Wood furnace as backup heat in power outage

Sfair

Good day everyone,

This is my first post. I live in an area that gets occasional power outages in climate zone 6. Our Geothermal died summer of 2023. I have purchased a large mini split to replace it.

We do not have a generator and I would like to have a solution for long term power outages. I would also run it as supplemental heat especially when it gets to very low temperatures. I purchased a used Clayton 1600M furnace. It has a draft cap on a screw and does not require power for induction of combustion air. The garage and basement are quite leaky so I'm not too worried about combustion air.

The 1/2 basement and 1st floor garage are not part of the houses conditioned space. My idea is to install the furnace and run a 14"x20" plenum directly up into the living space. The furnace hot air plenum opening is 13"x18". The net free area of the floor register is very close to the area of the opening on the furnace. The cold air return would be a short horizontal run of 20"x30" then a vertical run to the living space in the same 14"x20" plenum. I would want the filter box to accommodate a 20"x30"x5" filter for maximum filter surface area to keep restrictions and static pressure down. In the event of a prolonged power outage I would remove the filter to allow it to move as much air as it can with convection.

My concerns are how hot the air will be coming out of the furnace and if the duct and floor register will be too hot. With power the blower has 3 speed settings and the temperature it kicks on can be adjusted. With that short of ductwork and minimal static pressure I'm thinking the lowest speed should be more than enough. With that setup I suspect there will be enough convection the blower may not hit temperatures to come on as the heat to dissipate quickly.

In the event of a prolonged power outage the furnace could be throttled down with the damper and draft controls.

I was also planning to insulate the duct plenum with an air gapped double bubble reflective insulation. I also have concerns if it can be used so close to the furnace. It looks like it has a temperature rating of -50 to 180 degrees Fahrenheit. Do I instead how to switch to fiberglass or mineral wool if the temps are going to be too high? I can do a test run for temps before adding the insulation.

If anyone sees and red flags or code violations I would appreciate if you could let me know.

Does anyone have any suggestions on improving the idea?

Thanks in advance!

hot_rod

You risk over-heating and cracking a furnace heat exchanger if you do not have adequate air flow. The fan high limit safety switches are critical as is air movement.

Unless it was specifically designed for gravity flow?

It should not take much generator power to keep the fan spinning.

WMno57

That's one way to do it.
Basic propane furnace with one or two 12 volt deep cycle batteries and an inverter would be another. Advantage with that is you don't have to feed the forest eater. You don't even have to be home.
Off grid home in Michigan's Upper Peninsula

https://offgridcabin.wordpress.com/

Jamie Hall

Go over the codes, particularly spacing between duct work and combustibles, very carefully with your building inspector or fire marshal -- or both. In my area clearance of ductwork -- not just the flue -- to any combustible material -- which includes that bubble insulation -- must be a minimum of 2 inches.

Clearances from the boiler itself and the flue are likely to be larger.

Does that have a fusible link on the damper for when it gets over ambitious? It should.

I personally would never leave a forest eater unattended, or at least without someone on the premises at all times... seen too many chimney fires.