Hot-water skid from the rear. The blue buffer tank stands on its own legs with a service valve at the base, and pumps, dirt separators and plate exchangers behind

Skid, front left — Inzonex Modular Insulation, hot-water skid

Hot water / Vessels

Vessels in a plant room

Two blue tanks in a plant room, and they do opposite jobs.

A buffer vessel adds water to the circuit. An expansion vessel gives water somewhere to go. They look similar, they are often the same colour, and confusing them produces a system that either short-cycles or lifts its relief valve. The vessel on this skid is a buffer tank and it is not insulated. It carries no figure anywhere on this site for that reason.

Buffer

Volume, so the plant can run properly

What it is for

Heat-generating plant has a minimum sensible run time. A circuit holding less water than the boiler or heat pump needs reaches setpoint in a couple of minutes, stops, and starts again. Short cycling wears plant out and wrecks the efficiency it was bought for. A buffer adds the volume the pipework does not have.

Heat pumps especially

A heat pump has a narrow flow temperature and a long minimum run, and it also needs somewhere for defrost energy to come from. Almost every heat-pump question about buffers is really a question about volume and minimum run time.

Two-pipe or four-pipe

Two-pipe puts the buffer in series and every litre passes through it. Four-pipe puts it in parallel, so the buffer only takes the difference between generation and demand. The four-pipe arrangement keeps the return temperature low, which matters to condensing and heat-pump plant.

Expansion

Somewhere for the water to go

Water expands and the pipe does not

Heat a closed system and the water grows. With nowhere to go the pressure rises until the relief valve lifts. A diaphragm expansion vessel holds a gas cushion behind a membrane, and the water pushes into it instead.

Charge pressure is not system pressure

The vessel is pre-charged on the gas side, cold and isolated from the system, to suit the static height of the installation. Setting the charge with the system pressurised measures nothing, and a vessel with the wrong charge behaves as if it were the wrong size.

The service valve exists to be used

A capped valve at the base lets the vessel be isolated and drained so the charge can be checked without emptying the circuit. It is the part that has to stay reachable, and it is routinely the part buried behind something.

What sets the size

The volume of water in the system and how far its temperature moves, against the static height and the relief setting. It is a calculation, not a rule of thumb, and it is redone whenever plant is added to a circuit.

Calorifier

Stored hot water, heated indirectly

What it is

A vessel of hot water heated through a coil carrying the primary, so the stored water and the heating medium never meet. On a commercial circuit it is the answer to a demand that peaks far above what the plant generates continuously.

Against a plate exchanger

A plate unit makes hot water at the moment it is drawn and stores nothing, so it must be sized for the peak. A calorifier stores, so the plant is sized nearer the average. Which is right depends on how spiky the draw-off is, not on which is more modern.

Stratification is the point

A calorifier works because hot sits on cold. Anything that stirs the contents - an oversized return, a badly placed connection - destroys the usable capacity without changing the volume on the nameplate.

For insulation

A vessel is a large surface at one temperature

No gradient to help you

Unlike a pipe run, a vessel is at line temperature across its whole shell and both heads. It is the largest single uninsulated area a plant room usually has.

The connections are the awkward part

Service valve, drain, immersion or coil access, the gauge and the relief connection all have to stay clear. A covering that has to be cut to reach any of them is cut once.

Why this one carries no figure

The blue buffer tank on this skid is not insulated. It is excluded from every total on this site rather than estimated, because a number for something we have not modelled is not a number.

Reference

The two blue tanks

Vessel	What it does	What sets its size	What happens without it
Buffer vessel	Adds volume to the circuit	Minimum run time of the plant, and the demand profile	Short cycling, lost efficiency
Expansion vessel	Absorbs the water that heating creates	System volume, temperature swing, static height	The relief valve lifts
Calorifier	Stores hot water heated through a coil	Peak draw-off against continuous generation	Plant sized for a peak it rarely sees

Vessel · What it does · What sets its size · What happens without it

Asked about vessels in a plant room

Questions.

Answers distinguish modelled heat-loss estimates from equipment selection and service guidance.

What is the difference between a buffer vessel and an expansion vessel?

They do opposite jobs. A buffer vessel adds water volume to a circuit so the heat source has enough to work against and does not short cycle. An expansion vessel gives the water somewhere to go as it heats and grows, so the pressure does not rise until the relief valve lifts. One is about volume, the other about pressure.

How do you size a buffer vessel?

From the minimum run time the heat source needs, and the demand profile it is working against. Start from how much water the circuit already holds. Then add enough to keep the plant running for a sensible period, rather than reaching setpoint in two minutes. On a heat pump the defrost requirement comes into it as well.

What pressure should an expansion vessel be charged to?

To suit the static height of the installation, set cold with the vessel isolated from the system. Checking or setting the charge while the system is pressurised measures the system, not the vessel. A vessel with the wrong charge behaves exactly like one of the wrong size.

What is a calorifier?

A vessel of stored hot water heated indirectly - the primary passes through a coil and never mixes with the stored water. It lets the heat source be sized nearer the average demand rather than the peak, because the store covers the peak.

Should I use a calorifier or a plate heat exchanger?

A plate exchanger generates at the moment of draw-off and stores nothing, so it has to be sized for the peak. A calorifier stores, so the plant can be smaller. The spikier the draw-off profile, the more the store earns its space.

On this skid