Spending time in the middle of nowhere is a fantastic experience. Sometimes it could be a requirement to escape from the city, zombies, or ... whatever is hurting you.
Regardless the reason one will move in the middle of nowhere, everyone needs a little bit of comfort specially in a cold winter.
So I've deiced to build a hot shower in my car to enjoy both the winter sky and a warm shower.
To heat up the water a tank installed in a car there are at least three options:
- put a tank on the roof (maybe black painted) and let sun do the job
- build an heat exchanger that use the hot water coming from the engine to heat up the shower water.
- use an electric heater (that is basically a resistance)
- Tank on the roof works fine in summer, but it doesn't work too much if the air temperature is under 20 degrees C, specially if the car moves all the day or in windy weather. In addition, there is no way to control the temperature. In summer it can reach an uncomfortable level.
- Heat exchanger need to stay close to the engine. So we need a pipe to bring the water from the tank (that normally is on the car back) to the engine. A little messy and and it can be dangerous for the engine: in case of break of the exchanger coolant can exit from the cooling circuit. Finally, control the temperature is not easy.
So I've done my job using a electric heater.
Actually this is not a very complex operation: you just need a tank, an heater, a pump, some wires, relays and a switch. So I'm not going to show you the exact building phases.
What I'll show you is the result of some experiments about how to control safely the water temperature. Because this is not so simple how it can appear.
In details we need to take care about:
- how much power we need: your car is not a nuclear plant
- how to control the water temperature: not necessary to make the water to boil.
- Safety and reliability: working with power and heat in a car running off road can be dangerous.
Let's go ...
Step 1: Calculate the Energy You Need to Get the Required Temperature in a Reasonable Time
First of all we have to calculate how much power we need to heat up our tank.
These are the basic formulas (comma is the decimal separator):
- It takes 1 Kcal to raise 1kg (~1 litre) of water by 1 degree C.
- 1.000 Kcal = 1,163 KWh
If we have a 100 litres tank and we want to rise the temperature from 5 to 35 degrees C, we need:
- 100 x (35-5) = 100 x 30 = 3.000 KCal = 3,489 KWh
Now let assume we want this happen in no more than 6 hours. We need almost
- 3.489 KWh / 6 h = 0,588 KW, that simplifying is 600 W
Now we have to take care about the amount of current we are going to ask to our car:
- If our car has a 12 V electric power and we want to drain 600 W, we need 600 W / 12 V = 50 A.
Be sure to use the right cable size. Here a clear document: Cable size calculation
Take care about the fact that the cable itself heat up because the copper resistance is not zero. To avoid a cable heating double the size required.
Even the cable length is a critical parameter: much longer is the cable, much higher will be the loose of voltage along the cable. This fact reduce the power available to the heater.
More over, be sure the engine's current generator is able to produce at least the double of the current amount you need because engine ECUs and lights need current too.
If not, you have three options:
- reduce the tank size.
- be patient and increase the desired time to to reach the required temperature
- be content of a colder shower.
Note that reducing the tanks size do not means that you can't bring 100 liters of water in your car. You can use two tanks, one small (i.e 20 liters) with the heating system, one big (i.e 80 liters) for reserve. A small pump can help to refill the small tank from the big one when it is required.
Of course we are considering the worst situation, when the tank is full. When the tank will be half empty it will require half time.
My configuration is:
- 70 liters tank
- 300 W heater, draining 25 A
- 90 A engine alternator
- 16 mm2, 3 mt copper cable (2 cable, one positive, one negative)
Step 2: Control the Temperature ... This Is the Problem...
Electric heaters are widely available. Note that some product on the market are designed to be installed on boats and caravans to avoid icing. This product are "temperature limited" that means that will stop to heat when temperature is higher than a predefined temperature. This products are not suitable for our job.
In the first pic you can see the one I've used in my tank. In the second pic the heater installed viewed from inside the tank. The small one is first test, too weak for the winter when water temperature can go under 10 degrees C. I left it while looking for a cap.
Here on Ebay a nice 200 W 12V heater from a German seller: Heater
Anyway, to avoid to to overheat the water in the tank and enjoy an warm shower we need a thermostat.
A thermostat is a combination of two devices: a probe that transform the water temperature in a micro mechanical movement or electrical resistance and a switch operated. A scale allow to fix the temperature limit that when reached let the switch to open (or close) a circuit to winch it is normally connected the heater.
The most common thermostats are probes that have to be installed on the tank and "feel" the water. One immersion thermostat common model in last the picture. It is intended to be installed not close to the heater to avoid the false evaluation of the water temperature.
The heater have to be installed as lower as possible, to grant to be always in the water, at least until the water is almost finished.
It seems enough but in this scenario we have a big problem: when the tank is almost empty, the thermostat will feel the air temperature, that will never reach the limit and will not stop the heater.
The most common tanks are mad of PVC and PVC melt down around over 150°-200°C.
So if you do not want your tank to melt down, with a risk of fire in the car, it is important to have a more reliable control on the heater.
Step 3: Better Control on the Water Temperature
To increase control on the heater we must add a "water level sensor" that stop the heater when the water level is to low. The level sensor will avoid to heater to melt the tank.
So on our tank we will have:
- water temperature probe
- water level probe
Unfortunately this signal coming from the probe (normally a simple resistance) have to amplified to drive a relay that will switch the heater on and off. So we need to build a small electronic relay driver.
All this things in a car traveling on uncomfortable roads, do not convince me. Too complex and too dangerous.
So I've started experimenting and I've discovered a interesting fact: the external heater's metal flange reach the same temperature of the water inside the tank. Obviously (now).
This is a nice thing, because to control the water temperature we can just measure the flange temperature with a contact thermostat.
Contact thermostats are probes designed to measure the temperature of a liquid inside of a metal pipe without being in direct contact whit the liquid (when of course the pipe conducts heat).
One common model in the last picture.
In the other pictures you can see my contact thermostat fixed on the heater flange with a plastic wrapper.
Of course if your heater has a small flange, you need to enlarge the contact surface putting a thin metal plate in the middle between the flange and the tank.
Step 4: Happy Shower !
What happen using a contact thermostat?
- If the tank is full or anyway the water is enough to absorb the energy produced by the heater, both the water and flange temperature will increase slowly at the same level.
- If the water inside the tank is not enough to absorb the whole energy produced by the heater, the heater itself will heat up potentially reaching very high temperature. The flange heat up too reaching quickly the same high temperature.
In both cases, reached the chosen temperature limit the contact thermostat will click interrupting the current powering the heater, avoiding dangerous overheating of everything.
So, with just one probe we have a safe and efficient control of the water temperature in any condition.
In the attached schema you can see all you need to do the job:
- one fuse
- one main switch that have to be connected to the engine key
- one main relay that is driven by the switch
- one second relay that is driven by the thermostat and power the heater
- the contact thermostat
- the heater
Note that the main relay interrupt the circuit insulating both the heater and the thermostat. The second relay interrupt the circuit according with the thermostat. This is the most safe configuration.
Just some additional suggestions:
- Be sure that the thermostat can't move away form the heater's flange. If this will happen the heater will be totally out of control leading to a very dangerous situation.
- Do not connect the electric heater directly to the battery. A relay controlled by the main key (or an "active engine signal") is required to avoid the heater to discharge the battery if forgotten on while the engine is off.
- Use appropriate fuses to avoid any risk of short circuit. Remember that you are using electric current to heat up water. Two elements that are fine to play together.
- Use relays to open and close the circuit. Do no use switches or the thermostat directly. Normally they are not designed to support huge amount of current.
- Regarding the tank: 100 liters of water is a very heavy load. Be sure to fix its strongly to the car chassis to avoid dangerous movement.
Last suggestion: showering in the middle on nowhere is fantastic, but you have limited amount of water. It is useful to use a shower pipe with an integrated stop button.
Happy hot shower !