Can the 24 VAC x- former be replace with 24 VDC x-former in either a heating or AC system. I could see if it were DC to AC the relay may chatter but I see no problem AC to DC, except RMS voltage of the AC would be lower and the DC relay may overheat. In that case can the 24 VAC coil Relays be replaced with 24 VDC coil relays with the same contact rating.

Regardless of lables, transformers are only AC, they don't work on DC.

Regardless of lables, transformers are only AC, they don't work on DC.
There are 120 VAC to 24 VDC transformers, (think of a car battery charger) they just include rectification to reduce ripple on the DC secondary side of x-former.

OK, I'll confuse you. Probably the term x-former means "power supply". So the question might be re-stated as can a 24 VAC power supply be replaced with a 24 VDC one?

DC power is more expensive and thermostats are designed to expect a 24 VAC input. Many times though you may see other equipment allowing either an AC or a DC supply voltage. This is generally because the first thing the equipment does is rectify the AC and internally it runs on DC.

There are "universal" relay which will operate on 24 VAC/DC. They cut down on inventory necessary.

Cool I think I will invest in the universal instead of replacing x-former or relay. Thanks, I really appreciate it. What is a good universal Brand and model?

One specifically for HVAC.

I'm still confused with what you want. On the last page here, http://www.clrwtr.com/PDF/Weidmuller/Weidmuller-Relay-Couplers.pdf there is order data for relays with 24 vac/vdc coils.

Fan Centers have been used for HVAC. They usually mount in a 4 x 4 box and contain a 24 VAC transformer and a relay for the fan motor.