We’ve recently had a new bathroom put in. As part of that, we had a new pressurised hot water tank installed, which came with lots of new assorted pipework, including pressure blow off valves and a heating filling loop and pressure gauge. There are also a couple of what look like electronic valves. I asked our bathroom fitter what they were, and he said he had no idea, it was his plumber that fitted them (!).

Now, before all this was installed, the central heating worked fine. I presume the filling loop and pressure gauge were added to comply with current standards – we didn’t ask for them, and they weren’t discussed with us.

Anyway, since all this has been put it, the central heating pressure drops every couple of days. I know this is the sign of a leak, so I’ve checked all of the central heating pipework I can and there’s no sign. There’s also no obvious air in the system (all radiators hot including the highest one).

I’d topped it up using the filling loop on a couple of occasions, but the first we noticed it really being a problem was when the pump was ‘grinding’ as seemingly pumping dry. Once I topped up the pressure, it was fine.

Now, I’m fairly sure I know the answer to this, but this isn’t normal, is it? My limited understanding of central heating is supposed to be a closed system, so unless there’s a massive leak (which I don’t think there is), the water is escaping somewhere it shouldn’t.

Any ideas how this could happen?

The bathroom fitter and plumber have been less than receptive to our follow-up queries (for example, we had a water softener fitted at the same time, and on testing the water, none of it was being softened – it turns out the fitter had installed it without actually plumbing it into the system, and only when I refused to pay for it would he come back and rectify it) so I expect the brush off to begin with – any tests/things I can do to ‘prove’ an issue?

It could simply be the air from the fresh water that has been added when it has been drained and refilled being driven off via the auto air vent, and probably collecting in a few high spots.

Main place to check would be for a discharge from the safety valve though if you’ve no signs of a leak. If a magnetic filter has been added then that is a great place for the air to collect too.

We had a complete new system installed, with a new Worcester Bosch boiler.
After about four months of trouble free use, we started getting exactly the symptoms you describe. I was having to top up the water every day in the end. Searched everywhere for a leak, nothing obvious.
To cut a long story shorter, our plumber diagnosed a leaking heat exchanger, the water was going out through the condensate trap. Phoned Worcester Bosch., was told flatly by the engineer that that couldn’t be the problem, they came out anyway and…it was the heat exchanger. Changed under warranty. The fan failed five months later so within a year two of the major components had been replaced.
It did take about two weeks to get all of the air out of the system after the heat exchanger was replaced, in our system this was collecting in a heated towel rail as the highest point in the loop.

First thing I’d check would be the pressure relief pipe, which should be poking out of an exterior wall somewhere close to the boiler. If there are any signs of water dripping from there it is very likely the expansion tank is goosed. About £40 for a replacement to fix it yourself or a couple of hours labour for a plumber inc. draining and refilling the system.

Or the expansion vessel wasn’t properly pressurised when the system was filled. With no pressure in the system the expansion vessel should be about 2 bar. Could also be a split bladder in the expansion vessel. The expansion vessel should have a standard Schrader valve, if water comes out when the centre pin is pressed the bladder has failed, if not try pumping up to 2 bar with a bike pump with no pressure in the system, then pressurise the system to the correct pressure, probably about 2 bar.

What Stumpyjon says. Mine was doing the same as he describes, but it also had a corroded auto bleed valve that wasn’t bleeding.

We had the same problem and it was the expansion vessel

Tap it- should sound hollow. Ours sounded dull as it was filling with water

The two valves are for two different circuits (guessing you might have a lounge thermostat and a rest of house thermostat?)

With no pressure in the system the expansion vessel should be about 2 bar.

That much? Thought it was normally about 1 bar?

Read your installers manual first in any case, should have the correct pressure.

Your two valves will be for the heating and HW circuits.

1 bar is about normal for the air side of the expansion vessel, when measured when the liquid side of the system is sitting at 0 bar. Your boiler PRV will be set around 3 bar so the higher you pump up the expansion vessel the less of a pressure “buffer” you’ll have to cope with thermal expansion when the system is in use. 2 bar is too much. If it does need replacing it should already be at the correct pressure when you buy it.

One of the motorised valves will be on the central heating loop, the other will be on the supply to the heating coil inside the hot water cylinder.

Thanks all, certainly things to think about. I’ll take a look at the expansion vessel, though it should be fine as it was brand new.

The two valves are for two different circuits (guessing you might have a lounge thermostat and a rest of house thermostat?)

Nope – no thermostat at all, only TRVs.

Will just be for HW and CH circuits then, so you can have them on independently.

I have the same problem with a gas combi boiler. In the last 12 months it’s constantly lost pressure. Sometimes it takes 2 months to lose it or 2 days. Cannot see any evidence of a leak.
It’s in a rental property,which is 2 hours away from me,so the agent keeps sending round plumbers who just top it up at £40 a time. So far it’s been topped up 7 times.Every time they go and top it up I ask them to investigate it and the last one said to the tenant ” it’s normal,think of it as a tyre going down”.
The flat is only small with just 3 radiators so could it be getting just too hot. The tenant said the temperature controls on the boiler are both at maximum. It’s not old,only just over 2 years.

It will lose pressure when central heating is running not hot water which may explain the losing pressure in 2 days or 2 months. As a rule pressurise a combi expansion vessel to 0.75 bar. Water coming out of the Scrader valve doesn’t mean its always gone. This can be condensation that has collected in there. Lots if vessels are changed unnecessarily because of this.

I was losing pressure over time in my combi system and suggestion was pressure relief valve. Realised it was doing it when boiler hits max temp and cuts out, then there’s water dripping out. I turned down the central heating water temp on the boiler and problem solved.

Used to turn it up thinking I get warmer house but doesn’t work like that. It’s just based on the temperature of the return water pipe I believe on my boiler (as that’s where the sensor is). With thermostat in the house set the same as normal and boiler temp lower (more around what the manual recommends) it comes on more often but for shorter periods while heating to thermostat temp and temp is still maintained right and I don’t lose pressure. Also might be why the temp sensor kept failing. So far it’s working fine.