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Hey, I have this problem with with the heater only blowing warm air when with temp turned to max.

1.4 petrol 2010 polo
Fan works ok.
There's a slight change in pressure and temp of air when temp dial turned from cold to hot.
If I leave dial turned down for a while then turn up then a small amount of hot air blows out before it returns to luke warm again.
The coolant is full although has not been changed for about 4 years and a small amount of slime has built up on the expansion tank, about pea sized.
The top one of the two pipes that enter the bulkhead onto the heater matrix is hot, the bottom one is hotish, not as hot as the top one.

Not sure where to start, any help would be appreciated thanks!!

I don't really have a single answer for you, but, any slime appearing in the expansion tank is not good, without any external sources of contaminants being into the coolant system it is hard to see why the original type of coolant which is G12+/G12+++/G13 would not keep "good" for many more years even if it had not been replaced, if it has been replaced, what spec of coolant was put into this system, I'm guessing by your comments that the cam belt was replaced at around the 4>4.5 years point and that the coolant pump was also replaced.

Have you owned this car so some time now and did the cabin air ever get hot when hot was selected?

If not maybe there is an air lock in the cooling system, and that air lock has been there since the coolant system was refilled - these engines need to get refilled with the engine coolant temperature sensor removed and filling continued until coolant comes out of the open hole where the coolant temperature sensor is located free of any bubbles, then while continuing to filling via the expansion tank, refit the engine coolant sensor then continue to fill the expansion tank up to between the MIN and MAX marks. I have been there done that with a late 2009 SEAT Ibiza with that engine.

Edit:- can you confirm if that car is now filled with VW Group coolant G13 - and so is purple/red/pink in colour? Using the wrong type of coolant can end up with a gel forming which can cause blockages within the engine's coolant system - which is not good, can cause engine damage and maybe even block the water ways within the heater matrix, not all "other" coolant will do this, just some it seems.

Thanks for taking time to reply.
Main dealer replaced water pump and cambelt in 2012, I assume they used the correct coolant. it was pink at one stage but is a brownish colour now.
A couple of years ago however the car was loosing a small amount of coolant and needing topping up every few weeks. However I only used water to top up.
I did use a bottle of Radweld that cured the loss of coolant.

I think you should carry out a compression test, have a good look at the oil and coolant and test for contamination.

Are there water droplets in the exhaust gas?

I suspect you may have a failed head gasket which in certain circumstances will create the symptoms you describe.

I’ve seen it on Vauxhall Slant fours and Jaguar XK engines.

One last thing that you have not commented on, does the engine coolant temperature ever reach the usual 90C position? Without that happening you will not get as much hot air as you might expect, ie thermostat failed open along with cooler weather.

One other issue not connected with this at all, you said that the cambelt and water pump was replaced in 2012 probably due to a failure or very high early life usage - now it is near end of 2018, I'd be getting that cambelt etc replaced very soon as 5 years is all you are meant to leave that cambelt on for.

Very much appreciated for taking time to reply. I'll work my way through those ideas. Not sure how to see the coolant temps as does not show on dash.