Topping-up Expansion Tank - How Often is Too Often?

[endaxi]

Hi All.

I have a heat-only gas boiler downstairs (pump is upstairs) with a Megaflo Tank upstairs that has a red expansion tank and pressure gauge above it. The expansion tank is about 12 inches (30cm) tall and 9 inches (22.5cm) wide.

The gauge has a red line set at 1 bar which, rightly or wrongly, I take as a setting point for the pressure. Every month or two I notice that the pressure drops gradually to around 0.75 bar. At this point I open the little tap for a few seconds and top the tank up until the gauge reads just under 1 bar. I have been doing this for somewhere approaching 20 years.

Several years ago a BG engineer (I have a maintenance contract with them) said I shouldn't be touching anything and he used a foot pump to pump some air in but it never solved the issue so I kept topping-up as required.

I have a few questions.

1. There has never been any visible leaks in the CH/HW system so where all the water is going?

2. I had a new boiler several years ago and that was the last time inhibitor was put in the system. Should I have some more added as I must be diluting it every time I top-up?

3. Am I doing the right thing by topping-up every now and again or is there a fault in my system that can be remedied so the pressure doesn't drop so much?

Any explanations or advice would be much appreciated.

Thanks in advance.

 

[terryplumb]

Sounds like you are losing water from the sealed central heating system. Possibly a slight weep/ leak somewhere on the system. Any heating pipes buried in concrete or in voids under ground flooring ?
Constantly adding water will dilute the inhibitor eventually.

 

[Johntheo5]

Hi All.

I have a heat-only gas boiler downstairs (pump is upstairs) with a Megaflo Tank upstairs that has a red expansion tank and pressure gauge above it. The expansion tank is about 12 inches (30cm) tall and 9 inches (22.5cm) wide.

The gauge has a red line set at 1 bar which, rightly or wrongly, I take as a setting point for the pressure. Every month or two I notice that the pressure drops gradually to around 0.75 bar. At this point I open the little tap for a few seconds and top the tank up until the gauge reads just under 1 bar. I have been doing this for somewhere approaching 20 years.

Several years ago a BG engineer (I have a maintenance contract with them) said I shouldn't be touching anything and he used a foot pump to pump some air in but it never solved the issue so I kept topping-up as required.

I have a few questions.

1. There has never been any visible leaks in the CH/HW system so where all the water is going?

2. I had a new boiler several years ago and that was the last time inhibitor was put in the system. Should I have some more added as I must be diluting it every time I top-up?

3. Am I doing the right thing by topping-up every now and again or is there a fault in my system that can be remedied so the pressure doesn't drop so much?

Any explanations or advice would be much appreciated.

Thanks in advance.

If the EV is 20 years old? then time to renew it as the air might just be seeping through the membrane or leaking out the air end schrader valve or around it and you may have no leak.
You appear to have a 12L EV, IF this has been precharged to 0.75bar and you are topping it up to 1.0bar then that will take 1.5L, 1.5L X 12, 18L/annum, would think your CH system, rads etc would have well rotted away or be full of sludge in 20 years, normal top up should be ~ 1L maybe every 6 months or so.
Would suggest getting the EV renewed then ensure that the installer informs you of the precharge pressure, its quite easy to calculate the topping up amount then knowing the EV capacity and the filling pressure.

 

[endaxi]

Thanks for the prompt replies.

All CH pipes are behind plasterboard so it's impossible to tell if there is a hidden leak. All I can say is that there is no evidence of any leaks.

As to topping-up... I would guess I increase the pressure by about 0.15 - 0.2 bar each time I top up the EV. I haven't logged how often I do this but probably more in winter than in summer so maybe every couple of months if I average it out. I open the valve for 2 -3 seconds each time so I'm thinking it would be less than 18L/annum but probably too much nonetheless.

I topped-up to about 0.9 bar this morning but now a family member has put the heating on (why?) I can see it is reading 1.1 bar now. Maybe I top it up too much and should just leave it alone.

I will be having an annual service some time in the next few months so I will enquire about getting a new EV and what I should be doing as a householder to ensure all is as it should be.

Thanks again for the help.

 

[endaxi]

I should have mentioned that I had a power flush when the new boiler was fitted and was told it all looked clean. I also have the magnetic filter checked every year and that is always clean so no sign of any sludge in the system.

 

[flameport]

I topped-up to about 0.9 bar this morning but now a family member has put the heating on (why?) I can see it is reading 1.1 bar now. Maybe I top it up too much and should just leave it alone.

Multiple different things going on here.

If the pressure when cold falls, this is because water has leaked out somewhere. Topping up by opening the valve(s) puts more water in to replace what was lost.
Ideally there should be no leakage, but if it's taking months for any noticeable difference then it's a tiny insignificant amount.

The pressure will increase when the system is hot because water expands when heated. Any adjustments must be done when the system is cold.
Higher pressure when hot is entirely normal.

The red vessel is for expansion, and has air in it. It's there so that the expanding hot water has somewhere to expand to, by compressing the air in the vessel.
These may need to be recharged with air occasionally, but that should be a rare event, as in once every few years or never.
If these fail there is nowhere for the expanding hot water to go, the pressure will rise significantly to 3+ bar, and a relief valve will open to let water out.

 

[endaxi]

Thanks for the explanation. The EV seems to be doing its job but maybe at 22 years old it would be prudent to get a new one fitted in any case. It may cure the gradual loss of pressure if indeed the Schrader Valve is leaking a bit but some more inhibitor at the same time won't go amiss for sure.

 

[Johntheo5]

If the pressure loss is from the air end then the final pressure after cold top up and then heating will gradually keep rising until the pressure reaches 3.0bar and PRV lifting.
As stated above, its quite normal and a must for the pressure to rise when the system heats up.
Assuming a 12L EV with prefill & charging pressures of 0.75/1.0bar then there will be 1.5L reserve water in the EV which would have to be lost for the pressure to fall to its original 0.75bar, if you assume a system vol of 85L and a hot average system temp of 60C then the EV must accomodate a additional 1.38L of expanded water resulting in a final hot pressure of 1.3bar. If the prefill/charging pressures are 1.0/1.5bar then the reserve will be 2.4L and the same expansion vol of 1.38L will give a final hot pressure of 1.92bar.

 

[endaxi]

Thanks for that info. I had been mulling it over and thought about loss of air or a leak in the diaphragm causing a high pressure due to no expansion room. I guess the EV is doing its job and the problem is a gradual loss of water somewhere in the system of an amount that probably evaporates away rather than gathers somewhere. In any event, the inhibitor in the system must be reducing every time I top-up so whatever I do I need to get some more put in before long.

 

[Johntheo5]

If interested, you can do your own simple calcs in the attachment.

 

[endaxi]

If interested, you can do your own simple calcs in the attachment.

Thanks for that. It was all a bit much for me tbh. I don't know the system capacity but have 17 radiators spread over a decent size 4 bed house. I can see that the EV capacity is 18 litres with a Factory Pre-charge of 1.5 bar. The Set Pre-charge isn't recorded so I only have the red line (which is set at 1.0 bar) to go by. I don't know if the needle should be on the red line when the system is cold or when it's hot. Currently it seems to be at around 0.9 bar cold and 1.1 bar warm.

 

[Johntheo5]

Just ensure that EV is connected to the CH system, your Megaflo cylinder if unvented will have its own internal air bubble but sometimes a external EV (charged to 3.0bar)is added to eliminate replinishing the air bubble, this would normally be a white vessel with its outlet connected into the cold water supply to the Megaflo, but if so, then there will be two Evs, if only one, then that red EV must be connected into the heat only boiler.

All I can say without knowing the actual precharge EV pressure is that topping up a 18L EV by 0.2bar should mean that 1.2/1.5L of water is required each time.

 

[endaxi]

Just ensure that EV is connected to the CH system, your Megaflo cylinder if unvented will have its own internal air bubble but sometimes a external EV (charged to 3.0bar)is added to eliminate replinishing the air bubble, this would normally be a white vessel with its outlet connected into the cold water supply to the Megaflo, but if so, then there will be two Evs, if only one, then that red EV must be connected into the heat only boiler.

All I can say without knowing the actual precharge EV pressure is that topping up a 18L EV by 0.2bar should mean that 1.2/1.5L of water is required each time.

Thanks for all your input. The Megaflo has no EV connected and I periodically regenerate the air gap. I've just checked and I last did it in Feb 24 so that's due to be done again I guess.

 

[Johntheo5]

Thanks for all your input. The Megaflo has no EV connected and I periodically regenerate the air gap. I've just checked and I last did it in Feb 24 so that's due to be done again I guess.

Over a year is pretty good between regenerations, do you wait until the expansion relief valve lifts or are you one in 100,000 who has a pressure gauge on the UVC?

 

[endaxi]

Over a year is pretty good between regenerations, do you wait until the expansion relief valve lifts or are you one in 100,000 who has a pressure gauge on the UVC?

The Megaflo doesn't have a pressure gauge and I never think to look in the tundish to see if the relief valve is letting-by. The HW comes on for an hour three times a day so the water temp never goes from really cold to really hot (just tops up the temp a bit) so I guess the expansion is not as much as it could be.

I just get to a point where I think it's time to regenerate but sometimes it can be a couple of years if I'm a bit off the ball. I reckon many householders don't even know these things have to be done.

Edit... I have just looked in the tundish whilst the tank is heating up and lo and behold there is a steady drip so high time to regenerate.