Help - System losing pressure after oil boiler was replaced

Hi folks,

About a month ago, I replaced my oil boiler/burner with a used one that is about a year old. All seemed to be well after it was replaced however, within a week or two, the whole system has started losing pressure - approximately 0.2 bar every 2-3 hours. It seemed to happen a week or so after I bled the rads, but I've gone round every single one of them and I can't see any signs of a leak. I'll explain below what I've tried already.

Checked all rads, removed TRVs to check for any leaks. All good.
Checked expansion vessel located beside unvented cylinder. No loss of pressure (1-bar approx)
Checked expansion vessel for show. No loss of pressure (2 bar approx)
Checked combi boiler enclosure for signs of leaks but all looks ok.
Thoroughly checked all pipework around the house and in the attic (bungalow). Especially checked new pipework where the boiler was replaced. No sign of any leaks.

Things I maybe should check?
Would like to isolate the combi boiler, but there doesn't appear to be any valves installed near it.
The instruction manual says the combi oil boiler has an expansion vessel, but upon checking, it just looks like there is a burner and the boiler enclosure.
One of the bedroom rads was making a small hissing sound which I only noticed yesterday when the heating was on. Thoroughly checked all round the rad and no leaks.

Equipment
Warmflow INDIRECT Unvented Stainless Steel Hot Water Cylinder 150 LITRE (hot press)
Turco Countryman Slimline Outdoor Combi Boiler with Bentone BF1 Burner (outdoor - external housing)
Zilmet 18L expansion vessel (hot press)
Zilmet 12L show expansion vessel (hot press)
4 large, 6 small and 1 towel rail radiators (TRVs on all rads except towel rail)

The system was all working fine for over 2 years before we had the old boiler replaced , so that is why I am suspecting it has something to do with the replacement oil combi boiler. It is massively more efficient than the old one it replaced, but is losing pressure.

I would really appreciate your input/suggestions. Thank you!

What's the installer had to say about it?

Nothing yet. I can't get the hold of him. Have rang and texted him over the past few weeks. He's always busy it seems.

Was the whole boiler replaced or just the burner?

The whole boiler was replaced with a combi boiler. It was an old rubbish thing that was inside the garage. Basically it's on the other side of the wall in an outdoor housing. The different in efficiency is unbelievable. I reckon it is easy 3/4 times more efficient than the old one. Just a shame about the pressure loss. The heat doe have some way to travel with the hour being a bungalow. I can't see how that would be a reason for the pressure loss though.

Is it a brand new boiler, I thought you mentioned above something was second hand.
Oil boilers that are sealed have an expansion vessel but that is only sized for boiler content and a little extra I was told, often an additional expansion vessel is needed. The volume required is calculated using formula in OFTEC books and many other places I assume and I suspect this might be the problem or not helping. What is the expansion vessel charged to? To check and get an accurate reading you would need to drop pressure out of boiler and check. Also what pressure is on gauge when the boiler is working?

Thanks for the reply.

The boiler is about one year old and I am told that it was in good working order. It seemed to work ok the first week or two I had it, or maybe I didn't notice there was an issue.

The expansion vessel in the hot press is at 1 bar. I took a reading of this with a pump and initially it seemed low. Certainly below 0.5 bar. This might have been partly my fault because when I checked it a few days ago to see if it was leaking, I left the valve open a little too long and it lost a bit of pressure. I have pumped it back up to 1 bar and it has remained there. I think the expansion vessel is OK in the hot press.

There is a second expansion vessel in the hot press that I assume is for the shower and the pressure on that is around 2 bar. The reading on the vessel states 3 bar maximum, so I think this is OK too. I haven't touched this vessel before, so it has been like that for around 2 years now.

The one thing I have no idea about is the expansion vessel in the combi boiler housing. I don't know how to access it, or even if I need to.

Incidentally, the unvented cylinder is actually 150L capacity. I initially made a mistake in the original post which I have now corrected.

Thanks!

If you have an unvented cylinder then I'm assuming the second vessel is for that, unless the cylinder had a built in air gap. When the boiler is running what is the pressure on the boiler gauge.
Having a second hand boiler installed sounds alarm bells and you may have set yourself up for endless problems with know access to warranty etc.

Here's a picture of the boiler specs taken from the actual boiler.


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Thanks for the reply. I'm not sure where the boiler guage is. All there seems to be in the housing is a themostat. The pressure gauge next to the expansion vessel doesn't do anything when I switch the heating on. I've heard that it can sometimes increase significantly when the heat is turned on, but mine doesn't do anything. It doesn't go up or down, except after 2-3 hours or so which is consistent with the pressure loss whether the heat is on or not.

The boiler does say there is a 5 year warranty, but I'm not sure if they will honour this, even if I get a proper service carried out. I'm not the original owner.

The boiler it replaced was a very old piece of junk. It had a service sticker from 1992 on it. I thought a one year old boiler would be significantly better and I got it for a decent price.

When the boiler is running the pressure will increase slightly but not significantly like you said, unless there is a problem with expansion vessel which is what I was getting at above. If you're sure the expansion vessel is ok (sized properly and charged to roughly .3 - .5 bar under system pressure) then it is most likely a leak somewhere. Have you checked whether the pressure relief valve is sitting properly and not letting by, a bit of muck of something could cause this. I hope for your sake you don't have a split in water jacket around combustion chamber