Author Topic: Gravity flow furnaces  (Read 6423 times)

Ben Stallings

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Gravity flow furnaces
« on: October 07, 2010, 08:11:07 AM »
In a few weeks I'm taking a recertification test from Efficiency Kansas, and they're letting me choose the house I walk through.  I chose a neighbor's house which, among other quirks, has the original 1930 gravity-flow furnace.  I've never worked with one of these beasts before and don't really know what I'm getting into.  I plan to look it over before the test, but I'm not even sure what to look for.  Are there any gotchas peculiar to these furnaces that I should be on the lookout for, so that I can wow the examiner with my arcane knowledge? :-)


Bud9051

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Re: Gravity flow furnaces
« Reply #1 on: October 07, 2010, 08:36:06 AM »
Depends upon how old the examiner is, he may have cut his teeth on those old beasts :)

I'll be watching to see what the HVAC folks say, although it sounds like an automatic candidate for replacement.  The test could be interesting.

Bud

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stnick

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Re: Gravity flow furnaces
« Reply #2 on: October 07, 2010, 03:41:51 PM »
Ben,

Gravity Flow, originally coal fired furnace.  Probably converted to gas in the 1960s when gas was piped into towns across Kansas.

The principle is heat rises.  So the original thermostat did not require electricity. It just opened and closed dampers.

Large ducts in the basement to the 1st floor.  Many times you will see the supply hit a wall and a register on each side.  Sometimes a wall cavity to second floor.  Sometimes just a supply grill in the ceiling and another directly above it on the floor of the 2nd story.  Balloon framing.  Where does the heat go?

The returns are usually large floor grills.  Many times they are actually made of wood.  Large, 20x30 is not uncommon.  Beautiful!  2nd Floor return is the stair case.  Look for a return on 1st near the bottom of the stair case, and one more.  I've run duct blaster tests on two of these.  Both were tight, less than 6 CFM per 100 sf of conditioned space. Energy Star standards. You don't have to do that for Efficiency Kansas.

Be careful with the furnace and the ducts.  If they have not be dealt with since the conversion to gas, they are probably insulated with Asbestos. UGH!  $$$$$$$. The burner, if the size can be found is probably 100K or 125K or 150K.  They didn't care back then.  I would also wonder about the pilot valve and associated plumbing.

You may have associated coal handling equipment in the basement.  If present, probably stored. But look for it.  Automatic stoker, coal shovels, coal bin.  Coal Delivery Doors in the Foundation Walls.
All sorts of opportunities for mischief and upgrades.

You will have two costs on the replacement furnace.  Removal of the monster (with or without asbestos).  Then installation of a forced air unit and the new plenums that go along. I doubt you have someone in town to deal with the asbestos, but perhaps. Otherwise Wichita or KC.  $$$$  If they remove the basement and crawl ducts,  make sure you are ready to spec galvanized and sealed ducts, not the flex or duct board.

Think about the orphaned hot water heater.  You will be doing a condensing furnace.  The water heater to heat up that old masonry chimney or a new liner for the chimney.  Perhaps you can make a condensing DHW work. 

Then you have a chimney space to run an actual supply upstairs to second floor for air movement. Comfort issues on 2nd will probably be a big complaint  If you can't use the chimney, maybe there is an old dirty laundry chute you can use, or both.

What kind of kneewalls will you have?  Be prepared to measure them, and measure them, and measure them.

Infiltration!  Is there existing cellulose that has been blown?  Maybe not to bad then.  If it wasn't done properly, then you will probably need the CRF factor.  Be familiar with Krigger and Dorsi on balloon framing upgrade techniques.  There will be k&t holes new elect. holes, there will be open places where they pieced top plates. You may well find, boards that have finally dried out in the attic and the shrinkage or warping opened spaces for leakage.  Don't forget your original porch roofs for leakage.  Be prepared to be drilling a number of holes for your manometer probe or to find a number of cracks or holes large enough to probe.

Check the attic areas that you can. Be prepared for some that you can't get to.  Have your drill and plugs ready.

OK, guys and gals!  What did I miss?

Bud,  Trayce probably did not cut her auditing teeth on these types.

John   ;D
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David Meiland

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Re: Gravity flow furnaces
« Reply #3 on: October 07, 2010, 04:02:31 PM »

The principle is heat rises.

Just to pick a nit.... warm air rises. Heat itself doesn't care which way is up.
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stnick

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Re: Gravity flow furnaces
« Reply #4 on: October 07, 2010, 05:44:47 PM »
David,

Yep!  Heat will go to the place of lower heat.  The air warmed by the heat rises.  A nit.  An important nit among professionals.

John  ::)
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SLS Construction

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Re: Gravity flow furnaces
« Reply #5 on: October 07, 2010, 07:30:37 PM »
Ok call me stupid - what exactly is a gravity fed furnace - is this a steam, hot water or air system? Are we talking about how it is fed? Thanks

stnick

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Re: Gravity flow furnaces
« Reply #6 on: October 07, 2010, 09:00:16 PM »
Gravity hot air furnace.  No fan or blower. They were built and installed from the 1890's to 1940s.  Big huge asbestos insulated monsters.  I've seen foot prints at are 8 feet in diameter.  The ceiling was 9 feet and the floor joists were trimmed to make it fit.  (2x 12s real stuff).

When I grew up in Idaho, we had one.  My chore was to shovel coal into the stoker.  I was always real faithful about it.  I remember once when I had the chicken pox and my dad forgot to do my chore, how upset he was and how cold the house got.  I also got to remove the clinkers.

Down south there you may not have ever run across one.  I know Bud has seen a few.

My wife's folks moved off the farm in 1979 to a 1924 Craftsman Style house with one of these. It had been converted about 1960 to gas.  No blower.  Just the burners.  6 big huge ones that I remember.  Gas valve and pilot light.  They looked at one place that didn't have a gas valve, just a regulator and pilot light.  UGH!!!

It was always warm in the winter in that house.  After paying the propane bills and the REC electric rates on the farm, they thought the costs in town were low.  :o

John  ;D ;D

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SLS Construction

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Re: Gravity flow furnaces
« Reply #7 on: October 08, 2010, 06:10:25 AM »
Got it - thanks John

Ben Stallings

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Re: Gravity flow furnaces
« Reply #8 on: October 08, 2010, 07:45:04 AM »
Thanks for all the information, John!  I'm a little intimidated... have to remember that Trayce doesn't want me to fail the test... if I do, there will be another...

As long as we're picking nits, warm air doesn't rise, either; it's just that cold air pushes it out of the way as it sinks.  I mean, there's no anti-gravity force that warm air can tap into.  So on a gravity-flow furnace, the density of the return air is what's actually causing the warm air to flow up the supply ducts.  This implies that I should be on the lookout for any restriction to the return air flow, since that would inhibit the whole thing from working.  Correct?

As you described the duct characteristics of these old furnaces, John, I recognized a lot of the peculiarities of my own house (which was built 10 years earlier than the one I'll be inspecting).  Makes me wonder if the furnace I've got is the second or third the house has had.

I'm not sure what you mean by "What kind of kneewalls will you have?  Be prepared to measure them, and measure them, and measure them."  Do you mean the dimensions, or the construction?

stnick

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Re: Gravity flow furnaces
« Reply #9 on: October 08, 2010, 03:10:35 PM »
I have a 1912 craftsman style on my desk right now.  I've heard it called a 4 square.

The 1st floor is 10 feet ceilings.  The 2nd floor is  7 in the center.  The house is square.  There are 4 gables.  The valleys run from a corner to the center of the roof.  So, every room has knee walls.  The gable ends are topped with triangles. The kneewalls end with the roof forming sloped triangles or polygons.

Each room with have a flat ceiling.  It will have two to 4 sloped ceiling pieces and the walls will be interior and kneewall.  The one I am working has 2 attic accesses and you can't get to all of the attic.  i had to bore some holes.

Trying to get the wall square footage down is a real %^$#*!

Now! Don't let the house intimidate you.  You know how to measure and what to measure.  Spend your time doing the outside then move inside.  Use a tape and have a folding carpenter's rule.  Write the numbers down as you go.  If it is this complicated.  You might want a page for each room with a page to show the whole 2nd floor.  Just show her you know you have to take all those weird measurements.  You will probably only have to do a couple of the rooms.

I would provide the questionaire to the HO prior.  Have them leave it or give it to your when you come.  Then use any concerns they have to focus on.

I have also been in these types of houses with 10 foot 2nd floors ceilings and no knee walls.  So, good luck.

John   :)
« Last Edit: October 08, 2010, 03:12:06 PM by stnick »
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