Heat Pump Cylinders: Do You Need a New Hot Water Tank?

Why a heat pump needs a different cylinder

A gas boiler sends water round the cylinder coil at a high temperature, well above the temperature the stored water needs to reach. That big temperature difference drives heat into the tank quickly, which is why an ordinary indirect cylinder only needs a fairly small coil.

A heat pump works most efficiently at a much lower flow temperature. The gap between the water in the coil and the water in the tank is small, so heat moves across it slowly. The only way to get a sensible reheat time is more surface area, which means a much longer coil. That is the main difference between a heat pump cylinder and a standard one, and it cannot be added later. The coil is built into the vessel.

Put a heat pump on a standard cylinder and one of two things happens. Either the tank takes a very long time to reheat and runs out during the evening rush, or the heat pump is pushed to a higher temperature to compensate, which costs efficiency, and the immersion picks up the rest at electric prices. Neither is what anyone bought a heat pump for.

• Much larger coil to move heat at a low flow temperature
• Usually unvented, running at mains pressure
• Often a larger capacity than the cylinder it replaces
• Fitted with an immersion for the regular Legionella cycle and as a backup
• Sensor pocket and connections laid out for heat pump controls

Can you keep your existing cylinder?

Check the data label on the side of the cylinder, or search the model number. If it is described as heat pump ready or heat pump compatible, and it is in good condition, it may well be suitable, and the heat pump installer will confirm it against the heat pump output.

If it is an ordinary indirect cylinder, the honest answer is nearly always no. The same goes for an old vented copper cylinder with a loft tank. A twin coil cylinder bought for solar thermal is sometimes asked about too. Its second coil is usually sized for solar panels, not for a heat pump, so do not assume it will work without checking the maker's data.

A thermal store, such as a Gledhill Pulsacoil, is a different kind of appliance. Some heat pump systems use a thermal store deliberately, but you cannot assume the one you already have is suitable. That needs the heat pump designer and the store manufacturer's data, not a guess.

What size heat pump cylinder do you need?

Heat pump cylinders are usually sized a step larger than the boiler cylinder they replace. The reason is recovery time. A heat pump reheats more slowly than a boiler, so the tank has to hold enough to get through the busiest part of the day, and the heat pump then reheats it gently, ideally when electricity is cheapest.

The starting point is the same as any cylinder: the number of people, the number of bathrooms and whether showers run at the same time. Our guide on what size hot water cylinder do i need gives the bands for a boiler system. For a heat pump, most designers go to the upper end of that band or the next size up.

Physical space is where London homes run into trouble. A bigger cylinder has to fit the cupboard, including room above it to pull the immersion out, and a full tank is heavy. A loft or an upper floor airing cupboard in an older house is worth having checked before anyone orders the cylinder.

Legionella and the weekly pasteurisation cycle

Stored hot water should be held at 60 degrees to control Legionella bacteria. A heat pump running efficiently will often store water below that. To cover the gap, heat pump systems normally run a pasteurisation cycle at set intervals, usually weekly, using the immersion or the heat pump itself to lift the whole tank to 60 degrees or above for a period.

Check the cycle is set up and running. Controls that were never commissioned properly, or a failed immersion that the cycle depends on, mean the tank never gets hot enough, and you would not notice from the tap. Our guide on what temperature should a hot water cylinder be explains the setting in more detail.

If your heat pump cylinder depends on the immersion for its Legionella cycle and the immersion has failed, the cycle is not running. Get the element tested rather than leaving it.

Should you buy a heat pump ready cylinder now?

If your current cylinder has failed and a heat pump is a realistic prospect within the next ten years or so, it is worth pricing a heat pump ready cylinder alongside a standard one. A heat pump cylinder works perfectly well on a gas boiler in the meantime. The large coil simply heats the tank quickly on the boiler's higher flow temperature. When the heat pump arrives, the cylinder is already there.

The trade offs are size and cost. The cylinder is physically larger for the same capacity in some ranges, and the vessel costs more than the equivalent standard unit. Whether that is worth paying now depends on how firm the heat pump plan is. Our guide on how to choose a hot water cylinder covers the other choices you will make at the same time, and direct vs indirect hot water cylinder explains the coil options if you are unsure what you have.

Grants, regulations and who can fit it

In England and Wales, the government Boiler Upgrade Scheme gives a grant towards replacing a fossil fuel boiler with a heat pump, and the cylinder is normally part of that installation. The grant goes through the installer, who has to be MCS certified, so the cylinder is usually specified by whoever designs the heat pump system. Check the current rules on the government website, because the scheme terms change.

Because nearly every heat pump cylinder is unvented, the work falls under Part G3 of the Building Regulations. The installer has to hold the unvented hot water qualification, notify the installation, and fit the safety kit properly: the expansion vessel, the pressure reducing valve, the temperature and pressure relief valve and the tundish with a correctly run discharge pipe. The same qualification applies to anyone who services or repairs it later. A G3 qualified plumber can do that work whichever brand of heat pump sits outside.

Common problems with heat pump cylinders

Most of what goes wrong is the same as any unvented cylinder: a dripping tundish, a relief valve letting by, an expansion vessel that has lost its charge, or an immersion that has failed. Some problems are specific to heat pumps. Hot water that runs out early often comes down to a control set too low, a sensor not seated properly in its pocket, or the tank being heated at the wrong times rather than a fault with the cylinder itself.

If you have a heat pump cylinder that is not delivering, or you are replacing a cylinder now and want to know whether to go heat pump ready, call 020 4652 5111. Tell us the make and model of the cylinder and what it is doing. For unvented cylinder repair, the cylinder side is our work. Anything inside the heat pump itself needs the heat pump installer.