Moving Warm Air

Discussion in 'The Garage' started by Hootowl, Oct 4, 2012.

  1. Hootowl

    Hootowl Long timer

    Jul 2, 2002
    Bend Oregon
    I've got a green house (garden room) that's part of our house. It follows the roof lines of the house and has vertical glass windows and double wall translucent plastic roof.
    It faces south. In the winter, with the sun shinning the room heats up into the 70's. At the high end of the roof the air gets much warmer. With the door between the greenhouse and living area open, the warm air helps to heat the house via convection.

    I would like to capture some of that warm air at the high end of the roof and pump it into the living area but I don't know how to calculate the size of fan I would need.

    Here are the dimensions of the sun space
    In plan view its 11x23 feet
    In side view its 9 feet x 13 feet

    The question is: what size fan and what size duct would I need to move air into the living area of the house?
    I don't have much money for this project so there won't be a temp sensor to turn the fan on/off, I'll do that manually
    I don't want to run 220v for the fan, just 110v

    I look forward to the replies

  2. DSM8

    DSM8 Where fun goes to die....

    Aug 1, 2005
    Glendora, Ca
    Think HVLP

    High Volume Low Pressure.

    A large bladed fan would move a lot of air at low RPM = Low noise.

    A simple ceiling fan in the green house would keep the air moving to more uniformly heat the space, that warm air should increase the rate of convection into your house since now the cooler air of the house would flow into the space and displace the warm air.

    Just a thought.

    Wont need any ducting unless you are adamant about physically blowing the air from one space to another. Then you could use a good size fan like they use in a large bathroom etc. Once again cause price is an issue. Otherwise talk to an HVAC guy since they have squirrel cage fans which are small and move alot of air.
  3. jules083

    jules083 Long timer

    Jan 26, 2011
    Richmond, Ohio
    I believe you're over-thinking this. I understand what you mean about the heat being in the peak of the roof, and I think the ceiling fan idea would work. If you really want to do it on the cheap, get a piece of flexible ductwork, a small fan, and some duct tape. Run the duct from the peak to the top of the doorjam, and put the fan at one end or the other. Seal with duct tape.
  4. P B G

    P B G Long timer

    Mar 7, 2008
    Greater Chicago
    There are inexpensive fans sold as "booster" fans for inline ducting, commonly they are hooked up to the furnace so that long runs of duct (spare bed rooms at the far end of the house) get sufficient airflow,

    One of these set to run when the air temps were above XX degrees in the greenhouse - thermostat/relay?

    Would circulate that warm air into the house, and turn off when the greenhouse cooled down after sunset, to avoid "heating" the green house.
  5. Blackalac

    Blackalac Adventurer

    Jul 30, 2010
    Rochester NY
    Btu=1.08*cfm*temperature difference
  6. Tim McKittrick

    Tim McKittrick Long timer

    Dec 29, 2003
    Wasilla Alaska
    Why not just use a new low sond bathroom exhaust fan of 100CFM or so? they are reasonably cheap, quiet, available, and 110v.... and they already have a 4" outlet so you can use standard ducting and a beauty grill to make it all look like you intended it to be there all along.
  7. anotherguy

    anotherguy unsympathetic

    Jun 18, 2009
    the hills
    Vent high,vent low. Convection will do the job for you. Just make the vents adjustable. I have a friend who has a wall of black 55 gallon drums full of water in a greenhouse like addition that heat his small cabin well in winter. Just some slots in the separating wall top and bottom is all it takes. During the summer there are outside vents and he covers it up.
  8. zeeede

    zeeede Long timer

    Mar 6, 2012

    So Btu = 1.08*100CFM*20ish degrees of temperature difference means you're only getting around 2000 BTUs out of this.

    You should look at getting a really energy efficent fan, otherwise you might be spending more in electricity running the fan than you'd be saving with running the fan instead of just getting an energy efficent space heater or something.
  9. Hootowl

    Hootowl Long timer

    Jul 2, 2002
    Bend Oregon
    I appreciate all the replies.
    Yesterday I ran a fan with 18 inch blade for 3 hrs and it raised the living room temperature 2 degrees. The fan was sitting on the floor in the doorway between the sun room and the interior.
    I'll try the booster fan with duct if I can find them for a low price.
    With all the hot air near the peak of the sun room it would seem that I should be able to use that to warm the house but perhaps the math doesn't "compute"

  10. eepeqez

    eepeqez Long timer

    Sep 8, 2009
    Melbourne, Australia
    There's a limited amount of warm air in the greenhouse and a limited supply of heat warming it.

    The 70ish degrees is the end state where the heating effect of the sun equals the heat loss from the space without removing warm air and adding cold air.

    Once you've replaced your volume of about 2500 cubic feet of warm air with cooler air from the house, it will take time to recover, and in fact what you're effectively doing is to increase the cooling of the greenhouse and reduce the steady state temperature the greenhouse will reach. If you blow too much air through to the house, the steady state temperature of the greenhouse will be colder than you want the house and you'll just be using the house heating to heat the greenhouse!

    So the $64 question is how long it takes for the air in the greenhouse to warm up to a useful temperature again. You could try to estimate this, or you could just measure the temperature of the air in the greenhouse.

    You could do this with:

    • A fan that blows air from the greenhouse to the house
    • A thermostat that senses the temperature of the air
    When the air in the greenhouse is warmer than in the house, blow air into the house. When the temperature of the air coming in from the greenhouse falls to less than say 5degF above the house heating thermostat temperature, the fan stops.

    For maximum efficiency, you want to:

    • Draw the air from the greenhouse up high where it is warmest.
    • Draw the air from the greenhouse at only slightly above the required temperature in the house, to maximise heat transfer from the sun to the greenhouse contents and minimise heat loss in the greenhouse.
    • Have a return path at ground level (colder air) for the air from the house to the greenhouse so you're not sucking colder outside air into the system.

    Now back to the $64 question; how fast does the air heat up? Let's make a wild guesstimate of about an hour in ideal conditions.

    The volume of the greenhouse is about 2500 cubic feet. 50cfm (cubic feet per minute) would transfer 3000 cubic feet an hour, so that's probably ample.

    Low voltage cooling fans of around this capacity are cheap, as are 12V plug packs (you call them wall warts I think?) and a thermostat which will switch 12V. If the fan runs continuously, you can add a second one.

    You could cobble up just the 12V fan of known CFM rating initially and manually measure the greenhouse temperature over time at intervals of say 15 minutes on a cold sunny day and you'll soon know if it's cooled the greenhouse down too much to be useful within 10 minutes and takes 2 hours to recover when you stop it (too much air movement), or if it barely affects the temperature (not enough).

    Remember if the greenhouse is drawing in cold outside air to replace the warm air, this will dramatically reduce the efficiency. Very roughly, if you're aiming to draw air at 75f into the house which is maintained at 70f and instead of letting house air at 70f back into the greenhouse you let outside air at 50f, then it will take (75-50)/(75-70) = 25/5 = 5 times as long to reheat, or you'll get 1/5 as much heat from the greenhouse part of the system. It's not actually that simple, but it's a good approximation.

    Once you know whether you're getting enough heat to be worth bothering, buy the thermostat and automate it.