Bleeding radiators - CH on or off

[londonman]

Reading various websites it seems to me that it's a bit like an urban myth. Switch off the CH system before bleeding ....as air can get in.

Yet no-one can explain how this particular myth has gained credence. I can understand how it might ...nb might..happen if the pump has been incorrectly fitted in the return but not if it is in the flow.

If the pump is fitted in the flow then there is no need to switch off, I maintain.

 

[Agile]

Well its up to you how you choose to do it !

 

[londonman]

LOL....


So you don't know the answer either !!

 

[45yearsagasman]

View media item 32248
Interesting londonman. The only way I can see for air being drawn in on (say) a system setup like this, is if there were restrictions in the cold feed, and the pump is able to suck down the open vent during the venting process.
Generally speaking, venting is done fairly slowly and therefore should not present a problem. I've done it both ways(no smutty remarks required) and have had this problem only rarely.

 

[londonman]

Thanks, Pete

You're confirming what the conclusion that I have come to...namely it's an urban (plumbers?) myth.

 

[D_Hailsham]

Reading various websites it seems to me that it's a bit like an urban myth. Switch off the CH system before bleeding ....as air can get in.

There are two reasons for turning the boiler off when bleeding the rads:

1. Hot water can hold more air than cold (so you should really let the system cool down first).

2. You shouldn't have the pump working as it tend to force water out of the bleed valve rather than letting the air exit naturally.

You should always bleed downstairs rads first.

 

[londonman]

Reading various websites it seems to me that it's a bit like an urban myth. Switch off the CH system before bleeding ....as air can get in.

There are two reasons for turning the boiler off when bleeding the rads:

1. Hot water can hold more air than cold (so you should really let the system cool down first).

I'm sorry but it is the other way round. Think about it...

The amount of gas a liquid will dissolve depends upon the vapor pressure of said gas, which in turn depends upon the temperature.

To simplify things a bit, suppose we have pure water. That water has a vapor pressure, and that vapor pressure depends upon the temperature of the water. At 25 deg C, the vapor pressure is 3.1690 kPa (101.323 kPa = 1 atm). The vapor pressure increases with temperature up to (and beyond) the boiling point of the liquid. The boiling point is that temperature at which the vapor pressure of the liquid becomes equal to the pressure above the liquid (in most cases, atmosphere). The vapor pressure of water at 100 deg C is 101.32 kPa, and thus 100 deg C is the temperature at which water boils.

So basically as the liquid cools, so does the gas, and thus the vapor pressure of the gas goes down.

So hotter water stands more chance of releasing any air than cold water.

2. You shouldn't have the pump working as it tend to force water out of the bleed valve rather than letting the air exit naturally.
.

Why does that matter? It's not as if it's going to squirt out in a 10ft jet!

 

[D_Hailsham]

So hotter water stands more chance of releasing any air than cold water.

The problem is not air dissolved in the water but air bubbles trapped in the radiators, usually at the top, which is why a cold rad top is a sure sign that a rad needs bleeding.

It's not as if it's going to squirt out in a 10ft jet!

I know that! The problem really is that, as the water is exiting the bleed valve under pressure, it is not so easy to tell when the air, if any, has come out.

 

[kbdiy]

My understanding was that it was to eliminate any possibility of aeration that the turbulence from the pump could cause.

But science may yet again prove me wrong!

 

[londonman]

So hotter water stands more chance of releasing any air than cold water.

The problem is not air dissolved in the water but air bubbles trapped in the radiators, usually at the top, which is why a cold rad top is a sure sign that a rad needs bleeding.

You said in your post "1. Hot water can hold more air than cold (so you should really let the system cool down first). "

Now you are saying something different.

The air at the top of the radiator is already out of the water(if it ever was in there to start with!)....it couldn't care less if the water was hot or cold.

But your OP was talking about air held in the water and which was what I was replying to.

 

[londonman]

My understanding was that it was to eliminate any possibility of aeration that the turbulence from the pump could cause.

But science may yet again prove me wrong!

But if that were the case then how is the pump running when bleeding the radiators any different from the pump running normally? !

 

[xr4x4]

It's a circulator...not a pump.

There isn't a pump pumping anything.

 

[londonman]

It's a circulator...not a pump.

There isn't a pump pumping anything.

Bit pedantic?