Conduction vs convection, probably. The floor is heated by conduction through the solid material with maybe a bit of radiant transfer; you want the air that is being convected so you need to go where the hot air is.

A good solution is a hollow fireplace rack that takes air from inside the room and is ducted out of the fireplace. It is directly heated by the fire and this allows closed doors on the fireplace to limit the chances of a fire escaping the fireplace.

It's a Aranbe 220IB Convection wood heater so a hollow fireplace rack won't work for us unfortunately. Would a metal floor duct in front of the wood heater heat up enough through radiant transfer to heat up the air travelling past like a heated element to make underfloor heat transfer more efficient than a ceiling based system?

Studying Social Science: Psyc major, Comms minor. Took a year off work, Film School teacher (ask me questions about image capture devices and image capture pipelines eg bitrate and codecs) to travel around Australia with my partner and four kids. Toyota LandCrusier 4.5L V8 twin turbo, Jayco Sterling.

It sounds like your ducting is only for moving heat from the room with the fireplace to the rest of the house. If that is the case, you want to take air from near the ceiling. The coldest air will be at the floor. Your floor maybe warm near the fire due to radiant heat, but that is not the air temperature.

By the way, open fireplaces are extremely inefficient for heating. A wood stove or fireplace insert will give you several times the heat output from the same fuel. An open fireplace draws a lot of excess air into the chimney, and this air must be replaced by cold outside air coming into other areas of the house, making them cold. Is this the problem you are trying to solve with your ducting? At least have a duct supply outside air directly directly to the fire rather than to the occupants of the house.

Oh - so that's already got the hollow rack installed and already has a fan to circulate the heat from the unit. That outlet could be directly routed to a duct with a booster fan.

It is a fireplace insert. I don't think it has a hollow rack but it does have a fan with intakes underneath that blow air up the back, over the top and out the front of the firebox.

@Compositepro you're right I'm trying to move heat from the room with the fireplace insert to the rest of the house. I assumed that an intake in the floor that was closer to the fireplace insert would transmit more heat but in my tests I found an intake near the ceiling transmitted more heat than one near the floor. So I wanted to know why, thanks for all the answers so far.

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Temperature of the actual air would have given you the picture from the start. I've used my kitchen Thermopen a few times to check air temp in the house and I have every confidence in its readings.

This reminds me of the problem of mounting a thermostat on a wall that is not naturally the same temperature as the room. Hot wall, ice cold room no matter what the air temperature is.

One of these if anyone is interested further https://aranbeheat.com/aranbe-inbuilt

The IB bit seems to mean inbuilt or recessed into the fireplace already.

Doesn't sound like you're still travelling around Oz in a Landcruiser though....

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So what was the highest temperature of the wood floor from your laser thermometer. Convection heat transfer from the brick wall of the chimney and from radiant heated floor are the reasons for the elevated temperature of the wood floor.

Chicopee the floor was 81F and the ceiling was 71F. I was planning on using a metal vent for the duct intake and I was curious if the additional heat radiated by the heated metal of the duct would make a difference.

Studying Social Science: Psyc major, Comms minor. Took a year off work, Film School teacher (ask me questions about image capture devices and image capture pipelines eg bitrate and codecs) to travel around Australia with my partner and four kids. Toyota LandCrusier 4.5L V8 twin turbo, Jayco Sterling.

If 71dF is an accurate measurement, then the warm air being at 71dF entering the intake of the metal duct,there will be not a significant radiative temperature to the surface wood floor. "it has been suggested that 212dF is the highest temperature to which wood can be continuously exposed without risk of ignition" per information obtained from the Fire Protection Handbook, NFPA 17th edition, pg3-26. At best the wood floor surface temperature over your duct will not be higher than 71 dF. Another mention is that your OP would have been best posted in the NFPA (fire) code issues.

Quote:

Why is the intake for heat transfer better near the ceiling if the floor directly in front of the fireplace is hotter?

Hot air rises; the air on the floor is likely noticeably cooler than than near the ceiling. You are measuring surface temperature, not air temperature. The atmosphere is nearly transparent in the wavelengths that your "thermometer" uses. Your head is probably a better indicator of air temperature than your thermometer. Place your head near the floor and near the ceiling and you'll easily tell the ceiling air is warmer by far.

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Chicopee I want to radiate the air not the wood.

Thanks IRstuff I will try this.

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