HRV Defrost Airflow Routing Without Negative Pressure
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Solution Overview
Problem
Conventional HRV/ERV defrost systems face issues such as negative pressurization of interior spaces, heat energy loss, longer defrost times, and the need for additional ductwork or re-circulation of stale air, which are not effectively addressed by existing fan shut-down, 5th port, or recirculation methods.
Innovation Solution
A defrost system that redirects supply air through the HRV/ERV core during defrost mode, using dampers to block fresh air intake and exhaust air output, allowing warm interior air to defrost the core from both sides without negative pressurization or re-circulation of stale air, thereby maintaining neutral interior space pressure and efficient heat transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fan shut-down defrost system is used, then the HRV/ERV core can be defrosted by exhausting interior air through the core, but negative pressurization of the interior space occurs and heat energy is lost
Solution Approach 1:
A defrost coil is introduced as an intermediary component that condenses moisture from the exhausted air stream. This mediator allows the system to maintain the beneficial exhaust air flow through the core for defrosting while capturing and removing moisture that would otherwise cause freezing, thereby preventing energy loss and negative pressurization issues
Solution Approach 2:
The exhaust air stream is given multiple functions: it simultaneously defrosts the HRV/ERV core by passing through it, and provides moisture for condensation on the defrost coil. This multi-functional use of the same air stream eliminates the need for separate defrost air intake, avoiding negative pressurization and energy loss
2Productivity
If 5th port defrost system is used, then warm air can be fed to both sides of the HRV/ERV core for faster defrosting, but additional ductwork and space are required
Solution Approach 1:
The existing exhaust air duct and coil are made to serve dual purposes: their primary function for normal ventilation and an additional function as a defrost coil during defrost cycles. This eliminates the need for a separate 5th port and its associated ductwork, reducing system complexity while maintaining fast defrost capability
Solution Approach 2:
The defrost function is merged with the existing exhaust air handling system. The exhaust air stream and defrost coil are combined to perform both normal exhaust and defrosting functions, eliminating the need for separate defrost infrastructure
3Productivity
If recirculation defrost system is used, then defrost air can be recirculated through the HRV/ERV core, but stale and exhaust air is delivered to the interior space during defrost cycles
Solution Approach 1:
The harmful moisture component is extracted from the exhaust air stream through condensation on the defrost coil. This allows the system to retain the beneficial warm air flow for defrosting while removing the harmful moisture that would cause freezing and poor indoor air quality during defrost cycles
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution prevents core freezing, maintains neutral interior space pressurization, reduces energy loss, and shortens defrost time by using interior air to defrost both sides of the HRV/ERV core, eliminating the need for external ductwork and avoiding the re-circulation of stale air.
Implementation Method 1
a heat exchange core for transferring heat from the exhaust air to the fresh air
Implementation Method 2
which transfers sensible and latent energy from the exhaust air to the fresh air
Implementation Method 3
supply air, after passing through the first fan, is redirected by the first and second dampers through the ERV core
Data Source
AI summary
A defrost system for a heat recovery ventilator/energy recovery ventilator (HRV/ERV), uses the interior space supply air of an integrated fan coil for defrosting a HRV/ERV core without creating negative pressure in the interior space, which wastes energy, without need of an external fifth port from which to draw defrost air from the interior space, which increases costs, and without re-circulating exhaust air into the interior space. During the defrost cycle, automatically controlled dampers close off the fresh air and exhaust air inputs, and exhaust output, and circulate supply air through the heat exchange core and into to the living space.