Heater stopped working, car runs fine (Astra)

This is a 2005 Vauxhall Astra, petrol.
It has always run quite hot, with the fan cutting in frequently.
Now the internal heater is not working. Fan is ok, just blowing cold.
What's the first thing to check?
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On 20/11/2018 10:56, snipped-for-privacy@gmail.com wrote:

Water level in radiator?
I once stopped to offer help to a car with steam emerging on a motorway hard shoulder. They said "the heater stopped working a few miles back". Unfortunately what actually stopped them was a completely seized engine caused by their optimistic assumption that it didn't matter much.
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On 20/11/2018 11:14, newshound wrote:

+1 The in car heater is the highest point in the system. If the engine coolant water level has dropped the first thing that stops working is the heater.
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On 20/11/18 10:56, snipped-for-privacy@gmail.com wrote:

Check the coolant level, if low top up' Some cars are need a special technique when filling the coolant system to ensure that air isn't trapped, my Espace was like that. I don't know the Astra but, if it is prone to trapped air, it is possible that is the problem. Low coolant and/or trapped air would explain the car running hot as well.
In the heaters I've had to work on, the mini rad in the heater has always been in the water circuit, whether you get hot or cool air is determined by vanes which are moved by either cables (older cars) or a motor I suspect in newer ones. I've only worked on the cable ones, where the cable has come adrift etc. In the newer ones the motor could fail, as could the electronics that control it, including the switch. I've also known the vanes get jammed by debris- muck, leaves etc. The bad news is, getting at the heater is generally a pain in the bum.
If no one on here knows the Astra heater, it would be worth trying to speak to someone perhaps in a local garage who does. It isn't unusual for particular cars to have known problems. My first car, an Escort, had a heater problem and a tip from someone who knew Escorts saved me a lot of work.
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If the coolant level has been low at any time, perhaps an air lock? Some makes do require careful bleeding.
Other thing is the heater matrix itself becoming blocked. You can sometimes check this by measuring the temperature of the flow and return to it. If the heater is working, the return will be at a lower temperature. If partially blocked, little or no difference.
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On 20/11/2018 10:56, snipped-for-privacy@gmail.com wrote:

First check coolant level, the header tank can crack and leak, pressure cap fails -- oh and check the bottom of the radiator, they rot away. If thats all ok, check the thermostat - failure (jammed open) is common. The thermostat housing has to be replaced (model dependant YMMV) Google will provide the info you need.
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Update:
First, thanks to all of you who have replied. I appreciate your help.
I feel a bit daft now, because the water level was indeed low (none in the expansion bottle). I have no idea why that might have happened, because I've checked it in the past and it's been ok.
So, I've topped up with some new G13 anti-freeze mix and surprise, surprise, the heater works ok now. Not brilliant, but working.
I'm going to keep an eye on the level, because I'm old enough to remember the days when adding anti-freeze almost guaranteed a leak being 'found' somewhere. It could well have been an air-lock, as some of you say. Seems poor design if so.
Thanks again.
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On 27/11/2018 13:44, snipped-for-privacy@gmail.com wrote:

more likely that you have a leak, fan switch on the radiator or water pump are most common these days.
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On 27/11/2018 13:52, MrCheerful wrote:

As Mr Cheerful says, unless you've forgotten to top up for a very long time, you've almost certainly got a leak. I generally put a Sharpie mark on the header tank when the engine is cold and use that as a guide. Over a year, I doubt the level drops by even 1cm. Obviously how much that represents depends on the shape of the header tank but, at most 250cm^3 (1/4 pint or so), even on a big tank.
While the systems are sealed, I believe they are designed to vent if they over pressure/over heat. (Perhaps Mr Cheerful can confirm), that must cause some loss of steam and therefore water volume.
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On 28/11/2018 08:19, Brian Reay wrote:

yes the caps have a spring loaded pressure release, but if it has been in that sort of area then there is a definite problem to get fixed.
the op should take the car for a full warm up, then park it over a big clean bit of cardboard, that is a pretty good way to find a leak. or get a garage to pressure test the system, preferably with dye in and use a UV lamp, at the same time they can do a block test to show up any internal head gasket leakage.
Some cars do tend to 'lose' or use a bit of coolant, but only up to about a half litre in a year, anything more needs investigation.
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If the level had dropped considerably, you may well have an air lock. Without knowledge of your particular car, can't really say how to bleed that out. One car I owned has two bleed nipples, one for the heater. And if you didn't bleed that, no heat.
You could try parking the car on a hill etc so the header tank is as high relative to the heater as you can achieve, and running the engine with the filler off until the thermostat opens.
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On 27/11/2018 14:29, Dave Plowman (News) wrote:

Check the expansion tank. If there are a number of small bore hoses on it then it self bleeds. My bother's Chevy Kalos is like this, 2 small hoses discharge above the coolant level. Modern cars are being made idiot proof but they need a level sensor in the expansion tank (some have them in sump so an oil warning comes before the engine f'ed light).
Look for tell tale coolant stains in the engine bay, pink/blue/green heat shields etc zinc plate is grey. Suspect all plastic parts, rad header tanks etc. My bother's Chevy Kalos has a plastic thermostat housing, it had a hair line crack. He complained it sort of stuttered when changing to 3rd first time in the morning. I found the header tank near empty, topped it up, then it lost about 1/2L of coolant in 2 months. The exhaust heat shield was pink.
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