Hypermiling the furnace

Discussion in 'Environmental' started by warthog1984, Oct 11, 2009.

  1. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I'm looking into that.
  2. Airbalancer

    Airbalancer Well-Known Member

    if your crawl space is insulted it is not a bad thing to have leaks in the duct

    Systems that are engineer will have an outlet discharging air into the crawl space
  3. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    As someone who is not a HVAC expert , I don't understand. Why would you want to heat an area that people don't occupy ?
  4. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    We've got a couple of ducts venting into the crawl space, basically elbows coming off the rectangular trunk runs, with dampers. They're both shut as tight as possible right now, but the crawlspace is still pretty warm, getting it's heat both from leakage and just radiation, the ducts aren't insulated.

    After the sealing is done I may have to crack them open, though; I'll see how it goes.

    My rational is that it's better to have both the return and heated air circuits as tight as practically possible, and the insulated crawl space sealed from the living space.
  5. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    You do that when the insulation is out at the perimeter wall of the crawl space, not below the main floor. There's several advantages: there's likely less (and less complicated) insulation, the main floor floor will likely be warmer and more comfortable, you don't need to insulate ducts and pipes in the crawl space, there's less chance of exterior hose bibs freezing up, and so on.

    The crawl space doesn't have to be heated as much as the living space (though ours is, up to now :rolleyes:), but you do want some heat.

    Just kinda wondering now if a return air intake in the crawl space would make sense, especially with improved sealing between living zone and crawl space?
  6. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    Thank you. I learned something.
  7. 44 mpg by 2010

    44 mpg by 2010 Active Member

    1) Wall mounted electrical outlets are often major leaks. Gaskets can be purchased and installed behind the cover plates. Then install "child proofing" seals in the receptacles to prevent leaks where a plug would be inserted.

    2) There are flexible adheasive backed plastic gasket tape/seals (weather stripping) that can be used arround doors and the bottom of windows that is easily installed.

    3) Floor to ceiling wall to wall drapes with thermal liners on the north or windward walls can help further insulate those surfaces.

    4) Insulate above ceilings to a minimum of 10 inches. 20 inches? Do NOT use vapor barriered insulation above existing insulation.

    5) If there is a fireplace ... make sure the damper is closed when not in use. Typically fireplaces draw COLD air into and through the house to the fireplace for combustion and exhaust ... UNLESS ... venting is provided directly to the firebox from outside, not common practice in the past.

    6) Caution: taking inside temperature too low ... may result in FROZEN water pipes in walls and/or under the floor. Overhead plumbing is also a very serious risk depending on installation/insulation. Outside faucets are also a problem as interior temperature is reduced. Of course the magnitude of these risks are dependent on how far below freezing outside temperatures are. My guess is taking the indoor temperature below 60°F when outside is in the 10~20°F is probably the lowest I would go. IF temps are below 0°F, I would set for a minimum of 65°F maybe 70°F. Considerable widespread plumbing damage occurred in Austin TX when temps dropped into the teens.

    Hope this is helpful ...
  8. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    I still haven't sealed a number of seams, but one thing they have in common: they're all "shop" seams, not on-site seams. They look much tighter and uniformly so; I'm not sure how much sealing them would improve things. Now...., to maybe find out where the remaining weak spots might be:

    My wife picked up a Fog Machine for Haloween, a few years back. I'm thinking to start with the heating circuit:

    1. Shut off the furnace.

    2. Wrap the furnace filter in poly film, to more-or-less isolate the heating and return circuits.

    3. Remove the registers, wrap the registers in poly, from the bottom up, then push them back into the boots.

    4. Open up the damper on one (of 2) of the crawl space heated air outlets, hook up the blower side my wet/dry to it and seal the union, start the wet/dry, then shoot the smoke machine into the wet/dry suction a little, and see where it comes out.

    There's a couple of crawl space vents, at opposite ends of the basement, so I could repeat this at the other end.

    Then, repeat with the return air circuit, hooking up at one of the return air inlets.
    Last edited: Jan 12, 2012
  9. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    An update:

    I finished off my tub of mastic, sealing a lot of secondary "shop" seams in the ductwork, then sealed any remaining longitudinal seams with aluminum tape.

    Then I took the plunge: purchased a roll of ductwork fiberglass insulation, 1" fiberglass with FSK (Foil-Scrim-Kraft) backing, and rolls of FSK tape.

    I decided to clean the crawl space first, before getting into the insulation. There was a heavy plastic vapour barrier on grade, in pretty good shape, a few tears, but there was a lot of sawdust, dirt, and 30 plus years of grime and cobwebs.

    Then, I took a couple of utility tables down there, and got to work on the insulation. There's some info online, guidelines on how long to cut pieces, depending on the duct diameter. Also guidance on cutting 2" wide flaps.

    The insulation rolls are 4' by 100' and comes with one such 2" flap, running the whole length on one side. Then when you cut a piece you're supposed to create a second such flap at the cut edge, so that the FSK backing is always overlapping seams. There's guidelines for how long to cut a pice for wrapping. In a nutshell:

    Round ducts: circumference plus 7"
    Square ducts: perimeter plus 6"
    Rectangular ducts: perimiter plus 5" (near square? Split the difference?? ;))

    (All of the above include 2" for flap. For the round ducts for example, you need 5" overlength plus 2" flap allowance. These values stretch/compress the insulation slightly, theoretically to around 3/4" thickness.)

    I used 3" wide FSK tape. It comes with a backing paper that you peal off as you apply it. It's good practice to go over after with a spatula edge to press it firmly in contact.

    I wore old clothes, gloves and used a good quality dust mask, without exception. The gloves have to come off though, when applying tape, and finessing the insulation into tight quarters.

    I got into a routine: get the insulation roughly positioned, then "tack" it with thin strips of tape at intervals along the seams. Finally apply tape for full length of all seams. Transition pieces were tricky, required making some templates.

    Anyway, here's a few pics, before insulation with mastic and some tape applied, and after.

    Here's the main plenum, sealed:


    A round duct, coming directly off the plenum, sealed:


    A couple of locations where electrical wiring ran in/out of the return air circuit, now sealed:


    (This one I filled the big void with styrofoam, then caulked around. The caulk is clear, not all that visible)


    A typical rectangular trunk line seam, sealed:


    One of two registers for warming the crawl space, as needed, just sealed. The lever opens/closes the register via an internal metal damper disc.


    A round duct, sealed:


    (Note, in cases like this, with an elbow, I disassemble the duct, so that I could the mastic right into the joint. Where not possible, I used fiberglass mesh to reinforce the seal.)

    The roll of insulation, about half used up:


    (I ended up buying a second roll as well, and got about half way through that one)

    The plenum, after insulating:


    Note the tuck tape patches on the ground vapor barrier. I took the time to clean the whole crawl space, and patched the tears in the vapor barrier, both to tighten it up and to prevent dirt sifting through again.

    Insulated rectangular trunk line, with a vertical deflection, and a round duct coming off:


    A couple of round ducts coming off a trunk line, insulated:


    Close up of round duct meeting boot at underside of register:


    A bit of sprayed-foam is noticeable in this pic. I sealed all the sub-floor boot junctions with foam-in insulation (also caulked the boots against the sub-floor, from above). First I used Dow Great Stuff, then switched to a similar DAP product, one that was latex basedj. The latter was much friendlier to use, easier to clean up. The can can be used multiple times without clogging. Also, I was abple to put a flexible extension tube on it, to improve reach.

    Bottom line:

    Prior to starting the insulation, if I set a temperature probe in a particular outlet, with the thermostat raised several degrees centigrade, so that the furnace would do a sustained, fully warmed up run, the temperature got up to around 38~39 Centigrade. After the insulation, the temperature levels out at 47~48.

    This page has a number of relevant publications:


    (in particular, the second one down: "A Guide to Insulated HVAC Systems (AH121)"
    Last edited: Mar 3, 2012
  10. 08EscapeHybrid

    08EscapeHybrid Moderator

    Because your walls are insulated, and the floor above is not, your crawlspace is considered controlled space, and does not require insulation on the ducts. It certainly doesn't hurt to insulate it, but your gains will be relatively small unless you insulate the floors as well.
  11. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    Well, there's quite a large volume of air down there, I'd hazard a guess at 20% of the house's total air volume. Previously it was basically at the same temperature as the rest of the house, due both to vicarious heating through the thin sheet metal walls of the ductwork and the abundance of air avenues between the two air spaces. Now the crawl space heating is more controlled. So far I've left the 2 outlets down there almost tight shut. The temperature difference is still not much, maybe 2~3 degrees centigrade. I've thought a bit about insulating the underside of floor, but think I'll leave it as is for now.

    And the air coming out of the registers gets upwards of 10 degrees centigrade hotter now, so that's got to be good.

    One thing I've got in back-of-mind: this duct insulation would also help if we were to upgrade our furnace to a system with air conditioning.
    Last edited: Mar 3, 2012
  12. jcp123

    jcp123 Caliente!

    Hypermiling the furnace? I live an an attached home (duplex), and have had the HVAC swystem turned off for the last couple of days. My wife even had opened the patio doors for about an hour and a half last night and it is still quite cozy this morning. Temps have hung out around 60 or below the last few days, not hugely cold but enough that I would have thought that we would turn on the heating side of the central air by now. Either the place is well-insulated (which I think is the case, the college parties on the block are scarecely audible, plus opening and closing a door seems to cause a great change in air pressures throughout the home), and/or we are receiving a lot of heat from the neighboring unit.

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