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Discussion Starter · #1 ·
It occurred to me today, when running HOLD for heat, that this is more complicated a question than it seems.

There must be 4 different temperatures (at least 4), go with me here and see where it ends up:

A - the temperature at which the heater blend valve to cabin shuts off coolant from the engine block (as coolant cools)
B - the temperature at which the engine blend valve opens (as coolant warms)
C - the temperature at which the thermostat to/from the front radiator closes (as coolant cools)
D - the temperature at which the thermostat to/from the front radiator opens (as coolant warms)

Obviously, A<B and C<D.

It's not obvious if B<C.

It's also not obvious that these temperatures are fixed, though I'd guess at least C and D are.

I'll explain why this is significant.

Say you get into your cold Volt with the intention of heating the cab up with the engine in HOLD. You don't put the electric heating on. As the engine warms up, first it passes temp A and nothing happens, it is still warming up. Then it passes temp B and the valve to the cabin matrix opens and engine coolant flows around the matrix and delivers heat.

A must be some reasonable difference to B because if A and B were the same temperature, almost as soon as you turn the engine off, the valve would close and you'd be back with cold through the matrix. There must be some margin at which the engine heats up past A before opening B, and then allows the engine block coolant to cool down, back to A.

If A is too high, then you end up wasting warm coolant in the engine block, but if A is too low then if you turn the electric heating on you'll end up warming the engine block up with the electric heater.

You want D to be a good bit higher than B if you are engineering it for efficiency. You want to be able to warm the engine coolant up past B (so you can heat the cabin with the engine off, all the way back to A) but if D weren't much higher than B then you may end up wasting useful cabin heat out of the front radiator.

Let's say you have been running the engine for a while and gone all the way to D and now the engine is being cooled by the front radiator. You turn the engine off. What happens now? As the coolant cools, the system is still wasting heat out of the front radiator, because the thermostat won't close until it drops to C. But is it an electronic thermostat, and as soon as the engine stops the thermostat closes at D (or more)?

If the thermostat stays open all the way down to C, then that's just wasted heat that could have gone into the cabin.

Am I anywhere close to figuring out fine details of the heating system? There are some behaviours here that would help improve my efficient use of HOLD if only I could know.

For example, if D was very high, then it would make sense to run the engine for some time after reaching B (the start of getting cabin heat) right up to just under the thermostat temperature, then turn the engine off. This would give the maximum ratio of coolant heat into the cabin, and probably means the engine also ran more efficiently as it would have had less stop-start.

But if the engine reaches D almost as soon as it has reached B, then the most efficient approach would be to turn HOLD off as soon as you feel heat coming through.

Also, if the blend valve from the engine coolant operates only when that coolant is warmer than the electric heated side, then if you are using both electric and engine heating, it would make sense to try to avoid using electric heating any time towards the end of the journey because then the engine coolant circuit will stay 'open' and you can draw off as much of the engine heat as possible. If you electrically heat up the circuit on the cabin side and it causes the blend valve to close, then you'd have wasted the residual heat in the block.

Has GM thought about all of this, or is it a more 'dumb' and wasteful system?

What are the temperatures A, B, C & D?

... and you thought you knew how engine heating worked!!! :)

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Discussion Starter · #5 ·
Thanks, that's top information!

Unfortunate you can't see it the other way. The key to sucking all the heat out of the engine block is to know when it is inappropriate to turn the electric heating on during the block cool-down.

I had presumed ERDTT got up to temperature prior to rad opening, It'd make no sense to run it higher and lose heat to the rad. HOLD won't do this and will simply waste heat to the rad, unless you turn it off at the right moment.

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Discussion Starter · #6 · (Edited)
As far as I can tell from hanging out here for 7 years, it's just a conventional engine/heater core system with a supplemental electric resistance heater and electric pump for EV only cabin heat. There is no fancy footwork to conserve block heat.

If it's like most systems, there is no liquid valve between the block and the heater core. It's all air control flaps. The engine temperature control (mechanical thermostat) has nothing to do with cabin heat. It opens and closes to keep the engine at operating temperature.

Take LLninja's advice.
This can't be quite true though, because at some point the engine coolant and the heater circuit have to merge and de-merge.

If it was as simple as you suggest, then the electric heater would also heat the engine block up, and that would be a pure waste of electricity.

There's a reason the i3 doesn't use REx coolant for heat. It adds a huge level of control complexity.

Might have been easier to simply use two separate systems and two separate heater matrices!

Might have been even easier just to use dry direct-to-air electric heating elements?

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Discussion Starter · #10 · (Edited)
Thinking on it again, I am not sure why ERDTT doesn't go up to 70C or so. This would result in longer engine runs followed by longer heat delivery.

It would make sense to run HOLD right up to the point just before where the thermostat is about to open, but it would seem best to run it to the 'close' temp just to ensure you don't accidentally run the engine long enough to open the thermostat, where it'll stay open until it has lost the heat. I tend to burn gas until I feel heat, then leave it for just two or three more minutes of running, then turn the engine off. Looks about the right thing to do, based on your graph.

Electric heating while the coolant flow is still around the engine block would make sense. You can then shut off the electric heating just where you want the temperature to be, then it can cool down.

I guess they stopped that for the same misguided reasons they didn't implement a block heater software-option for engine heating during preconditioning.

So I think the best thing is to turn the engine off around 8 miles from destination, and heat will flow until around 4 miles. Once it is too cold for comfort, then and only then turn the electric heater on for a burst, and turn that off a few miles from destination. In that way, you'd have sucked as much heat from the block circuit as you could have, without 'trapping' warm coolant (that is slightly cooler than the electrically heated circuit) on the block side.

Conclusions for max efficiency when using HOLD for heating;
Run HOLD to 70C coolant temp if you can, then switch off.
Don't leave any warm coolant in the engine block; to do this [for the last run on HOLD you plan to do] let the heater run cold after running HOLD before running electric heating, if desired for just a little more top-up heating.
Turn off electric heating a few miles short of destination, then you arrive with no 'wasted' heat in either coolant circuit part.

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Discussion Starter · #14 ·
I started doing this last point over last winter.
I have an extremely short commute and it shaves probably 25% off the total energy use on extremely cold days.
Makes no sense to arrive at work with a pool of hot coolant to bleed energy to the parking lot. Better to use it all for me.
I hit 'fan only' about 3-4 mins before I arrive at work or home.
As you'll have probably found, though, if you use the electric heating last then it builds up a pool of humidity (as the AC is forced to run) which may start misting quickly, whereas if you use OFF/MIN with engine heating until the last few miles and shut that off, then you don't get that as the engine heating will have displaced the evaporator humidity. However, you may waste warm coolant left in the block.

I just wish there was a manual AC ON or OFF. Both ON and OFF would be valuable to maximise efficiency in cold weather.
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