I own a two story with two thermostats - a 5 wire thermostat downstairs and 7 wire upstairs. When I set the temp downstairs to 75, I can hear the furnace come on but no heat blowing downstairs but warm upstairs. When I set the thermostat upstairs to 75, it starts baking in the rooms upstairs but still no heat blowing at at all downstairs. On a rare day, it would blow heat downstairs. I thought it was a faulty thermostat downstairs but I just replaced it and there's still no heat. I just can't seem to figure what's wrong as I've been left with some cold days downstairs. PLEASE HELP!

Please give a description of units.
Is this a gas ,oil or electric furnace or is this a boiler system?
Is this a heat pump system?
What are the model numbers of the furnaces or what are the model numbers of the furnace and zone control.
What were the old thermostat model numbers and brands?
What are the new thermostat model number and brand.
Do you have more than one furnace ? OR
Is your house zoned using two thermostats one for the furnace and the other for the zone control ?

These are just questions that have to be answered for us to begin to help you with your problem.

1-it is a gas furnace
2- I'm not sure what a heat pump system is
3- on the furnace the OPERATING INSTRUCTION STICKER has this number 414410A and HW SV9500. There's another sticker printed REFRIGERANT/ EVAPORATOR model no. BHL 3460 and S/N 177 16849

4- old thermostat - robert shaw brand.
Downstairs model cm 200 heat/cool 34976 9650 5-wire
Upstairs model cm 200 heat/cool 34976 9708 7-wire
5- new thermostat - robert shaw brand.
Downstairs model 300-252
Upstairs still got the old model
6-one furnace
7- I've got 2 thermostats but I'm not sure how they work but I know either one can turn on the furnace.
Thanks for your help.

((I've got 2 thermostats but I'm not sure how they work but I know either one can turn on the furnace.))

Usually the above statement means that you have a zone type system and one of the thermostats controls the zone system and when the zone is open for heat it causes the furnace to operate.

Zone systems can be difficult for a ordinary HVAC repair person to work on. There are many different brands and that also helps confuse the problem.

The number you posted SV9500 is not the furnace model but a number off a control. The other numbers you posted are also not valid for search purposes.

It really does not matter cause I cannot tell you how to fix your problem from here. The problem is with the zone control I believe and that is going to take a service call from a knowledgeable professional on site so they can test all the components and make a proper diagnosis. Anything else than this is strictly a guessing game and that gets real expensive real quick with the price of parts.

The unit could be miswired also and they will discover that on the service call to your house.
Sorry but I can be of no help with this repair except to give you sound advice on what to do.