HUUM UKU Ext Box UKU Extension Box for Heaters over 11kW
The HUUM UKU Extension Box is a required component for all HUUM electric sauna heaters over 11kW. It is not a standalone control - the UKU control unit (Local Black or WiFi) must be purchased separately. The Extension Box mounts on the wall outside the sauna room and serves as the electrical intermediary between the high-load heater elements and the UKU control panel, managing the current draw that exceeds the UKU control's direct load capacity. Without the Extension Box, HUUM heaters over 11kW cannot be connected to a UKU control and cannot operate. This listing is for buyers purchasing the Extension Box separately - for example, replacing an existing Extension Box or sourcing it independently from a heater package. Most HUUM heater packages at or above the 12kW threshold include the Extension Box in the bundle.
Which HUUM heaters require the Extension Box
The Extension Box threshold is over 11kW. HUUM heaters that require the Extension Box include the HIVE 12STU (12kW), HIVE 15STU (15kW), HIVE Mini 10.6STU (10.6kW), CLIFF 11STU (11kW), and STEEL 10.5STU (10.5kW). The HIVE 12, HIVE 15, HIVE Mini 10.6, CLIFF 11, and STEEL 10.5 packages all include the Extension Box. When ordering heaters in this range as standalone heaters (without a package), or when replacing a damaged Extension Box in an existing installation, this listing provides the required component. HUUM heaters at 9kW and below - including the DROP series, HIVE Mini 6 and 9, CLIFF 6 and 9, STEEL 6 and 9, and all other models under 12kW - do not require the Extension Box and operate directly from the UKU control.
Installation - outside the sauna room, with the UKU control
The Extension Box is mounted on the wall in the electrical installation area outside the sauna room - typically in the changing room, mechanical room, or adjacent wall space where the heater wiring terminates. It is wired between the heater and the UKU control panel and does not need to be inside the sauna room. The UKU Ext Box Manual 2022.pdf (available in the Installation Manual section below) covers the wiring diagram and mounting instructions. A licensed electrician is required for all connections. Do not connect to a GFI/GFCI breaker. The UKU control unit is sold separately at BathingBrands.
Required component for all HUUM electric sauna heaters over 11kW. Mounts outside the sauna room. Works with UKU Local Black and UKU WiFi controls. Required for: HIVE 12kW, HIVE 15kW, HIVE Mini 10.6kW, CLIFF 11kW, STEEL 10.5kW. Not required for heaters 9kW and under. UKU control sold separately. 5-year warranty. Made in Estonia by HUUM.
Required for all HUUM heaters over 11kW HIVE 12 and 15 - HIVE Mini 10.6 - CLIFF 11 - STEEL 10.5 Not required for heaters 9kW and under Mounts outside sauna room Works with UKU Local Black and UKU WiFi UKU control sold separately 5-year warranty Made in Estonia by HUUM
ℹ️ Extension Box included in most high-output HUUM packages. The HIVE 12STU, HIVE 15STU, HIVE Mini 10.6STU, CLIFF 11STU, and STEEL 10.5STU packages all include the Extension Box. This standalone listing is for buyers replacing an existing Extension Box or sourcing it separately from the heater. If ordering a new installation, confirm whether the heater package already includes the Extension Box before purchasing this listing separately.
⚠️ UKU control is not included and must be purchased separately. The Extension Box is not a sauna control - it is a required intermediary component between the heater and the UKU control. A separate UKU Local Black or UKU WiFi control unit must be purchased to operate the heater. A licensed electrician is required for all wiring connections.
Specifications
Compatibility
| Required for |
All HUUM heaters over 11kW |
| Not required for |
HUUM heaters 9kW and under |
| Compatible controls |
UKU Local Black and UKU WiFi |
| UKU control included |
No - sold separately |
Installation
| Mounting location |
Outside sauna room - wall-mounted |
| Position in circuit |
Between heater and UKU control |
| Electrician required |
Yes - licensed electrician for all connections |
| GFI/GFCI |
Do not connect to GFI/GFCI breaker |
Heaters requiring the Extension Box
| HIVE 12STU (12kW) |
Ext box required - included in PKG |
| HIVE 15STU (15kW) |
Ext box required - included in PKG |
| HIVE Mini 10.6STU (10.6kW) |
Ext box required - included in PKG |
| CLIFF 11STU (11kW) |
Ext box required - included in PKG |
| STEEL 10.5STU (10.5kW) |
Ext box required - included in PKG |
Heaters NOT requiring the Extension Box
| All DROP models (4.5-9kW) |
No ext box needed |
| HIVE Mini 6 and 9 (6-9kW) |
No ext box needed |
| CLIFF 6 and 9 (6-9kW) |
No ext box needed |
| STEEL 6 and 9 (6-9kW) |
No ext box needed |
| CORE series heaters under 12kW |
No ext box needed |
General
| Warranty |
5 years |
| SKU |
H2003012 |
| Origin |
Made in Estonia by HUUM |
What's included
HUUM UKU Extension Box
Mounting hardware
Installation manual
Required (not included): Compatible HUUM electric sauna heater (over 11kW); UKU control unit (Local Black or WiFi, sold separately at BathingBrands). A licensed electrician is required for all wiring connections. Do not connect to a GFI/GFCI breaker.
Why the HUUM UKU Extension Box
Required for heaters over 11kW
The UKU control panel manages heater operations up to a direct load limit. For HUUM heaters over 11kW - the HIVE 12, HIVE 15, HIVE Mini 10.6, CLIFF 11, and STEEL 10.5 - the element load exceeds this direct capacity and the Extension Box is required as the intermediary component. Without it, the heater cannot be connected to the UKU control and cannot operate. It is a non-optional component for these models.
Mounts outside the sauna room
The Extension Box is installed in the electrical mounting area outside the sauna room - the changing room, mechanical room, or adjacent wall - where it is not exposed to the sauna environment. This placement protects the component from the high heat and humidity inside the sauna and keeps the sauna room itself free of electrical enclosures. Wiring runs from the heater through the sauna wall to the Extension Box and then to the UKU control panel.
Compatible with UKU Local Black and UKU WiFi
The Extension Box works with both UKU control variants - the UKU Local Black for on-site digital operation and the UKU WiFi for HUUM app remote preheat and scheduling on iOS and Android. The UKU control must be purchased separately. For high-output installations where remote preheat of a large sauna room is wanted, the UKU WiFi with the Extension Box delivers the complete control solution.
Included in most high-output HUUM packages
The HIVE 12STU, HIVE 15STU, HIVE Mini 10.6STU, CLIFF 11STU, and STEEL 10.5STU packages all include the Extension Box alongside the heater and UKU control. Buyers ordering these packages do not need to purchase this listing separately. This standalone listing serves buyers replacing an Extension Box in an existing installation or sourcing it independently from the heater.
Simple installation with licensed electrician
The Extension Box installs between the heater wiring and the UKU control panel. The UKU Ext Box Manual 2022.pdf in the documents section below provides the wiring diagram and mounting instructions. All connections require a licensed electrician. Do not connect to a GFI/GFCI breaker. The installation procedure is covered in the heater package IOMs for models that require the Extension Box.
5-year warranty - made in Estonia
The HUUM UKU Extension Box carries a 5-year warranty from the date of purchase covering defects in manufacturing and materials under normal use conditions. It is manufactured in Estonia by HUUM and is the official replacement part for any installation requiring the Extension Box. Proof of purchase from an authorized HUUM dealer is required for warranty claims. We are an authorized HUUM dealer.
About HUUM
Made in Estonia - 5-year warranty
HUUM UKU Extension Box. Required for HUUM heaters over 11kW. UKU control sold separately.
HUUM is an Estonian sauna heater company. The UKU Extension Box is the required intermediary component for all HUUM electric sauna heaters over 11kW - the HIVE 12, HIVE 15, HIVE Mini 10.6, CLIFF 11, and STEEL 10.5. It mounts outside the sauna room and connects between the heater elements and the UKU control panel. The UKU control unit (Local Black or WiFi) is sold separately. Most HUUM packages for these heater models include the Extension Box - this standalone listing is for replacement purchases or independent sourcing. We are an authorized HUUM dealer.
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Required for HUUM heaters over 11kW - HIVE 12/15, HIVE Mini 10.6, CLIFF 11, STEEL 10.5
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Not required for HUUM heaters 9kW and under
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Mounts outside sauna room - between heater and UKU control
- Compatible with UKU Local Black and UKU WiFi
- 5-year warranty - made in Estonia
Manuals and Documents
Warranty
5-year warranty
The HUUM UKU Extension Box is covered by HUUM's 5-year warranty from the date of purchase. The warranty covers defects in manufacturing and materials under normal use conditions. The warranty does not cover damage caused by improper installation, connection to a GFI/GFCI breaker, use with non-HUUM heaters, or failure to follow the installation guidelines in the Installation Manual. A licensed electrician is required for all electrical connections. Proof of purchase from an authorized HUUM dealer is required for warranty claims. We are an authorized HUUM dealer.
