DAFT QUESTION OF THE DAY: How DOES air get in.....

[Bunker]

I know its a daft question but I have to go to a job next week where they reckon they are bleeding the same 2 rads almost every day. Its a pumped system with water on gravity. Im all over boilers but often get stuck on simple CH design problems..

Thanks

 

[PlumbBobBob]

might not be air, could be nitrogen produced by corroding system.

dur should read hydrogen of course, thanks Whn1

 

[Whn1]

Could be anything from corrosion to incorrect pipe layout/pump positioning.

 

[Whn1]

Hydrogen

 

[kay-jay]

Could be corrosion. Check by trying to ignite the gas from the rad vents if it lights then it is hydrogen a by-product of corrosion. Needs a flush and a good dose of inhibitor.

Or bad positioning of pump/vent/feed where pump is drawing air in. Possibly convert to a wet vent feed(combined feed and vent) check boiler mi'sfor system compatibility.

 

[Blake]

Dirty system, air sep blocked, lack of water in header tank. Stick a pint glass full of water under the open vent and turn the system on to see if it's sucking in.

 

[Bunker]

Thanks all, I did wonder about the corrosion but thought the frequency was unusual - apparently its only recently started happening.

 

[kay-jay]

I don't know.....type a meaningful and lengthy reply and offer solutions and a bunch of half a job harry's jump in before ya

 

[Whn1]

I don't know.....type a meaningful and lengthy reply and offer solutions and a bunch of half a job harry's jump in before ya

Hahah

 

[Bunker]

Stick a pint glass full of water under the open vent and turn the system on to see if it's sucking in.

I like that idea, never thought of that although youd need long arms...

 

[Blake]

if you are slow at typing you are stuffed on here hey!

 

[kay-jay]

I like that idea, never thought of that although youd need long arms...

Not if your in the loft

 

[kay-jay]

if you are slow at typing you are stuffed on here hey!

One has to give a reasoned and well considered reply when one is a trusted advisor don't you know

 

[Deleted member 25704]

If it is pumped central heating and gravity hot water, then the pump will be mostly likely positioned on the return, which puts the heating system under negative or sub-atmospheric pressure.

If there are small micro leaks on say the pump valves (look for signs of corrosion on the inlet side of the pump) then air is pushed into the system from atmosphere due to there being a sub-atmospheric condition (created by the pump running).

Micro leaks are small enough to allow air to be pushed into a system, but small enough for water not to leak out. However, signs of corrosion are sometimes observable on the pump valves.

Ask customer if air is present in summer months when heating not on, this will help you link the problem to the pump and heating use.

Look for red oxide on pump, in system water or expansion cistern, which indicates air in system, which rules out hydrogen or corrosion issues. If its air then micro leaks, and positions of cold feed and vent should be investigated.

Note: diagnosing problems such as these are impossible to be certain, no matter how much experience or knowledge you have. There is always something we do not know!

 

[Deleted member 25704]

I would also consider a leak under the floor, with the regularity of the bleeding, which I didn't read properly the first time round. Ask if they hear the cistern filling at night.

...in addition, the very fact that they are venting so often, may link the problem to a blocked cold feed and expansion pipe. Ask the customer if they see the expansion cistern overflowing - it probably is dripping away and they don't notice it.

What happens is the blockages on feed and expansion pipes are often pourous, and water may perculate through very slowly to fill system up, but when the system is expanding the same 4% volume of water cannot expand into the cistern in the same slow way. So water expands in the system and goes up the vent and into the cistern, leaving a 4% volume loss of water in the system experienced in the need to bleed the radiators, which are always filling slowly through the partialy blocked feed and expansion pipe.