PTC (Auxiliary) Heater for Quicker Heating in Winter

[piggoy]

My bad. I understand. The hot exhaust gases heat the coolant via the EGR cooler, so in theory the newer BLT engine should warm up quicker than the older ASZ. My vRS (BLT engine) already has the EGR cooler fitted and it still takes fecking ages to warm up on a cold icy morning. I'm hoping to discover a real benefit of retrofitting the PTC heater. Hopefully the windows will de-ice and de-mist within a minute. Once I've worked everything out and fitted successfully I'll post a proper guide.

[Breezy_Pete]

[snip] When originally fitted it will only draw what the alternator is able to supply after supplying all the other bits....[/snip]

I'm not sure that's true (but happy to be educated if there's something I'm missing). 

The wiring diagram I linked to above suggests that the three heater elements within the PTC heater are either on or off, as dictated by their relays, via ECU control. As far as I can see, there is no mechanism of giving them more or less current, just all or nothing, on or off.  What is available to give when switched on depends on the battery and its state of charge.

Sure, in the factory install the ECU knows the battery voltage, and so can inhibit the heater if it decides that's a good idea, but that is what piggoy's voltage-sensing comparator circuit is doing too (if it works as intended). As I understand it, if current draw from the battery exceeds current supply from the alternator, the battery terminal voltage drops in proportion, due to its internal resistance. This is why battery terminal voltage droops significantly when cranking from a cold start.  Starter current is huge, alternator is doing nothing or next to nothing, so lots of volt drop occurs across the battery's internal resistance, leaving less to appear across the terminals and starter motor.

[Phil-E]

Ah. I'm thinking of the other heater aren't I.

 

There are two. One that is controlled through relays by the ECU and another that is just wired with a + and - then wires into the CAN system. The car then tells it how much power it can draw.

 

Phil

[piggoy]

I measured the resistance to GND of each of the three heater elements using an calibrated resistance meter, taking into consideration probe contact resistance -

 

White wire - 0.7Ohm

Black wire - 0.7Ohm

Red wire - 0.5Ohm

 

I am presuming this is the cold resistance which can only increase as the temperature of the elements climb. Therefore maximum transient current on startup (@14.5 volts) -> IMax = (14.5 / 0.7) + (14.5 / 0.7) + (14.5 / 0.5) = 70.4A

 

I intend to test this theory with a 200A current source at my workplace, when I have access to it.

Edited 11 November, 2014 by piggoy

[BO60BYS]

Problem solved

[Breezy_Pete]

Those resistance numbers sound broadly consistent with dieselV6's stated 80A max current.  Will be interesting to see how fast and how much this drops off as they heat up (in a draught/fan ventilated). 

The red wire goes to the element that is used on its own fed by  the 'low heating capacity' relay, rather than the two other elements which are in parallel fed by the 'high heating capacity' relay, in the factory set-up, so not surprised that it may be slightly different to the other two elements. 

 

On the other hand, It may just be measurement issues with the low resistances involved. As you've found, probe wire/contact resistances become (relatively) very significant at these low resistances.

 

I've got a server PSU that'll chuck out over 100A at 12V if it should be useful at any stage of bench testing.

[piggoy]

I think I might order ones of these bad boys -

http://www.ebay.co.uk/itm/310248391691?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

 

Quite expensive at £20 but it's the only 100A relay I can find that has stud contacts - quite necessary for the large ring crimps that'll be required for the 2AWG or 4AWG supply cable. Multiple smaller relays don't work out any cheaper.

 

Thanks for the offer of the PSU. I'll let you know if I can't sort one out at my end :-)

Edited 11 November, 2014 by piggoy

[piggoy]

The heater matrix enclosure is proving very difficult if not impossible to remove. This may be a show stopper :(

 

http://www.briskoda.net/forums/topic/334257-fabia-vrs-heater-matrix-removalaccess/

[Breezy_Pete]

Sounds like what dieselV6 was talking about in post #43, but he was referring to Mk2 Fabias and other models. I wonder what ECU it is? Far too many of the pesky things in these modern cars.

[TMB]

 I wonder what ECU it is? Far too many of the pesky things in these modern cars.

 

I'm pretty sure it's the airbag module.

[piggoy]

I've found a Fabia workshop manual that explains the procedure. You have to remove the enclosure along with the heating matrix by clamping and detaching the coolant hoses in the engine bay. Difficulty will depend on access in the engine bay, maybe better from underneath.

 

http://workshop-manuals.com/skoda/fabia-mk1/heating_air_conditioning/heating_air_conditioning/heating/removing_and_installing_parts_of_the_heater_unit_part_1/removing_and_installing_heat_exchanger_and__heating_element_for_additional_air_heating_z35/

[dieselV6]

Found this on Interwebs, hope this helps. If you manage to lift and remove the airbag ECU,  the heater housing should have enough clearance to be undone without touching heater matrix hoses. Wish I'd known earlier it was airbag ECU.

 

Edited 17 November, 2014 by dieselV6

[Breezy_Pete]

^ That looks like less grief than having to dis-plumb the matrix, cos  aren't the matrix hoses alloy at that point on turbo cars, and with a reputation for being difficult to re-seal into place once disturbed?

 

The note about recoding the airbag module presumably only applies if you are replacing (fitting a new/different module), as opposed to re-fitting after removing?

[piggoy]

I agree with you guys. However I'm thinking that the airbag control box might not be the only thing preventing it from dropping out. That enclosure has to drop a long way low enough to get it to slide out past the matrix. There's also lots of carpet and sound deadening in the way, as well as a curved bulkhead to the rear. Also, vice versa, the control box might not slide past the enclosure. I'll certainly give it serious consideration next weekend.

 

Another snag I've discovered is that the enclosure for the with and without PTC heater options are different.

Without PTC heater 6Q0819875B

With PTC heater 6Q0819875C

As far as I can tell the only difference between them is a slot which is cut into the backside for the with option. I'll take a risk and cut a slot in mine with a Dremmel :-)

http://autopartmaster.com/en/?action=catalog_show&cat=skoda&t_id=8833&added_=SKODA%20Fabia%202005%20%D0%9A%D1%83%D0%B7%D0%BE%D0%B2

 

Edited 17 November, 2014 by piggoy

[TMB]

The note about recoding the airbag module presumably only applies if you are replacing (fitting a new/different module), as opposed to re-fitting after removing?

 

Yep

[Phil-E]

The note about recoding the airbag module presumably only applies if you are replacing (fitting a new/different module), as opposed to re-fitting after removing?

 

Probably just need to clear the fault code.

[piggoy]

After reading about the nightmares of other members regarding leaking coolant pipes feeding the matrix I'm not too keen to play around disconnecting them.

 

I'm gonna attempt to remove the airbag control box this weekend. He's a picture with the carpet peeled back. That enclosure has to drop at least 35mm so it can be removed.I fear that the curved bulkhead may be the next restriction. Fingers crossed.

[Phil-E]

Looks like there's enough room to remove the bolts.

 

Fingers crossed!

[piggoy]

A question for Wino about the electrics for this heater that I'm trying to work out in my head. I have always been taught that when jump starting a car you should NEVER attach the jump leads whilst the boost car is running. This is because the sudden draw on the boost car's alternator can burn out it's diode pack. If this PTC heater is suddenly able to draw up to 100A from the system that why cannot it damage the alternator too?

[Breezy_Pete]

I've not heard that about jump starting (the alternator aspect anyway).  I'd think the main benefit of not connecting to a running system is that when non-running the voltage is lower (12.something rather than 14.something when running) so there'll be a smaller spark. Trouble is, if you connect up when both cars are not running, the donor car may not start!

 

Do you have a Haynes manual? If so, look at the very first wiring diagram on page 12.22. Aren't you proposing to connect into terminal 30 junction, item 6?  Wired direct to battery via fuse F2, and only indirectly wired to alternator via F1 as well.  So anything the alternator can't 'offset' with its contribution will come direct from battery via fuse F2.  This is the fuse you may need to uprate, after having a good glance at the cable which links it to item 6, to be confident that it can cope with the extra current. It would be handy to know what that fuse is uprated to when the PTC heater option is fitted, and whether this cable is the same/bigger gauge. Any Scandinavian members know?

 

As stated earlier, measure (on the bench) the current drawn when the heater has been on for a second or two; I think it'll surprise you how much it drops away.  As far as I can see, the switch-on surge only hits the battery in the same sort of way, but much less so, as when you turn the key to crank the starter. Alternator will just do what it can, no more.

 

 

The factory installation may possibly switch the heater elements on in stages (at separate times), via the two relays, to reduce surge loads and so reduce the 'excess' fuse rating that might otherwise be needed at F2? If the ECU switched on the single element first, it would take 1/3 of the surge, and would passively warm the other two elements by proximity, reducing the surge they took when the second relay was energised?  Just thinking aloud, as you can probably tell.  

[piggoy]

Thanks very much for your help Wino, always appreciated :sun:

 

The plan at the moment is to connect directly to the battery using a 100A rated subwoofer kit. You can buy these kits on ebay for a tenner and they include all the cable, fuse and crimp connectors I'll need. I scrapped my original plan to connect to the T30 junction after I realised the fuse F2 and the cable beyond may not be suitably rated to drive the heater. It seems that Skoda weren't too good on making the wiring loom compatible with the non-fitted options.

 

I'm currently torn at the moment on how to do the switching -

1.) I can switch all elements of the heater via a single 100A relay. This would simplify the comparator circuit since it only has to switch a single relay. My worry about this configuration is the effect on the system voltage when all the elements are connected. The comparator will switch the relay on when the system voltage reaches a threshold level only to be switched off immediately as the system voltage is the pulled down by the sudden addition of the load. Unless the comparator has some hysteresis or damping it may oscillate between on and off.

2.) I can switch the elements sequentially with three 30A relays. These are cheaper than the single 100A relay. I haven't figured out how to do the comparator and timing circuit for this yet without the use of a microcontroller. I'd rather avoid programming a PIC chip if I can. I want this circuit to use simple off the shelf components that people on this forum can easily replicate from a guide.

 

Unfortunately I haven't had a chance to do the bench testing. The 200A current source I was promised by a colleague turned out to be non-existant! Anyhow the whole project hinges on whether i can fit it into the heater assembly this weekend! I'm gonna assume its transient startup current based on the cold resistances I measured.

[Breezy_Pete]

I must admit, I had wondered about the comparator circuit and how it would work out, but bear in mind that your 317 regulator will still be able to produce 2V right down to about 5V system voltage, and you could put a biggish local capacitor on the other comparator input ( I think?) to isolate it from short-term variations in system voltage during switch-ons, but it would still detect gradual drops in batt voltage corresponding to the battery really struggling.

 

I guess the bench testing isn't essential; if you have some way to measure tens of amps DC, you could just connect the heater one element at a time to your car's battery to see what it draws at switch-on, and running.  Happy to supply server PSU (and current monitor stuff) though, here, at  yours, or somewhere in between!

 

I agree on avoiding a microcontroller, overkill unless you like programming for the hell of it.  555 timer circuit or similar could do delayed action though? 

 

Somehow I have a feeling you'll succeed at the heater box aspect of the project.  Fingers crossed. :sun:

[piggoy]

The more I think about the circuitry to drive this PTC heater the more I realise the importance of switching each heating element on sequentially. Based on what you say Wino the sequential activation of each element has the potential to massively reduce the overall transient load, so much so that we could connect to the terminal 30 junction without any need to uprate the F2 fuse or it's supply cable. Specifically if the centre element was switched first it would "pre-warm" the outer two elements such that their transient start-up current could be massively supressed. I'm becoming increasingly confident that the whole thing could be switched by two 30A relays, one to switch the centre element, the second to switch the outer two elements.

 

The problem is how to do the comparator/timing circuit. A 555 timer clocking a 4 bit binary counter chip could be used where the relay transistors are switched by the two most significant bits (01,10,11 in sequence). However I subsequently realise this would not work since the counter would only reset itself as the clock pulses from the 555 timer continue to arrive. I discounted the idea of a PIC microcontroller yesterday but I am now thinking otherwise. The PIC12F629 is a 8 pin DIP and includes an onboard comparator and configurable internal voltage reference! The use this chip would only require minimal external components (5v regulator, two transistors, a handful of diodes, resistors and capacitors. It would allow an amazing level of control complexity and could do multi-level system voltage monitoring if needed. I'm pretty experienced with PIC microcontrollers. If any Briskoda member wanted to replicate this project I could simply send them a programmed PIC chip at minimal cost. The amount of soldering and component cost they would need to create the circuit would be minimal. Food for thought :-)

 

http://ww1.microchip.com/downloads/en/DeviceDoc/41190G.pdf

Edited 19 November, 2014 by piggoy

[Breezy_Pete]

Can you see that the red-wire element is the centre one? If it is, that may support the idea that it might be being used as a pre-warmer for the other two, for the reason we're speculating about.  On the other hand, it may just be about switching on less/more heating depending on sensed ambient temperature versus demanded/set temperature.

 

It would be very handy to be in touch with a member who has this heater factory-fitted, so they could look at fuse values, wire gauges, relay ratings etc. I'm wondering if Canadians on vwvortex forum might be a source of info?  Don't use that one myself. I dare say there are some Skoda techs around this forum who could access this info; anyone?

 

I can see where you're coming from with the micro, if you're happy that way then it almost certainly simplifies hardware design, reduces component count and increases flexibility.

[piggoy]

Yes red wire is the centre one.

 

Your second point is also correct according to the following source -

http://workshop-manuals.com/skoda/fabia-mk1/heating_air_conditioning/heating_air_conditioning/air_conditioner/repairing_the_air_conditioning_system_passenger_compartment/summary_of_components_of_heat_exchanger_and__heating_element_for_additional_air_heating_(ptc)_z35/

The load is dynamically switched depending on a variety of conditions including ambient temperature, coolant temperate and alternator load.