MVHR in Summer: Why It Fails & How to Fix It
The Hidden Problem with MVHR in Summer—And How to Fix It
MVHR systems are great in winter—but in summer? They might be doing more harm than good. Here’s what you need to know, and how to fix it.
Why MVHR Doesn’t Automatically Work in Summer
MVHR is a winter hero—but in summer, without cooling or dehumidification, it can make things worse. Most systems in NZ use sensible-only heat exchangers that can’t handle moisture.
A Real-World Example
Summer conditions:
- Outdoor air: 35°C at 50% RH
- Indoor air: 25°C at 60% RH
- MVHR supply air: 27°C at 80% RH
Even with 90% heat recovery, supply air ends up hotter and more humid—comfort declines fast.
Common Mistakes in Summer MVHR Design
- No summer bypass or humidity control
- Assuming passive cooling is enough
- Supply ducts running through unconditioned spaces
- MVHR not integrated with any cooling element
How to Fix It: Cool & Condition the Supply Air
COLD.W – Post-Cooling with Hydronic Coil
Perfect for homes with radiant heating/cooling. Conditions air before it reaches the room.
COLD.X – Direct Expansion Coil
Connects to split AC units to cool and dehumidify the supply air. Great retrofit option.
COLD.R – Reheat Coil
Raises the temperature of cold, dehumidified air to prevent condensation on ducts.
Dehumidification: The Unsung Hero
High humidity doesn’t just feel bad—it also reduces cooling efficiency and promotes mould. Integrated dehumidification is key, especially in airtight homes.
Smart Use of Summer Bypass
Use it only when outdoor temps and humidity are lower
