60,000 input Gibson Furnace 93% seven years old bad HX

EBEBRATT-Ed

My neice bought a forclosure property. Has 7 year old furnace. Furnace was running ok this winter and started locking out on roll out sw two weeks ago. I went their (first time on the property). Found HX bad. Fire looked ok on start up but went to **** when the blower started. Removed hx and found huge hole in it. Ordered new HX (under warrenty) replaced this past Friday night and got it back on line. I need to go back and check gas pressure (propane) my gage was broken. Manual calls for 11-14" wc inlet and 10" manifold, want to make sure it's not over fired. It does have the correct orfices #55 and reg spring changed looks like the orig. installer converted it to propane correctly. Have to check temp rise too I suspect it is high, duct undersized.

The supply house tells me they like a 50-55 temp rise even though nameplate says 40-70. I just want to make sure I am sizing the duct correctly. 60,000/(1.08 X 55) deg=1010cfm X .93 efficiency =940 cfm Is this correct?? I have 20 X 8 return drop and only 4 6" round return ducts. Reg boots are 2 1/4 X 12".

Supply branch ducts are 8 6" round ?? why the diff?? I was told a 6 " round is only good for 100-120 cfm.
Yesterday I ripped out 3 of the supply reg 2 1/4 X 12 and installed 3 4 x 12" reg. I also am going to install a 14 X 20 return grill in a central location

Just want to see if I am on the right track. Not a duct expert. Do you size based on velocity or friction? The supply house told me to size the registers at 6-700 fpm on the supply and 4-500 on the returns

If I shoot for 950 cfm on this does that look right? What size duct for the new 14 X 20 return grill.

I need to get this done don't want the new hx to over heat

Ironman

The lack of returns (four 6") is your main problem. They're good for about 400 cfm. At 14x20 filter grille will handle about 560cfm comfortably. That would be a 14x8 duct or a 12" round pipe. If you use a short piece of flex with no bends, 12" is okay. If otherwise, go to 14" flex.

Your calcs look correct. The main thing: get the temp rise within specs.

GW

If you know how to check static pressure, you will find your problems. 60000 btu with 4 6" returns, wow. Your returns suck. It's no wonder your hx has taken a nap.

Your math looks good but your numbers are hypothetical. I've sized 90 plus furnace/ ducts with 140 cfm per 10,000 Btu input, per the great Jim Davis. Although I think he's been recommending 150 as of late. Same basic numbers you're coming up with.

But it's all theory until you check your rise and adjust the blower.

Yes, sure, shoot for the middle of the name plate rise specs.

I routinely downsize furnace installs; it's like people oversized for pleasure years ago...many still do.

Gary

SWEI

GW said:

I routinely downsize furnace installs; it's like people oversized for pleasure years ago...many still do.

I continue to be amazed by the number of 100-125k furnaces (80%) we see in <1400 square foot houses here (10-14°F design temp.) I'm currently working to convince an architect not to accept a proposal for a 3T heatpump in a 1,000 sq ft apartment.

EBEBRATT-Ed

I haven't done a heat loss but my gut tells me it's not oversized.(the furnace that is). The ductwork is another story. I found some sizing information and a "ductulator" The trunk duct (supply and return) and mains are not too bad, a little undersized but not much and not enough to bother reworking. This thing needs 950-1000cfm to be happy. It's mostly the 4 6" returns that are killing this thing. It has 7 6" supplies so its about 300 cfm short on supply and about 800 short on the returns.

John Mills_5

My 60K Nordyne VS is on a fairly low setting and running about 1100 on high fire which puts it just over 60° temp rise. At 940 it would certainly hit limit. Today's condensing furnaces don' t take high temp rises like the old relics with their heavy heat exchangers. Actually mine is so oversized I have a switch to shut off high. I have more ducting that your house but still gets a bit noisy on high.