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Re: Why hot water heating?

Posted: Wed Feb 15, 2012 12:02 pm
by zowland
tesla500 wrote:Why did Nissan choose to use hot water based cabin heating? It seems to make much more sense to heat the air directly, and avoid wasting energy and time heating up the thermal mass of all that water.

My Pontiac Firefly electric conversion uses a 1.5kW ceramic direct-to-air heater, which is hot 10 seconds after turning the car on. The LEAF is a downgrade for me in this respect!
I think the use of hot water heating also keeps the cabin temperature from varying as much as might happen with a direct electric element to air heating design.

When I look at the climate control system energy use on the nav panel, it looks like maximum heating energy use may go as high as 4 kW. If the direct heating coil was using that much energy and the fan failed, the heating element would probably get very hot in just a couple of seconds, even if a thermal safety switch was installed in the system. Might result in a melted heat chamber, or even a fire. Wouldn't want to have that if the car was unoccupied, preheating in the garage.

They also tend to smell when they are first turned on, particularly if dust has collected on the coils.

B>

Re: Why hot water heating?

Posted: Wed Feb 15, 2012 1:21 pm
by Ingineer
I think safety is the prime reason they opted for the hot water system. This makes the possibility for fire almost zero, say if something flammable got sucked into the system, or an accumulation of dust, etc. Secondly, it keeps 400 volts out of the cabin, which, say if there was water intrusion, could be a really bad thing.

Thirdly, it keeps EMI/RFI from being radiated from the heater coils, as they'd effectively become a big antenna for broadcasting the inverter's switching noise.

And finally, it meant they can re-use an off-the-shelf existing climate control block with a standard heater core.

There are a few other benefits as well, such as thermal mass in the coolant to retain heat after pre-heating on grid power.

Still, I think the addition of a reversing valve and a electronically controlled expansion valve would have been a good idea. All the rest of the controls and sensors are already there, so all it would take is software. Ideally, put the PTC heater elements in the refrigerant loop instead of in a separate coolant system. This would be your heater boost system in colder weather, and also aid in evaporator defrost if it ever became needed. Given the range limitations, evaporator icing might not ever be an issue anyway.

As for retrofit, this isn't going to happen. The integration cost is too high without access to Nissan's source code for the HVAC ECU.

-Phil

Re: Why hot water heating?

Posted: Wed Feb 15, 2012 1:36 pm
by LTLFTcomposite
Ingineer wrote:I think safety is the prime reason they opted for the hot water system. This makes the possibility for fire almost zero, say if something flammable got sucked into the system, or an accumulation of dust, etc. Secondly, it keeps 400 volts out of the cabin, which, say if there was water intrusion, could be a really bad thing.

Thirdly, it keeps EMI/RFI from being radiated from the heater coils, as they'd effectively become a big antenna for broadcasting the inverter's switching noise.

And finally, it meant they can re-use an off-the-shelf existing climate control block with a standard heater core.

There are a few other benefits as well, such as thermal mass in the coolant to retain heat after pre-heating on grid power.

Still, I think the addition of a reversing valve and a electronically controlled expansion valve would have been a good idea. All the rest of the controls and sensors are already there, so all it would take is software. Ideally, put the PTC heater elements in the refrigerant loop instead of in a separate coolant system. This would be your heater boost system in colder weather, and also aid in evaporator defrost if it ever became needed. Given the range limitations, evaporator icing might not ever be an issue anyway.

As for retrofit, this isn't going to happen. The integration cost is too high without access to Nissan's source code for the HVAC ECU.

-Phil

-Phil
Those sound like a lot of good reasons, all of which beat "because they are stupid". I had wondered too if a direct air resistance heat strip would be less durable than a liquid immersion heating element, particularly when subjected to vibration.

Re: Why hot water heating?

Posted: Wed Feb 15, 2012 3:26 pm
by Ingineer
LTLFTcomposite wrote:Those sound like a lot of good reasons, all of which beat "because they are stupid". I had wondered too if a direct air resistance heat strip would be less durable than a liquid immersion heating element, particularly when subjected to vibration.
They use a PTC (Positive Temperature Coefficient) element in the water heater. They are also used in many small portable space heaters now because of their durability and safety. I don't believe durability would be a factor, as I suspect an air cooled unit would last longer than the liquid cooled unit will. This is especially true if you are using filtered air, as it would be in the Leaf.

-Phil

Re: Why hot water heating?

Posted: Wed Feb 15, 2012 3:57 pm
by LTLFTcomposite
Ingineer wrote:an air cooled unit would last longer than the liquid cooled unit will
-Phil
So a coil of wire glowing orange will outlast a water heater sort of element immersed in coolant? I find that surprising... after seeing so many my wife's (admittedly el cheapo) hair dryers crap out in a puff of smoke.

Re: Why hot water heating?

Posted: Wed Feb 15, 2012 6:03 pm
by Ingineer
LTLFTcomposite wrote:So a coil of wire glowing orange will outlast a water heater sort of element immersed in coolant? I find that surprising... after seeing so many my wife's (admittedly el cheapo) hair dryers crap out in a puff of smoke.
PTC heaters are not "coils of glowing wire", they are a variable resistive substrate bonded to or mixed with a ceramic substrate. You wife's hair dryers are no comparison, but what the most common failure mode for those is the brushed DC cheap fan motor or the switches. The fan motors die most often from hair wrapped around their shafts which cause overload and burnout.

-Phil