Ducting for blown air heating

Lodesman77 said:
+1 But as said before, insulating the first 0.5 - 1.0m after the heater may cause the unit to overheat.
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Rubbish. How does the heater know whether the outlet duct is insulated or not. The is no feed back mechanism nor restriction in air flow either way. The air leaves the heater @ say 125°C and travels through the duct work at a fixed rate. In the meantime it loses heat through the duct wall. If the first 0.5~1m is left uninsulated there will be considerable loss of heat at that section heating your aft locker or under bunk locker or wherever you mounted the heater. if this section is insulated there will be minimal heat loss and the air exiting the duct will be noticeably warmer. The heater does not know if the duct is insulated or not. I have insulated the duct work on my eber all the way from the heater outlet to the cabin outlets and the heater has never over heated. I had to change the outlet grills from plastic to stainless as the temperature of the air exiting the duct was measured at 100-105°C. I would much rather the heat was in the cabins rather than in the heater locker
 
PowerSlapper said:
Rubbish. How does the heater know whether the outlet duct is insulated or not. The is no feed back mechanism nor restriction in air flow either way. The air leaves the heater @ say 125°C and travels through the duct work at a fixed rate. In the meantime it loses heat through the duct wall. If the first 0.5~1m is left uninsulated there will be considerable loss of heat at that section heating your aft locker or under bunk locker or wherever you mounted the heater. if this section is insulated there will be minimal heat loss and the air exiting the duct will be noticeably warmer. The heater does not know if the duct is insulated or not. I have insulated the duct work on my eber all the way from the heater outlet to the cabin outlets and the heater has never over heated. I had to change the outlet grills from plastic to stainless as the temperature of the air exiting the duct was measured at 100-105°C. I would much rather the heat was in the cabins rather than in the heater locker
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Interesting argument. I think there's something missing in your logic though, because empirical data shows that what you say is - to use a phrase - also "rubbish" (as far as many other users are concerned)

ANy thermodynamicists out there? I suspect its something to do with the heat capacity and cooling rate of the unit itself.
 
skyflyer said:
Interesting argument. I think there's something missing in your logic though, because empirical data shows that what you say is - to use a phrase - also "rubbish" (as far as many other users are concerned)

ANy thermodynamicists out there? I suspect its something to do with the heat capacity and cooling rate of the unit itself.
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That was my thoughts that there would be a build up of heat which is influenced by the heat loss of the immediate output area and quickly reaches an equilibrium once you change the heat loss in that area in fact reduce it the temperature in the combustion area will increase as it is no longer leaching so much heat energy to the immediate outside area to attain the equilibrium.
 
Would be an easy thing to test... I guess someone would get one of those laser temperature probes and point it at the casing of the heater and measure the temperature with and without insulation on the first metre of so...

One theory would suggest an insulated pipe would cause the increase in temperature at the heater.
And the other theory would suggest that the temperature doesn't increase significantly...

Out of curiosity, what are we trying to protect from getting too hot and how much temperature is that part rated for?
 
Like a hairdryer or hot air gun or electric fan heater, the slower the mass of air per second then the hotter it gets for a given heat input in the same time.

So anything that restricts airflow will mean the air leaving the heater is hotter than for an unrestricted flow. Length of run, bends, joins, splitters etc all restrict the airflow.

The casing of an Eber is plastic I think? Some wiring insulation certainly is. Solder will melt too so there has to be a limit to how hot the unit will get after which it shuts down. If the Unit IS getting too hot it needs to be cooled. You can do this by blowing cold air on it (theoretically) or by removing insulation from the ducting near the heater which allows the ducting to cool so that more heat is transferred from the heater itself

Think of it as a large copper rod with a flame at one end and a ball of wax half way along. If you apply enough heat to just melt the wax then by applying cooling to the opposite end of the rod you can once again make the wax solidify
 
skyflyer said:
If the Unit IS getting too hot it needs to be cooled. You can do this by blowing cold air on it (theoretically) or by removing insulation from the ducting near the heater which allows the ducting to cool so that more heat is transferred from the heater itself
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The overheat sensor is on the heat exchanger inside, not on the casing, so blowing air on the unit wouldn't solve the problem.

If the unit is correctly installed, there shouldn't be a problem. However, if the duct resistance is too high, reduced airflow can cause overheating, which is the circumstance David2452 was referring to in the post linked to above.
 
pvb said:
The overheat sensor is on the heat exchanger inside, not on the casing, so blowing air on the unit wouldn't solve the problem.
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Three methods of hreat transfer: Convection, Conduction and Radiation. If the case is cooled then the heat will transfer outwards by all three of those methods - same as if in a car with the heater on any given setting, you (the sensor) will feel colder on an icy cold day than on a summer day!

But this is semantics - the point is that far from being "rubbish" as PowerSlapper claims, there are valid physics to explain why the removal of some insulation may prevent an overheat.

Its not rocket science but it IS science.
 
peterf said:
Would be an easy thing to test... I guess someone would get one of those laser temperature probes and point it at the casing of the heater and measure the temperature with and without insulation on the first metre of so...

One theory would suggest an insulated pipe would cause the increase in temperature at the heater.
And the other theory would suggest that the temperature doesn't increase significantly...

Out of curiosity, what are we trying to protect from getting too hot and how much temperature is that part rated for?
Click to expand...
It has already been done and guess what? removal of the insulation made NO difference to the heater temp though it did result in cooler air from the ducting outlets. Replacing the insulation did NOT result in the heater running hotter but did increase the air temperature at the outlets.
While people are talking of rods etc imagine a steel rod say 12" long and apply an oxy-acetylene torch to one end. the rod heats up and the heat travels along the rod. Next insulate the rod. Does the flame get hotter? NO. The rod will get hotter until equilibrium is reached.
The other way to look at it is the fan blows X cubic feet of air through the duct capable of transferring Y Joules of heat energy/min (Z watts)now if the heater produces z watts and heats the incoming air to say 115°C and assuming perfect insulation (no heat losses through the duct) the outlet air temp will be 115°C. Now remove some insulation (adjacent to the heater. The heater is still producing X cubic feet of air at 115°C but now some heat energy is lost at the uninsulated section resulting in cooler air emerging from the outlet, The thermodynamics within the heater have not changed and hence the temperature within the heater remains constant. Now if one restricts the air flow the temperature within the heater will rise as the reduced air flow will not be able to transfer the Z watts produced by the heater so the temp will rise. Likewise a higher air flow will result in cooling the air at the heater outlet. Heater outlet temp is controlled by either reducing or increasing energy input (burning more or less fuel) or by controlling the volume of air flowing over the combustion chamber and through the outlet duct. The temperature within the heater is not influenced by insulation on the duct. Thermodynamics 101!
 
What you state is true for YOUR set up. If there is no significant back pressure (ie flow restriction) then yes there will not be an issue.
Take it to an extreme. Block up the outlet. Obviously the unit will then overheat
Common Sense 101
but i cant be bothered to continue the argument. My Eber works fine (since i removed 30cm of insulation)
 
PowerSlapper said:
I had to change the outlet grills from plastic to stainless as the temperature of the air exiting the duct was measured at 100-105°C. I would much rather the heat was in the cabins rather than in the heater locker
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I would not want to be sitting anywhere near an outlet with air at 100degC, it would burn your legs pretty rapidly I would imagine!!
 
When we bought our Malo, the boat took ages to heat up. After the first winter, I insulated all the ducting with Webasto insulation, and it made a HUGE difference to the warm-up time, particularly in the forward cabin.
Yes, the temperature from the main vent in the saloon is fairly hot at full fan speed, but rarely runs for long at high speed as the thermostat cuts in, but you don't sit with your feet in the way?
 
PowerSlapper said:
Rubbish. How does the heater know whether the outlet duct is insulated or not. The is no feed back mechanism nor restriction in air flow either way. The air leaves the heater @ say 125°C and travels through the duct work at a fixed rate. In the meantime it loses heat through the duct wall. If the first 0.5~1m is left uninsulated there will be considerable loss of heat at that section heating your aft locker or under bunk locker or wherever you mounted the heater. if this section is insulated there will be minimal heat loss and the air exiting the duct will be noticeably warmer. The heater does not know if the duct is insulated or not. I have insulated the duct work on my eber all the way from the heater outlet to the cabin outlets and the heater has never over heated. I had to change the outlet grills from plastic to stainless as the temperature of the air exiting the duct was measured at 100-105°C. I would much rather the heat was in the cabins rather than in the heater locker
Click to expand...

At the recommendation of Eberspacher I spoke to a very knowledgeable person at Krueger about this and other installation technicalities . The answer to my question “can we insulate the ducting right up to our D4 heater?” was “Yes”.
 
I realise this is an old thread, but having just bought a diesel heater I am wondering what is best practice installing heating, I wish to have 3 ports from the heater, and was wondering if I could just have them all inline one after the other. I realise more air will come out of port 1, but is this feasible to do.

TIA

Ump
 
Umpa said:
I realise this is an old thread, but having just bought a diesel heater I am wondering what is best practice installing heating, I wish to have 3 ports from the heater, and was wondering if I could just have them all inline one after the other. I realise more air will come out of port 1, but is this feasible to do.

TIA

Ump
Click to expand...
You could use adjustable vents to balance the output. But you're supposed to have one fixed outlet to make it impossible to accidentally run the heater with all the outlets closed.
 
An old thread I know but I've been having issues with my Planar d4 heater cycling down to idle from startup when the set point is way above the panel temperature. I've got insulation right up to the heater on the outlet duct and it does bend down slightly too (unavoidable unfortunately).

Think this could be the problem? The supplier suggested it might be an overheat issue. Not an easy job to remove to test so wondered about other people's experiences...
 
Na ... an overheat trip would shut down completely requiring some kind of reset or manually turning on. Your problem sounds more like temp sense reacting prematurely. Where is the stat sensor position in relation to hot air outlet ?
 
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