What rating furnace for what size house?


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Old 01-28-09, 07:26 PM
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What rating furnace for what size house?

We have moved into a 30 year old house that has a Payne 1900 series downdraft furnace. We have had only 2 nights above freezing since the middle of November. It cycles very frequently and while the blower is moving a lot of air it is barely warm. The filters are clean (I use the premium 3M anti-allergen ones). Have not taken the cover off the burner chamber to see what they are doing yet. Am also certain that the ducts underneath the house are not insulated
My bias for a replacement is Trane/American Standard and the basis for my question is that somewhere I have read that for a 1000-1300sf house a furnace greater than an 80% unit is overkill; is that true.
Also, when sealing ductwork, what is the experience with using foil tape vs mastic?
thanks all.
 
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Old 01-28-09, 07:50 PM
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Post the model number of the furnace obtained from the rating plate of the furnace in the burner compartment.

There is a reasonable arbument for replacing less efficient gas furnaces with 90%+ efficiency furnaces. The colder the temperatures you live with, the better the argument for a high efficiency furnace since greater heating demand equates with greater savings with a high efficiency furnace.


But it may be that there is a problem with your furnace that is causing the house to not be heated properly. Correct that and you may be more comfortable, even if you decide to replace the furnace.
 
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Old 01-28-09, 07:51 PM
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Never heard of a unit over 80% being over kill. I would get the most energy efficient unit you can buy with the funds you have. Fuel and electricity is going to keep going up. Duct seal would be the best thing to use because it is not going to get old and dry up.
 
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Old 01-28-09, 08:30 PM
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What rating furnace for what size house?

Efficiency is not the only numer to use to size a furnace. The driving force is the required heat provided. After that, it just to determine what type of fuel and then furncae efficiency is the best buy.

I am in Minnesota and could not justify a 90+% furnace over an 80% based on the furnace cost, installation and projected energy cost.

I went with an 80% efficiency, two stage gas furnace with a variable speed fan. I think it was a 65,000 BTU for a 2 story, 15000 sf townhouse (quad home if you know what that is). Even with -30F to -40F wind chills, the second stage never comes on except in the morning while catching up with our 58F evening temp to get to to the early AM temp of 63F. This usually comes after about 15 minutes on the low stage and the high never comes on again during the day or evening.

Our maximum monthly gas bill (recently) was $120 for heating, hot water and service charges. In the summer we have $20/mo for hot water and service connection charges.

Because of our situation a 90% system installation cost coupled with the furnace cost did not make any sense. We used our existing AC system and coils and our AC cost even went down.

The big saver is the variable speed fan that actually saved AC $s because we have an open stairway/floor plan so we need the uniformity.

Get a good analysis and calculation on what you need for your house.

Take a hard look at the filters you are using. They could be clogged or the MERV rating may cause decreased floe.
 
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Old 01-29-09, 10:48 PM
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The model of the furnace is a Payne 100D-19. The filters are less than 2 months old and looked fine two weeks ago. The anticipator is set on 4. Airflow at the register furthest from the furnace is quite good ... with a piece of yarn standing straight away.
The lower door of the furnace is missing and someone had taped a one time turkey roasting pan over the burner compartment ... which made me really suspicious because it did not go all the way down to the pedestal and created a much bigger chamber for that area. Resources up here are extremely limited ... even to sheet metal work ... but, today I finally found what I needed at the Ben Franklin store next town over; an oven liner that was absolutely perfect in size to cover the area completely.
Inspection of the burners and their chambers caused my credit cards and checkbook to run off into the fields ... the two outer chambers had water corrosion coming down from the outside top; those two burners are working at maybe 3/4 their length the rest is too corroded. The two middle burners look fine. I know the flames are supposed to be all blue, no yellow, with natural gas but trying to use the "mixture control" valves provided little change because there is so much dirt in the tubes. This system has probably not been serviced in 20 years, or ever.
I know the burners can come out but how the heck do you clean them. Do replacements still exist?
Just putting the oven liner in place of the missing cover has generated a perceptible improvement in the air temp at all registers.
Having just retired last summer and moved way up here it is going to take some doing to replace that beast ... which is another reason for my original question.
 
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Old 01-29-09, 10:58 PM
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A gas furnace that hasn't been serviced in twenty years and is missing a critical safety element (the fan compartment door) could be lethal.

In my opinion it needs to be serviced by a competent repairman, who might well shut it off as being unsafe to operate for good reason.

Can't really improve on the last paragraph for advice --- sorry.
 
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Old 01-30-09, 01:17 AM
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I had a Payne 60,000 BTU/hr counterflow (downflow) furnace in my house and at about 18 years of age the burners were completely shot. It had sheet metal burners with louvers as the gas outlets. Roughly two-thirds of the louvers were missing which caused an uncontrolled flame. The furnace would light off with a bang that caused me to always rush out to the garage to see if it had blown off the burner door or perhaps started a fire outside of the furnace casing.

I won't describe here what I did to make it through the final heating season of this furnace but I will state that I found new burners available on line for about $200 per burner. Needing three burners it was a toss-up between having an 18 year old furnace with new burners but no warranty and a very strong possibility of something else failing soon, a new similarly rated furnace for about $700 (me install) or going the smart route and getting a new two-stage variable speed furnace installed with complete warranty for about $3,000. It was a fair chunk of change but I opted for the last and I'm not a bit regretful of that choice.


As for sizing your furnace, the only truthful way is to do a heat loss calculation on your home. This will take into consideration the construction of your home, insulation, windows, age and other factors. Using a computer program for this will allow you to easily ascertain the benefits of increasing your insulation or additional weatherstripping, new windows or other energy saving considerations. More likely as not your present furnace is oversized for the home and if you have any energy saving additions since the home was built the furnace may be considerably too large. By going to a properly sized furnace you will likely save on your fuel consumption.
 
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Old 01-30-09, 12:04 PM
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Red face

SeaP, it was the lower (gas control/burner access) cover that was missing and that had been jury rigged with the too big (deep) roasting pan. The oven liner I replaced it with for an exact fit has made enough of a difference to take the edge off the chill in the house. The original upper door where you take off the covers to the filters is still there; the actual blower compartment is yet another complete panel and it is intact.
I agree that a two stage fan system is the best choice and that doing the measurements and calculations for heat load requirements will generate the data to determine what is the most efficient choice for replacement. A lot of that I can do myself except the pressure checking and that may take having someone come from Redding which is 70 miles away. Ugh.
My basic question is fairly well answered but I will leave the thread open for a while in case others choose to weigh in.
thanks again for the inputs.
 
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Old 01-30-09, 12:11 PM
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Having the fan compartment door in place is important for safety, and these days a door switch usually will shut off the equipment if it's not in place.

But the burner compartment cover isn't especially critical. Combustion air is needed for the burners and you want to keep critters out of the burners....
 
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Old 01-31-09, 06:30 AM
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mastic on all the joints is the way to go the wrap both supply trunk and return ducts if they are in an unheated foundation area going out or coming back.you need to pull the return heated air back as close to that space temperature as you can.if your return is uninsulated and being run thru a crawl space in the winter you are cooling it off before it reaches the furnace for another cycle.......the temperature rise on the furnace is whatever the return+the heat exchanger temp,so the warmer it is the hotter that supply air will be on those grills.how is the squirrel cage on the fan (rounds clear of dirt build-up),and do you have an "A coil" above the heat exchanger for the AC...might have dirt built up also there if the squirrel is dirty.check also that speed should be low to medium for heating thru that furnace with the stat in auto fan/heating mode.if you have a dial limit control within that furnace fan off/on should be 160F-180F.....with hi limit at 200F+
 
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Old 01-31-09, 06:46 AM
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Those filters may be reducing your air flow and causing the short cycling. Try some cheap fiberglass ($1.00) filters and see if there is a change
 
 

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