Monday, February 23, 2015

Practical Insulation

Like most, we seek to keep the cold out and the heat in. Many methods are used. I make no claim that our method is the best, it’s just the one we have adopted.

We were able to quantitatively determine how well our method works recently from the observation of how often the furnace turned on while the outside temp remained constant.

Prior to buttoning up, the furnace was turning on every 14 minutes. Afterwards, the interval was 70 minutes. We did not completely button up until after dark, so while the outside temperature remained constant, 34, we also lost the radiant heat of the sun. 

We put Reflectix in all vents and snap the factory supplied covers in place. The cab windows and the shower skylight are also covered with Reflectix. This slowed the furnace cycle somewhat, but the surprising thing to us was how much insulation the window shades provide. We are always reluctant to lower the shades, but now we know the difference they make.

Linley amplifies:

Saw the item on the Lazy Daze Companion about added insulation (in the windows, etc.) making a big difference in heat loss. The item's conclusion made by noting how often the furnace ran was interesting and, in fact, does indicate a good improvement.

However a much more accurate measure is obtained if the percentage of time the furnace is firing measured instead.  This can be found by measuring both the length of time the furnace flame is burning during each cycle (it makes a racket) and also the length of time between the start of each cycle, and then dividing.  

With the percentage of firing time in hand one can get an estimate of the actual heat loss.  Multiply the percentage of firing time times the number of cycles per hour and then take that times the heat output of the furnace in BTUs per hour. 

There is a problem about here because RV furnaces seem to be rated in BTU's per hour of input (i.e. propane consumption) instead of heat output.  A rough guess is that one gets something like 50% or 60% of the input heat as output inside the coach.