Does diesel heater 'downtime' mean they are poor at maintaing a constant temperature?

[crinklystarfish]

I have a question for people that use any kind of diesel-burning furnace for blown-air or hydronic heating.

I personally have a Webasto Airtop 2000 S blown-air diesel heater.

Assuming a cold cabin to start with; when I set my (standard dial) room temperature controller to its mid point the heater initially fires up and burns quite fiercely. As room temperature rises the heater gradually reduces output (what I believe is the ‘constant modulation' function referred to by manufacturer) until it eventually stops calling for fuel and cycles down.

However, unless I turn the room temperature dial to ‘off', the heater fan never stops running and pretty soon starts to blow cold air which actively - and irritatingly - reduces room temperature. Unless I raise the setting on the dial, the heater will not re-ignite until temperature has dropped considerably - way lower than is comfortable. I assume this operating cycle is designed to reduce the number of times the glow-pin is required.

Is this experience shared by other people with diesel-powered blown-air heaters and are hydronic systems the same? ie do they seem to be designed to have overly-long periods between cycling down and re-firing?

 

[Bbasso]

I'll be paying attention to your thread because I'm still installing my Espar D4 heater.

 

[Martyn]

We have installed Espar hydronic systems into numerous vehicles, most notable the E7 Series 70's, and have not experienced any of these issues. The fan will run after the required temps have been reached but only for a short while. As for not re starting until the temperature has dropped considerably, it sounds like your thermostat is having issues. I forget the technical term for it, but there is a small range say of 5' where the temperature will fall from the set temperature before the furnace will kick back on. Seems the range on your thermostat is too large. Does your unit display any error codes?

 

[fluffyprinceton]

are hydronic systems the same?

The hydronic systems are slower to heat up and slower to cool down - due to the thermal retention qualities of the medium being heated - water vs. air. The greater the water volume in your system the greater the "heat lag" will be. I've got about 7.5 gallons in my system and it takes about 20 minutes to noticeable warm the cabin - but then it's very steady - I would think a system with minimum water capacity (hydronic systems have a required minimum water volume) and big fans would cut the lag time considerably. But if you want heat NOW air heat is the way to go.

Sounds like you are experiencing the "nature of the beast" at least in part...Moe

 

[crinklystarfish]

Thanks for the responses.

My unit has been reliable apart from once when I drowned the exhaust in a deep river fording but I soon sorted that. Apart from during that episode it's not thrown up any error codes.

For info, I did a bit of an experiment yesterday evening with the truck on the drive. The habitation area interior temperature was 12.5 deg C (54.5F) with all heating off. I slowly cranked the thermostat dial up to the point where the heater fired (a very low setting). Over the course of about an hour the interior temperature was raised to about 14 deg C (57.2F) whereupon the heater cycled down. I then opened up windows and hatches to allow the interior to cool as quickly as possible.

The fan kept running and did not ever turn off before - after about 3 hours and a temperature of 11.7 deg C (53F) - the heater eventually deigned to re-fire and begin the heating process again.

May be I'm expecting too much but 3 hours with no heat input and a drop of circa 4 deg F doesn't seem like an optimal automated routine to me. More than anything, I don't see the logic in the fan never turning off.

Conversely, my Truma LPG powered blown-air heater that I run as well as / instead of the diesel heater will readily cycle on and off several times an hour and this holds internal temperature to +/- one degree without any input from me.

I've tried to establish via Webasto tech-help whether the Airtop's operation is deemed 'normal' but so far no response. I'm going to ring them rather than rely on emal response and will report back if I find anything useful out.

 

[crinklystarfish]

OKAY, as promised, I've done some research and essentially this is what I've established:

To avoid unnecessary 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.

Some blown-air diesel furnaces, having reached required room temperature and cycled down, will not turn off all their functions completely and are designed to continue to circulate cold air. This purportedly allows internal sensors to monitor the recirculating air temperature and will actually speed up the re-firing process. Of course, the hours that the heater is noisily blowing cold air do not help with conserving battery power or the uncomfortable feeling of a constant draught.

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 its physical functions completely when it's not actually burning fuel.

Hope this helps if anyone is considering buying a heater anytime soon. To be honest, the more I've used and read about diesel heaters the more I'm persuaded to use LPG.

 

[nick disjunkt]

My eberspacher airtronic D2 doesn't work like this. It seems to have at least two heat settings; it start up full blast and unless you have the temperature control on full, it will cycle down to a lower heat mode. Depending on the ambient temperature, and the temperature setting you choose, the heater will switch up to the higher power mode more or less frequently. It never blows cold air unless you put in ventilation only mode. On the lower heat output, it is practically silent and just releases a gentle trickle of warm air.

The problem I have with the D2 is that the ambient temperature reading, which it uses to regulate the temperature, is taken from the air input. If the heater is mounted inside the vehicle this is fine, but I have mine mounted under the truck, extracting fresh air from outside. It therefore cannot regulate the temperature well as it constantly thinks that the temperature inside the vehicle isn't changing. The temperature regulation works fairly well considering, except when the temperature outside is very cold; in this case it will leave the heater on full power, regardless of what setting I leave it in, thinking that the temperature inside the vehicle is uncomfortable. Eberspacher sells a remote temperature attachment to get around this, but I just haven't had the time to extend the loom yet.

 

[crinklystarfish]

Hi Nick, is it the case that the D2 will just keep on running, albeit with a very low output, until you physically switch it off then?

 

[nick disjunkt]

yes, it'll run indefinitely if your controller has that option.

 

[mhiscox]

A very helpful thread, by the way. Thanks to the Crinklystarfish for the question and the research, and to the other posters for the helpful information.