Heating system losing pressure

[Octoberfishing]

We recently had our boiler serviced, and the engineer mentioned the boiler pressure was low, and reminded us about how to top it up, which we did.

A day later, the pressure had dropped down almost to zero again. I topped it up again.

24 hours later, it's dropped right down again.

There are no obvious signs of a leak / radiators are getting warm and we haven't recently bled any radiators. Very occasionally (as in once a month) the boiler will make a dreadful banging sound, turn itself off and on again, then all is well!

Does anyone have any suggestions (I remember @pigletjohn was an expert)? We have British Gas cover, but there's a £60 excess fee, and I'm loath to call them out if they just say "oh yes, your pressure's low, I've topped it up for you".

The boiler is an Ideal, 7 years old.

Ours was only covered for 5 years. It came with our new build house.

@Octoberfishing

Obviously you know your Boiler guarantee but I thought Ideal Boilers were always guaranteed for 10 years.

Even if it is out of guarantee it might be worth getting them out to look at it. My very experienced Plumber couldn’t work out what my problem was but the Ideal engineer knew immediately. They were very efficient.

i think they offer fixed price repair options and have an annual service plan.

idealheating.com/search/results?q=Warranty

Sorry, I'm not the op but have been following this thread as I have the same problem.

The pressure drops to 0 when the heating is off and rises to normal/1.5 when the heating is on. The plumber who fitted our new system said it would be a leak in the existing plumbing so not his problem essentially, would that likely be the case?

He said it was a lot of disruption to check if we wanted to be sure, and since it's working despite the fluctuation in pressure we decided to leave it.

My description is of expansion vessel fault. Other symptoms have other causes.

Thanks @PigletJohn

I probably should have said that I'm not usually topping it up anymore, I do if I remember but it just falls and rises again without me doing anything and the system never shuts off. Still the expansion vessel?

Yes.

Might just need pumping up.

If you have an unusually large house the one inside the boiler may be too small but an extra one can easily be added.

If I am right your boiler man is a numbskull.

Perhaps I am wrong.

To be fair to the plumber, I didn't know that the pressure was falling and rising on its own at the time, I was topping it up whenever I noticed it had dropped to start with. But he definitely didn't want to put too much effort into finding out the problem. It's an ashp system, tonight is going to be it's first big test -4 forecast here tonight and -8 later in the week!

The gas engineer came yesterday and reinflated my expansion vessel, which sounds a lot more fun than it actually was.

However, pressure today has already dropped from the 1 he topped it up to, to 0.5. Could this be because it's very cold today, or does the vessel need replacing, do we think? Or could it be something else?

Did he say it might not last? My expansion vessel was knackered and had to pay £180 for a new one- expensive but I have concrete floors and my last two underground leaks costs tens of thousands in insurance to find and rectify. So the question is which is cheaper to try first, a new expansion vessel or do you have floor boards to lift up/ pipes easily accessible to check for a leak?

Can't rell. If he inflated it because the vessel had run down, and the vessel has run down again so soon, the vessel is probably leaking and needs replacment or repair, perhaps it just has a faulty valve.

It is possible, but I'd think it less likely, that there is some other fault leading to loss of pressure, as well as the faulty vessel, and you have two different faults with the same symptoms.

If the vessel is faulty, you can add a new one on a convenient pipe, it does not need to be inside the boiler like the original.

Have you looked at the pressure relief valve going through the wall behind the boiler, and is there any sign water has squirted or dripped out if it?

@PigletJohn Silly question, but where exactly should I be checking? The boiler with the controls is in our kitchen. But the expansion valve is on a big tank in our airing cupboard upstairs. Where should I be checking the external wall - by the boiler in the kitchen or the tank upstairs?

I've had this problem several times.
Most likely is that the boiler expansion vessel needs replacing. Plumbers can check a vessel to see if it's broken, fairly sure of that. Much less likely is that the heat exchanger in the boiler has a failure but I suggest leaving this last.
Fairly likely especially if any building work has been done is a leak in the system somewhere. So go round every point that you have access to and look for leaks. Better to leave the system cold for this as evaporation from a hot system will hide leaks quite well.

If you find that pressure is still dropping then there is a hidden leak somewhere. I have found leaks as follows. Add a cheap bottle of perfume to one radiator; look up on YouTube if you don't know how. Give it about 5 days and you should smell the leak. I have done this twice and it worked for me. It's better than asking a plumber to lift floors to find a leak and lots of people do this and it's expensive. Good luck.

The pressure relief valve vents outside the house, through a small copper pipe that bends round so that in the event of a gush, hot water will squirt against the wall, and not onto any passers-by.

It comes out of the back of the boiler unless there is some pressing reason why it must go somewhere else.

I sniffed out our very small leak without the perfume! Fishy/oniony smell in the bathroom. There was a small leak from the heated towel rail valve, but the rad was drying it before it became noticeable. I'm like a bloodhound 😂

Love the perfume idea though! Far nicer than fish!

I would be checking the little pipe that lets overpressure out the system, putting a rubber glove on it to make sure any water is from the system (should be none) check all the silly things like radiator valves are not dripping unnoticed. Otherwise probably need a leak detection carried out. Have a read of Heating tips and leak seal , as it has some information on what can go wrong and how to find it.