Improving airflow on a diesel air heater

[Hunterlees]

I have an MV Airo 5kw heater, same as eber/webasto/CDH. It is mounted near the stern and feeds 5 outlets in total. 3 are in 75mm ducting, but due to structural constraints the head and forward cabin are fed from the same 60mm ducting, about 4mtr long. The result of this is poor airflow and lack of heat. Closing off the other 3 does push more air forwards, but seems to confuse the heater which turns itself down, probably due to back pressure and/or a rise in temperature within the heater unit.

My solution was to buy a 3” inline bilge fan, together with a PWM fan controller. I had to reduce both ends of the fan to 60mm with adaptors, and inserted the fan into the existing ducting close after where the ducting is T’d off. I can now push more air forwards, and can control the flow using the fan controller. I am now getting a good supply of air at 70C to both forward outlets. More than happy with the result.

My only concern might be if the bilge fan can take the heat long term, but it seems to be made of the same material as the adapters in the system, so hopefully it will survive.

Total cost - fan £18, Controller £7.

 

[simonfraser]

yes good move, i have done similar with an 80mm PC Fan to take the heat from a double skinned exhaust away from the deck and to somewhere more useful

 

[dankilb]

Thanks for sharing - this is is interesting for me as I'm currently working on a similar heater install and one run of ducting is necessarily long (approx 10m) and will feed 5 outlets with maybe 7m between the first and last. I plan to use 75mm ducting throughout (using 2-ply silicone hose and closed cell foam insulation - hopefully to reduce heat loss) and smaller 60mm vents. I was hoping to get lucky with this combo of wider ducting and smaller vents and strike a balance between air flow/back pressure/heat loss etc.

I appreciate this is somewhat of challenge and before anyone asks 'why not fit another heater?' - a second heater is already part of the plan (it will feed the opposite side of the boat with a more modest duct run to 3 outlets)!

I've recently wondered about adding extra fans to boost airflow and thought of posting to ask whether anyone had tried this, so it's good to hear the OP's and other's experiences.

My initial impression is, even if the fans don't last that long, it could well be worth it to be. I've agonised about other installation options - but the above plans means the heaters can be mounted in what is by far the best (and many ways only sensible) location.

 

[dankilb]

Something like this claims to operate up to 120 degrees C (not in-line but I'm sure could be modified to fit): FD1240-D0042M2AL | ARX CeraDyna Series Axial Fan, 12 V dc, DC Operation, 28.30m³/h, 3.72W, 40 x 40 x 28mm | RS

 

[PetiteFleur]

I had a similar problem - heater in stern and outlets in stern, saloon, loo and forecabin. The stern cabin was roasting, so fitted a diverter valve which reduced heat to the rear, and reduced the pipe diameter to loo & forecabin. Works on my setup.

 

[jon and michie]

Something like this claims to operate up to 120 degrees C (not in-line but I'm sure could be modified to fit): FD1240-D0042M2AL | ARX CeraDyna Series Axial Fan, 12 V dc, DC Operation, 28.30m³/h, 3.72W, 40 x 40 x 28mm | RS

Dont forget to insulate the cables to the fan or they could possibly melt/catch fire

 

[ChromeDome]

I've got two heaters and must say it is a very efficient and flexible solution as it obviously produces more heat when so desired.
Different reason but also solves the problem with long runs and many turns.

This manual sect. 9.4 is about resistance in ducting (Webasto, they state a max sum of resistance points for each heater model, and the points for each component), see pg. 23
https://www.butlertechnik.com/downloads/Webasto_Install_Instructions_Marine_heating_systems.pdf

If you measure the actual temperature inside the ducting you'd know what the fan has to cope with.

I've fitted a cabinet fan to ventilate the area behind the fridge, controlled by the fridge thermostat so not a constant run, and realized that the noise level from these needs to be considered. Some of them turn pretty high rpm and may be loud (which btw a bilge also blower is).

Could you fit this type of fan at the outlets, optionally with individual switches (thermostats), to be able to turn on/off when needed?