I’m bracing myself for a ‘nuclear’ reaction.
The more I think about these on demand diesel heaters the more I realize that the majority of users are going to find that they are an expensive liability.
Webasto are a well respected manufacturer of these heaters and when used in their correct environment are first class performers. They are designed as block heaters to preheat diesel and petrol engines in freezing climates. The heater switches on and then runs flat out for 30 or 40 minutes heating the engines coolant water up. It can also be wired up to kick in the cars internal blower fan and defrost the car, ready for your journey.
The diesel ones work well when using ordinary road diesel, but even Webasto are advising that red diesel isn’t suitable to use.
When these units are supplied as water heaters for our industry, they are adapted to suit our requirements. A small internal water circuit is created with a small header tank and 1 or 2 heat exchangers to warm the water being pumped to the brush depending on the number of operators.
Initially the single operator systems favoured 5kw Webasto Thermo Top heaters, but these were quickly upgraded to 9kw as they didn’t produce enough heat. Whilst only 2.6kw is required to heat a flow of 1.5LPM by 25 degrees, a 5kw could barely cope when taking into consideration heat loss through the exhaust and other heat transfer inefficiencies. At 2LPM the same unit will only raise the water temperature by 18 degrees, which to all intense and purposes is luke warm.
A Wesbasto 90s is more than capable of heating these afore mentioned water flow rates to a much higher temperature for a single operator. However as soon as another operator is also drawing heat, the 90s will not keep up with both of them. The heater will rely on intermittent use by both users so heat in the internal circuit can be ‘shared’ between the 2.
The other issue the supplies have to decide on is how the hot water is distributed between the 2 heat exchangers. The early Heatwave Thermo2 divided the hot supply line into 2 so the hot water was equally distributed (in parallel). The later model I saw was linked in series. So if the first operator was drawing most of the heat, there is not much left for the second operator. This issue has been raised by several different owners on this forum complaining at a poor heat output from one of the outputs where the other is acceptable.
Another problem that some have had is that of cycling on and off. If an operator is like me and stops to chat, the heater will still operate and it will raise the water temperature in its internal circuit to 80 degrees C. It will then kick into reduced heat mode until the water reaches 84 degrees C. It then switches off. If you switch your tap back on the heater will restart, but if you only use the system for a short burst, the heater will switch off again. This cycle of on and off is heavy on battery power and causes coking up of the burner and glowpin with costly premature failure.
For users like me the manufacturers of these systems need to add another heat exchanger and circulation pump controlled with a 12v digital temperature controller. Once the Webasto reaches 80 degrees C and reduces to half heat, the controller should activate this extra pump and bled the hot water from the heater into the WFP tank. As the temperature drops to 64 Degrees C the controller should deactivate the pump. This will allow heat to slowly build to 80 degrees C when the pump is again activated. This will keep the heater running on idle mode.
When I see how easy it was to install an 8 LPM Meny gas heater, even with fitting a Broan vent through the roof, and how effortlessly it runs, I can’t understand why these diesel heaters are still being bought.
This is my opinion.
