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Now that it's winter time I'm again not very happy with the cabin heating of Touran. I installed a 2 kW heater unit that is powered by a Chinese inverter board. It sort of works down to 5°C, below that the cabin isn't exactly cosy.

So since I have quite a lot of space left in the engine compartment in front of the motor and gear box I though about installing some sort of heat pump there. The volume available if I move the cooling radiator is 76x50x30 cm³ (WxHxD).

Cheapest of the pack seem to be pool heatpumps, some of which work down to -10°C. They also come in a size compact enough to fit said volume. It seems they're limited to about 45°C output though, not sure if the heater matrix will work well with it.

They also allow cooling, so I'd get A/C in summer as well. Provided the condensation will drip out.

Any other ideas anyone? Ever tried this?

In the winter the water temp would need to be 55C or 60C so your heat pump wouldn't cut it.

I suspect I'm loosing a ton of heat by having the heater in the engine bay. The heater itself and the pipework are exposed to the cold and any heat coming off them is lost. I could have insulated them if I'd had the space but sadly not.
I was thinking a PTC heater in the heater matrix location would be more efficient and simpler than a heat pump.

Alibro wrote: ↑Thu Dec 04, 2025 2:36 pm In the winter the water temp would need to be 55C or 60C so your heat pump wouldn't cut it.

Not necessarily and it could also work in tandem with the 2kw heater to be sufficient when really needed.

45degC is certainly warm enough for most of the year, just the warm up time would be a bit slower than you'd want. If you have a way of plumbing a second matrix in parallel this would further improve the output.

A PTC will never be more efficient than a heat pump either. A heat pump will be around d 200% efficient at least, even with a poor system the PTC will.never be more than 100%.

I like this idea a lot. Johu, do you have any idea of what size heat pump would be required? If I go to my local pool shop website they are all quoted by kW capacity. I know a car aircon runs at about 2kw, but I’m not sure if this translates to pool heat pumps.

Typical compact heatpumps output about 2.5 kW of heat and use like 500-800W of electricity. The slimmest one I found is 260mm. Also currently looking at caravan air-to-air heatpumps (e.g. https://www.dometic.com/de-de/produkt/d ... 9620013224) as they would skip the tiny heater matrix and losses in the engine compartment. Not sure if I can find one that fits though

Jacobsmess wrote: ↑Thu Dec 04, 2025 2:54 pm Not necessarily and it could also work in tandem with the 2kw heater to be sufficient when really needed.

45degC is certainly warm enough for most of the year, just the warm up time would be a bit slower than you'd want. If you have a way of plumbing a second matrix in parallel this would further improve the output.

A PTC will never be more efficient than a heat pump either. A heat pump will be around d 200% efficient at least, even with a poor system the PTC will.never be more than 100%.

All true but a lot of extra work and I'm waaaaaay to lazy for that.

johu wrote: ↑Thu Dec 04, 2025 10:52 am ....
Any other ideas anyone? Ever tried this?

I actually kept OEM AC system on Peugeot and Megane and i am using Mitsu AC compressor and drive it via its CAN telegram.
I use a crude closed loop to keep compressor from overspin and reset.
In Peugeot there is not other consideration because it uses pressostat switch which will stop AC clutch signal when evaporator valve will freeze or pressure is too high.
This is safety issue because normal AC works at 6bar with condenser fans operating. If there is too hot outside and cooling fan cant remove heat pressure in the system could raise even to 25bar!!!! At that time you must stop compressor to dump heat out of the system and gas can get liquified.

Megane has different logic with only a pressure sensor on the condenser and a temp sensor on the evaporator. I am still struggling with safety control with using AC in winter.

I have sensors in place and simple logic works. But i dont dare to use AC in winter because i dont have cabin sensors figured out yet...
I imagine you could have some more piping and reverse the flow for heating with existing installation.

I remember VW EGolf was using two valves and evaporators (one as condenser) in order to get heat pump to work in reverse.
This means you would need another temp sensor to warn the system of icing on the outside condenser/evaporator when in winter and high temp in summer. See this document for reference... https://static.nhtsa.gov/odi/tsbs/2016/ ... 1-9999.pdf

A heatpump is basically just an A/C with one extra valve.

I would think of it more like "I want to get A/C, and maybe some heat for free".

I think a heat pump is the opposite solution for you though, as, they work less and less good as the temperature drops. So, you're saying the heater works until 5'C... well, the colder you get the less you'd even notice a heat pump running. It can't make up for too large of swings in temperature.

I wonder if you could use it to get part way though. Heat up something from outside temp to, I dunno, 15'C warmer than ambient, and then use that heat with electrical heat to carry it the rest of the way. That'd require some kind of awkward heat exchanger though.

The best and most practical solution I've seen for heating is to just use a diesel heater. I forget who here is a proponent of that.

When you want motion out of combustion, it's highly inefficient. But when you just want heat out of combustion, you're getting pretty good value.

Yeah it's not the perfect environmental solution, but, it's such a small amount of diesel used in an entire season, in lieu of chopping your range so significantly, I think it'll be what I go with.

MattsAwesomeStuff wrote: ↑Thu Dec 04, 2025 10:56 pm ....

The best and most practical solution I've seen for heating is to just use a diesel heater. I forget who here is a proponent of that.
...

That would be me. I see the fault in using motive energy (battery) to heat up the space inside. It is convenient as the heater is already there, but you are reducing the range for comfort.... and you shouldnt as you are buredoning the network because you need to charge more.
For temps less than -3degC i would argue electric heaters are not viable. 2nd law of TD will get at you allways!

The burner does introduce some added complexity and hazard of fire with the fuel. But if you must drive in -10degC i am sure you wont think twice after using diesel heater, its so warm .

How far is a Nissan leaf heat pump system from dropping in?
(For reference my wife's 2019 leaf with a heat pump drops about 3 miles with the heat on (62kwh pack, about 220 miles fully charged.)

The interior heat exchanger fitment and tubing/ fittings are the tricky bits.
May be able to reuse the VW evaporator.

arber333 wrote: ↑Thu Dec 04, 2025 11:28 pm That would be me. I see the fault in using motive energy (battery) to heat up the space inside. It is convenient as the heater is already there, but you are reducing the range for comfort.... and you shouldnt as you are buredoning the network because you need to charge more.
For temps less than -3degC i would argue electric heaters are not viable. 2nd law of TD will get at you allways!

The burner does introduce some added complexity and hazard of fire with the fuel. But if you must drive in -10degC i am sure you wont think twice after using diesel heater, its so warm .

I couldn't agree more with this. Not only to the points above but with the added points of being able to condition the battery if you use a diesel water heater. It is almost trivial to plumb into the existing heater matrix and the battery cooling loop (assuming you have one). As much as I love batteries for motive power, heating directly with electricity doesn't make much sense to me, there is no comparison in power density between even the best NMC and diesel, petrol or gas.

I'm coming from the domestic side were air-to-X heatpumps are currently making huge steps forward. They work down to -15°C. I will also install a R290 A/C in my living room next month, at mild temperatures that we usually have around here it has a COP above 5.

Now even a suboptimal one with a COP of 2 or 3 would still be better then COP<1 of the PTC.

If I could fit something in the spare wheelwell (18 cm heigh, 60 cm diameter) that would directly heat/cool the cabin without passing through water I could add some efficiency, too and leave the old PTC system in place in the front.
I'm not super worried about loosing range in winter as the longer trips usually happen in summer. It's more about general energy usage on short/medium trips in winter. Plus having some more heat output.

In my world fossil fuels ceased to exist. Recently dipped my finger into a puddle that happened to be Diesel. It smelled for hours and I couldn't wash it off. Let alone the smell when burning the stuff. Not playing around with that ever again.

A car style A/C sounds tempting but am hoping for a solution that doesn't involve working with the coolant loop itself, getting it gas-tight and stuff.

Have you had a look at Camper/Caravan heat pumps?

I realize the diesel predicament and I also think dino juice is a smelly sh...t.

What to use as alternative?
I saw in history logs of ww2 locals here used wooden gas stove to fuel converted delivery trucks since fuel was reserved for military. They had range of 40km and they needed a preheating of 20min for the gas to collect.
It was basically a CNG kind gas.

I think in future fuel burners methanol could be used. it is liquid, doesn't just explode and it is already a part of our industry. Nothing new here...

And of course there is propane. I think they make propane heaters for campers as well...

Ironically my car came with a diesel heater that I refuse to use. It would be trivial to plumb it in as I never removed it but It is a horrible noisy smelly thing that I don't like.
In Ireland the temps rarely drop below -5C except overnight and I don't use the car in the winter all that much so it's not a big issue for me.

tom91 wrote: ↑Fri Dec 05, 2025 10:47 am Have you had a look at Camper/Caravan heat pumps?

Yes I have. I found 3 models on ebay: https://www.ebay.de/sch/i.html?_nkw=sta ... &_osacat=0

Perhaps I could make them fit in the trunk but they are rather bulky.

There is also split versions: I have space below the wheelwell because the muffler used to sit there. Just a whole lot of debris there

This can be likely reworked to run off HV DC. I've converter some of their other units to run off of HV DC. This is actually made by Midea. It uses a inverter driven variable speed compressor.

https://www.highseer.com/products/pione ... onditioner

Please correct me if I am wrong but are these pretty much the same as so called "mono block" heat pump heaters/air conditioners?
I am looking for things for my Triumph Spitfire project but I think this is too big and heavy...
Still, an interesting idea!

johu wrote: ↑Thu Dec 04, 2025 6:26 pm Typical compact heatpumps output about 2.5 kW of heat and use like 500-800W of electricity. The slimmest one I found is 260mm. Also currently looking at caravan air-to-air heatpumps (e.g. https://www.dometic.com/de-de/produkt/d ... 9620013224) as they would skip the tiny heater matrix and losses in the engine compartment. Not sure if I can find one that fits though

Hm...

I have given this some thought and it seems you already have some components that were made for the car enviroment...
The DC to AC inverter which you can use to power climate module in your trunk
https://openinverter.org/forum/viewtopi ... 142#p86142

Yuo would have to be carefull as 7.2A fuse means about 1600W which is just out of range for that Mitsu unit... Maybe it can work with a little overhead... I bet you can have a remote to switch the unit ON and set it to heating or cooling...

Wouldn't be a problem to roll with the Chinese board for now which happily supplies 10A all day long, as long as none of it's shortcomings stop it from working alltogether.

johu wrote: ↑Fri Dec 05, 2025 9:45 amI'm coming from the domestic side were air-to-X heatpumps are currently making huge steps forward. They work down to -15°C. I will also install a R290 A/C in my living room next month, at mild temperatures that we usually have around here it has a COP above 5.

Unrelated to cars, but related to homes, I think this new A/C design will basically make everything else obsolete. It's amazingly more efficient for a small modification to existing A/Cs.

(skip to around 8 minutes if you want to avoid the "what is heat" background blah):



Bear with it, he's just awful at jumping into the useless details without introducing the basics. It'll come full circle.

MattsAwesomeStuff wrote: ↑Mon Dec 08, 2025 11:54 pm Unrelated to cars, but related to homes, I think this new A/C design will basically make everything else obsolete. It's amazingly more efficient for a small modification to existing A/Cs.
.....

Hm... for car i think those units are a bit shady. They are not meant for harsh enviroment that a car is. Namely vibration and temperature variations.

Motorhome units however would do i think... need to try one . They would only work with air to air exchange though.

johu wrote: ↑Fri Dec 05, 2025 11:15 am
Perhaps I could make them fit in the trunk but they are rather bulky.

I have space below the wheelwell because the muffler used to sit there. Just a whole lot of debris there

If you find a unit that has the specs you want, but you can't fit it in its current physical form in the space you'd like to utilize. I've taken units such as these before and "repackaged" them into a form that'll better physically fit the new purpose. You can take units apart and relocate major parts such as the evaporator, condensor, compressor without invalidating the design and control electronics of the unit. You just have to have some HVAC service tools. Which I have anyway because they are so useful on all kinds of projects.

The only tricky part is recalculating the new refrigerant volume. Because this kind of repackaging typically involves lengthening refrigerant lines. But it's not that hard to do if you measure superheat and sub-cooling on the unit at a reference running condition and then use that data and the same running condition on the re-packaged setup to get the refrigerant charge right. Plus, most of the new variable compressor speed system have to account for such a wide range of operating conditions, and have such a large refrigerant accumulator, that if you get the refrigerant charge in the ball park, they'll work fine.

arber333 wrote: ↑Tue Dec 09, 2025 6:01 amHm... for car i think those units are a bit shady.

Yes, I meant for Johannes' home.