Oversized furnace ?


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Old 12-18-18, 11:03 AM
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Oversized furnace ?

I just replaced a 90+ 125,000 BTU furnace with a 100,000 92% and am having trouble dropping the heat rise within specs. I have changed thr blower speed to high. No Acoil heat only.
Was never happy with the old 125k as it never ran more than 5-7 minutes, the new runs longer but when I did some of the online load calcs they called for 50-60k or less, did I really oversize it that badly, the 125k was specified by a HVAC contractor who also designed the duct systems.
I did that install 30 years ago as well as the one last week.
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Anyway to reduce the BTU output to bring the heat rise within range?
 
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Old 12-18-18, 03:44 PM
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It sounds as though you are a bit oversized. Some people love to sell oversized units and the larger ductwork to go with it. It makes for a better Christmas for them.

About your short cycling, unfortunately unlike oil where you can most time lower your firing rate by lowering the nozzle, atmospheric gas simply comes into a pipe through regulators and into the burners with really no way to adjust it.

The reality is because of the furnace being oversize you are simply making more heat faster than it can be delivered so the furnace shuts of on high limit.

Another change made from days gone by is the fan/limit switch. The older units were temp controlled and could be manually adjusted. Today's units cannot.

Just my thoughts.
 
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Old 12-18-18, 03:59 PM
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How about the "low fire" range ?
 
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Old 12-18-18, 05:34 PM
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Check the simple stuff: Are all of your supply registers open as well as the dampers? Change to the cheap filters that won't restrict airflow. Is it over-firing? The installation manual should tell you how to check.
Otherwise get more airflow to the unit. Returns are usually undersized.
 
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Old 12-18-18, 06:04 PM
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Low fire would be good if the furnace was 2 stage but not all have that option unfortunately.

As far as less restrictive filters or more returns or even more registers or larger ducts for that matter or fully open dampers. All this to get the air from the plenum to the living space are options but the bottom line is if you have to go through all of that than I would guess the furnace started out to be oversized in the first place and the increased air flow or higher velocity could become uncomfortable.

Also, the quicker you get the warm air into the living space by any of the above changes, the quicker the stat will get satisfied and chances are you will still be short cycling.

Just my thoughts.
 
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Old 12-19-18, 06:27 AM
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Unfortunately it is only a single stage unit. Starting to consider going to smaller orifice to reduce BTU output?
 
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Old 12-19-18, 11:59 AM
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You can't under-fire the furnace by changing orifices because doing so messes up the fuel to air mix.

You should clock the gas meter to see if it's over-fired. The gas pressure may need adjustment.

You should also check the filter. If the slot is the standard 16x25x1 or smaller, you pretty much can only use fiberglass. The pleated filters of the standard size are only good for like 800 to 1000 cfm, above that they're too restrictive.

What's the temperature rise now? What's the furnace model number?

Is it tripping the limit?

Newer units need more airflow for the same btu output even if the old one was high efficiency; your old one may have been rated 40 to 70F wheres many new ones are rated 40 to 60, some with large blowers, 30 to 55.

If everything checks out, fuel input is correct, using fiberglass filter and it's overheating you pretty much have to put the right size furnace.

50 to 55k heat loss, you should have a 60 to 70k btu input furnace if 95%+. Even 80k is too big.

Most manufacturers do 40, 60, 80k. Lennox has a 66k which is nice if 60k is borderline yet 80k is too large.
 
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Old 12-19-18, 08:09 PM
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My thermostat allows me to set the drop in temperature before it calls for heat - the larger you make that setting, the larger the range of temperature swings in the house but it allows for the furnace to run longer each time and get closer to an efficient burn.
 
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Old 12-20-18, 01:15 PM
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Sorry it took so long Goodman GMSS921004CN , Itís a 14 x 24 RA plenum , was using a high eff filter so rise was 74 degrees. Checked all dampers to fully open removed the filter and rise is down to 47 Degrees with blower on high speed
 
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Old 12-20-18, 04:14 PM
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Was the filter 1"?

Return drop 14x24 or actual filter.

See what the rise is with the filter slot closed and no filter.

47f rise on high speed is good for this size furnace - static pressure is within normal limits. (charts put it around 0.5 to 0.6")

At the end of the day, if you want to use something better than fiberglass, you'll have to have a media filter installed. Would probably need to be 25 x 25 or something to take the 1700ish cfm this furnace should move - they make right angle air cleaner cabinets which can take a larger filter than the furnace's return opening.
 
 

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