Hot Chimney, Cold Chimney

On the 15th January 2013 it was -13°C. On the 15th January 2014 it was plus 13°C. 26 degrees difference! Bulbs are growing, birds are singing…is that the end of the winter?

But even in these mild temperatures, heat is shooting up chimneys across the country. Have you tried the stack effect calculator?

With a fairly standard flue, say, 11 inches square, with the fireplace opening on the ground floor of a three storey house and an indoor temperature of 20°C, with a night time temperature of 5°C the air flow rate is still 354 cubic feet per minute. Enough energy to run a tumble drier for an hour. Or a hair drier for 3 hours. Grill something for two hours. Or run an electric heater for an hour! In mild conditions!

I’m not claiming that the Chimney Sheep™ will save this much energy, I’m quite careful about making claims about its efficacy and if anything will understate it so as not to make promises I can’t back up with evidence. The stack effect calculator uses established data from engineering principals – have a look at Chuck Wright’s version if you want to check. But put a 13″ x 13″ sheep in that gap, and most of the heat is going to stay in the room. The thermal image below illustrates it perfectly.

In the top one, you can see the warm air heating the stack and if you look closely can even see it escaping out of the top of the flue. The second photo was taken the following evening with a sheep in the flue – no glow to the stack and a noticeable difference in the house! These photos were taken recently, on both occasions it was around 7 degrees outside. All we want is some cold weather so we can go out and play with the thermal imaging camera a bit more!

An infared image showing the effectiveness of a chimney sheep in preventing heat loss