Quote Originally Posted by fixitman View Post
I'm anticipating using some Viconics 'stats to provide proportional control of some electric duct heaters. The heaters in question use pressure switches to enable them when air flow is present. As currently configured, when the fan starts, the safety contactors pull in and stay in during the entire time the fan runs. So they close once a day.

A controls consultant is telling me that SCRs (brand new, also Viconics, in this case) can sometimes leak and waste energy. He advocates using a DO in the 'stat to enable the heater in addition to the fan switch. This means that the contactors might close many times a day, rather than just once. Unfortunately, Viconics tech support says the DO in question would close after the proportional signal is applied, meaning that the contactor will sometimes close when the SCR is turned on. This will shorten contactor life, in addition to the noise-nuisance of the contactors closing in occupied spaces.

I've seen a few leaking SCRs in my time - but wonder just how common it is these days, given the present state of the technology. The devices I've found leaking were built 20 or more years ago. It seems like today's devices would be expected to perform a good deal better.

Comments? TIA ...
Leakage current during the heating season is not a serious issue because the waste heat does go towards heating the building and is unlikely to drive temps beyond their control setpoints. During the cooling season open the safety contactor to eliminate the loss of energy.