Ducting for blown air heating

Kelpie

Kelpie

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I've been a bit surprised not to be able to find this information out, but what sort of temperature does a blown air diesel heater actually chuck out?
Just asking because I have a bunch of insulated ducting left over from a ventilation system, and it's rated to 70degC, but I'm not sure if that is OK or not? Sounds like it should be OK but better safe than sorry...
 
Kelpie said:
Just asking because I have a bunch of insulated ducting left over from a ventilation system, and it's rated to 70degC, but I'm not sure if that is OK or not? Sounds like it should be OK but better safe than sorry...
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It's not OK. Air temperature in the ducting can easily be 90degC, and the overheat sensor on Eberspacher Airtronics trips at around 110degC. Use the correct ducting, otherwise you may need to do it all again.
 
pvb said:
It's not OK. Air temperature in the ducting can easily be 90degC, and the overheat sensor on Eberspacher Airtronics trips at around 110degC. Use the correct ducting, otherwise you may need to do it all again.
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And that's why you ask :D
Thanks, I genuinely had no idea they ran as hot as that.
 
I second that comment about insulation. Just finished installing D2 on boat. Four air ducts were luke- warmish without insulation and very hot with insulation- made a massive difference. I used 20mm rockwall with foil heat reflective outer on 60mm air ducting..
 
pvb said:
It's not OK. Air temperature in the ducting can easily be 90degC, and the overheat sensor on Eberspacher Airtronics trips at around 110degC. Use the correct ducting, otherwise you may need to do it all again.
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That seems extraordinary to me. 90 (let alone 110) deg c will take the skin off your flesh.

I know there is a loss between the unit and the outlet, but mine comes out nearer 30-40 deg c.

Are you talking about the exhaust duct rather than the hot air or even muddling deg C and F.

PS My Eber manual is on the boat so I can't check!
 
peterf said:
I second that comment about insulation. Just finished installing D2 on boat. Four air ducts were luke- warmish without insulation and very hot with insulation- made a massive difference. I used 20mm rockwall with foil heat reflective outer on 60mm air ducting..
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Doesn't all the heat just go into the boat anyhow?
 
Spirit (of Glenans) said:
Doesn't all the heat just go into the boat anyhow?
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The ducting that runs along the back of lockers will heat up the air which in turn dissipates / conducts through the hull into the water, so a loss of heat that would otherwise be in the air as it exits from the duct vent. The air in the saloon is somewhat insulated from the hull by cushions, fittings and the air in the lockers, so it does matter as far as efficiency takes place. If the water is very cold then the heat in the locker may never contribute much to heating the saloon. That's how I see it. Small gaps could be left between insulation to allow lockers to be warmed, if this is wanted.
 
Note also that the bigger the temperature gradient the higher the loss. ie The hotter pipes nearer to the heater should be prioritised for insulation over the cooler pipes at the end of the run.
 
The only bit of ducting that needs the super high temp rating is within a metre or so of the unit which should also not be insulated. The Eberspacher documentation has the details some where.
 
This has been covered before (unsurprisingly) and the wisdom from the professional is that any insulation near to the heater itself should be rated to 140C...

http://www.ybw.com/forums/showthread.php?385708-Insulating-Heating-trunking&p=4585378#post4585378


Even a small amount of insulation makes a huge difference to the temperature at the outlet. Being a cheapskate I've used rockwool wrapped in tin foil which is fine for the high temperatures and then lower grade insulated ducting around that further away from the heater.
 
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Thanks all for the extra info.
I think insulation must be worth doing- yes there is an argument that the heat will diffuse into the boat anyway, but the whole point of these types of heaters is to get heat where you want it, when you want it. Otherwise you wouldn't bother with ducting at all, and would just wait a few hours for the entire boat to warm up.

The insulated duct that I have is just foil and rockwool construction- hard to see how it would fail at high temps, but I'm not going to risk my boat by finding out. OTOH it is oversized, at 100mm, so it's very tempting to simply sleeve it over normal ducting, which would be 80mm.
I wonder if SWMBO will let me stick a bit in the oven and see what happens to it as I crank the heat up...
 
Kelpie said:
Thanks all for the extra info.
I think insulation must be worth doing- yes there is an argument that the heat will diffuse into the boat anyway, but the whole point of these types of heaters is to get heat where you want it, when you want it. Otherwise you wouldn't bother with ducting at all, and would just wait a few hours for the entire boat to warm up.

The insulated duct that I have is just foil and rockwool construction- hard to see how it would fail at high temps, but I'm not going to risk my boat by finding out. OTOH it is oversized, at 100mm, so it's very tempting to simply sleeve it over normal ducting, which would be 80mm.
I wonder if SWMBO will let me stick a bit in the oven and see what happens to it as I crank the heat up...
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Rockwool will be fine at the outlet temperature .
 
petehb said:
Depends on how many bends and constrictions there are in the run, if it's pretty straight and open probably won't be a problem.
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Of course it will vary depending on the installation - my 1.6kW is OK with insulation up to about 30cm from the heater (the rest is difficult to get at anyway) but it may cause problems with newer heaters which have lower cutout temperatures.
 
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