Greetings experts,

I am retrofitting a poorly designed multiple-zone hydronics/forced air
system at my house in Oakland, CA. In its original conception, the air
handler fans are always on. That is, there is a manual switch for
every fan. And the handlers are not that quiet either. I am adding
relays to control fan on/off. I bought a few 30A mechanical contactors
(the fan motors range from 1/3 HP to 3/4 HP) for about $10 each and
installed one, and it works fine, but it is wicked noisy! THUNK!

This one isn't close to a bedroom so it doesn't matter too much, but I
have to do better for the bedrooms. I am considering these three
choices.

1. Small 10A relays. They click too, but they are much quieter.
However, I don't know if the contacts can stand the inrush current. I
bought 3-pole relays and connected the poles in parallel, so that
gives me 30A in theory. Is this a good idea?

2. Mercury relays. These can handle high currents and cost about $25,
but are they quieter than the small relays? Does anybody know?

3. Solid-state relays. I am looking at the Carlo Gavazzi models, which
seem very nice. They have models with 24VAC input, and a 50A relay
costs about $30. The voltage drop across the relay is 1.6V, and at
steady state the currents I am dealing with are low enough that I
won't need to install heat sinks.

Thanks for any help!
Luigi Semenzato