A German test crowns a radiator thermostat that works without the cloud

The winner and the runners-up

As heise.de reported in its top-10 test, the best smart radiator thermostat is the Homematic IP Evo from the German maker EQ-3. It costs 78 euros. The testers liked that it combines efficiency with independence: it balances the heating water between radiators automatically, and it can be run without the cloud.

The Tado X is a close second at 65 euros. The testers call it the most modern device in the field. It connects through Thread and Matter, two standards that let gadgets from different brands talk to each other. It also has a rechargeable battery instead of disposable ones.

For value, heise.de picks the TP-Link Kasa KE100 kit, offered at 33 euros on eBay. The Sonoff TRV Gen2 is only slightly dearer at 35 euros. It uses Zigbee, a low-power wireless standard for smart homes, and fits well with Home Assistant, a popular do-it-yourself smart-home program.

What 'without the cloud' means here

A cloud-based gadget depends on the maker's servers to work properly. If the company changes its rules or ends a service, you can lose features and cannot do anything about it. The test says nine of the ten devices can run fully locally. The cloud is mostly needed for remote access, voice assistants and location-based triggers, and sometimes for first setup.

With Homematic IP, the control box decides. The cheap Access Point 2 (53 euros) sends everything through the maker's cloud. The Home Control Unit (from about 284 euros) or a CCU3 keeps settings, automations and measurements at home. No user account is needed either way.

The one real exception is the Tado X. Without a connection to Tado's servers, you only get classic heating control. The smart features, including the automatic balancing, stop working. The test also notes that Tado introduced rate limits on its cloud interface in 2025, which slow down third-party integrations.

Hydraulic balancing, explained

Hydraulic balancing means each radiator gets just the amount of hot water it needs. In many houses this is not set up properly. Rooms near the boiler get too warm, while radiators at the far end stay lukewarm.

Several brands now do this in software. The thermostats watch each radiator over a heating season and limit how far the valve opens. No valve swap or draining is needed. The testers say it pays off especially with heat pumps, because a lower water temperature makes them more efficient.

Homematic IP, Tado X and Danfoss Ally can all do this. Tado charges for it in most cases, through a subscription of about 4 euros a month or 30 euros a year. The heise.de testers add a warning: with a heat pump, avoid deep night-time temperature drops. They force the system to run hot in the morning and can cost more than they save.

How much will you really save?

The savings are real but often oversold. Each degree Celsius of lower room temperature cuts energy demand by up to 7 percent. Since the average room in Europe is 22 °C and many people find 19 to 20 °C comfortable, a drop of 1 to 2 degrees is barely noticeable. heise.de calls 5 to 10 percent a realistic result.

Estimates differ. EQ-3, together with window sensors, assumes 10 to 30 percent. Stiftung Warentest arrives at up to 8 percent. Either way, the testers say the purchase usually pays for itself within a few years.

Window sensors matter because the built-in open-window detection only reacts after a sharp temperature drop. Magnetic or optical sensors tell the thermostat immediately.

The subsidy trap and practical snags

In Germany the 2026 subsidy rules for this kind of balancing have become messy. The grant covers 15 percent of materials and labor, but only a trained tradesperson can fill out the required form, and your own labor is not funded. By heise.de's math, a professional's bill would need to stay near 18 percent of material costs to beat doing it yourself. On 500 euros of hardware, that is under 90 euros, which the testers call unrealistic. Their advice is to buy for efficiency, not for the grant.

Fitting is simple. Most thermostats screw onto the common M30 × 1.5 valve thread, and Danfoss valves usually need an adapter, which most kits include. Renters can generally swap thermostats without permission because the valve itself stays untouched, but should keep the old heads. If a radiator sits behind a cover or in a niche, the thermostat measures trapped heat instead of room temperature, so you need a model with an external sensor.