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Old 13th June 2006, 19:28   #1
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Default Why can't we shut the heater off?

I have a few queries about car's heating, ventilation and air conditioning system (HVAC). It seems most of the manual systems have a "temperature" knob or slider which essentially mixes cold and hot air, obviously comming from AC and heater (better known as heater core, which is nothing but a secondary radiator system). AC can be switched on or off independently but I have never seen such a switch for heater counterpart! If you are wondering why should one need it, here is the reason-

Just for simplification, assume that we are using re-circulate mode.
Temperature of the air comming out of AC is much below ambient (cabin) temperature, and it is too cold. Whereas the air from heater is too hot. The idea is to create right misture to achieve a temperature little below the ambient, but above what AC produces and maintain it so as to make it as much comfortable as we can. Let us assume temperatures are-

T1 - cold air from the AC.
T2 - the ambient (cabin).
T3 - hot air from the heater core.

Clearly the relation is, T1 < T2 < T3

When we set the knob to a desired level, what happens is this:
- when the AC compressor is cut in (engaged to the engine), extreem cold air from AC gets mixed with extreem hot air from heater as per the knob setting. The net effect is that the cabin temp starts decreasing. So, T1+T3 = cooling.
- when the AC compressor gets cut off (disengaged), air from AC becomes "ambient" air, which is nothing but air from the cabin itself. This air gets mixed with extreem hot air and the result is hotter than the ambient temp, which result in fast warming up of the cabin temp! So, T2+T3 = heating.

As a result, the system cycles through cooling and heating alternately. I think there are two disadvantages with this-
1. The cabin temperature fluctuates too much around the desired level, making it either too hot or too cold most of the time. Hence its a compromise on comfort.
2. heating is not required, rather it is undesirable because AC gets unnecessarily forced to work harder than needed, to compensate for the extra heat.

Both these problems could have been overcome by having ability to shut off the heater. It will lead to less temperature deviations and saving in power (hence the fuel) required to run the compressor. Instead of cooling and heating cycles, it will result in cooling and "just ventilation" cycles. Similar reasoning applies for "fresh air" mode as well, the only different is that outside air is used for mixing, which would still be closer to the cabin temperature than extreem hot and cold.

I think it can easily be done by bypassing the heater core by connecting its inlet directly to the outlet. But this would be a permanent change which is not really a good option. We need heater in cold or rainy wheather for demisting (luckily most cars, at least my baleno, allow us to use AC and heater both at same time, which is requred for demisting). Better solution is to have some kind of valve control to select between heater and bypass duct paths.

What say guys? has anybody tried this?




Other queries about the compressor on-off control itself:

How is it controlled? some people say that there is a dedicated thermostat for this function, whereas others say that it has fixed dealys... confussing

If it is a thermostat type, where is it generally located and what is commonly used temperature at which it switches?

Is it linked to the "temperature" or blower fan speed controls in any way? I am still talking about manual systems, not climate control
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Old 13th June 2006, 20:39   #2
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Quote:
Quote:
Originally Posted by santosh.s
Other queries about the compressor on-off control itself:

How is it controlled? some people say that there is a dedicated thermostat for this function, whereas others say that it has fixed dealys... confussing

If it is a thermostat type, where is it generally located and what is commonly used temperature at which it switches?

Is it linked to the "temperature" or blower fan speed controls in any way? I am still talking about manual systems, not climate control
Sensor probe of the Thermostat ( in most cases it is the regulator switch on the Dash) is fixed on the cooling coils of your blower unit.The blower just throws ambient air and when this passes through the cooling coils or the heating element it becomes cold or hot as per the setting. When the coils have cooled enough the regulator cuts of the compressor. Thats all the Thermostat does. Sounds simple, but to do that millions of times it has to be a very sensitive and reliable piece of device.

In order to make sure that AC system is working well the grille temperature should read +4 degrees +/- 1 degree. The ideal cabin Temp should be 22-27 degrees and the relative Humidity should be 55-70%.

So long....
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Old 13th June 2006, 21:06   #3
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Varun, thanks for the information! a couple of clarifications required, though....

Quote:
Originally Posted by varunroy
( in most cases it is the regulator switch on the Dash)
you mean the mixer control itself, or is there any separate control for thermostat?

Quote:
In order to make sure that AC system is working well the grille temperature should read +4 degrees +/- 1 degree.
I guess by "grille" you are referring to the cooling coil?
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Old 13th June 2006, 22:29   #4
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[quote]
Quote:
Originally Posted by santosh.s
you mean the mixer control itself, or is there any separate control for thermostat?
Its the circular swiitch like the Regulator of a ceiling fan with marking in blue and red bands or in some cars it is shown as High, medium & low. Its the knob you turn clockwise or anticlockwise to control the temperature in your car.

In some top end model you have a seperate knob to direct the Airflow to the lower levels or at your feet, to the middle or either from the sides of your car. This switch does not have a Thermostat but has a simple mechanical flap that direct the air through different ducts leading to this vents.

Quote:
I guess by "grille" you are referring to the cooling coil?
No, its not the cooling coil but the vent openings on the dash from where the cold/hot air gushes out. These vents have flaps that move up & down, left & right that help you direct the air flow to the required area.

So long....
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Old 14th June 2006, 21:34   #5
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Hey Santosh....I think many of the assumptions you have made are not correct to start with.

"It seems most of the manual systems have a "temperature" knob or slider which essentially mixes cold and hot air"

IMO this is not correct. I presume you are saying a mixing happens so as to control the temperarture of the air. I can explain further how temperature is contolled but for a quick explanation think about it this way.....what about cars that dont have a heater (Indica Base models for example)....what about a home window a/c which does not have a heater.....the temperarture is controlled in these devices also. The way it is done is by controlling the amount of time the compressor runs (in case of cooling) and by controlling the amount of air that passes over the hot fins (in case of heating).

"but I have never seen such a switch for heater counterpart!"

Simply because heating is free in a car. There is no power hungry compressor to run...just the blower fan.

when the AC compressor is cut in (engaged to the engine), extreem cold air from AC gets mixed with extreem hot air from heater as per the knob setting.

As explained above this is incorrect. Cabin temperatue is maintained by controlling the running time of the compressor not by mixing hot and cold air.

This air gets mixed with extreem hot air and the result is hotter than the ambient temp, which result in fast warming up of the cabin temp

False. When compressor gets cut off...just the cabin air keeps recirculating. It does not get mixed with anything.

As a result, the system cycles through cooling and heating alternately.

It doesnt.

The cabin temperature fluctuates too much around the desired level, making it either too hot or too cold most of the time.

The way to get arounf this is by selecting the right combo for the temperature switch (sliding knob with red/blue markings in the baleno) and blower speed. In cars with climate control this is achieved by varying the blower speed.

heating is not required, rather it is undesirable .......

Yes it is.....and heating does not happen.

I hope this will clarify.
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Old 15th June 2006, 10:35   #6
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Quote:
Originally Posted by ritik123
Hey Santosh....I think many of the assumptions you have made are not correct to start with.

"It seems most of the manual systems have a "temperature" knob or slider which essentially mixes cold and hot air"

IMO this is not correct. I presume you are saying a mixing happens so as to control the temperarture of the air. I can explain further how temperature is contolled but for a quick explanation think about it this way.....what about cars that dont have a heater (Indica Base models for example)....what about a home window a/c which does not have a heater.....the temperarture is controlled in these devices also. The way it is done is by controlling the amount of time the compressor runs (in case of cooling) and by controlling the amount of air that passes over the hot fins (in case of heating).
My concerns are about systems with heaters only (as the title itself implies). I agree regarding systems without a heater, there has to be some other kind of temperature control.




Quote:
"but I have never seen such a switch for heater counterpart!"

Simply because heating is free in a car. There is no power hungry compressor to run...just the blower fan.
Yes, I am aware of that. In fact using heater aids in engine cooling...
Alas... overheating doesn't happen in winters, though!

I am bothered about heated air being used for temperature control below outside temperature, not otherwise. That is the reason why I wish there was a heater on-off switch available. Similarly, for getting temperatures above outside, we don't need AC (unless for dehumidifying).





Quote:
Quote:
when the AC compressor is cut in (engaged to the engine), extreem cold air from AC gets mixed with extreem hot air from heater as per the knob setting.
Quote:
As explained above this is incorrect. Cabin temperatue is maintained by controlling the running time of the compressor not by mixing hot and cold air.
May be different car systems use different methods. But at least for the the baleno it seems like hot-cold mixing method is used. The temperature slider also seems to be "all mechanical", doesn't seem to have any electrical control associated with it. As per my knowledge the same is the case with most other cars (which have both AC and heater).

I tend to support Varun's explanation about compressor on-off being controlled by a thermostat, which just maintains cooling coils at low enough temperature. But that thermostat doesn't directly monitor the cabin temperature (unlike automatic climate control, which uses additional temperature sensors inside and outside cabin)






Quote:
Quote:
This air gets mixed with extreem hot air and the result is hotter than the ambient temp, which result in fast warming up of the cabin temp
Quote:
False. When compressor gets cut off...just the cabin air keeps recirculating. It does not get mixed with anything.
Quote:
As a result, the system cycles through cooling and heating alternately.
Quote:

It doesnt.
As explained earlier, I believe it does. The problem does not arise when there is no heater in the system at all. BTW, which car(s) do you use and do they have heaters?... I think most cars do have.







Quote:
Quote:
The cabin temperature fluctuates too much around the desired level, making it either too hot or too cold most of the time.
Quote:

The way to get arounf this is by selecting the right combo for the temperature switch (sliding knob with red/blue markings in the baleno) and blower speed. In cars with climate control this is achieved by varying the blower speed.
I think the fact that we have to play around with blower speed in addition to temperature control itself, in a way, indicate that the problem exists! If there was no mixing of hot air, then we wouldn't have faced the problem of large deviations in temperatures in the first place (it would be relatively lesser)... irrespective of blower speed.







Quote:
Quote:
heating is not required, rather it is undesirable .......
Quote:

Yes it is.....and heating does not happen.

I hope this will clarify.
Need I say any more... my initial argument still holds!
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Old 15th June 2006, 11:35   #7
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Rather dumb question here: Why can't you keep the a/c setting on MAX cool? It will completely cut off heater. On other end of same switch, it will cut off a/c and keep only heater on. So, where is the problem?
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Old 15th June 2006, 11:57   #8
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Quote:
Rather dumb question here: Why can't you keep the a/c setting on MAX cool? It will completely cut off heater. On other end of same switch, it will cut off a/c and keep only heater on. So, where is the problem?
Spot on RX 135 !
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Old 15th June 2006, 12:57   #9
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If we were to use only MAX settings for both cold and hot then you guys mean you don't want/need the temperature control at all?? IMHO, it will make the cabin either a freezer or an oven, unless its peak summer or winter .

Relatively, MAX COOL is rather okay. Actually thats what I am doing right now...I put it at MAX COOL position and manually keep pressing the AC on-off button as per the need. If I don't do that then it gets too cold, even at lowest blower speed! In this case here is what happens-

compressor ON - it gets too cold, need to switch off the AC button (and once again later to restart it)
compressor OFF - No problem, it warms up slowly

But shouldn't temp. control free you from this trouble of having manual intervention every now and then? So instead of manually switching the AC button, one should be able to change the temp control slider to a higher temp (though still on the COOL side). But another problem that I noticed with this is that it becomes noticeably hot when compressor is off. So in this case-

compressor ON - No problem, it doesn't blow too cool air
compressor OFF - It gets uncomfortably hotter than desired

In short, we are not able to get comfortable temp in both compressor on and off periods. If we try to set one, the other gets spoiled! I have started this thread because I think if we have ability to shut off the heater, or more precisely convert it to "normal" air when in off position, then this problem can be solved. We will have much better and smoother control over the cabin temperature while using AC. But then there won't be any motivation for people to go for climate control

You can appreciate my point by trying this out: just turn the AC OFF and keep the temperature control in between MAX COOL and MIDDLE position, such that it is little closer to the MIDDLE (just to aggravate the problem). You should notice that the air blown is warm! (of course, here I am assuming that the system in your car uses similar mechanism as in mine)
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Old 15th June 2006, 15:37   #10
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Default Is this how the manual HVAC really works ?

I remember reading somewhere that the manual HVAC systems (like in a Baleno)
are rather simple mechanisms with no user accessible thermostat controls.
With the AC switch turned on, the compressor is always on (it never turns off
unless the temperature falls rather low, below a certain temperature preset on
a thermostat). The only control accessible to the user (apart from the fan) is the
cool/heat lever/knob which affords a desired mix of cold/hot air, to adjust the
cabin temperature (rather like adjusting the temperature of water in a shower).
If you turn the AC on/off using the switch frequently, you are trying to control
the temperature in a manner it was probably not designed for. The benefit is
perhaps a few percent fuel saving, at the cost of added aggravation to yourself,
and also, perhaps, more cycling stress imposed on a compressor designed for
continuous duty. The automatic climate control mechanism (like on a new Baleno)
is, of course, more sophisticated.
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Old 15th June 2006, 16:43   #11
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Quote:
Originally Posted by santosh.s
If we were to use only MAX settings for both cold and hot then you guys mean you don't want/need the temperature control at all?? IMHO, it will make the cabin either a freezer or an oven, unless its peak summer or winter .
It surely will, which is why the adjustable slider/rotary control is there.

So, I must be missing some point here, but your original question was how to cut off heater. This is possible with MAX cool setting, isn't it? This is what you'll achieve by closing heater vents/closing heater core (if I understand you correctly).
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Old 15th June 2006, 17:07   #12
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Meer, thats exactly how I understand it!

With AC on, the "hidden" thermostat actually runs and stops the compressor after every 10-40 seconds (roughly). Its preset temperature might be just above freezing as mentioned by Varun. So, I hope the system should be robust enough to bear that. Manual switching by me is done after longer periods, minimum being a few minutes and that shouldn't be an issue (?)

Moreover, the AC switch and all other air-flow controls in baleno are electronic type, which just need a soft push to operate like computer or ATM keys. So I think even those will not wear so easily. Blower control is a 4 step electric regulator knob (old type) whereas air intake and temperature "mixer" are mechanical.

Quote:
The benefit is
perhaps a few percent fuel saving, at the cost of added aggravation to yourself
That is not my primary concern... don't get me wrong! More that fuel saving, I am trying to get rid of the irritation caused by temperature deviations (in spite of having "better than merc xyz class AC" as advertised by maruti ). I remember some other people also complaining about too hot - too cold problem.

As of now, I prefer to switch the AC button. It is very convenient and has a LED indicator in the switch itself.
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Old 15th June 2006, 18:18   #13
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Quote:
Originally Posted by RX135
So, I must be missing some point here, but your original question was how to cut off heater. This is possible with MAX cool setting, isn't it? This is what you'll achieve by closing heater vents/closing heater core (if I understand you correctly).
You are right, if you go only by the title... but I hope you might have read the whole 1st post

Let me put it this way- I am not satisfied with the temperature control while using the AC. Because there is considerable fluctuation of temperature as the compressor automatically passes through ON and OFF stages. I am thinking that it is because of heater always being in ON state. If we are able to stop the heater whenever required, then IMHO, the temperature control will be easier, more accurate and most importantly the amount of temperature fluctuation will be considerably reduced.

Here are further details (sorry for repeating...) :

Let us say you are using only heater, AC is switched OFF. When the "temperature" or "mixer" knob is at MAX-HOT position, you get highest temperature. As you keep pushing it towards the other side, more and more "normal" air will be mixed with it, thereby reducing the temperature. By "normal" I mean the air already inside the cabin or outside air depending on the circulation mode. When you reach the other end (MAX-COOL), you will get only normal air, and hence the ambient temperature. You can not reduce the temperature further.

Now consider the case when you switch ON the AC. Instead of hot-normal air mixture, you will get hot-cold air mixture. When the knob is at MAX-COOL, you get the lowest temperature. When it is at MAX-HOT, you get the maximum temperature. Obviously somewhere in the middle, you would have observed no change in the temperature, where hot and cold air balance each other. This point need not be exactly at the middle because the difference between ambient temperatue and cold and hot air respectively may not be equal, though the volume of hot and cold air may be exactly equal. Therefore, the temperature control will be less accurate (less precision and granularity) as compared to earlier "heater only" case. Moreover, while AC in ON, compressor will be automatically turned ON and OFF. So, actually you will have hot-cold mixture when its ON and hot-normal mixture when its OFF. Due to this there will be considerable difference between the temperature during each period. The difference diminishes to minimum when the knob is at MAX-COOL position (or MAX-HOT for that matter).

My argument is that if we turn the heater OFF, then you can have normal-cold and normal-normal mixtures during compressor ON and OFF periods respectively. As the knob moves from MAX-COOL to MAX-HOT, we actually get temperatue from minimum to ambient. It will be more easy (more granularity), precise control with much reduced temperature deviations. Overall a more desirable/comfortable "mixer" with "AC only" operation, as compared to the default hot-cold mixing which is rather crude (relatively)


BTW, Meer had given a perfect analogy of shower mixer! I hate "just for the sake" kind of things...have similar opinion about normal (two taps) mixer versus single lever (rotary cartridge type) w.r.t. their ability to control water temperature and ease of operation. So I have actually used single lever mixers everywhere in my house
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Old 15th June 2006, 18:51   #14
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Default Aaaaaaaaarrrgghhhhh...

Quote:
Originally Posted by santosh.s
I have a few queries about car's heating, ventilation and air conditioning system (HVAC). It seems most of the manual systems have a "temperature" knob or slider which essentially mixes cold and hot air, obviously comming from AC and heater (better known as heater core, which is nothing but a secondary radiator system). AC can be switched on or off independently but I have never seen such a switch for heater counterpart! If you are wondering why should one need it, here is the reason-

Just for simplification, assume that we are using re-circulate mode.
Temperature of the air comming out of AC is much below ambient (cabin) temperature, and it is too cold. Whereas the air from heater is too hot. The idea is to create right misture to achieve a temperature little below the ambient, but above what AC produces and maintain it so as to make it as much comfortable as we can. Let us assume temperatures are-

T1 - cold air from the AC.
T2 - the ambient (cabin).
T3 - hot air from the heater core.

Clearly the relation is, T1 < T2 < T3

When we set the knob to a desired level, what happens is this:
- when the AC compressor is cut in (engaged to the engine), extreem cold air from AC gets mixed with extreem hot air from heater as per the knob setting. The net effect is that the cabin temp starts decreasing. So, T1+T3 = cooling.
- when the AC compressor gets cut off (disengaged), air from AC becomes "ambient" air, which is nothing but air from the cabin itself. This air gets mixed with extreem hot air and the result is hotter than the ambient temp, which result in fast warming up of the cabin temp! So, T2+T3 = heating.

As a result, the system cycles through cooling and heating alternately. I think there are two disadvantages with this-
1. The cabin temperature fluctuates too much around the desired level, making it either too hot or too cold most of the time. Hence its a compromise on comfort.
2. heating is not required, rather it is undesirable because AC gets unnecessarily forced to work harder than needed, to compensate for the extra heat.

Both these problems could have been overcome by having ability to shut off the heater. It will lead to less temperature deviations and saving in power (hence the fuel) required to run the compressor. Instead of cooling and heating cycles, it will result in cooling and "just ventilation" cycles. Similar reasoning applies for "fresh air" mode as well, the only different is that outside air is used for mixing, which would still be closer to the cabin temperature than extreem hot and cold.

I think it can easily be done by bypassing the heater core by connecting its inlet directly to the outlet. But this would be a permanent change which is not really a good option. We need heater in cold or rainy wheather for demisting (luckily most cars, at least my baleno, allow us to use AC and heater both at same time, which is requred for demisting). Better solution is to have some kind of valve control to select between heater and bypass duct paths.

What say guys? has anybody tried this?




Other queries about the compressor on-off control itself:

How is it controlled? some people say that there is a dedicated thermostat for this function, whereas others say that it has fixed dealys... confussing

If it is a thermostat type, where is it generally located and what is commonly used temperature at which it switches?

Is it linked to the "temperature" or blower fan speed controls in any way? I am still talking about manual systems, not climate control


The heating cycle of the cars work differently. They do NOT consume Engine power unlike AC..

The way to shut the heater is by the thermostat knob / button only..

We should try and keep points simple rather than elaborate on the same thing many times over!!

No offense to anybody please!

Last edited by Rehaan : 13th July 2008 at 10:59. Reason: 4 smileys
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Old 15th June 2006, 20:26   #15
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[quote]
Quote:
Originally Posted by santosh.s
Now consider the case when you switch ON the AC. Instead of hot-normal air mixture, you will get hot-cold air mixture. When the knob is at MAX-COOL, you get the lowest temperature. When it is at MAX-HOT, you get the maximum temperature. Obviously somewhere in the middle, you would have observed no change in the temperature, where hot and cold air balance each other.
Are you trying to imply that the compressor and the heater are both working at the same time.


[quote]
Quote:
Originally Posted by santosh.s
BTW, Meer had given a perfect analogy of shower mixer! I hate "just for the sake" kind of things...have similar opinion about normal (two taps) mixer versus single lever (rotary cartridge type) w.r.t. their ability to control water temperature and ease of operation. So I have actually used single lever mixers everywhere in my house [/quote
]


I am sorry but a shower mixture works in a different way. There is no thermostat in the mixture that controls the temp ot the water. The Hot and the cold water comes from two different sources and the mixture just blends the 2 to the desired requirement. While In the AC/Heater system the two never ever work together. The inbuilt system switches off either of them depending on the Cabin temp required.

[quote]
Quote:
Originally Posted by headers

The heating cycle of the cars work differently. They do NOT consume Engine power unlike AC..

The way to shut the heater is by the thermostat knob / button only..
While the AC relies on the compressor which is driven by the Engine the Heating element/coil derives its huge power requirements from the battery.

[quote]
Quote:
Originally Posted by santosh.s
Yes, I am aware of that. In fact using heater aids in engine cooling...
Alas... overheating doesn't happen in winters, though!
Therefore for you to say that the heater aids in engine cooling would be a wrong statement.

So long....
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