Diesel water heater for showering and climate

type: note | domain: technology | topic: mobile-office | lang: en | pub: 2021-03-07

Under construction, draft.

Abstract approach

You can do many things with a diesel water heater, like frost protection of the water tank, engine pre heating and so on. Both the engine and the diesel heater produce hot water and the question is what you will do with that water, regardless of the source of heat.

Before you know it, the project becomes too complex, so limitation is the order of the day. No preheating of the engine is required because trips are mostly long. So the diesel heater is only required for:

• Heating of the interior with a radiator fan combination.
• Heating of household and shower water with a heat exchanger.

Finally it is good to state that the heater is only connected to the house electrical system, it is smart to keep the car and house system apart from each other as much as possible.

What is the best place to install a diesel heater in a Fiat Ducato? Under the hood there is simply no space. Under the loading floor means exposure to wind and weather (and salt). This could be solved by an extra housing.

Choosing a heater model

• You can choose expensive, I'll skip these (Webasto, Eberspacher)
• You can choose relative cheap, typically for sale on Asian sites. The latter does mean there are different qualities. For example:
  • Choose steel or...
  • Choose aluminium models.
  • Built quality, beside steel and aluminium, will remain an adventure.
    • It is strongly advised to take precautions in a wet and salt environment, use at least 304 fasteners, shrink tube with glue inside for electrical connections, use zinc spray and epoxy coatings at weak spots.
• The capacity is an aspect you want to consider. Models have a minimum and maximum power:
  • These devices typically have a maximum performance but also a minimum mode of operation, consuming a minimum of diesel, before turning off.
  • Obvious, not enough power will be a disappointment.
  • However, too much power will result in a lot of off-on operations because the minimum setting delivers simply too much heat.
  • Typical models are maximum 5 kW, 10-12 kW and 16 kW.
  • With a minimum of 40% of maximum, this translates to minima of 2 kW, 4-5 kW and 6.5 kW.

Capacity

Theory

• Water must be heated to shower by ΔT (Kelvin).
• The heater has a capacity P (kW, kilo Watt).
• It takes 4184 J(oules) to heat 1 kg of water 1 K(elvin)
• The number of liters of water per minute is the capacity:

So we have a formula:

• C=14.3*P/ΔT (l/min)

An example, heating water from 15 degrees Celsius to 40 degrees means heating 40 - 15 = 25 degrees. A heater supplies 10 kW. That gives a capacity of:

• 14.3 * 10 / 25 = 5.7 liters of water per minute.

This is theory, in practice there are losses, but it is enough to say safely: "We can take a shower".

Practise

• How much water must be heated?
  • A minimum of 3 liters per minute is reasonable and not a luxury.
• Wat is the temperature of water in the tank?
  • It depends on where you travel.
  • Temperatures below freezing point are not considered.
  • I find a temperature of 5 °C reasonable during cold nights.
• How efficient is the system? For example, only 80 % of the advertised heater power is available and losses in the heat exchanger and tubing may be considerable. Let's departure from 65 % efficiency.

This is enough to transform our formula into something practical and do a calculation.

• First the prior formula:
  C = 14.3 * P / ΔT =>
  P = C * ΔT / 14.3 =>
  P = 0.07 * C * ΔT
• In practise, our losses are a number, a factor f. So 65 % means f = 0.65. Now our formula becomes:
  P = 0.07 * C * ΔT / f

So let us fill in some values...

• C = 3 liter/min, ΔT = 40 - 5 = 35 °C, f = 0.65
• P = 0.07 * C * ΔT / f = 0.07 * 3 * 35 / 0.65 = 11.2 kW
• So we can conclude:
  • A heater of 10 or 12 kW probably does the job.
  • A heater of 5 kW will probably under perform.

A last word about this, you'll have to fill in your own expectations. For example, when you expect your tank water to be ~ 20 °C, then you may want to consider a 5 kW unit.

Heat exchanger

A mean stream exchanger should be okay. It should have large connectors, at least 3/4", and a minimum of 30 plates, in order to avoid too much flow resistance.

Radiator

So far I have not found a proper solution so my plan is to do it myself. Ingredients, a radiator of a Suzuki Swift II Sedan (AH, AJ), a 30 W DC motor and a 12" fan blade. This way I can tune fan speed with a PWM controller, have a lot of air flow with a minimum of noise and power usage.

In fact, the roof ventilation is build this way, it is a proven technique, very satisfying.

Heater fuel tank

Some things to consider first:

• Fuel prices are differing all over Europe.
• A tank for the heater can also serve to overcome larger distances without refuelling and or serve as a spare supply for the car.
• Modding the car tank is an option but I like to keep things strictly apart.
• Diesel should not be in a tank for long periods without usage.

Some challenges:

• Finding a tank that fits onder the floor is not easy:
  • Search the net and buy or...
  • Make a tank yourself.
• Steel 304 or aluminium versus plastic. Plastic is often preferred.