How to find leak in pressurised CH system?is it possible?

[John D v2.0]

In a sealed CH system though 'the weight' isn't enough to alter the pressure to any real extent and therefore the pressure would effectively be the same across the system.

Even in a manometer the weight of a column of water is enough to create a pressure difference. 1cm per millibar.
If you add that up to a metre that is 100mb or 0.1bar.
That 10m per bar (per kg per litre) is pretty much constant on earth, actually it's called acceleration due to gravity.

 

[Wildcat1961]

Even in a manometer the weight of a column of water is enough to create a pressure difference. 1cm per millibar.

Dependant on which type of fluid used.
Are you familiar with the different types of manometers used?

 

[John D v2.0]

Dependant on which type of fluid used.

Firstly no it's not dependant on the type of fluid. If it didn't create a pressure difference it wouldn't be a working manometer.
Secondly even if it did, I doubt many people fill their domestic heating system with anything other than water, and that's what the whole thread is about.
Thirdly I actually said water explicitly.

weight of a column of water

 

[HERTS P&D]

I wonder if the OP has found the leak yet?

Andy

 

[Wildcat1961]

Firstly no it's not dependant on the type of fluid. If it didn't create a pressure difference it wouldn't be a working manometer.
Secondly even if it did, I doubt many people fill their domestic heating system with anything other than water, and that's what the whole thread is about.
Thirdly I actually said water explicitly.

SG/HG.

 

[John D v2.0]

SG/HG.

That'll wreck your hex and pump for a start

 

[Dan Robinson]

Oh dear.

 

[fluff29]

Hi everyone, just a quick update.

We have checked all joints we can get access to. 2 joints that the plumber created have a slight weep, not even a drip. Just a small bit if dampness on a bit of blue roll. all other joints that we can access (originals and from recent works) are dry.

1) could 2 very small weeps (not even leaks) equate to 0.5 bar loss over 4 weeks? I will be contacting the fitter about these plus a couple of other no related issues.

Engineer at work has a leak as well and tops up every month or so as he cant find the leak either. He has advised just to continue to top up. I know we run the risk at some point of the leak getting worse but at least at that point it will be more visible. We have 8'x4' sheets of chipboard flooring which are nailed down and it will be difficult to check all joints with some being inaccessible due to location or in pipe box.
1) will this approach cause any issues for the system that we haven't thought of?
2) if yes what are the consequences?
3) we have air in the (upstairs towel rail) that we can't eradicate as there is no bleed plug, so another think to speak to fitter about. Im assuming this is because of having to continue to top up the system.

Thanks again for all your help and please be gentle with your replies as we are trying our best and I am taking on board your thoughts.

Cheers
Fluff

 

[fluff29]

Anyone?

 

[fixitflav]

Anyone?

Expansion of water between 4°C and 100°C is about 4%, that’s probably a little too conservative, so if we say 3%, the expansion of a typical 100 litre system = 3 litre.

That causes a pressure rise say from 1 barg to 1.5 barg, 0.5 bar rise. So, if your leak causes a pressure drop of 0.5 bar, the volume leaking out is of the same order, 3 litre, rather more if you do the calcs in more detail. I’d expect that quantity of leakage in 2 – 3 weeks would be readily visible.

Maybe CH experts will query my figures, but I don’t think it will change the conclusion much.

 

[fluff29]

Expansion of water between 4°C and 100°C is about 4%, that’s probably a little too conservative, so if we say 3%, the expansion of a typical 100 litre system = 3 litre.

That causes a pressure rise say from 1 barg to 1.5 barg, 0.5 bar rise. So, if your leak causes a pressure drop of 0.5 bar, the volume leaking out is of the same order, 3 litre, rather more if you do the calcs in more detail. I’d expect that quantity of leakage in 2 – 3 weeks would be readily visible.

Maybe CH experts will query my figures, but I don’t think it will change the conclusion much.

Thanks for the reply. I didn't realise that 0.5 bar would equate to 3l of water. We are assuming no leaks in the upstairs as there are no water marks.

Other than ripping up 8x4' sheets of nailed down chipboard, we don't have access under the bottom floor. What are the consequences if we just keep topping up the water? Will it damage the system or cause other problems? Although if we are losing 3l of water I would rather find it.

Cheers
Fluff

 

[John D v2.0]

, if your leak causes a pressure drop of 0.5 bar, the volume leaking out is of the same order, 3 litre, rather more if you do the calcs in more detail. I’d expect that quantity of leakage in 2 – 3 weeks would be readily visible.

Maybe CH experts will query my figures, but I don’t think it will change the conclusion much.

I'm not an expert in ch but your conclusion is not correct, physics states that the relationship won't be linear in a system containing an incompressible fluid as well as the compressible one where the compressible one is retained to a certain volume of the system (the ev).
As an example if your system is at 1.5 bar and the ev is 99% full of air and 1ml of water, then as soon as you lose more than 1ml of water your pressure will drop in the system below the ev pressure (it will match the static height of the system)
This is why your ev charge is critical.

Someone will come along complaining my terminology doesn't match what they learn in heating circles but from a physics point of view it's correct.

 

[fluff29]

I have no idea what you are talking about John d, so I will defer to the experts on this forum. Not sure if this is important but we have about 1/2 of the heated towel rail full of air. It is at the top of the house with no vent plug so we need to get the plumber to fit one.

Will this air be having an impact on losing water? I would think not until it is removed and we top up with water but thought I better mention this.

Thank you for your reply

 

[fixitflav]

I'm not an expert in ch but your conclusion is not correct, physics states that the relationship won't be linear in a system containing an incompressible fluid as well as the compressible one where the compressible one is retained to a certain volume of the system (the ev).
As an example if your system is at 1.5 bar and the ev is 99% full of air and 1ml of water, then as soon as you lose more than 1ml of water your pressure will drop in the system below the ev pressure (it will match the static height of the system)
This is why your ev charge is critical.

Someone will come along complaining my terminology doesn't match what they learn in heating circles but from a physics point of view it's correct.

But in your example, the EV precharge pressure is almost 1.5 barg (2.5 bara). There’s no reason to use that. I did some calculations, attached if you’re interested. Let me know if you think I've made any mistakes. If you have Mathcad you put them in and have a play! According to that, the lower the precharge pressure the smaller the total EV volume. And the same for pressure rise on initial (cold) fill. Somebody is welcome to correct me, but my assumption is the expansion vessel is shipped precharged with air to about atmospheric pressure, with obviously atmospheric pressure, 1 bara on the water side. Then top-up water added to give pressure say 1 barg (2 bara). That takes care of static head up to about 10m less a bit for safety, which covers most domestic situations.

Of course boiler manufactures probably use different figures, and have it down to a fine art to minimise cost, but I think that’s the general idea.

But does anybody here know what a typical time is for a sealed system to lose say 0.5 bar and need topping up? 6 months? 12 months? I’m sure 2 – 3 weeks is too short. Or the volume needed to top up?

I accept that if everything is fine there is still some loss, or there’d be no topping up, so that would need to be subtracted from the estimated leakage figure.

On to the physics, the incompressible part is irrelevant to the relation between volume and pressure of the air. For the air, assuming isothermal conditions, volume varies as 1/pressure (Boyle’s law), so reduction in volume is proportional to increase in pressure. But whether it’s linear or not, I don’t think you can get away from saying that if 3 litre water input raises the pressure 0.5 bar, water outflow to cause a drop of 0.5 bar is of the same order. My 3% ignored the expansion of the rads. Linear coeff of expansion of steel = 0.037*10-4, giving change over 80°C = 0.037*10-4*3*80 ~ 0.001 per system volume, or 0.1 litre for 100 litre system. I think we can agree the change due to expansion of the pipes, and compression of the water at the increased pressure, are negligible.

 

[dilalio]