Air in central heating system


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Old 03-29-24, 04:41 AM
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Air in central heating system

I live in a bungalow which has an open vented hot water and central heating system.
About three weeks ago a registered gas accredited plumber found it necessary to replace the heat exchanger in our Worcester Bosch gas boiler. This involved a partial drain down of the heating system. The central heating has five separate pairs of pipes that drop from the loft to tne various radiators throughout the bungalow. One pair of downpipes supplies two radiators, one in a bathroom and one in an adjoining on-suite shower room. This particular pair of pipes have, what I think are called, thumbscrew air bleed valves. These are at the top of 15 mm upstand pipes which are connected to the aforementioned radiator feed pipes in the loft.
I now find that, every other day, these two rads go cold. So I am having to go into the loft each time to let air out from one of the air bleed valves to enable the two rads to heat up.
Can anyone please provide me with some guidance as to why this problem has arisen and what may need to be done to cure it?
 
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Old 03-29-24, 07:27 AM
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I am not as familiar with UK systems as I am with U.S.

Not sure what "open vented" systems means. Does your system have a pump to circulate the hot water?

Generally, air in a hot water heating system can prevent the flow of water. Air can get into a system by draining (as in your case) and can also be released from fresh water as it is heated. Over time the air in the water will be depleted. Until then air must be bled from the system. That can be done manually through the bleed valves. There are also air vent valves that will do it automatically. The pump should not be running when the air is being vented. The pressure in the system will have to be restored after bleeding and it helps to increase it slightly above the normal running pressure before opening the bleed valves.

 
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Old 03-29-24, 09:06 PM
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Keep bleeding the air out. It should stop eventually when all the air is out.
 
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Old 03-30-24, 04:51 AM
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Thank you both. It sounds like I just have to keep venting the system until, hopefully, all the air is expelled. I did have thoughts that I should ask a plumber to replace one of the thumbscrew valves with an automatic vent. However, I am not sure if such vents are reliable over time. I would not want to find such a vent failing to hold back the water in a year or so and having a permanent leak in the loft. By the way John, by an open vented system I mean one that is not pressurised and has a small open feed/expansion tank in the loft.
 
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Old 03-30-24, 02:07 PM
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not pressurised and has a small open feed/expansion tank in the loft.
That is what I thought but in the U.S. most systems are pressurized and have pumps and a sealed expansion tank. My house built in 1912 originally had an open gravity system with an open vent tank in the attic.

It was converted to a closed system with a tank in the basement ceiling and a new oil-fired boiler in the 1920s but still gravity. Only the disconnected pipes from the basement to the attic remained.

A couple of years after I bought the house in 1975 that boiler failed and I converted to a pumped system with a pressurized expansion tank when the new boiler was installed.

Since your system is open vented, manual bleeding is probably the best way to eliminate the air since an automatic vent might suck air in when the water cools.
 
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Old 04-03-24, 05:52 AM
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Every time you bleed out air, add some more water to the system. Not sure how this works on a non-pressurized system, either there is a manual fill valve in the basement or a funnel on top of the expansion tank in the attic. (Autofill does not work on an unpressurized system)

The total amount of water in the system expands a little on every heating cycle and contracts between heating cycles. Using trial and error you need to find the water level that does not bring so much air down from the attic expansion tank into the radiators during the off cycle, requiring repeated bleeding, and does not send so much water up to the expansion tank during heating cycles to overflow onto the attic floor.

Suggested amount of water to add when you do add water: Have all zones (all thermostats) calling for heat and and the boiler flames are full on. Wait until the flames go off to end ithe boiler heating cycle (not necessarily corresponding to a zone heating cycle). Then fill the expansion tank almost to the brim.
 

Last edited by AllanJ; 04-03-24 at 06:43 AM.
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Old 04-04-24, 06:02 PM
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Correction -- You can have an autofill valve similar to a toilet tank float and fill valve in an unpressurized tank up in the attic.
 
 

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