How a Pulsacoil Works

What is inside a Pulsacoil unit

Open the front panel of a Pulsacoil and it looks nothing like the copper cylinder in a house. There is a tall insulated vessel, a small plastic cistern sitting on top of it, a vent pipe, two immersion heaters screwed into the side, a small circulating pump, a compact stainless plate heat exchanger about the size of a thick paperback on most models, and a control board. All of it is factory assembled into one white box, which is exactly why it fits in a cupboard in a flat.

The vessel holds the store water. It is filled once, kept topped up by the cistern above it through a float valve, and open to the atmosphere through the vent pipe. It is not under pressure, it is not drinking water, and its only job is to hold heat.

Because the store is open vented, a Pulsacoil has no tundish, no expansion vessel and no pressure relief valve discharging outside, and it does not fall under the unvented cylinder rules. People told their flat has an unvented system are often wrong on that, and it changes which faults are possible. If you are trying to work out what you actually have in the cupboard, the difference between an unvented and a vented cylinder is the clearest place to start, because a thermal store is neither of them.

• The store vessel, holding heated water that never reaches a tap.
• An integral cold water cistern on top with a float valve, keeping the store topped up and providing the open vent.
• Two immersion heaters, a larger one for off peak heating and a smaller boost element for daytime top ups.
• A circulating pump that moves store water across the heat exchanger when you draw hot water.
• A plate heat exchanger where mains cold water meets the heat from the store. Earlier models used a coil inside the store instead of an external plate.
• A flow switch on the cold mains inlet that tells the controls somebody has opened a tap.
• A control board, and on some models a thermostatic blending valve on the outlet to limit the delivery temperature.

How the Pulsacoil store gets hot

Almost every Pulsacoil is heated by electricity alone. The larger immersion element sits low in the vessel and heats the whole store, wired to run on the off peak rate through an Economy 7 meter or a landlord controlled time switch. By morning you have a tank of heat that should carry the flat through the day.

The smaller boost element sits higher up and heats only the top portion of the store. That is deliberate. If the store runs down in the evening, boosting only has to reheat the top of the vessel to give you a bath, rather than paying peak rate to heat the whole thing. On most installations the boost is a switch on the front of the unit or on a wall plate nearby, and on some it is on a timer.

A thermal store runs hotter than a conventional cylinder. Where a cylinder is set to sixty degrees, a Pulsacoil store is normally held in the seventies or low eighties, because the hotter the store, the more usable hot water comes out of a fixed volume of tank. That is also why the outlet usually carries a blending valve and why scale is such a problem in a hard water area like London. An immersion heater not working is the single most common reason a store never charges overnight, and on these units the larger element is the one to suspect first.

Each immersion heater has a thermostat and, behind it, a manual reset overheat cut out. If the unit has tripped its overheat stat there is a reason for it, normally a failed thermostat or a scaled element. Resetting it repeatedly without finding the cause is how a store gets boiled and how the cistern on top ends up overflowing.

How hot water reaches your tap

This is the part that makes a Pulsacoil different from everything else in a cupboard. Nothing happens until you open a hot tap. When you do, cold mains water starts to flow into the unit and passes a flow switch. The flow switch closes, the control board starts the pump, and the pump circulates hot water out of the store, through one side of the plate heat exchanger and back into the store.

On the other side of that same heat exchanger, your incoming mains water is travelling in the opposite direction, separated by thin stainless plates. Heat crosses the plates. Cold mains water goes in at one end and hot water comes out at the other, a few seconds after you opened the tap. The store water and the tap water never mix and never touch.

Two things follow, and both come up constantly. The hot water at your tap is at mains pressure, because it is mains water, so hot and cold are balanced. And a Pulsacoil cannot improve a poor mains supply. If the cold flow into the flat is weak, the hot will be weak too, and no work on the unit changes that.

It also means there are moving and electrical parts between the store and your tap. A Pulsacoil needs a flow switch to notice you, a board to respond and a pump to run, so if any one of those fails the store can be full of heat while the tap runs cold.

Why the hot water runs out in the evening

A thermal store holds a fixed amount of heat and gives it away until it is gone. There is no boiler behind it firing on demand. Once the store has cooled below the point where the heat exchanger can lift mains water to a usable temperature, the hot tap goes lukewarm and then cold, and it will not recover on its own until the next off peak period unless you press boost.

What gets described as the unit failing is very often the unit working as designed while being asked for more than one night of heating covers. The tell is the pattern. A good quantity of genuinely hot water that then fades means the unit is heating and delivering correctly and the problem is capacity or timing. Never properly hot from the first minute is a fault. Water not getting hot at any tap, at any time of day, is a different symptom again and has its own order of checks, starting with the electrical supply rather than with the store.

The timing side is worth checking before anything mechanical. The off peak period is set by the meter or the time switch, not by the unit, and those change. If a meter has been swapped, if the hours have moved, or if a timer has lost its setting after a power cut, the store will simply never be charged overnight and everything else will look fine.

• Hot at first, then fading to cold: the store is being emptied. Look at usage, the off peak timing and whether the main element is still working.
• Never more than lukewarm, even first thing: suspect the heat exchanger, the pump or a scaled element rather than capacity.
• Stone cold at every tap, all day: normally electrical. Supply, controls, elements or the board.

How to tell which Pulsacoil model you have

Gledhill made several generations under the Pulsacoil name and they do not share parts or controls, so identifying yours matters before anybody orders anything. The model name is on the badge, usually on the front panel or inside the door.

The earlier units, including the Pulsacoil 2000 and the Pulsacoil III, use a mechanical flow switch and a control board that switches the pump and the elements directly. They are the ones most often found in older blocks and the ones where spares availability has become the real question rather than whether the fault is repairable.

The later A Class and Eco units moved to sensors rather than simple thermostats and behave differently when they fault. A unit flashing a code or refusing to run the pump is saying something specific, and what it means depends on the generation. The BP versions are fed by a separate or communal heat source rather than immersion elements, so on those the question moves to what is supplying the heat. Where a model is genuinely beyond spares, the conversation turns into a replacement, and a hot water cylinder replacement in a flat is decided by what the building can supply rather than by preference.

Common Pulsacoil faults and what each symptom means

The failure list is short and fairly predictable, which is the advantage of the design. An engineer works through it in roughly this order, cheapest and quickest first. Our page on Pulsacoil repair in London covers what each of those visits involves and what is useful to have ready before an engineer attends.

Scale deserves a separate mention in London. The store runs hot, the water is hard, and scale forms on the hottest surfaces it can find. On an immersion element it forms a crust that stops the element shedding heat into the water, so the element runs hot and eventually fails. Inside the plate heat exchanger it coats the plates and narrows the waterways, so heat stops crossing and the flow drops. Performance declining slowly over a year or two, with nothing obviously broken, is very often a scaled exchanger. London hard water is why that happens here faster than elsewhere, and it works on the elements and the exchanger at the same time, so a unit that has lost performance has usually lost it in two places at once. An immersion heater repair on a thermal store is therefore worth doing only once the exchanger has been assessed as well, because renewing one scaled component and leaving the other returns the same complaint within the year.

• No power, a tripped breaker or a blown fuse in the spur. Always checked first because it costs nothing.
• Immersion element failed or scaled. A common wear item in hard water, and the reason a store never charges overnight.
• Immersion thermostat failed, or the overheat cut out tripped and needing investigation rather than simply resetting.
• Flow switch stuck. The store is hot, the tap is cold, and the pump never starts because nothing told it to.
• Pump seized or failed. Same symptom as a flow switch fault, different cause, and the reason a hot store delivers cold water.
• Plate heat exchanger scaled. Weak, lukewarm hot water and reduced flow that got worse slowly.
• Control board failed. Often the point at which model and spares availability decide the outcome.
• Float valve in the top cistern stuck, causing an overflow that looks like the unit is leaking when it is not.

Water running from an overflow pipe outside, or pooling under the unit, needs attention rather than watching. Turn the hot water off at the isolating valve and get it looked at. A thermal store holds a large volume of very hot water and a flat above other flats is the worst place to let that develop.

What you can check yourself before calling anyone

These are worth five minutes, because they account for a fair share of the calls where nothing is actually broken. None involve opening anything electrical or removing a screwed panel.

• Check the fused spur or breaker feeding the unit, and check whether the off peak circuit is on a separate breaker that may have tripped on its own.
• Press the boost switch and wait. If you have hot water within the hour, the store was simply empty and the question is about timing and usage, not a fault.
• Look at the time switch or the meter and confirm the off peak hours are what you think they are. Power cuts and meter swaps quietly change them.
• Run the hot tap for a full minute. Some of the answer is in whether it is hot then fades, or never gets hot at all.
• Listen at the unit while the hot tap runs. You should hear the pump start within a few seconds. Silence is a useful piece of information to pass on.
• Check the cold mains flow at the kitchen tap. Weak cold means weak hot, and that is a supply question rather than a Pulsacoil question.

If none of those change anything, the fault is inside the unit and needs somebody qualified with the covers off. Beyond that point you are dealing with high current immersion circuits and a vessel of water hot enough to scald.