Is it possible for environmental test chamber to only control humidity, but no temperature control?

If you only want to test product in dry or wet environment, then without temperature control is acceptable. But you still have to measure the temperature to calculate the relative humidity. Relative humidity is actually relative to temperature. The higher the temperature, the higher the humidity the air can hold at a relative temperature. Relative humidity is the ratio of moisture content in the air to its capacity.

In another way,your product will face temperature and humidity at the same time during manufacture and use,it’s the best way to test them with temperature and humidity.