[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

For a mid-size office building with glazed facades on all sides and an internal core of meeting rooms, I need to decide between heat pump VRF (all units on a system in the same mode) and heat recovery VRF (simultaneous heating and cooling). Heat recovery costs more and needs branch selector boxes.

• Which building and zone patterns justify heat recovery?
• How can I estimate the energy benefit quickly?
• Does heat recovery change how the outdoor unit is sized?

[MEP Center Engineering]

Choose heat recovery when a significant part of the operating hours has rooms needing cooling and others needing heating on the same system at the same time – typically deep plans with internal zones, mixed facades, or server and meeting rooms next to perimeter offices. If all rooms on a system swing between heating and cooling together, heat pump VRF with sensible zoning is enough.

How to decide

1. Map the zones: perimeter by orientation, internal core, rooms with high internal gains (meeting, IT, kitchenettes).
2. Check simultaneous demand: in the mid-seasons and in winter, does any core or south zone need cooling while north or perimeter zones need heating? Hourly load simulation is best; a manual check of a winter morning, a sunny winter noon and a mid-season afternoon is a reasonable minimum.
3. Consider the alternative: could heat pump systems be grouped by orientation so that every system has one mode at a time? If yes and the core is small, heat recovery may not pay back.
4. Consider control expectations: heat recovery lets every room choose its own mode, which matters for hotels, offices with tenants, and clinics.

Idealised energy example

At one operating point the core needs 40 kW cooling and the north perimeter needs 25 kW heating. Assume EER 4.0 and COP 4.0 for a simple comparison.

• Heat pump VRF, two separate systems: cooling 40 / 4.0 = 10 kW compressor power; heating 25 / 4.0 = 6.25 kW; total 16.25 kW.
• Heat recovery VRF: the cooling side absorbs 40 kW and rejects 40 + 10 = 50 kW of heat; 25 kW of this goes to the heated rooms and the remaining 25 kW is rejected outdoors. Compressor power is roughly the 10 kW of the cooling duty plus some losses, say about 11 kW.
• Saving at this point: about 5 kW out of 16.25 kW, roughly 30 %.

This is the best case; the benefit falls when demands are unbalanced or when only one mode is needed. Multiply the saving only over the hours when simultaneous demand really exists, then compare with the extra capital cost of the selector boxes, the third pipe or larger pipes, and controls.

Sizing the outdoor unit

Heat recovery does not change the basic rule: the outdoor unit capacity must be at least the sum of the connected indoor unit capacities (connection ratio not above 100 %), with modules in a group equal or at most one size step apart. In addition, each branch selector box has its own maximum connected capacity and number of ports, and indoor units that must change mode independently need their own port. Check refrigerant charge and the room leak-concentration limit; the selector boxes and extra piping increase the charge.

Common mistakes

• Putting units that need different modes behind the same selector port.
• Locating selector boxes above rooms without maintenance access or where their noise is a problem.
• Claiming heat recovery savings for hours when the whole building is in one mode.
• Using heat pump VRF for a mixed-mode building and then fighting complaints by changing the master mode.

Related: VRF heat recovery vs heat pump systems, VRF branch kit selection. For deeper study: VRF/VRV system design course.

Check the edition of the code that applies to your project.

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