Using and maintaining your hot tub's wood-fired heater

Short answer: the fixed order is always the same: first fill the tub well above the top heater connection, only then start the fire; afterwards remove the ash once it is cold, and only drain the tub once the fire has fully gone out. Following that order lets you fire an RVS316 wood-fired heater (internal or external, 38 kW) for years without trouble. The full usage instructions follow below.

Your heater: internal or external

• Internal heater: stands inside the shell, shielded by a wooden guard, and transfers its heat directly to the water. You feed the fire from inside or over the tub. Keep your distance from the guard and never let children sit on the heater side. With an internal heater, the Q-tub XXL retains 1300 litres.
• External heater: hangs next to the tub and is connected by two pipes. The water circulates without a pump through natural convection: warm water rises in the heater and flows into the tub through the top, cooler water flows back through the bottom. Lighting, adding wood and removing ash are all done entirely from outside. The Q-tub XXL then holds the full 1400 litres.

Both versions are made of chlorine-resistant RVS316 and bring the water up to temperature in around 2 to 3.5 hours.

Firing safely, step by step

1. Fill the tub first, well above the top heater connection, up to the level specified in the installation manual. A heater that burns without water suffers irreparable damage; this is the one mistake beginners cannot undo.
2. Check the installation: flue pipe (pipe set up to around 280 cm), heat shield and rain cap according to the manual. With the installation service, this is delivered fully connected.
3. Fit a spark arrestor on the chimney, especially in wind, dry conditions or near trees and thatched roofs.
4. Light with small, dry wood and firelighters; never with petrol, methylated spirits or other flammable liquids.
5. Burn dry, split wood (less than 20 percent moisture) and control the fire with the air damper: fully open to heat up, closed down to hold the temperature. The complete firing plan with heating tips is in hot tub heating time.
6. Let the fire die down around 35 to 36°C: the water mass has thermal lag, so keeping the fire going until the target temperature is reached will overshoot it.

After firing: ash and cleaning

• Remove the ash after every firing session, only once it is completely cold. With an external heater this is done from outside through the door; with an internal heater, carefully over the tub.
• Keep the grate free of ash residue; a smothered air supply makes the next fire slow and smoky.
• Have the chimney swept annually, as with any wood-fired heater. Deposits in the flue reduce draught and are a fire risk.

Draining the tub via the ball valve

1. Wait until the fire and embers have fully gone out and the heater has cooled down. A hot heater left to run dry still suffers damage.
2. Open the ball valve at the bottom of the tub. With a hose connection you can drain the water in a controlled way via a garden hose to a drain or flower bed.
3. Clean the shell and waterline before refilling. You can read how often it is wise to change the water in how often to change the water.

Winter and frost

Continuing to use the tub in winter is not a problem: as long as you fire it regularly and keep the cover on, the water in the tub and heater will not freeze. If you leave the tub unused during a frost period, drain everything completely, including the heater and pipes: frozen residual water is the most common cause of winter damage. The complete schedule is in winterising a hot tub.

Common mistakes

• Firing with too little water: always fill above the heater connections first.
• Draining while the heater is still hot: wait until everything has gone out and cooled down.
• Burning wet, painted or treated wood: produces little heat and pollutes the heater, chimney and bath water.
• Leaving a full ash tray in place: restricts the air supply.
• Forgetting the spark arrestor in dry conditions or with overhanging vegetation.