Hydro stove and pellet boiler: the basic hydraulic layout

The principle

A hydro stove or a pellet boiler heats water that feeds radiators, underfloor heating or a cylinder. The difference from a gas boiler: the appliance does not modulate as fast and does not like short cycles. Hence the buffer tank.

The parts of the circuit

• Buffer tank. Count 20 to 30 litres per kW of appliance output, so 300 to 500 litres for a 14 to 18 kW hydro stove. It stores the heat, smooths out the cycles and lets the appliance run at a steady rate.
• Anti-condensation valve. It keeps the return to the appliance above roughly 55 °C to prevent the flue gases condensing in the heat exchanger. Some appliances have it built in; check the manual.
• Circulation pump. Often built into the hydro stove; otherwise allow for one on the distribution side. On a boiler, one pump per circuit (radiators, underfloor, DHW).
• Expansion vessel and safety. 3 bar safety valve, vessel sized on the total water volume (buffer tank included), air vent, pressure gauge. On a boiler, overheat protection (thermal discharge valve or discharge heat exchanger) is mandatory.
• Hot water production. DHW cylinder with a coil fed from the buffer tank, or a combination cylinder (buffer tank with built-in DHW). The choice depends on the space available and the number of occupants.

Temperatures and radiators

A hydro stove delivers water at 60 to 70 °C, suited to the existing radiators of an oil-heated house. For underfloor heating, allow for a mixing valve that brings the temperature down to 35 or 40 °C. In a renovation, keep the radiators in place and just check that their output at 60 °C covers the heat loss of each room.

Controls

The hydro stove controls its own water temperature; the house controls (room thermostat, outdoor sensor) drive the pumps and the valves. For a pellet boiler, weather-compensated control with an outdoor sensor and a heating curve gives the best results. Connected (Wi-Fi) appliances let you monitor the buffer tank temperature remotely.

The Girolami Soft and Biotec

The Soft (14 to 26 kW) is a wood and pellet hybrid hydro stove: the layout is that of a standard hydro stove, with a slightly larger buffer tank to absorb the output of a log fire. The Biotec (26 to 34 kW) is a boiler: plant room, silo or pellet store, and ash removal to plan for.

Before you order

Send us the planned layout or a description of the existing circuit. We go through it and flag what is missing. The manufacturers' standard layouts come with every hydro appliance.