Monobloc or split during power cuts: frost protection must be designed in

When choosing between a monobloc and a split system, it is easy to focus on price and COP. But for a building that may experience prolonged power cuts, there is another question: what happens to the water side when the equipment stops?

The ability to operate in freezing weather and the ability to withstand a power cut safely are not the same characteristic.

Where the water runs

In a typical outdoor monobloc, the refrigerant circuit is contained in the outdoor unit, with water pipes running to the building. In a refrigerant split system, refrigerant pipes connect the outdoor and indoor units, and heat is transferred to water indoors. Vaillant describes this difference in construction. (Vaillant — monobloc heat pump construction)

But the description ‘two units’ is not enough: the system diagram needs to be checked. An outdoor unit with water connections still means water is present outdoors, even if there is a separate hydraulic module inside the building.

A refrigerant split system eliminates the risk of outdoor water connections freezing by having none. However, the indoor water system can also freeze if an unheated building cools down for a prolonged period. (Vaillant — split heat pump construction)

Electronic protection needs electricity

If protection relies on running a pump or electric heater, it cannot operate after a complete loss of power. An ‘anti-freeze’ option in the menu does not explain how the system behaves without power.

Thermal insulation slows cooling but does not generate heat. The issue must therefore be addressed through system design, a power source or an approved method of fluid protection, not insulation thickness alone.

Mechanical frost protection valves are available that respond to water temperature without an electrical supply. Caleffi describes this principle in its iStop range. However, their compatibility, positioning and provision for water discharge must be checked for the specific system. (Caleffi — iStop: the principle of mechanical frost protection)

There is no single solution for every building

At the design stage, we would consider a manufacturer-approved antifreeze heat transfer fluid, backup power for essential equipment or a purpose-designed arrangement for draining the outdoor circuit. Do not add antifreeze or install valves yourself on the basis of generic advice: material compatibility, concentration, flow rate and controls must comply with the documentation.

The restart procedure must also be defined. After draining, the system cannot be assumed to return automatically to normal operation without checking that it is filled and that flow is established.

A buffer tank is not an unlimited heat source

Consider an idealised 200-litre tank. If its water cools uniformly from +35 to +20 °C, the heat released is:

E=200×0,001163×15≈3,49 kWh of heat

With a building heat demand of 8 kW, this is mathematically enough for approximately:

t=3,49/8≈0,44 h,

or 26 minutes. In practice, not all of this stored heat is necessarily usable for space heating: radiators may need a higher temperature, and circulation requires power.

This is not a calculation of time to freezing. It does not mean the outdoor heat exchanger is protected for 26 minutes: it has its own cooling conditions.

What must be clear before installation

We would document which parts may cool below zero, what protects them without mains power, which scenario the backup power supply is designed for, and who restores the system after a prolonged shutdown.

A monobloc is not the wrong choice for a cold climate. What is wrong is an outdoor water circuit without a properly considered power-cut strategy.