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Joined: Nov 28 2016
Posts: 67

My audi heater core isn't getting hot at idle. After leaving the engine to idle for 15 minutes the heater core temperature sensor was only reading 25 degrees. I've got the feed from the engine going through the LPG vapouriser into the core and back out. Unfortunately it was so long ago that I can't remember whether the top or bottom pipe is the feed or return.

A few questions. Does it make a difference which side is the feed and which is return? Which is best?

Do you have the temperature sensor attached to the rubber pipe or somewhere else?

In addition I've got a leak from the pipe connections to the core. I have to keep going in and nipping the jubilee clips up every few months. What style of clips have you guys used? Whilst I'm on changing the clips I'm also wondering if it's worth replumbing it so the vapouriser is fed from the return rather than the feed?

Thanks

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Joined: Dec 30 2015
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It shouldn't make a difference which way round the flow through the vaporiser is. In theory it should be inlet at the top but in practice it makes virtually no difference. With them plumbed in series, then you will get full flow through both. If they were in parallel I would say that was the problem as the coolant would take the easiest route but in series? How high up is the vaporiser mounted? If too high you could be getting an airlock in it or there may be too much restriction to the flow. What size hoses have you got going to the vaporiser? What vaporiser do you have? On both mine I've got the 19mm hose from the inlet manifold to a reducer down to 16mm, though the vaporiser and then another 16-19mm adapter to the heater and they are both working fine, even in this weather. The sensor will be reading a bit low if it is on a rubber hose rather than the original alloy pipe but I wouldn't have expected it to read that much lower. Do you have an infra red thermometer to check the temperatures of the flow and return hoses to the heater at the bulkhead? Mine read the same within a couple of degrees.

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Joined: Jan 16 2017
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I'd have a look at the vapouriser temp if you can, Also are you sure your thermostat actually works?

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Joined: Feb 24 2017
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Do you get hot air from the air vents when the engines RPMs are above 1500RPM? I’m no expert on anything, but if you do get hot air once the RPMs increase then I would guess that you have air trapped in the system.....
Of course I may have misread the problem you are having altogether and if so, then disregard my post...

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Joined: Nov 28 2016
Posts: 67

Once the revs are up the air temperature increases. The engine and the vapouriser both get hot.

I was refering to the top and bottom hoses on the audi heater core, sorry that I wasn't clear about that. I was just wondering if one was flow and one was return and I had them mixed up.

The coolant for the vaporiser is plumbed in series. Going out the inlet manifold, through the vaporiser, into the heater and back out as usual. It's located next to the brake booster pump. It's a Tomasetto Antartic and I believe it has 16mm water connections. Like you I have a reducer from the inlet manifold to the vaporiser then another back to the heater core.

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Joined: Jan 05 2016
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Honestly I'm not a fan of the Audi heater core anymore - I'm sure it worked fine on my first P38, but on my current one, I ended up taking it back out.

My main reason for removing it was trapped air - if you look at the way the pipes go in, their will always be air trapped in the top of the heater core. The inlet/outlet are on the bottom compared to the P38 core, which has them on the top. I thought this was contributing to my excess pressure issues - but it seems more likely I have a slight crack in the block, as it remained after going back to the standard heater core. What was improved however was the heat available inside the car.

And really, the o-rings aren't that big of a deal to change every couple of years. They were actually the one place that didn't leak under excess pressure in the cooling system.

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Joined: Dec 30 2015
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Just to be contrary, my Audi heater core works fine - even plumbed in parallel.
Of course it'll explode now I've mentioned it.

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Joined: Dec 30 2015
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No idea on the Audi core but on the original the flow goes in at the bottom and the return out of the top. I'm not much of a fan of the Audi core mod either. The original O rings lasted over 15 years and while being a bit of a pain aren't that difficult to change whereas to change the heater core involves either a dash out or butchery to the metalwork to allow it to be got out. As Sloth says, change the O rings every so often (although I'd say nearer every 10 years would be adequate) and leave things as they are.

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Joined: Nov 16 2016
Posts: 616

On the subject of parallel versus serial reducer plumbing I think which route is best comes down to the model of car and model of reducer. Some cars heater circuit designs dictate a certain routing.. some with water control solenoids would have no or severely limited water flow with the heater turned off/down, some with electric pumps in the heater circuit (and/or various shunting valves etc) would see most water flow go through the matrix instead of the reducer especially at low rpm (when the pump kicks in). Two aspects of water flow, volume and pressure - Could have a heater circuit with high pressure where pressure decreases dramatically as flow rate increases in which case series routing might be best, or could have low pressure but capable of retaining that pressure at high flow rates in which case parallel routing would be the best. Both the reducer and the matrix will give some resistance to water flow - with average pressure and volume capabilities there probably isn't much difference between volume and series routing, in terms of advantages for one way or the other what you gain on the roundabouts you lose on the swings. If the river Ouse flowed hot water and I wanted to use it's flow to heat multiple small items where each item had limited internal flow I'd stick them side by side in the river (use parallel routing) / If I wanted to heat the same two small items using a tiny little jet of hot water leaking at high pressure from a boiler I'd probably go with series routing. Some cars and reducer positioning can make airlocks more of a potential issue with one way of routing over the other, usually series routing would be most likely to prevent airlocks but it can go the other way. Different model reducers have different water flow properties, wouldn't want to use series routing with (say) a Bigas reducer that uses 10mm pipe when usually the matrix is fed with 19mm pipe. Reducers and matrixes can furr up reducing flow, with series routing if one gets blogged it reduces flow to both... I've 'fixed' a few factory fitted Vectras plumbed in series where besides the LPG system not working the heater also didn't work.. because the matrix was blogged - by re-plumbing in parallel (at owners request after I'd explained the real cause of the problem and the owner decided he'd fix the matrix later... in summer or the owners might have gone for swapping the heater matrix straight away lol).

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Joined: Jan 05 2016
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On that point, my OMVL reducer restricts the inlet down to what looks like about 8mm ID. Given the heater matrix has 21mm pipes feeding it, I wouldn't/haven't put the reducer in series. On a nice cold day like today, having the matrix and reducer in parallel is working well, and I get heat inside the car nice and quickly.

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Joined: Dec 30 2015
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Mine was plumbed in parallel when I first got it and noticed the heater dropped down to vaguely lukewarm while siting at idle waiting to get into a car park to do my Christmas shopping, but fine once the revs were up due to lack of flow at idle. The Classic I had before was also in parallel and that would ice up the vaporiser within a couple of hundred yards due to lack of flow through the vaporiser. Swapping both to series cured both problems and allowed the pipework to be a lot neater. As Simon says though it is going to depend on the car and how clogged up things are. Might be worth reverse flushing the heater and vaporiser and seeing how much water flows and how close to oxtail soup it looks.