Unvented Cylinders in Barnstaple

Knowing what's actually going wrong is half the job. Here's what I see most on Barnstaple call-outs.

Discharge pipe dripping or flowing at the tundish. The tundish is that little funnel on the discharge pipe, usually on the wall near the cylinder. It's there so you can see whether the safety valves are letting go — an air gap that stops back-siphonage and tells you at a glance what's happening. Water dripping or running steadily from it means the system is discharging through either the pressure relief valve (PRV) or the temperature and pressure relief valve (T&P valve).

The PRV lets go on excess pressure, usually because the expansion vessel can't take up thermal expansion any more. Heat water in a sealed cylinder and it expands. The expansion vessel, with its pressurised air charge normally set to 3.0 bar, is what soaks that up. If the membrane has failed or the charge has gone, the vessel waterlogs and soaks up nothing. Pressure spikes on every heating cycle, the PRV lifts, and out it comes through the tundish. That's the number one cause of a dripping discharge pipe.

The T&P valve lets go on excess temperature (above 90–100°C) or excess pressure. If it's the T&P discharging rather than the PRV, that usually points at a thermostat or overheat thermostat failure — the cylinder is overheating rather than over-pressurising.

No hot water. Could be a dead immersion heater, a failed thermostat, a motorised valve stuck shut, or something wrong with the cold supply. Each one has its own route to the answer.

Temperature fluctuations or intermittent hot water. Thermostats drift out of calibration on older cylinders. The dual-stat on an immersion heater puts the operating thermostat and the overheat cutout (high-limit cutout) in one unit, so if either trips you lose heat. Heavy scale on the element also drags down heat transfer, which shows up as slow recovery and temperatures that won't sit still.

Slow hot water recovery. Scale on the immersion heater element is the likeliest cause in Barnstaple, though the South West Water supply here is fairly soft. It can also be a cylinder that was undersized for how much hot water the house actually gets through.

Rumbling or banging noises. Scale on the element makes a noise very like boiler kettling: the element overheats in one spot, vapour forms, and you get that rumble. Thermal expansion noises as the cylinder heats and cools are common too.

Cylinder leaking. Corrosion round fittings, a PRV seat that weeps instead of discharging cleanly, or fittings that have worked loose. If the cylinder body itself is leaking, it needs replacing.

Every unvented cylinder wants servicing once a year by a G3-qualified engineer. That's the recommendation under Building Regulations Part G3, and most manufacturers' warranties require it. Some — Megaflo among them — say outright that annual G3 servicing is what keeps the warranty alive.

Here's what I go through on a Barnstaple annual service.

Expansion vessel pre-charge pressure. I test the charge with a pressure gauge. On most domestic cylinders you're looking for 3.0 bar cold, with the vessel off the system. Low, I recharge it. Membrane failed and the vessel waterlogged, I replace it.

Tundish and discharge pipework. I check the tundish is clear, in the right place and visible for inspection, and that the discharge pipe falls continuously as it should. A standard cylinder wants 22mm copper; bigger cylinders or longer runs need 28mm. The pipe has to end somewhere safe outside — not over a path, not by a door, not at face height.

PRV test. I lift the PRV by hand to see it opens and reseats properly. One that won't reopen, or that weeps afterwards, gets replaced.

T&P relief valve test. Same manual lift. The T&P is your last line of defence against a dangerously hot cylinder, so it has to be proved working.

Thermostat calibration. I check the setpoint against an independent thermometer. Cylinders need to run at 60°C to kill Legionella. A thermostat that's drifted low is a health risk; one that's drifted high wastes energy and wears components out faster.

Overheat thermostat (high-limit cutout). This is the safety device on the immersion heater that doesn't reset itself. I check it's working and hasn't already tripped.

Anode inspection. Barnstaple's fairly soft South West Water supply means less scale than in hard water areas, but the anode still matters. Stainless steel cylinders use either magnesium sacrificial anodes or titanium impressed-current ones. A magnesium anode more than half gone needs replacing, because it's what protects the cylinder body from galvanic corrosion.

Cold water inlet group. I check the lot: the check valve, the pressure reducing valve (usually set to 3.0 bar), the expansion relief valve and the strainer. A blocked strainer chokes the cold feed and ruins hot water performance.

Motorised valve operation. Where a motorised valve runs the hot water zone, I make sure it opens and closes as it should.

Cold water supply pressure measurement. I measure the incoming mains to confirm it sits inside the cylinder's operating range.

A vented cylinder depends on a cold water tank in the loft, and your pressure comes from how high that tank sits above the tap. In a typical house that's 0.1–0.3 bar at the shower head, which is why gravity-fed systems usually end up with a pump bolted on.

An unvented cylinder feeds straight off the mains. At 2–3 bar working pressure, every outlet in the house gets proper pressure without a pump, and two showers running at once still behave themselves.

The trade-off is cost up front and who's allowed to touch it. Vented cylinders are simple, and any competent plumber can work on one. Unvented cost more to install and need G3-qualified servicing. For most houses with more than one bathroom, the difference in performance is worth it.

There are times when vented still makes more sense. If your incoming mains pressure is below about 1.5 bar, an unvented system won't give you what you're hoping for, and you'll have spent the money for nothing. I'll measure the mains before I recommend switching anyone over.

Megaflo (Heatrae Sadia): the market leader in the UK and all over North Devon. Parts are easy to get, the factory-fitted valve set is dependable, and the stainless steel construction lasts. Megaflo components are readily available, and I know the range well.

Gledhill — one of the most common brands in existing installations. The Stainless Lite Pro and Torrent ranges are well made. On older Gledhills I check the plastic tundish for UV damage and go over the combined T&P valve carefully.

OSO: Scandinavian-built, good cylinders, less common here than the big domestic names. I've worked on plenty and I carry the service documentation for them.

Telford: a popular choice, with the Tempest and Hurricane ranges specified on a lot of new builds. Reliable units. On the larger Telfords I pay attention to discharge pipe sizing, to be sure 28mm kit is fitted where it should be.

Range Tribune: a UK brand fitted widely in social housing and private homes alike, so it comes up on service calls often. On older Range Tribune units the dual-stat and immersion heater thermostat wiring are worth a careful look.

Joule: turning up more and more on new builds, stainless steel, well made. Newer to the UK market but taking a bigger share every year.

Under Building Regulations Part G3, a new unvented hot water cylinder has to be notified to Building Control. That applies whether it's a straight like-for-like replacement or a brand new installation.

Being G3-qualified, I can self-certify under the Competent Person Scheme. That means I notify Building Control for you and give you a compliance certificate saying the installation meets Approved Document G.

That certificate is worth having. When you come to sell, your solicitor will want building regulations paperwork for any notifiable work. An unvented cylinder with no compliance certificate becomes a problem: the buyer's solicitor spots it, and you're into retrospective approval or a Building Control inspection. Much easier to get it right on the day.

Servicing doesn't need notifying. Only new installations and major component changes.

You'll get an answer. Ring or message during the working day and you'll hear back the same day; evenings and weekends, it'll be the next working day. And if it isn't a job for me, I'll tell you that and point you at someone who can help, rather than going quiet on you.

When you book, I'll confirm it and give you a two-hour window rather than a whole day sat at home waiting. On the morning you get a text with a tracking link, so you can see where I've got to instead of wondering. Traffic and the job before yours are real, so it's a window rather than a promise to the minute, but if it slips you'll hear it from me first.

I work across Barnstaple and North Devon, with EX31 and EX32 as home territory. Annual service, fault diagnosis or a new installation — call 01271 444410 or email info@tarkaboilersbarnstaple.co.uk.

G3-qualified. Gas Safe registered. Same-day response available for urgent faults.