# NG furnace duct temp



## mtaccone (Jan 28, 2009)

How hot should ducts get with a NG furnace? Seems like with oil there is always plenty of hot air coming from the vents. I have metal uninsulated ducts but still. My furnace has to run half hour before all my vents get warm air. This furnace doesnt seem to keep up when it is cold and I was firing off 300 therms last month in my 1500sq.ft. drafty house. Any ideas?  I am about ready to go back to oil, I never had so much usage with oil in anyplace that I lived as I have had with NG.


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## Redox (Jan 28, 2009)

Furnaces are rated by temperature rise and should have this rating on the nameplate.  Most gas furnaces will have a design rise of 50-90 degrees F, that is, if it is returning at 60, it will put out 110 to 150 degree air.  The lower number is more efficient, but not as comfortable.  You can slow down the blower to increase the rise, but sacrifice efficiency.  Oil furnaces run notoriously hot, 150-180 degree discharge is not abnormal.  If the air is really that cool, you should consider insulating the ductwork.

Efficiency and comfort level don't always go hand in hand.

Chris


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## mtaccone (Jan 28, 2009)

The blower is on its lowest setting. What is recommended to insulate the duct work? Insulating may be 1 hell of a task as I can't fit in the same space as the ducts in the attic..... Man I miss oil!


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## Redox (Jan 28, 2009)

If your ductwork is in the attic and has no insulation then that's where all your heat is going.  I think I would be looking into blowing more insulation up there to cover the ductwork up.  It should be wrapped with fiberglass and a vapor barrier when it's installed, but now that it's in...  Maybe you could hire some skinny young helpers to do it for you?

Just curious; Do you have AC on this system?  Attic ductwork is very unusual in the northern US.

Chris


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## mtaccone (Jan 28, 2009)

No A/C My main ducts are in the basement but the ducts that run to half of my upstairs run along my attic. I have the old floor joist type of return ducts which I am sure are not the best sealed.  first floor registers don't get very warm either and the basement ductwork some gets to be 100 degree some do not. I think that this furnace is too small for the application as it was just thrown in so they could sell the house fast as the old one died the day we came to look at it back in March 2007.. It is 80% gas fired 70,000btu output. Worst heating system I have ever seen... 


I am a little wary of insulating the ducts as I may add on a wood furnace in the future. Can they still be insulated??


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## Redox (Jan 29, 2009)

I don't see why not, especially the part that is in the attic.  There's basically no insulation between that ductwork and the outside if I'm reading this right.  Definitely check the clearance requirements on the furnace you pick, but I think it all should be insulated, unless you deliberately want to heat the basement.  You might wanna post this question in the Boiler Room; there may be a reason that I am not aware of.

How big is the house?  Will it hold 70 degrees inside during a cold spell?  It's beginning to sound like the furnace is undersized.  It wouldn't matter if the furnace were oil fired or not; 70K isn't a lot of furnace for a loose house in NY state.  It's probably time to do a heat loss calculation to see how big a furnace you need.  This can be rather complicated and there are calculation programs and guides online; ask again in the Boiler Room.  I have an old DOS based program for this, but there are probably better ones out there.  Knowing your actual heat loss is going to be the first step.

Chris


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## mtaccone (Jan 29, 2009)

Yeah I agree with ya, the attic ducts are going to get coverd with blown in insulation in the spring that are in the attic. I do want some heat in my basement and I am pretty sure this furnace is undersized I may have a contractor do a heat loss on this one. Like I say they just stuck a furnace in as fast and cheap as possible to get this house sold to us. I am not sure that a heating person even installed it. I think it was "the best $400 can buy".  It will only maintain 70degrees on a 0degree day if it runs almost constantly hence the $3-400 gas bills/month.


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## Corey (Jan 30, 2009)

Some things I would look at:

- Is the air filter clean
- Are the flames translucent blue and relatively steady burning (should not be yellow and/or 'dancing')
- Is the heat 'balanced' in the house.  You may need to partially close some of the registers close to the furnace and leave those further away wide open.
- How is the cycle time?  You should hear the gas burner kick on, warm up the heat exchanger for a minute or so, then the blower kicks on.  Shut down would be the reverse...burner goes off, blower runs for a minute or two to extract the heat, then shuts off.

IMHO, you're going to get better efficiency running the blower at a higher speed.  As Redox mentioned (and maybe contrary to what you'd think) the lower the temperature rise, the higher the efficiency.  If you have a higher temp rise, that means your heat exchanger is hotter and more heat is going up the flue.  Also, moving a large amount of cooler air through the ducts is more efficient than moving a small amount of super hot air.  If your duct work is heated to 190º, that radiates a lot more heat away than say 110º.

The reason I mention all this is when I bought my current house, the previous owners gas bill was ~$320+ per month in the winter.  I replaced the clogged air filter, closed two registers which were directly in the main plenum (and venting a huge amount of air into the basement), adjusted the pilot light from a 5" long blowtorch down to the standard ~1" tall flame, and changed the cycle time - for some reason the burner would kick on, about three minutes later - (after the heat exchanger was super hot!) the blower would kick on, the whole show would run for about 3 minutes, then the blower and burner would practically shut off at the same time - leaving the HX super hot.  Then about 15 minutes later the whole cycle would repeat.  This was putting a tremendous amount of heat in the HX and up the flue, but not much in the house.  After the tune-up, the bill dropped to about $110. (Then I put in a wood insert and it dropped to about $40!)

Either way, between gas and oil, I wouldn't think there should be a huge difference in heating ability.  If the appliances are rated at roughly the same efficiency, then you should use about the same amount of btu's (though there may be a big cost difference between gas and oil)  If you're using substantially more btu's with gas, then there must be something going on with the actual furnace.


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## mtaccone (Jan 31, 2009)

I have tried all of what you have mentioned except for the pilot as this is hot surface ignition. With my wood insert going 24/7 my bill went dow only by $100 was 340 gas alone last month and not is 244 what the hell! the furnace only runs 2.5 hours a day with the wood insert going. This is rediculous for this size house insulated or not. I lived in an apartment that was brick all the way around and half the walls inside were just brick as well and I only used 275 gals of oil in the winter.


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## humpin iron (Jan 31, 2009)

How big was the apt.??   Your describing your house as: drafty-not well insulated-a 70,000 BTU furnace for 1500 sq ft is too small-the best $400 can buy.  I don't think your problem is oil vs gas.  NG is the best dollar value in the heating world right now


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## mtaccone (Feb 1, 2009)

The apt. was about 1,000 sq.ft. I am just not a fan of NG in my neck of the woods right now gas is close to the same cost of oil right now which coould change in a week. When I say drafty I meen constant air movement from air infiltration. I am thinking of replaceing this unit and my water heater in the spring with something just not sure what yet. I want a wood add on furnace but only have 1 chimney in the basement so I would have to power or direct vent the new back up heat whether it be gas or oil. I do like the gas water heater being that if the power goes out we still have hot water. What btu range should  I be looking in I was thinking 90-100k output?


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