Engine-heated (Hydronic) hot water?


Well-known member
Hi, planning to build my pop-up with a small, insulated hot water tank for cleaning and bathing, principally heated with the engine coolant or a diesel engine pre-heater, & possibly 110v &/or 12v from my solar. Before I start I want to make a long-winded post to try and keep warm.

I'm gathering parts and hoping to make a solid plan before I cut into my coolant lines. If I can make it work, I'd like to add a radiator/blower type heater for the camper to the coolant loop as well. This will be permanently under and in the bed of my pickup, so it will need to survive being in the snow and altitude in winter, and functional then would be even better.

I have a few questions and am open to design suggestions-- getting this working on the first go would be ideal.
I have been building and tinkering my whole life, and 15 years with this truck in particular, so there's a fair chance that could be possible.


The main components are a marine water heater like the Isotherm Spa or a Shure-Just Horizon 20 liter tank, a Chinese Webasto Thermo Top knockoff and some sort of fan-blown radiator for heat in the camper. The tank will live in a front corner of the bed, the heater underneath the bed and the radiator wherever it fits in the camper. The system will have at least one circulation pump, probably between the engine and the diesel heater, and enough valves to choose what operates or cut-out the auxiliary coolant loop all together.

The parking heater wasn't in my original design, but the price was good enough to risk the added complexity, and if I can add forced-air heating without running the truck, that seems worth the effort of integrating it. Using a water heater instead of the common diesel air heater lets me mount the heater outside, cut smaller holes for the water tubes, and run those to wherever I want to put the tank and blower. It also means those systems can work off engine heat alone even if the heater craps-out.

I plan to tap into the engine coolant loop somewhere along the cab-heater return hose, which seems to be a common takeoff for OEM heat in the back of vans. My hoses are 3/4" there, and I have a number of 3/4" manual and electronic valves and fittings that incline me to stick to that size, though many of the common fittings are 5/8". The domestic water will probably be run in 1/2" or 3/8" PEX, both the hot and cold will be drawn from the same storage tank & pressurized with a 12v. pump. The parking heater will run on diesel or gas, apparently, but I'll probably stick to a dedicated tank instead of tapping into my vehicle gas tank.

View attachment 834193
In the picture above, the thing in the right corner is the OEM heater and the part below it the 12v aux pump. left of that is the heater and the tank and camper heater are not shown.

With the coolant moving clockwise, the tank and forced-air heater will fit after the parking heater, but still in a loop that could be isolated from the engine. I have a few "Tee" valves that should work for this, and If I park for a time and don't care about the engine or cab, I can limit the heating to the back of the truck & the water. If I get a leak or a break, hopefully I can isolate it and keep the truck functional. One can find a "heater bypass" valve assembly that interrupts both legs of the coolant loop, which might be a clean way to segregate the Aux loop.

So, a few sticking points;
Unless I go through the firewall and the cab, the coolant lines to the back have to run really close to the exhaust system, and I'm not totally comfortable snaking heater hose around the cats or along the transmission tunnel... how hot can that get? Can a regular hose withstand that?

I think there's room to fit some 3/4" annealed copper from the engine to behind the transmission for the supply and return, possibly insulated with a high-temp fiberglass wrap. I'm not totally confidant in my soldering skills, so I will probably use flare fittings I can torque down to resist shocks and vibrations, is there a better way I haven't considered?

Is it worth having a powered valve to close in an emergency if something in the system gets damaged or leaks?

I'm just basically hoping that the 3/4" tubing and supplied pump will overcome the friction losses from the tank, heater and all the various fittings. Costs dictate I limit the number of valves and fittings, but is there a better method for designing circulation loops? I'm guessing the pump capacity and duty-cycle are the most significant factors...

Lastly, the control functions of the heater are limiting, because the Germans who designed it had a different use in mind. The heater basically circulates for at least 10 minutes and up to an hour, because the object was to preheat the engine and cab, not a living area. Apparently the run-time is also bounded by a low temp of 140ºF (60ºC) and a high of 212º F (78ºC), it isn't clear if time or temp overrides one or the other, I'm hoping it's the temp.

If that's the case and I want to prolong the cycle by having the return water cooler than 140º, is it worth using a second pump to move the coolant through the vehicle radiator when the engine is off to bleed the excess heat after it has been through the water tank and forced-air heater?

Thanks for getting this far, any input is appreciated, there's a ton I've learned from this place already.


I have been running something like what you have in mind for 7 or 8 years.
I used a Webasto Thermotop E and a Surejust 30L calorifier with twin coils (one for the diesel heater and one for the engine) https://www.surejust.co.uk/30-litre-horizontal-single-coil-surecal-calorifier.
I used 5/8" auto heater hose for all plumbing and a 12V Davies Craig EWP80 auxiliary in-line pump.
I designed my own radiators (one for the bathroom and one for the living area) because commercial ones tend to be too noisy.
The short story is that it has been absolutely brilliant and totally trouble free.

I am now building another vehicle and it will have a similar system, with some minor modifications.
1. The diesel heater will be an Eberspacher D5E, because it can run at lower heat output without cycling.
2. The calorifier will be a 10L Surejust (single coil) because the reheat is so fast that 30 L is not required and I want to keep the weight down. Single coil means that the engine will be in series with the diesel heater instead of in parallel. 3 way valves will provide the option of including the engine or not. Often it will be included to provide hot water whenever we drive and because it provides an easy way to "dump" excessive heat.
Note that the only time the auxiliary pump is required is when preheating the engine using the diesel heater on the current system. At other times, the circulation of hot coolant is performed by the diesel heater pump or by the engine water pump. The new build will also have an auxiliary pump, but it may not in fact be required.
Note that the calorifier also has a built-in 1,000W A/C heating element that I can run via solar/inverter.
3. 3/4" heater hose for all plumbing instead of 5/8" to reduce losses.

4. Coolant to air radiators.
Single core copper automotive radiator material and high quality (German) 120mm fans.
The new ones will be the same except that the 120mm fans will be high speed and PWM controlled to give the option to increase the heat delivery if required.

One 'cool' thing we can do is to heat the bathroom without heating the habitat area which lets us use the bathroom as a cloths drying space. The heating system can of course run while driving with the engine providing all of the heat.

ps... the Webasto can be operated without the supplied control with a simple ON-OFF switch.

OKA196 motorhome
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Well-known member
I'm really glad to hear the details of your system and the upgrades, I feel like I'm on the right track. I know adding valving adds weight and cost (so many fittings!), but the ability to choose heat sources and zones is really appealing, even if I ultimately settle on just a few patterns.

I was waiting to hear if they could send me the heater with different fittings, so the news of you downsizing is timely. I'm just occupying the bed of my Ute, so more space and less weight sounds great! ++for the 3/4" tubing, less friction/noise and less surface to volume seem worth the marginal up-charge for parts.

I don't have a big inverter, but the Shure-Just has 12v & 120V elements, so I want to use it for solar dump-loads (my batteries are usually charged before noon when I'm parked). How would you integrate a thermostat to cut off the electrics before the relief valve opens?

With your custom radiators, are you just modulating the heat with the fans or do you adjust the valving as well?

With valving in general, if you close one end of a dedicated loop, like to the bathroom heater, can you leave the return open or do you shut-off both ends to fully isolate it? I'm thinking you still get some conductive transfer through an unmoving loop, but is it worth considering, or basically insignificant?

Thanks again for the input, it makes the designing the system more clear, even if no more simple.


Well-known member
The Webasto will also run controlled by a 12v Heatmiser thermostat. Forget the Webasto controller.
Hadn't seen these before, is there a particular recommendation?
I see 2 different 12v. versions, one discontinued and the other with a bunch of communication and wiring protocols I've never heard of.

I think air & water temp, manual on/off, a temperature hold &/or a timer would be useful, remote operation or weekly scheduling not so much.

The basic controller shipped with my ThermoTop copy is a 4-wire, and they caution one to let the heater cycle-down to cool before cutting the power, looks like the fan & pump both need to run for 3 seconds or 3 minutes (?) once it stops combusting because of the temperature ceiling or a shutdown command.

...I should probably bench-test this thing before I think about installing it:unsure:


For central heating, I have no thermostats in the system except for the one in the diesel heater itself that cycles it on/off if the glycol reaches top temperature, just diesel heater on/off and radiator fans on/off (plus speed control on the new one). I do open windows to reduce temperature inside or more usually just turn the system off.
For hot water I monitor water temperature and turn the system off when it is hot enough. If it is a couple of showers, I switch off after the first shower and before the last one. Does not take long.

Turning a valve to isolate a section is sufficient to control the heat. I also run the hot coolant via the radiators when no heat is required. If the fans are off, the heat from the radiators is minimal.

This is the control plumbing for the new one. Radiators not there yet.
RHS goes thru the floor then up to the calorifier.
Blue and one black goes through the wall to the engine and diesel heater.
3 x 3 way valves. For simplicity I can run the bathroom heater alone, or the bathroom plus the house together, not the house on its own.
Accessibility via a hatch in a false floor section.

This is the engine connection via the heater hoses.
This is the new build which is identical to the old one.
The (2) valves (one not visible) would remain open and are for emergency isolation only.

Calorifier (new 10L).
Black heater plumbing comes up through the floor.
Water plumbing (black for cold supply and white for hot plus a calorifier drain) connections in place but not plumbed.
You need to tell Surejust what connections you want for the Calorifier. Matthew is great to deal with.
The calorifier is right next to the bathroom to the left. The kitchen sink will be right next to it on the right.
Above the calorifier will be a tea towel rack (under the kitchen bench) to make use of the little bit of lost heat.
The plumbing end of the calorifier will be accessible via a door and the opposite end (the electrical connection) will be accessible from a hatch to the outside.

NOTE: 212F is 100C not 78C (your original post).
The diesel heating tends to shut down when the coolant going IN is about 70C, but the pump continues to run and circulate coolant until the temperature drops and the diesel heater starts up again.
When driving, the coolant temperatures get quite a bit higher - typically 85-100+C. It takes less than 30 minutes driving to heat the 30L calorifier, so on the road there is always hot water.

OKA196 motorhome
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We had the Webasto on an on/off switch originally. At -20C it is no fun jumping out of bed in the morning to start the heater. The thermostat is a perfect solution. It is just a temperature controlled on/off switch. The heatmizer has a lot of adjustability of cut in and out temp range. It also has a frost setting that will keep the camper above 5C, great for leaving the truck without having to winterize it.


Our Webasto on/off switch is reachable without getting out of bed :)
Very important feature.......
We don't have the sub zero conditions like some of you guys though.
OKA196 motorhome


Well-known member
Thanks for the build details, very helpful, especially with all the labels!

I bench-tested my Hicalory Webasto knockoff today and had to spend a lot of time getting air out of the system, but once I did, it worked well, so I'm willing to find a place under the bed for it. I think I'll keep the spare pump I have handy though.

My truck has one bleed-valve at the high point of the system by the firewall, if I add another high-point in the back, should I put a second bleed-valve there?

I'm still not sure exactly where to include a thermostat for the domestic water, but it looks like they can be tapped into a fitting once I choose one--still looking for the controls that will be most useful. Luckily, my bed is in reach of most everything, so it won't be too complicated where it goes.


You might notice a bleed valve in the top corner of the radiator, next to the outlet fitting in the pic in my first post. This is the highest point in the old circuit. I did put additional bleeds around the circuit, but I doubt they were needed.
The new radiators are similar except the other way up.

I don't have a thermostat for the domestic water. The water really can not get hotter that the engine cooling water and it is under the same pressure in a single circuit. The calorifier does have a mixing valve to prevent domestic water being too hot, but we set that to maximum for the dishes and we have a digital water mixing valve for the shower which has been very accurate.
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Found my very sophisticated plumbing layout schematic for the new build. :)
"ESP" is the diesel heater (with integral pump).
"E" is the engine.
"Engine" is the 3 way valve for including the engine in the circuit.
"Cal" is the calorifier.
"Bath" is the bathroom radiator.
"House" is the habitat radiator.
"P" is the auxiliary pump which would be manually switched.

All valves are manual control with easy access from inside the house (except engine isolation which is in the engine compartment).
Electrical switches:...............all accessible from the bed.
Diesel heater on-off.
Bathroom radiator fans on-off plus PWM control.
House radiator fans on-off plus PWM control.
Auxiliary pump on-off.

23-08-12 HeaterPlumbing c.jpg

OKA196 motorhome

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