About a month ago I had the engine out and completely rebuilt inc RCM head gaskets and extreme11mm studs. It all went in OK and worked well. I filled the coolant slowly into the rad until it was full then top up the header tank slowly.
Subaru heaters are not marvelous but it worked OK.
Yesterday it went a 400 mile trip. On the way back the heater output got cooler and cooler. The heater was set to max hot and the fan ran at max on auto. Top and bottom hoses are hot and it doesn't overheat. I've bled the heater at the top connection. I've spent all evening running the engine on and off at various revs with a partially filled funnel in the header tank filler squeezing top and bottom hoses. Still only lukewarm heater pipes. Any ideas?

 

As it is a full flow bypass heater, first off check the heater hoses to/from the matrix at the bulkhead are hot, if not it could be your waterpump/blocked matrix.
Try connecting the ends of the heater hoses (bypassing the heater matrix completely) and see if they then get hot. Back flushing the matrix.

If they are....
The hot/cold air is mixed/controlled directionally, using a number of flaps(doors) operated by electric actuators. I would then suspect one of these is not working or has become detached/stuck in some way.

Attached is the auto diagnostic procedure for the HVAC system
Attached is the HVAC components diagrams

 

Thanks Don. Heater pipes are both lukewarm and water pump was replaced at engine rebuild about 1500 miles ago.
I will try bypassing the heater as you suggest and see if the problem is engine or heater.

 

You may find the attached document useful.
Note that water flows through the heater matrix from the main water x-over pipe back to the water pump inlet.

 

Hi, before you bypass anything, Did you leave the header tank cap off and let the fans kick in twice to bleed the system, its the way i have found and used for 10 years on Impreza`s and never had an air lock after.
hth

 

Yes, did that more than once.
Tried the bypass. It got hot so problem was within heater. Flushed that with garden hose and all's OK now.
One word of caution to anyone flushing a car heater or radiator, don't connect the hose directly. Car systems are designed for 1.5 bar. Mains water pressure is about 5 times that. The result would be not nice, very not nice.
Thanks for the suggestions.