Engineering practice · Air conditioning
Article 04 / 10
Simultaneous heating and cooling: a control error or useful heat recovery?
The server room needs cooling while the neighbouring offices need heating. The BMS screen shows both modes at once. This is not necessarily wasteful.
The question is different: does the system use heat that already needs to be removed, or simply reject it outdoors and generate new heat separately?
Do not confuse heat recovery with systems working against each other
If a radiator heats a room unnecessarily while an air conditioner cools that same room, the coordination of setpoints and control algorithms should be checked.
A different situation is when separate zones have different, justified demands. Heat recovery VRF can transfer heat from a cooling zone to a heating zone. This capability is available in specific configurations, not in every multi-zone air conditioning system. 1
Permitted combinations of indoor units, controllers and operating modes must also be checked. The Heat Recovery label is the starting point for selection, not a substitute for it.
Where the useful heat comes from
For a simplified steady-state refrigerant circuit balance without heat losses:
Suppose that, at a specified calculation condition, the system removes 12 kW of heat from the server room and compressor work adds 3 kW to the balance. This gives approximately 15 kW on the condensing side.
If the simultaneous space heating demand is 18 kW, these 15 kW can cover part of the load, provided the selected system can transfer them at the required conditions. The remaining 3 kW must come from another specified source or a permitted system operating mode.
This is the balance for a defined scenario, not a rated specification for a particular unit.
Why this is not ‘heating without electricity’
The compressor continues to run. Moreover, the need to deliver heat at a particular temperature may change its operating conditions compared with simply rejecting heat outdoors.
Savings are therefore determined by comparing two complete arrangements: separate cooling and heating, and combined operation. Using condenser heat does not, in itself, guarantee a percentage saving. The refrigerant circuit architecture and the heat recovery temperature level matter. 2
The same energy must not be counted twice: the 12 kW of server room heat and the 15 kW at the condenser are not two independent sources that can be added together.
Coincident loads determine the seasonal benefit
Suppose all 15 kW are usefully recovered for 1500 equivalent hours. Then:
This is the quantity of heat usefully delivered. If it replaces a boiler with an assumed efficiency of 90%, the displaced fuel equivalent is 25 000 kWh based on its calorific value. To calculate the financial benefit, changes in compressor electricity consumption, auxiliary energy use and energy prices must also be considered.
In summer, the server room will continue to release heat. But without a user requiring heat at the corresponding temperature level, this part of the seasonal benefit disappears. That is why we would start with combined load profiles.
Heat recovery is not valuable because it produces an impressive sum of kilowatts. It is useful when actual heat finds an actual user at the same time.