H99 on a Dimplex LA MI inverter outdoor air-to-water: Poor water flow during defrost (anti-freezing protection of indoor heat exchanger)
Short answer
On a Dimplex heat pump, H99 means: Poor water flow during defrost (anti-freezing protection of indoor heat exchanger). The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Poor water flow during defrost (anti-freezing protection of indoor heat exchanger).
Part involved: Refrigerant liquid temperature sensor / flow.
When it shows: During anti-freezing control in defrost, refrigerant liquid temperature < 0C.
Likely causes:
- Poor flow during defrost.
- Refrigerant charge incorrect.
- Liquid temperature sensor misreading.
- Main water PCB.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: unknown.
Manual reset: Clear via control panel per Dimplex article (method shown as image).
What you can check
- Check the heating system pressure gauge reads about 1.0-1.5 bar.
- Note the code and contact the installer or Dimplex service.
What the engineer will check
- Isolate the heat pump from mains before any work.
- Flow checks as H62.
- Weigh out refrigerant charge (F-Gas).
- Check CN-TH1 pins 3&4 probe resistance.
Who to ask for: refrigeration / heat pump service engineer.
Certification: F-Gas (refrigerant handling) certification for any refrigerant-circuit work.
What you might notice
Sources
- GDHV Help Centre article: i have h99 error on my la mi heat pump what does this mean ↗ · GDHV article (modified 2023-02) · page n/a (web article) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026