Can a combi boiler diverter valve leak central heating water into domestic hot water?

15 Mar 2021
Reaction score
United Kingdom
Hi there. [I'm new to combi boiler problems, so I might not be explaining myself correctly.]

Over the past three weeks I've started losing central heating pressure. Starting from 2.0 bar, it drops to 0.5 bar over 24 hours.

I've checked the radiators and all seems okay. I know that everyone will say I've a leak, but I'm desperate to rule out all other causes before I have to rip up the hardwood flooring.

I've had a bag tied tightly to the PRV outlet pipe outside for two weeks and no water has collected.

To see if the domestic hot water was reducing central heating pressure, I tried the following:

Boiler: Ideal Logic Combi 30
Preheat: off
Tap Temperature: 2/3 anticlockwise
Radiator Temperature: e=economy
Function: Tap only

1. Central heating gauge = 0.6 bar
2. Run hot tap for 30 seconds, central heating gauge immediately jumps to 0.95 bar
3. After tap has closed, boiler rumbles for 60 seconds, then central heating gauge drops back to 0.6 bar
4. Steps 1 to 3 repeated 10 times, pressure always returns to 0.6 bar

Although the central heating pressure didn't drop any further, is it normal for the central heating gauge to jump to 0.95 bar when Function is set to Tap only?

I thought the central heating and domestic hot water were separate circuits, so it seems strange the pressure jumps at all.

Looking at the combi boiler instructions, this model only has a single heat exchanger so a diverter valve is needed to keep the water circuits separate.

If my steps aren't telling me anything, is there any other way I can rule out problems with the diverter valve or a leak in the exchanger?

Sponsored Links
Your boiler has two heat exchangers, one for central heating and a secondary one to generate hot water. That's not the issue though, the diverter can't allow central heating water to pass into the hot water, and if the secondary heat exchanger had failed you'd most likely see a rise in heating pressure, not a fall.

Given the model of your boiler, the most likely cause is a failed main heat exchanger allowing central heating water to drain away out of your condensate. It's a common problem with this boiler. If it's out of warranty, call Ideal direct to see if they'll offer a fixed price repair on it. If not, expect to pay an independent heating engineer somewhere in the region of £600 to repair it
Thanks for that.

I'm covered by a British Gas service contract. However, when I called them out about the lack of pressure, the engineer just topped up the pressure, had a feel of the radiators and said I had a leak somewhere.

Is there any non-specialist test I can do to show that the heat exchanger has a problem? Or is there any specialist test that British Gas can do?

Lifting the floor will be costly and it would be good to rule out everything beforehand.

You can turn off the boiler, disconnect the condensate drain and leave it running into a bucket. If it's still dripping after an hour the heat exchanger is knackered
Sponsored Links
Okay, it took me a while to get round to disconnecting the condensate, but I've now done it.

I could not see any leak.

I then thought that it might only leak when working hot. So, I depressurised the system, bled out 3 or 4 litres from the drain valve, added 100ml of eau de toilette (****, "him", £13 from Tesco) through a radiator bleed valve, then repressurised the system (making sure radiators have been bled).

Two minutes after switching on the central heating system, the outside vent is pumping out distinct perfume smells! Also, there's absolutely no smell of perfume inside the house.

So, am I right in thinking this means I've not got a leak in my pipework/radiators, but the heat exchanger has a fault when working at temperature?
Yep sounds like it. Interesting diagnosis method, might try that as said before, these boilers heat exchangers have a habit of falling in this manner so pretty good odds that that's the problem
[I've just realised the forum must have a clever spell checker as it's used **** for the trademark used by French Connection UK Ltd!]

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local