Buffer tank and hot water
Why a UNI 60 needs thermal storage, how the buffer tank fits the scheme, and how the TIS Tronic 260U prepares domestic hot water in Summer and Winter modes.
A UNI 60 running without thermal storage spends most of its time in supervision mode, cycling the fan to stop the water boiling — which wastes fuel and tars the chimney. A buffer tank turns the boiler's natural behaviour, a hot fast burn, into something the building can use over the following hours.
Why storage
A full 58-gallon firebox releases far more energy than a house absorbs at that moment. Without somewhere to put it, the boiler:
- reaches setpoint quickly and drops into supervision;
- burns at low air for hours, which is exactly the condition that deposits tar;
- cycles the pumps and the fan repeatedly;
- and delivers uneven comfort at the emitters.
With a buffer tank, the boiler runs a clean, hot burn into storage, and the emitters draw from storage afterwards.
Sizing
Size storage from the firebox, not from the building. The practical rule for gasification boilers is to store the output of one full load so the boiler can finish its burn without being throttled.
Factors that change the number:
- Firebox volume — 58 gal on the UNI 60.
- Fuel density: oak stores far more energy per load than spruce. See Fuel properties.
- The temperature spread you can use between the top of the tank and your emitters. Radiant floor loops draw a wider spread than fan coils.
- Whether the tank also serves domestic hot water.
The MBtek recommended scheme uses a BF650 buffer tank with a DHW250 hot water tank; the UNI 60 product page pairs the boiler with the APOLLO DHW80 combined buffer and indirect tank for smaller loads. Confirm the sizing with MBtek or your heating designer against a real heat-loss calculation.
Domestic hot water on the controller
The TIS Tronic 260U drives a dedicated hot water pump on output Q4, reading tank temperature from sensor T2. The module is off until you switch it into Summer or Winter mode.
| Mode | Behaviour |
|---|---|
| Off | Hot water pump disabled; the home screen for hot water is greyed out |
| Summer | The boiler's job is to make hot water only. All other pumps are switched off — except when the boiler exceeds the maximum boiler temperature, when all pumps start to protect it |
| Winter | Central heating pumps and the hot water pump operate together |
Full configuration is on the hot water page, including hysteresis, priority, working hours and the temperature-increase function.
Priority explained simply
With HUW priority on, the central heating pump stops while the tank is being heated, and the tank's temperature target outranks the boiler setpoint. It is the right choice when the household runs out of hot water; it is the wrong choice in deep winter if it lets the house cool.
The sensor well
The boiler (CH) sensor goes into the immersion well, and the STB goes on top of it. Adding any liquid — oil, water, thermal paste — to the immersion well is forbidden.
Where no well is available, the CH sensor may be fitted to the supply branch pipe up to the connection nut and pressed against it with a metal clamp.
NextFuel and storageHow much wood you need, how dry it must be, and where to keep it.
Frequently asked questions
Hydronic system design
Design the UNI 60 hydronic system — closed loop, expansion tank, safety relief valve, return-temperature protection and the recommended MBtek installation scheme.
Fuel and storage
Which fuels the UNI 60 accepts, how dry wood must be, the calorific value of common species, and the 4 ft fuel storage clearance rule.