New Boiler Losing Pressure? Here’s What’s Actually Going On

If your boiler’s just been installed and it keeps losing pressure within the first few days, the cause is almost always the same: there’s a leak somewhere on the heating side of the system — and the fix is to call back whoever installed it.

I’d be confident that the installer will come back to trace it. It’s in their interest to — word about a bad job travels fast these days.

Why This Happens With New Boiler Installs Specifically

It’s more common than people expect right after a new boiler goes in, and there’s a straightforward reason for it.

Many older vented central heating systems worked perfectly well at a lower pressure. Today’s modern boilers are designed to run at a higher one.

When a new, higher-pressure boiler gets connected onto that old system pipework and radiators, joints and fittings that were fine under the old, gentler pressure can suddenly start weeping under the new one. The system itself hasn’t changed — but the pressure running through it has, and that’s often enough to expose a weak joint that was never a problem before.

A Story From The Job: Steve’s Boiler

I had a customer once — a really nice guy, very friendly — who wanted an estimate to replace his boiler and convert the system to a combi. A friend of his had recommended me.

With every job that involved replacing a boiler, I’d take a good look at the whole system first, just to get an idea of what I’d be up against — no two jobs are ever the same.

One thing I always put in the estimate, without fail: when converting from a low-pressure system, there’s a possibility of leaks occurring. Partly it was honesty, partly it was a get-out clause if the customer later wanted to take things further over a leak. I’d explain the same thing in person, and I’d build a small allowance into the price to cover it. If nothing went wrong, the customer got their first service free.

If I judged the existing system to be older than 15 years, I’d recommend against pressurising it at all — a view one of the trainers at a well-known boiler brand told me he shared entirely: 15 years or older, think about replacing the lot.

Steve said he understood completely and would come back to me once he’d seen the estimate.

About three weeks later — this was back when estimates went out by Royal Mail or hand delivery, not email — I had a voicemail from him asking me to call. I assumed he was ready to book the job in.

When I called, it turned out he’d gone with a competitor instead — cheaper — and wanted my advice because he was having problems with the new boiler.

I called round the next evening after work. He explained he’d found me a bit “over the top” about leaks during the estimate, whereas the other installer hadn’t mentioned them at all — told him straight out, “you don’t get leaks.” So he went with the cheaper option.

The other installer had been back twice already and refused to come again without being paid to find and fix the leak. Steve looked at me and admitted he should have believed what I’d told him — face to face and in writing on the estimate. That was exactly why he wanted me, not the other installer, to do the repair: he trusted what I’d put in black and white.

I went back two days later and found two causes: an old AAV (automatic air release valve), and a joint where the installer had connected old copper pipe to new copper using an imperial-to-metric fitting.

After that repair, I ended up servicing Steve’s boiler — and doing other work for him — for the next 20 years.

Where To Actually Look For The Leak

If there’s no visible leak around the boiler itself, it’s almost certainly somewhere on the radiator side of the system — a joint, a valve, or a connection point somewhere in the pipework.

Worth checking first: TRVs, pipe joints (especially under floors), and radiator valve connections.

What Normal Pressure Looks Like Day To Day

• Cold, the system should typically sit around 1 to 1.2 bar.
• As the water heats up it expands, and the pressure rises a little — that’s normal, not a fault.
• The safety relief valve is there to open if pressure ever climbs above roughly 3.5 bar, which it shouldn’t reach under normal conditions.

What To Actually Do About It

If your boiler’s only a couple of days old, this is a job for whoever installed it — not something to chase yourself. A new installation should be leak-free, and a competent installer will be able to find the source of the pressure loss and put it right. That’s part of what you paid for.