.


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