Thermostat wiring advice needed: digital for heat only

The white wire (to furnace) is 'power' not a ground.
12V DC comes thru Red then when contacts are made up 12V DC goes thru yellow to white. From t-stat power via white wire goes to timed delay relay.

So whatever that black wire (not OEM---somebody added it) goes to it is getting 12V DC when t-stat calls for heat. (Blue crimp with white wire and black together/size of black wire not OEM)