[paulgato]

Oh, so your winters there sound about the same as ours in England. It doesn't tend to get below -10C very often here.

Your experience is interesting. My 1800w coolant heater is also pumping warm coolant directly towards the heater matrix, but I found use of the car's cabin blower not very useful, and yet using it seriously slowed down engine warm-up.

The difference I guess is that my car's engine coolant pump is driven off the cam belt rather than being electrically powered. The coolant pre-heater's pump is quite small and weak in comparison. Yes, I can see that if you are using the main engine coolant pump then that can easily cope with a 5kw heater. A 240v electric shower, after all, will use 8kw and upwards. The coolant pre-heater I installed comes in a '3kw' or a '2kw' version. I went for the 2kw version so as to be able to run a battery charger and a fan heater off the same standard 13A RCD-protected outdoor power socket.

My feeling is that in the absence of a powerful coolant pump such as yours, I'd be inclined to distribute the heating power, so for example I could add a modestly-rated thermostat-free pad heater to the oil sump pan, and possibly a similar pad heater to the gearbox housing.

The pre-heated circulating coolant is maintained thermostatically at between 75 and 85 degrees C (10 degree C hysteresis) but as soon as the engine is turned over that coolant temperature drops immediately by between 10 and 15 degrees C. One reason for that will be cooling via the oil/coolant heat exchanger (AKA 'oil cooler') but also my secondary pump will probably not pump heated coolant around all parts of the engine as the primary engine coolant pump does with the engine running. Not much I can do about those unheated coolant pathways, but I can safely heat the 5 litres of oil in the sump to 150 degrees C without risk of degradation. The pan is aluminium so will spread any heat quite efficiently and not create a hot spot in one location.

The difficulty is that then I'd need a 2nd 13A outdoor circuit, and that's not entirely practical in this house, for reasons I won't bore you with. I guess I might be able to run a 300w pad heater without issue on the same circuit, but I'd rather not even do that. One possibility is to use the coolant preheater's thermostat to switch the oil pan pad heater and gearbox heater on whenever the coolant heater element switches off.

But it looks like you need a plumber there. Those fittings look like standard water or gas pipework fittings, and all those bushes you need will be easily available I would imagine in the plumbing world. At least they would be in the UK.