1998 Renault Scenic heater only blowing cold air

Start the engine and run to normal operating temperature. Turn blower on full and heat up full. Go under hood/bonnet. Locate the 2 pipes at the back of engine going through the firwall into passenger compartment. Check they are both getting hot. If only one getting hot flush the cooling system. If both getting hot repost for more information.

Did you have the heater blowing full heat?. Sounds like you have the correct pipes. 2 black pipes/hoses about 5/6 inch apart going through into dashboard from engine compartment. If none are getting hot/warm. You have a blockage in the cooling system. Let the engine cool down. Remove water tank filler cap. Remove and replace the thermostat. Take bottom hose off radiator to release coolant. Take both pipes/hoses off where they connect to heater core near firewall in engine compartment. There should be a clip on each pipe/hose. Remove the clips and pull the pipes/hoses off. They will be tight. You will lose more coolant at this stage so make sure you have let the car cool down. Put a hosepipe connected to a tap on the right hand side metal pipe you see sticking out of the firewall. Turn on your tap on and let water run through the heater core. You will see it coming out of the other metal pipe coming out of the firewall. When it starts to run clear and steady. Turn tap off remove hosepipe. Reinstall all pipes. Fill coolant system with 2 litre's of water/antifreeze. Start engine and fill coolant upto correct level . Put cap on coolant tank and let engine get upto temperature. Check heat is working.

Thats ok. Best to be safe than sorry.I'm working form memory here as I don't have a manual on hand. Are the 2 pipes about 1/2 inch diameter?. I think you have the correct pipes. The higher pipe is the inlet (hot in) the lower is the outlet back to the engine. Remove top hose and put the hosepipe on the top pipe. And remove the cars hose off the bottom pipe and flush through untill you get a steady flow of clean water from the bottom pipe. If you are not sure regarding any of this. Most library's will have a manual to lend. Let me know