Blocked Plate Heat Exchanger: Symptoms and the Fix

What the plate heat exchanger does

A combi boiler has two heat exchangers. The main one heats the water that goes round your radiators. The second, smaller one is the plate heat exchanger, a block of thin stamped metal plates stacked together. When you open a hot tap, hot water from the boiler runs through one set of channels between the plates, and your cold mains water runs through the other set on its way to the tap. The heat passes across the plates, and the mains water leaves hot.

The channels are narrow, which is what makes the part efficient and also what makes it easy to block. If you want the full picture of how the two circuits fit together, our guide on how does a combi boiler work goes through it. For how this part compares with the other heat exchangers in your boiler and cylinder, see what is a heat exchanger.

Signs your plate heat exchanger is blocked

The failure creeps up rather than arriving overnight, which is why people often live with it for a winter or two. Look for a pattern like this:

• Heating works normally, but the hot water never gets properly hot
• Hot water starts hot, goes cold, then comes back, while the tap is still running
• The hot tap runs noticeably weaker than the cold tap next to it
• The shower has got gradually worse over months rather than failing suddenly
• The boiler fires, cuts out and relights repeatedly while a hot tap is open
• Banging, crackling or kettling noises from the boiler when you run hot water, but not when only the heating is on
• Turning the hot water temperature up on the boiler makes little difference at the tap

Scale on one side, sludge on the other

Which side of the plate is blocked changes what you notice. On the mains side, the water coming into your home carries dissolved calcium. Heat makes it drop out as limescale, and the plates are the hottest surface it meets. Most of London is a hard water area, so this is the common one here. Scale narrows the channels, so the hot flow at the taps drops, and it insulates the metal, so less heat gets across. Our page on hard water and hot water cylinders in London explains why the problem is worse here than in most of the country.

On the boiler side, the heating water can carry black sludge, which is iron oxide from corroding radiators, plus bits of debris. If that clogs the primary channels, the mains flow at the tap may be fine but the water never gets hot enough, because not enough boiler heat is reaching the plate. The boiler then overheats on its own side and cuts out, which is where the firing and relighting comes from. Sludge in the plate usually means sludge elsewhere in the system too, so the radiators and the filter are worth a look at the same time.

Faults that look the same

Several other combi faults produce lukewarm or cycling hot water, and they are fixed in completely different ways. That is why this is not a part to change on a hunch.

• Diverter valve sticking or letting by: the hot water is lukewarm and the radiators warm up when a hot tap runs, even with the heating off. Our page on the diverter valve replacement cost covers that repair.
• Flow sensor fouled with scale: the boiler is slow to notice the tap, or does not fire at all on a low flow.
• Blocked filter on the cold inlet to the boiler: the hot flow is weak, but the water that does come through is hot.
• A thermostatic shower valve or tap cartridge: only one outlet misbehaves and every other hot tap is fine. Our page on no hot water in the shower covers that case.
• Low mains pressure into the property: the cold tap is weak as well as the hot.

Checks you can make without opening the boiler

Run the kitchen hot tap fully open for a couple of minutes, then the cold, and compare the flow. If the cold is strong and the hot is clearly weaker, the restriction is on the hot side, which points at the plate, the inlet filter or the pipework to the boiler. If both are weak, the problem is the supply, and our guide to low hot water pressure is the better place to start.

Next, try every hot tap in turn. If they are all lukewarm or all weak, the fault is in the boiler. If one is fine and another is not, it is the fixture. Then, with the heating switched off, run a hot tap for five minutes and feel a radiator. If it warms up, suspect the diverter valve rather than the plate. Finally, listen at the boiler while the tap runs. A burner that keeps lighting and going out, with a rumble or a crackle between, is typical of a plate that cannot shed its heat.

The plate heat exchanger is inside the sealed case of a gas appliance. Do not take the front off the boiler. Any work inside it should be done by a Gas Safe registered engineer, and you can check how to check a plumber on the Gas Safe Register before anyone starts.

Descale or replace?

A plate that is lightly scaled can be descaled. The engineer isolates and drains the boiler, removes the plate or connects flushing equipment to it, and circulates a descaling solution until the channels clear, then rinses it, refits it and refills the boiler. Where it works, the hot water comes back to full flow and temperature.

A plate that is badly furred is usually better replaced. Descaling can open up enough of the channels to improve the flow without cleaning the surfaces that do the heating, and a plate that has been overheating repeatedly may also have started to leak internally. Replacement plates are available for most current boilers, and on many models the plate is held by a couple of screws and seals and can be changed without taking much else apart. On a cramped or older boiler it takes longer, and on some discontinued models the part is hard to get.

Either way, the boiler is drained and refilled, so you will be without hot water and heating for part of the visit. Our hourly rate and call-out are on the pricing page. The total depends on whether the plate is descaled or replaced, the part for your make and model, and how easy the boiler is to work on.

Stopping it happening again

A new plate in the same hard water will scale up again. Guidance installers work to calls for scale protection on the cold feed to a combi in hard water areas, and a lot of boilers in London were fitted without it, or with a reducer whose cartridge has not been changed in years. Fitting or servicing one is cheap next to a second plate.

Setting the combi hot water temperature to what you actually use, rather than the maximum, also helps, because scale forms faster at higher temperatures. On the heating side, a magnetic filter and inhibitor keep sludge out of the plate and the rest of the boiler. If the boiler is old and this is one of several recent repairs, it is worth weighing up a boiler replacement rather than paying for the same kind of fault again.

Getting it sorted

If your combi hot water has faded, fluctuates or runs weak while the heating is fine, call us on 020 4652 5111 or send an enquiry. Tell us the make and model from the front of the boiler, whether the hot flow is weaker than the cold, and whether the radiators warm up when a hot tap runs. That tells us a lot before we arrive.

Hot water faults are our core work and we have 20 engineers across London. A proper boiler diagnosis tests the plate, the diverter valve and the flow sensor in turn and tells you which one has failed before any part is ordered. The full list of causes for a combi that has stopped giving hot water altogether is on our page on no hot water from a combi boiler.