Bottom line is: they can fight you a bit.
- Glow plugs. Diesel furnaces demand a heavy current draw at start up as an inbuilt glow-plug needs to pre-heat incoming diesel in order that it will initially combust. In some cases this is the case at shut-down as well where the glow plug can be called upon to help in a ‘self-clean’ routine. The glow-plug time is usually minimal but your wiring and batteries must be able to cope with the short-but-heavy current draw otherwise the furnace may refuse to start
- Hysteresis. To avoid too many start-up / shut-down cycles and thereby minimise reliance on battery-draining glow-plugs, many diesel furnaces are programmed to allow a relatively large temperature differential between cycling down and re-firing. As a result they can be poor at maintaining a constant room temperature. They will initially burn fiercely but reduce their output as they approach your chosen thermostatically preset room temperature. When the preset temperature is reached some heaters will stop burning fuel, cycle down, and not re-ignite until room temperature has dropped sometimes uncomfortably. As a battery-saving alternative, some heaters do not cycle down at all. Instead, they continue to burn fuel and produce heat but at a very low rate. In these cases it falls to users to simply switch off manually if the habitation area becomes too warm. These regimes are designed to conserve battery power not to optimise end-user comfort
- Zero idle. Some blown-air diesel furnaces, having reached the required room temperature and cycled down, will stop supplying heat but will not turn off all their functions completely and are specifically programmed to continue recirculating ever-cooler air. This is purportedly so that the recirculated air temperature can be monitored within the heater casing and when it’s sensed to be sufficiently cool, it indicates to the heater that it’s time to re-ignite. Unfortunately this has real-world negative effects. It results in the unwelcome cooling of a warm habitation area, unnecessary noise, and an unhelpful draw on battery reserves. If you are going to purchase a blown-air diesel heater, you’d be well advised to ensure it has a ‘zero idle’ capability ie that it will shut down all of its mechanical functions completely between cycles and not continue to blow cooler air around
- Metering pump tick. Diesel heaters require a metering (aka dosing) pump to deliver fuel to the furnace. These pumps ‘tick’ audibly and, notoriously, drive some people to distraction. Careful siting of the pump and the use of purpose-made pliable rubber mounts can help enormously in reducing this infamous noise
- Altitude issues. Diesel-powered furnaces do not operate well at altitude. Anything over a sustained 1,500m might start to cause trouble. At altitude, notwithstanding the thinner air found there, the heater’s dosing pump (unless modified) will continue to deliver fuel at its pre-programmed rate. This results, effectively, in an over-fuelling scenario that will eventually manifest as ‘sooty’ deposits and, ultimately, a clogged furnace. Different manufacturers have different solutions and some of these are specific to the particular furnace in question. The only answer for reliable operation at altitude is to seek manufacturer’s advice (usually, extra altitude-compensating hardware is required), or to utilise an alternative heating system.
- Waxing. Though petrochemical companies appropriately prepare fuel for different climates, in very low temperatures diesel can turn ‘waxy’ and will not flow. Diesel heaters may fail to work in very low temperatures unless the fuel supply is insulated / otherwise protected
- Exhaust noise. Exhaust noise from diesel heaters, even through a silencer, can be substantial. When combined with metering pump tick, your popularity amongst neighbours may border on ‘generator’ levels
- Over-cycling hydronic system. In some hydronic systems, a sensor in the diesel furnace monitors returning water temperature and if this is close to the temperature of the water leaving, the heater may compute room temperature has been reached and cycle down. Unfortunately, it might well be the case that hydronic circuit is ‘short’ or otherwise has insufficient opportunity to ‘lose’ heat so the habitation area may well not actually ever reach an agreeable temperature before the heater cycles down. In these cases builders need to find ways to reduce the temperature of hot water returning to the heater. This might mean incorporating a valve to reduce the volume of flow to one part of the circuit if components are plumbed in parallel (dedicated calorifier circuits are often a culprit), or, if all components are plumbed in series, extending the overall circuit and running it through extra heat exchangers. Of course, over-extending the circuit can also lead to poor performance or failures. Balance is the key
- Bad fuel. Poor grade fuel can quickly prevent diesel heaters from working and on-the-road maintenance may be required if diesel quality is very poor. To improve the chances of avoiding foreign objects and water entering the metering pump it’s best practice to draw fuel via a stand pipe that doesn’t reach all the way to the bottom of the fuel-supply tank