We summarize results of an ongoing effort to assess long-term ZTD trends for eventual use in climate models, either for assimilation or validation. We have been analyzing the ZTD time series estimated from the six REPRO3 IGS Analysis Centers (ACs) who made them available, and compared their long-term trends, here meaning 20 years or longer. A subset of the IGS global network composed of thirty stations were selected. After being homogenized, using ERA-5 derived ZTDs as reference, the estimated ZTD time series were averaged to daily values to minimize potential influences coming from different estimation strategies used by individual ACs. Two combinations, using weighted mean and (a robust) least median of squares are then generated from the six homogenized and averaged time series. The combinations serve as quality control to each ACs. Analysis of the generated trends is performed both in time and frequency domains. Results vary depending on the geographical location. For example, ALBH, in Canada, shows inter-AC scatter is 0.47 mm/decade for the trends, 0.11 mm for the annual amplitudes, and 0.29 degrees for the annual phase. The work to be presented is under the scope of JWG C.2: Quality control methods for climate applications of geodetic tropospheric parameters.