central heating advice sought please

[RogerS]

If you bleed a central heating system with the pump on you will draw more air in, see the link below

http://www.homeserve.com/help-advice/bo ... iator.aspx

Stew[/quote]

They are partly right. It depends on where the pump is fitted. If the pump is in the flow (where it should be) then it doesn't matter one whit. The system is under positive pressure and so no air can get in. But I'm guessing that they are covering themselves for the odd plumber who sticks the pump in the return.

The only other possible reason is if it is a major bleed and that there is going to be a lot of cold water going into the boiler and the boiler is hot then that 's not a good idea. Nothing at all to do with getting air into the system, though.[/quote]Roger

I have checked 10 different web sites and phoned 2 plumber friends (both are also Gas Safe) and all have said you must turn off the heating before you try to bleed your radiators, if you don't you will draw air into the system! I too thought tha same as you, but when given the advice of professionals I accepted they were right.

Stew[/quote]

LOL! Both you and I are correct. It's a bit like speaking to 100 electricians about supplementary bonding and finding that 99 get it wrong.

Ask any of them why. I bet that none of them will give an explanation. Ask them how is air going to get into the system when (assuming the pump is on the flow) the system is under positive pressure. I will donate £20 to a charity of your choice if any of them come up with a proper, sane, rational answer! :lol:

 

[Lons]

In my experience there are several reasons why regular bleeding is required in a c/h system depending on whether open vented or sealed design.

1). There is a leak somewhere, can be very minor
2). Air is being sucked into the system via the pump or one or more joints.
3). System is over running and water is discharged via the overflow.
4) There is excess sludge (iron oxide) in the system, usually corrosion from inside the radiators producing gas which is bled out believing to be air.

What compounds the problem is that whenever fresh cold water is introduced from the header tank for example it is oxygenated and when heated introduces more air into the system so it's a vicious circle.

This is typical of an old system which probably needs a chemical and power flush clean and then protected using the correct inhibitors. Unfortunately, powerflushing can cause leaks in old radiators.
You can buy a small leak fluid to add to the system which can seal small leaks and help but no guarantees.

As to the hot or cold issue when bleeding. I've done both and probably doesn't matter really but what I always do is turn off the radiator valve whilst bleeding to prevent air being pushed anywhere but out of the bleed nipple.

Just my twopennerth. I'm not a c/h expert or plumber but have many years experience fixing the bl**dy things

Bob

 

[devonwoody]

Sincere thanks to all, btw., I havent had to bleed the the bt since I posted.
But I will switch off first, cant do any harm can it.

 

[sparkymarky]

i`m a plumber so i`ll give it a try,

Ask any of them why. I bet that none of them will give an explanation. Ask them how is air going to get into the system when (assuming the pump is on the flow) the system is under positive pressure. I will donate £20 to a charity of your choice if any of them come up with a proper, sane, rational answer!

even if a pump is fitted on the flow it could still suck in air via a venturi effect this could be due to the velocity / head of pressure of the water being pushed through the radiator is greater than the static head of the system pressure.

i once experienced this with air in a water main with a static head of pressure of 5bar however every morning the customer would have air in the highest mains taps in the house, this turned out to be due to a small leak about 2 miles away at the bottom of a valley and the house happened to be the highest point on that supply line in the area it was only found after about a month when the small leak ruptured and caused a major flood, after repaired the customer had no more problems.

hope that helps explain how air could be sucked into a system with the pump fitted on the flow and the rads under positive pressure, it would be very rare this would happen and probably only happen with a conventional header tank system in a bungalow with a small amount of radiators (as a side note this could cause air to be sucked in to a system and means no water may considerably leak out of a very small pinhole leak on a CH system).

 

[DIY Stew]

i`m a plumber so i`ll give it a try,

Ask any of them why. I bet that none of them will give an explanation. Ask them how is air going to get into the system when (assuming the pump is on the flow) the system is under positive pressure. I will donate £20 to a charity of your choice if any of them come up with a proper, sane, rational answer!

even if a pump is fitted on the flow it could still suck in air via a venturi effect this could be due to the velocity / head of pressure of the water being pushed through the radiator is greater than the static head of the system pressure.

i once experienced this with air in a water main with a static head of pressure of 5bar however every morning the customer would have air in the highest mains taps in the house, this turned out to be due to a small leak about 2 miles away at the bottom of a valley and the house happened to be the highest point on that supply line in the area it was only found after about a month when the small leak ruptured and caused a major flood, after repaired the customer had no more problems.

hope that helps explain how air could be sucked into a system with the pump fitted on the flow and the rads under positive pressure, it would be very rare this would happen and probably only happen with a conventional header tank system in a bungalow with a small amount of radiators (as a side note this could cause air to be sucked in to a system and means no water may considerably leak out of a very small pinhole leak on a CH system).

Thanks for that Mark.

RogerS as I have spinal problems Arthritis Care is my chosen charity.

Stew

 

[graduate_owner]

Hi Devonwoody,
Fernox make a liquid that you can add to the header tank, and it solidifies in the presence of air (i.e. when it escapes as a leak). There are probably other makes. This may only be a temporary solution, but if the problem stops then it could indicate that the problem is indeed a leak. That means it is not gas being produced and flushing wouldn't be necessary.

Hope this helps.

K

 

[themackay]

I had a similar problem some time ago and traced the problem to the isolation valves on the circulation pump the seals on the valve spindles had fallen apart allowing pump to suck in air when system was running but no water was leaking when off this was a vented system.

 

[RogerS]

i`m a plumber so i`ll give it a try,

Ask any of them why. I bet that none of them will give an explanation. Ask them how is air going to get into the system when (assuming the pump is on the flow) the system is under positive pressure. I will donate £20 to a charity of your choice if any of them come up with a proper, sane, rational answer!

even if a pump is fitted on the flow it could still suck in air via a venturi effect this could be due to the velocity / head of pressure of the water being pushed through the radiator is greater than the static head of the system pressure.

Sparkymarky...I appreciate you stepping up to the plate! But sorry, the venturi effect doesn't work like that. You have to have a restriction in the flow...see here http://en.wikipedia.org/wiki/Venturi_effect

i once experienced this with air in a water main with a static head of pressure of 5bar however every morning the customer would have air in the highest mains taps in the house, this turned out to be due to a small leak about 2 miles away at the bottom of a valley and the house happened to be the highest point on that supply line in the area it was only found after about a month when the small leak ruptured and caused a major flood, after repaired the customer had no more problems.
.

No venturi effect here either.

 

[devonwoody]

Hi. folks, the small bungalow at the top of the hill is me.
(so another possible cause.)

How about if our system is running all day and the water is actually boiling, doesnt that produce bubbles?

(I have turned down the boiler thermostat valve by around 25% (economy measure) and I havent had to bleed since)

 

[Tom K]

Hi DW, sounds like your old boiler is a little bit scaled or sludged up creating a whats called "kettling" localised boiling within the heat exchanger producing gas. Turning it down is a short term solution a good flush once the weather warms up will probably do the trick.

 

[devonwoody]

Tom, that sounds promising, but how do I flush a boiler, just keep draining the system in the summer and refilling?

Or is it a flusher needed?

 

[Doug B]

Hi DW,

Lovin the thread, if you weren't confused before I'm guessing you could be now,.
I'm still trying to work out which side of a boiler is the wrong side :lol: :lol:

If you've still got my number give me a ring or pm me.

Best wishes,

Doug

 

[Harbo]

You could do it yourself with flushing chemicals but I would get a plumber in with a pressure flusher - a lot less hassle.
Then fill up with the appropriate system additive depending on your water?

Rod

 

[devonwoody]

When summer comes I shall get a clean out, the muck that is coming out of the towel rail radiator is pretty mucky, (the highest point of radiator system). I did put in a container of that Fern...? last year when a motorised valve was replaced.

 

[devonwoody]

I just went and did a bleed from the towel rail, no air in the rad at the moment but this is what the fluid look like that I was able to bleed out. urghhhhhhhhhhhhh.

so there is the sludge?

 

[RogerS]

Yes....it should be clear. I reckon a good power flush as suggested is called for. Basically they remove the pump and connect two hoses in its place to the power flush unit. Ideally they would empty the system first to get rid of most of the crud you've got. Then fill it back up and include some power flushing additive, bring the ch system up to temperature and then run the power flush which can send the heated water back the other way thus giving all the sludge a good shake-up.

But to be honest, I do recommend you ask him to fit something like a MagnaClean which will grab all the haematite that looks to be in your system.

 

[dj.]

A pump should always be turned off when bleeding a system to prevent air entering the system, usually via the vent pipe or in sealed systems via faulty automatic air vents.
If sufficient air is drawn into the system it can cause the pump to run dry & so burn out, the pump is particularly at risk when it is sited in a high position such as an airing cupboard & the vent connection is just under the pump, a common scenario in millions of homes where Y plan systems are installed.
The possibility of pump burn out happening is greatly exacerbated when the cold feed to the system is restricted in any way, such as debris in the header tank, poor cold feed lay out, corrosion in that part of the system or indeed as happens often in older systems the ball valve sticks in the off position & evaporation empties the header tank.



Regards.

dj.

 

[RogerS]

A pump should always be turned off when bleeding a system to prevent air entering the system, usually via the vent pipe

How? The water level in the vent pipe is going to be at the same level as that in the header tank. That's a huge amount of water you are going to be bleeding out of the radiators before you empty that pipe....if it actually ever does get emptied because it's going to get filled up from the header tank via the boiler.

 

[dj.]

How? The water level in the vent pipe is going to be at the same level as that in the header tank. That's a huge amount of water you are going to be bleeding out of the radiators before you empty that pipe....if it actually ever does get emptied because it's going to get filled up from the header tank via the boiler.

The possibility of pump burn out happening is greatly exacerbated when the cold feed to the system is restricted in any way, such as debris in the header tank, poor cold feed lay out, corrosion in that part of the system or indeed as happens often in older systems the ball valve sticks in the off position & evaporation empties the header tank.
.

I`d already given the answer, if the water in the header tank is restricted from getting into the system what I`ve explained can & does happen.
Very slow filling systems are the bane of a heating engineers life & are surprisingly common.
Turning the pump off when bleeding was, when I was at collage referred to as good practice & I still think it is, whilst this problem is more likely to be the exception rather than the rule, burning out a £70 pump is an expensive lesson to learn.

Regards.

dj.

 

[johnluc]

They are partly right. It depends on where the pump is fitted. If the pump is in the flow (where it should be) then it doesn't matter one whit. The system is under positive pressure and so no air can get in. But I'm guessing that they are covering themselves for the odd plumber who sticks the pump in the return.

this is only true if the open vent,cold feed and pump are ALL on the flow and in that order, and the open vent and cold feed are very close to one another then the whole system will be under a positive pressure, thus preventing pumping over or sucking in air.
johnluc.