How often should the central heating expansion vessel be topped up?

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I always wondered what is the normal amount of pressure loss with a sealed expansion vessel. All the manufacturers seem to state is that topping it up once or twice a year is normal.

I have 30x15ft of UFH, 15 double radiators, 4 towel rails.

I noticed when the whole system is first filled from a complete drain the pressure used to subsequently drop small amounts weekly over 3-6 months (i.e 0.2-0.4bar total). I presume this is all the air from the new water escaping?

It usually then settles down and will lose about 0.2-0.4bar over a year..

After fully draining 2 huge radiators, over the course of 3 months its dropped 0.15bar i.e 1.4 bar - 1.25, a tiny bit drops per month - again I presume this is the new water having the air escape, as there is no leak/weeping.

My question is, as plumbers how much pressure is usually lost per year on average (with no visible leaks anywhere obviously) on a system this size? and other than leaks, is some pressure always going to be lost from air being released/ oxidation etc?

Thanks
 
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What size vessel and what boiler model (ie. is it a system boiler?)
 
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Top up frequency?, once/twice per year after settling down should be fine, it can vary quite a bit though depending on the E.vessel pre charge and filling pressures, if you have say a 10L E.vessel with precharge/filling pressures of 1.0/1.5bar then there will be a reserve of 2L. if the pre & filling pressures are both 1.0 bar then no reserve so much more frequent top ups. I keep an eye on a few relations oil fired boilers and they require top ups ~ once/year at most. I have a OV oil fired system with a isolation valve on the make up to the F&E tank ballcock which I keep closed, I open it a few times/year and I have never seen it add a drop of water, its now approx 3 years since a partial drain down to renew a TRV and still abolutely no top up.
 
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System boiler, vailant ecotek 438, 18L vessel.

I wasn't looking for a comparison with exactly the same set up, just generally
 
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Top up frequency?, once/twice per year after settling down should be fine, it can vary quite a bit though depending on the E.vessel pre charge and filling pressures, if you have say a 10L E.vessel with precharge/filling pressures of 1.0/1.5bar then there will be a reserve of 2L. if the pre & filling pressures are both 1.0 bar then no reserve so much more frequent top ups. I keep an eye on a few relations oil fired boilers and they require top ups ~ once/year at most. I have a OV oil fired system with a isolation valve on the make up to the F&E tank ballcock which I keep closed, I open it a few times/year and I have never seen it add a drop of water, its now approx 3 years since a partial drain down to renew a TRV and still abolutely no top up.
How much pressure are you topping up per 6months /year?
How long does it take to settle down, 6 months?
Mine looks to have 0.5bar of pre charge as per the manufacturers spec. (Flexcon)
It says 6 bar- i presume i then have over 4 bar for expansion?
 
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The three 12L EVs that I replaced settled down in a few weeks at most, I precharged to 1.0bar and cold filled to 1.5bar, I top up at ~ 1.25bar which means around 1L after a year.

Precharge/filling pressures of 0.5/1.4 bar means (18L EV) a loss of 0.75L in falling to 1.25bar, nothing to worry about really if topping up say twice/year, if topping up say 4times/year then, theoretically, loosing 3L/year, would keep a eye on it.

Expansion: The boiler PRV lifts at 3bar so a 18L EV will allow for a system vol of 275L before lifting the PRV or more realistically, 215L as one doesn't want the final pressure after expansion > 2.5bar.

What is your final pressure with all rads on and up to temperature?.
 
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The three 12L EVs that I replaced settled down in a few weeks at most, I precharged to 1.0bar and cold filled to 1.5bar, I top up at ~ 1.25bar which means around 1L after a year.

Precharge/filling pressures of 0.5/1.4 bar means (18L EV) a loss of 0.75L in falling to 1.25bar, nothing to worry about really if topping up say twice/year, if topping up say 4times/year then, theoretically, loosing 3L/year, would keep a eye on it.

Expansion: The boiler PRV lifts at 3bar so a 18L EV will allow for a system vol of 275L before lifting the PRV or more realistically, 215L as one doesn't want the final pressure after expansion > 2.5bar.

What is your final pressure with all rads on and up to temperature?.
The vessel seems sufficient as I never get more than 0.3bar above cold even at full whack.
I.e if system is on 1.2 bar cold, with boiler at 38kw, 7m on pump and all radiators demanding it will sit at 1.40-1.45.

But what causes this loss other than leaks? Air releases from water so I suppose over time this continually releases? I never have issues with air in my system, I never have any air in towel rails or radiators, not even a tiny bit. My UFH system has an auto air valve thing, which with the boiler I presume is releasing the periodically and causing the drop?

I usually top up to 1.4 and then leave it to drop to 1bar, which before took around 1.5-2 years.
Each time I've filled after a significant drain down I always see small pressure loss for about 3-4 months as I presume more air is being released. I just wondered what was 'normal' and 'why'
 
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Based on your info, your system contents are ~ 70L.

A 18L EV in falling from 1.4bar to 1bar will loose 2.25L so 2.25L over 2 years is 1.13L/annum, perfectly normal IMO, as to why it looses this, I have no idea really, the same as I have no idea as to why my system never requires any top up.
 
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Anyone else got any feedback as to how much they generally top up per year and why it can go down (if there are no leaks)
 
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From the research I have done, there can be a small let by of water via TRV's/joints or air being drawn in, other than that air is produced when the radiators rust which can affect pressures.

In my example, my radiators are brand new and no visible water ever around joints or TRV's, but there is a lot of water volume inside and air gets released from the water all the time.
One would usually bleed that air a few times a year but my UFH has an open auto bleed valve.
Therefore I never get any trapped air to release but the pressure does 'drop' gradually because the air is being bled and pressure released automatically.

Once or twice a year I and others would put in a few more litres of water to top up the pressure and that new water also has even more air to release.
If the system doesn't ever get drained /radiators removed I would presume most system pressures would be pretty stable after a year or so as less air is released.

I don't believe there is a hard and fast rule, but with 20 radiators and a large UFH system that is a lot of water with air to release and unless one can see pressure dropping consistently over a few week its unlikely there is an issue.
Pressures also seem to fluctutate slightly i.e 0.01-0.20 of a bar even at cold based on temperatures.
 
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System boiler, vailant ecotek 438, 18L vessel.

I wasn't looking for a comparison with exactly the same set up, just generally
I thought the Vaillant 4xx series were heat only boilers?
 

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