# Thursday, April 27, 2006

The other day, my car overheated -- pegged the temperature gauge -- after about 5 minutes of driving. I limped it home and discovered it was out of engine coolant. After I added more I could drive for a few minutes but again it violently overheated and all the coolant was gone. With visions of expensive repairs dancing in my head I took it to my neighbourhood mechanic.

It turned out to be a cheaper repair than I expected, but a more frustrating one. You see, the minivan I was driving has a separate heating system for the back seats. Their own controls, vents, everything. We actually use this perhaps once a year, it's a low-value frill. But here's how it works: they pipe the engine coolant halfway back the car so it can go through a heat exchanger and warm up the air in the back. And of course it makes cars easier to build if all the pipes and wires are underneath the car. Over the years, kicked up stones and gravel weakened this coolant pipe until it finally broke, spewing precious engine coolant all over the road. I could have easily lost the engine or been stranded hundreds of miles from home. And for what? A separate heating system in the back? Who cares?

What could they have done instead? They could have piped the hot air instead of the hot coolant, so a leak would only make the back seat cold instead of ruining the car. (I know, from my former life as a chemical engineer, how much easier and cheaper it is to move liquid than air.) They could have installed a sensor that would detect low coolant levels directly, instead of indirectly when the car overheats. They could have run the piping in some protective shroud. (Ha! This is the company that blames me when the wiper motor breaks because the car gets snow and ice on the windshield! Three times so far on this van.) They could have skipped the whole feature on the grounds it made the car way too vulnerable for the benefit it offered.

I see a moral in this story for software developers. What features are you writing that turn out to be a heating system for the back of the van?


Thursday, April 27, 2006 8:09:49 AM (Eastern Daylight Time, UTC-04:00)  #    Comments [3]
Thursday, April 27, 2006 10:53:39 AM (Eastern Daylight Time, UTC-04:00)

It may be cheaper to move a gas than move a liquid, but you have forgotten about the most important characteristic. The coolant going to the back seat is hot! It needs to stay hot enough to be transferred to hot air through the heater in the back.

Liquid has a much higher heat retention ratio than any gas. If the hot air could stay hot all the way to the back of the vehicle inside the pipe, the engineers would certainly have done that a long time ago. If you did push hot air through a pipe, you would want it have the same heat level in the back as you currently get from the liquid/gas heat exchanger that is currently in the back. To do that the air in front would have to be a lot hotter because of the rapid heat loss. This would probably require heat that is higher than the heat generated by the engine.

Keep in mind that the manufacturers want to make the car as cheaply as possible so they can make more money when selling it. If it really were cheaper to push hot air the engineers would be doing it.

This same debate can be made with heating a house. Do you use forced air heat or do you use radiant heating (radiators)? Strictly from an efficiency standpoint, radiant heat systems are cheaper to operate. Of course, radiant system can't do air conditioning and humidification, which is why most homes today use forced air.

Don Benson
Friday, April 28, 2006 5:06:03 PM (Eastern Daylight Time, UTC-04:00)
"(I know, from my former life as a chemical engineer, how much easier and cheaper it is to move liquid than air.)"

I think you misread that Don.

Monday, May 01, 2006 8:45:41 AM (Eastern Daylight Time, UTC-04:00)
Yes, they took the easy and cheap way. If someone had said they couldn't pipe the coolant back there, but would have to run all the control wires forward to where the hot stuff is, and then blow the hot air back, in some sort of insulated thing so it would still be hot, that would have cost THEM and someone would have killed the feature. But nobody stopped them from costing ME with a vulnerability, so the feature went ahead. Why not? There are probably people who bought Van A over Van B because of the back seat heating system.

Me? I'm never buying from that manufacturer again -- every time there's a cheap option, even when that leads to defects for the owners later that cost hundreds of times the amount saved by the cheap option, these guys have clearly gone for the cheap option. That's their choice, but it's not the kind of car I want to pay over 30,000 Canadian dollars to own. I am really fed up of owning this car with it's ever-breaking wiper motor, door handles, and whatnot, but I count my blessings that it didn't overheat itself to death and make me run out and buy a new one over the weekend.

To tie this back to being an actual chemical engineer who designed bits of plants, we used to have a lot of meetings about whether to pipe the stuff you wanted to warm up or the stuff that was hot. Piping oil is easier than water, for example, because oil doesn't corrode pipes and doesn't freeze. But we weren't allowed to just pipe the hot oil all over the plant, in case a pipe broke. You had to consider what units would go down and also the environmental issues of the leak. In one case we pumped water about a mile roundtrip - in expensive moly pipe - because it was just a boiler preheat and we couldn't make the main unit vulnerable by taking a stream across the refinery and back. I had forgotten that until now.
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