Boiler showing heat demand; could a leak in the pipes cause this?

theonenonly

Vaillant Ecotec Sustain boiler; half a year ago had its heat exchanger replaced by British Gas cover.

Last 2 weeks I see the boiler firing up to heat water as if the taps are running, and then switching off after 10 seconds, and then coming back on after a minute for another 10 seconds etc.

Had BG come and check it, the nice bloke reckons it's a leak in my plumbing causing the demand for the hot water. He turned off the water going into the boiler, turned it back on, and there was no heat demand. Surely if there was a leak, there would be demand? Anyhow, started turning on taps around the house, and the usual heat demand but nothing from the earlier issue. About 10 minutes later, the boiler fires up again.

BG say there's nothing they can do.

I'm dreading the prospect of it being a leak in the pipes, it could be anywhere. Is there a remote possibility it could be something else inside the boiler?

ladruid

It doesn’t sound like a leak in your pipework, and the behaviour you’re describing doesn’t really match a leak anyway.

A leak would cause constant flow through the boiler, so the boiler would keep firing as if a tap was permanently open. What you’ve got is the boiler firing for a few seconds, stopping, then firing again later, that’s an intermittent demand signal, not a continuous one. That points much more towards something inside the boiler giving a false “hot water demand” reading.

On Vaillants, the usual suspects are:

• The hot water flow sensor This is a little turbine that spins when you open a tap. If it sticks or sends dodgy signals, the boiler thinks a tap has been cracked open even when nothing is running. Very common fault, especially after work has been done on the boiler.

• The diverter valve / microswitch If the diverter valve isn’t moving properly or the switch is playing up, the boiler can think there’s hot water demand when there isn’t.

• A temperature sensor reading wrong If the boiler fires, sees the temperature spike too fast, shuts down, then tries again later, that can be a dodgy NTC sensor or a partially blocked plate heat exchanger.

None of these require digging up floors or chasing leaks through the house.

Your own test with the BG engineer actually helps rule out a leak. If there really was a leak, turning the cold feed back on would immediately cause flow and the boiler would fire straight away. The fact it didn’t means the boiler only fires when its own sensor decides to misbehave.

The 10‑minute delay is another giveaway, leaks don’t wait 10 minutes to show up. Faulty sensors do.

If you want to push BG back, ask them specifically to check:

• the DHW flow sensor
• the diverter valve actuator/microswitch
• the DHW NTC sensor

These are all known causes of “ghost firing” on Vaillants.

Bottom line:Yes, there’s a very real chance it’s an internal boiler issue, and no, your symptoms don’t really fit a leak.

FreeBear

A Vaillant Ecotec has a minimum flow rate of ~1.7l/min to trigger the boiler. That quantity of water as a leak would be apparent very quickly. I'd go with one or more possible faults suggested by @ladruid .

theonenonly

Thanks both; another thing I've noticed is that surely, a leak would present itself with low pressure? There's been no noticeable drop in pressure at all.

Gone onto the BG website to book another callout and surprise surprise, no availability until next week!

daveyjp

Downside of BG contract, you need to hit on an engineer who knows one end of a Vaillant from the other. Going direct to Vaillant may be a wiser move.

theonenonly

Plotwist: on the advice of my father, went out to the digital water meter on the pavement and noticed the last digital moving every minute.

So indicating a leak perhaps.

FreeBear

https://forums.moneysavingexpert.com/discussion/comment/81981629#Comment_81981629

If the leak is on the domestic hot water side, and a little more than ~1.7l/min (to trigger the boiler).

https://forums.moneysavingexpert.com/discussion/comment/81981758#Comment_81981758

That kinda adds more weight to a leak after the boiler. You just need to go looking for wet patches on the floor or ceiling (or a badly leaking hot tap). First place to check is the kitchen tap and pipework under the sink. Bathroom/shower would be next.

WIAWSNB

https://forums.moneysavingexpert.com/discussion/comment/81981758#Comment_81981758

The three red digits are litres, I believe, so you are saying that the end one moves a whole digit every minute?

That is (Googles...) roughly a cupful every 15 seconds, so if it's an internal leak above ground floor level, it is surely either draining away safely down a waste or overflow, or will have exposed itself by now in a wall or ceiling.

If the meter movement is consistent every minute, then it should be easy to confirm whether it's internal to the house - close mains stopcock - and on the DHW supply - close mains isolator to the boiler.

Can you absolutely clarify that you've tried this?