PF100 Cold Air Return Question

[Ejectr]

What would be the result if the air going through the cold air return to the furnace never heats up? This may explain a lot of things that I have wondered about regarding my PF100 performance.

 

[SmokeyTheBear]

What would be the result if the air going through the cold air return to the furnace never heats up? This may explain a lot of things that I have wondered about regarding my PF100 performance.

Well provided there is sufficient air flow in the return air system the temperature at a hot air vent will become a constant over time (steady state) until you increase the actual amount of fuel fed to the fire or its equivalent.

What happens if the temperature actually warms up in the return air system is that the amount of heat extracted from the heat exchanger will decrease and more will go up the stack.

It isn't likely that the temperature in the return system never goes up at least some but it is possible. It can also go down.

Funny, isn't it?

 

[Ejectr]

What would be the result if the air going through the cold air return to the furnace never heats up? This may explain a lot of things that I have wondered about regarding my PF100 performance.

Well provided there is sufficient air flow in the return air system the temperature at a hot air vent will become a constant over time (steady state) until you increase the actual amount of fuel fed to the fire or its equivalent.

What happens if the temperature actually warms up in the return air system is that the amount of heat extracted from the heat exchanger will decrease and more will go up the stack.

It isn't likely that the temperature in the return system never goes up at least some but it is possible. It can also go down.

Funny, isn't it?

Here's why I ask. I have an extension on the back of my house that is heated with a pellet stove when we use it. When we don't use it, the door is blocked off by a bamboo curtain that we drop from the top of the door to block it off. Being bamboo, it is something less than a perfect seal until I put a door on it and admittedly, it is a little short at the bottom. So there is cold air that escapes through the cracks between the bamboo and the bottom, which just today I blocked up with a rolled up blanket.

I've noticed the furnace working kind of hard when it shouldn't be at times and couldn't figure it out. Today, I heated the extension with the pellet stove as I was going to watch the football game in there. During a break, I went down the basement to check the pellet level and much to my surprise, the furnace had not used its normal amount of pellets and the furnace was in a slow burn mode that I NEVER see it do only feeding pellets to the burn pot 4 seconds out of every minute.

It suddenly dawned on me that the cold air return grate is in the hallway floor just outside the extension and it was now getting warm air from the heated room instead of cold air escaping from the room. I figured that with this warm air returning to the furnace, it wasn't working as hard as it normally had to with the cold air returning and cooling down the heat exchanger after passing through the blower motor squirrel cage fan. There has to be a difference between this really cold air hitting the heat exchanger and this warmer air to cause the furnace to run like I've never seen it run since the warmer days of October.

I'm definitely putting a door in and closing that room off good when it is not in use. Am I on to something here with this cold air/warm air difference?

 

[SmokeyTheBear]

Well you are in that the heat loss is effectively higher without the help of the stove and that would account for the return air temperatures not also seeing a bit of an increase.

It is all in the heat loss. Anything that reduces heat loss helps.