Heater boxes OVER firewall insulation or under?

Hey folks,

I've read everything I can find about insulating the firewall with Fiberfrax and stainless foil, and I'm attempting to follow best practices here, but there are a couple of details that I just haven't been able to glean from all of the incredible research out there. I have two questions.

First - are folks bolting the heater boxes directly to the firewall and then nesting the fiberfrax around it, OR bolting the heater boxes through the foil and fiberfrax? I get that the point of the fiberfrax is to insulate the firewall, and having the heater boxes directly contacting the firewall creates a bunch of heat transfer. BUT, the plans call for sealing the heater boxes, and my worry is that the compressible fiberfrax won't allow a solid seal.

Second, for the oil cooler mount, same sort of question. Are y'all riveting the cooler mount THROUGH the fiberfrax and foil, or, say, shimming it the width of the frax and then riveting through the stainless foil? The former seems ideal, but it feels like riveting through the compressible frax would compromise the integrity of the rivet.

I guess I'm missing something key here since I could ask the same question of all of the fuel fittings too. (Should the nut clamp down over the frax?) For things like my ammeter shunt my plan was to use a washer to shim the thing the width of the frax, cut out a small hole for the washer and then bolt the ammeter through the stainless. At least I've got that part straight.

Thanks for your help thinking this through, all! Can't wait to get this firewall shielded.

At Oshkosh I purchased stainless steel boxes from Cleveland the recommendation was to mount them on top of the insulation to lower the heat transfer.

Answer to one of your question

I see that you are dealing with multiple issues concerning firewall insulation.
Seeing that you have already done some research on the subject, I am sure you have come across Dan Horton's firewall insulation write ups.

Not all of us insulate the entire firewall but confine the scope of insulating to
address a couple of areas, heater box/ tunnel face and possibly the firewall area behind the oil cooler.

My insulation is limited to a fiberfrax sandwich between the heater box and the firewall to reduce heat transfer into the tunnel. I sealed the perimeter of the heaterbox/fiberfrax sandwich with 3M firebarrier.
I also insulated the face of the tunnel as well as the floor of the tunnel, however this insulation is on the inside and made up of stainless covered firewall blanket. It is simply there to reduce heat transfer to the tunnel.

I have no insulation anywhere else. The oil cooler gets cool fresh air and I doubt that the the heat extracted from oil cooling significantly contributes to
heating up the firewall/cockpit area.

My tunnel temps stay cool to the touch.

BTW: My fuel pump and red cube is located in the tunnel. I routinely burn 91 Octane E10 mogas and have never had any heat related fuel issues or any other fuel issues for that matter.

Ernst, that's good info and that makes sense. I'm going to insulate the entire firewall, and the answers here have cleared up the approach for the heater boxes.

I'm still not sure if riveting through the fiberfrax is going to weaken the rivets. I think for everything else the answers here make clear that I should try and sandwich the fiberfrax wherever possible and not worry about the bolts that I tighten through the frax loosening up over time as the frax... compresses? Wears away?

Riveting??

Not sure I understand where you plan to rivet with fiberfrax insulation included.
The heater boxes are bolted and not structural. The oil cooler plenum is riveted
directly to the firewall.
I definitely would not rivet anything that included fiberfrax in the sandwich. You'll have to work around things like engine mount and bulkhead fittings as well as cables and wiring bundles if you plan on insulating the entire firewall.

N427EF said:

The heater boxes are bolted and not structural. The oil cooler plenum is riveted directly to the firewall.
I definitely would not rivet anything that included fiberfrax in the sandwich.

Click to expand...

Listen to Ernst.

Sandwiching fiberfrax doesn't make much difference when hanging bolted sub-assemblies. The major exception would be the motor mount.

However, riveted joints should always be metal-to-metal. Besides, it's best if the rivets are under the protective shielding...they're aluminum.

DanH said:

Listen to Ernst.

Sandwiching fiberfrax doesn't make much difference when hanging bolted sub-assemblies. The major exception would be the motor mount.

However, riveted joints should always be metal-to-metal. Besides, it's best if the rivets are under the protective shielding...they're aluminum.

Click to expand...

Thanks to you both! I'll rivet the oil cooler directly on to the firewall and shield the rivets with the fiberfrax + stainless.

Dan, on your plane did you end up shielding the rectangle inside of the oil cooler mount as well? Since I'm doing this on new install I imagine the best route here is to

- cut the fiberfrax to the entire shape of the firewall, but cut flaps in the places where the oil cooler needs to mount
- attach the frax and stainless
- fold up the flaps to expose metal for the oil cooler, rivet, then attach the folded-up-flaps back down over the newly riveted joints.

This would provide the most coverage. Am I on the right track?