Sizing circulation duct

[ergeorge2000]

I have a fireplace in the living room, which also happens to have the only thermostat in the house. I like to burn it alot in the winter, but the back bedroom gets pretty cold.

What I'd like to do is run a small duct with an inline fan through the crawlspace from the back bedroom to the living room, and suck the cold air out of the bedroom so that warm air can flow down hall and keep things a little more reasonble.

The run is about 50 feet, probably flexible duct. Two 90 degree elbows and a floor grate on each end. I'm considering either 6" or 8" duct. For the 6" I found inline fans claiming 160 & 250 CFM. For the 8" I have 500 & 210 CFM. I would like to estimate the total CFM for the installed duct.

Any suggestions? It's a ranch style house and the bedroom is about

20x30 feet, 8 foot ceilings throughout.

[SQLit]

Wont work very well. Heat rises cold settles, crawlspaces that I am familiar with are on the floor. You would be moving the coldest air in the room. turn the heater fan to on when your using the fire place. Might help some

[Joseph Meehan]

Just some notes:

Fireplaces tend to suck the warm air out of the house up the chimney. It is likely making your bedroom colder. The temperatures difference is not high, so the volume of air needed to be moved is very large. You are going to need same very large ducts and fans. Flex duct is much less efficient due to the friction looses of the rough sides. My guess is you will need several of the 8" jobs to do any good.

[Stretch]

To determine the amount of air needed you need to do a load calculation. You will also need to know the temperature of the supply air. The load calc will require square feet of walls, windows, ceilings, floors and doors exposed to the outside. Also insulation values for those items. How tight is the construction, as infiltration needs figured in. How powerful is your blower. I need a fan curve, inches of static at any CFM. Flex duct will have more resistance than sheet metal. Ducts should be insulated.

I think you are wasting your time. results will not be impressive unless you have a BIG blower and big duct, then the air will feel drafty.

Stretch

[nicksanspam]

Glass doors or woodstove inserts can make fireplaces a lot more efficient.

If warm air can flow down the upper part of the hall, cool air can flow in the other direction in the lower part of the hall, no?

A 20x30' room with 2 R20 exterior walls and 48 ft^2 of R3 windows and a window conductance of 48ft^2/R3 = 16 Btu/h-F + 176ft^2/R20 = 9 for walls = 25 Btu/h-F would need (70-30)25 = 1000 Btu/h to stay 70 F. If 70 F air thermosyphons through a 3'x6' door with 1000 = 16.6x9ft^2sqrt(6')dT^1.5, dT = 2 F (with 511 cfm of flow in the hall), so with the doors open, the bedroom might only be 2 F cooler than the living room. A window box fan pushing air into the lower part of the bedroom door could help, and storm windows and more bedroom insulation and airsealing could help.

Nick

[ergeorge2000]

Yea, that's the idea actually. Move the coldest air in the bedroom out to promote warmer air flowing in from the hall to replace it.

Yea, the fireplace (actually a fireplace insert) has a fan, which works great - but the heat isn't getting down the hall to the bedroom.

[ergeorge2000]

Yea, it actually is an insert - not an open fireplace.

In theory. In practice it doesn't seem to be enough even with a portable fan in the hall.

I know the difference is more then 2 degrees - I've seenabout 70 degrees on the thermostat, and had that back bedroom below 60 (the temp at which I jave a small electric heater set to kick on). The windows in the bedroom are new and have curtains. I think things are pretty tight.

Thanks for the suggestions folks!

[Stretch]

Nick,

R-20 walls are unusual, R-11 is more common. (Mine are R-24, but I have never seen anyone else do that). Probably figure average windows, actual sizes would help, single or double pane?? Don't forget ceiling and floor (Crawlspace) load. Also, don't use a propellor fan to push air through a duct, not enough static available to be very effective. A small squirrel cage forward curve blower would be much more efficient and more effective.

An 8" duct with .1" static per 100 feet would deliver about 230 CFM with metal duct, about 160 CFM with flex duct. To get 511 CFM ducted would require 12" metal duct. 12" flex would provide 480 CFM. No allowance was made for grille losses.

Otherwise I agree.

A window box fan in the hall would definately work the best for the least money.

Stretch

[SQLit]

cold air is a lot harder to move than warm air. I used to run my a/c fan when using my fireplace. It would warm the bedrooms some, ~2-3 F. Over the course of an evening. I live in a pretty temperate climate so heat is not really something to get excited about. The only fireplace heater I ever saw that worked was a heatalator fireplace that was installed with ducts and a fan. The seperate duct 18x10 ran down the attic and branched into the bedrooms It did the job, noisy as hell.

[nicksanspam]

Keep up, Stretch. That 511 cfm was a natural hallway airflow, with no ducts.

Nick

[nicksanspam]

There's very little difference at typical house temps, IMO.

Nick

[Stretch]

How did you measure that air flow Nick? Or was it just a guess? Who's hallway was it anyway?

My digital thermo-anemometer measures halway air velocities with heating and cooling off at well below 10 Feet per Minute, eg.: natural convection. I will try again when the weather is more extreme to see if that changes.

My halway has about a 24 square foot cross section.

Stretch

[nicksanspam]

It was a calc (not a guess :-) using an empirical stack-effect formula, cfm = 16.6Asqrt(HdT), so heatflow = cfmdT = 16.6Asqrt(H)dT^1.5 Btu/h, approximately.

Perhaps the OP's. See below.

Convection requires a temp diff. Try heating one room and not the other.

Sure. Try when it's cold outdoors. Since your house is very well insulated, you may not see much flow.

A 20x30' room with 2 R20 exterior walls and 48 ft^2 of R3 windows and a window conductance of 48ft^2/R3 = 16 Btu/h-F + 176ft^2/R20 = 9 for walls = 25 Btu/h-F would need (70-30)25 = 1000 Btu/h to stay 70 F. If 70 F air thermosyphons through a 3'x6' door with 1000 = 16.6x9ft^2sqrt(6')dT^1.5, dT = 2 F (with 511 cfm of flow in the hall), so with the doors open, the bedroom might only be 2 F cooler than the living room. A window box fan pushing air out of the lower part of the bedroom door could help, and storm windows and more bedroom insulation and airsealing could help.

[cfm = 16.6x9ft^2sqrt(6'x1.95F) = 511.]

Nick