Thread: Were our furnaces (2) sized incorrectly?

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Were our furnaces (2) sized incorrectly?

Hi! I’ve got a large home (approx. 6000 sq. ft.) with two furnaces that I don’t think are sized correctly. I’m looking for help to determine how they should be properly sized next time around. I have run the HVAC-Calc program and I believe I did it correctly but my calculations don’t match up with what actually seems to be happening in our home.

I live in the St. Paul, MN area which according to the HVAC-Calc program has a winter temperature design point of -12 degrees. Here is a breakdown of what I have calculated and what is currently happening:

Lower Level
The current heater is a Trane XE90 at 100K BTU. At -5 degrees today with the thermostat on hold at 69 degrees, the furnace would run for 3.5 minutes and then stay off for 11 minutes (on 24% of the time). The HVAC calculation is 70.8K BTU/hr loss for a tight home and 79.6K BTU/hr for an average home. There is in-floor (transite?) heating and the exhaust side of the air handler goes to 21” x 25” ductwork.

Upper Level
The current heater is a Trane XE90 at 120K BTU. At -5 degress today with the upstairs thermostat on hold at 73 degrees, the furnace would run for 3.67 minutes and stay off for 9 minutes (on 29% of the time). The HVAC calculation is 70.2K BTU/hr loss for a tight house and 85.5K BTU/hr loss for an average house. The air handler goes to 20” x 23” ductwork.

The HVAC-Calc manual states that the calculations are for actual heat loss and that usually you would want to oversize the furnace by 25%. The calculations would suggest that our lower level furnace is the right size but given the 24% run time, it seems that the furnace is substantially oversize. Similarly, based on run time and the HVAC-Calc, our upstairs furnace is also way oversize.

Ideally, when it comes to replacement time, we will replace with something that is a two (or more) stage and hopefully with a variable speed motor. Is there a way to “back into” the correct size given the run times on the existing furnaces? Is it possible the furnaces where chosen not so much for their BTU ratings but rather to be of the physical size or fan horsepower (blower capacity) to match up with the existing ductwork? Let me know if I need to add cooling information as well. We've never turned on the AC in the lower level and the AC on the main level seems to be sized properly at 4 tons (the total calculated heat gain for the entire house is 51K BTU/h with 15.8K BTU of that being for the lower level.).

Thank you!

2. They sound considerably on the oversized side of things. Ought to be runnin' pretty hard in this weather! Might want to chat with Don about your numbers and see if he can find anything done wrong. I don't know about going 25% over what the program calls for though. I sure don't.

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Well, I could talk to Don but - I did the calculation about 2 years ago with the residential one-time version. I think there is a 30 day limit on the user license.

Again, is it possible the furnace was sized more by blower capacity, duct size or AC size than by heating BTUs? If so, then slightly decreasing the maximum BTU output and betting a two-stage unit would likely solve many of the problems.

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Are you sure those are Trane units? Cause all the Trane contractors are factory trained and there's no way they would over-size a unit. Myabe you didn't build your house to the size of the furnaces.

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It was another very cold night last night – down to -16.1 degrees F. The lower thermostat was at 71 degrees and the furnace ran for 5.5 minutes on and 8.5 minutes off (39% run time). The upstairs thermostat was at 72 degrees and the furnace ran 4.75 minutes on, 5 minutes off (48% run time).

If going to another roughly 90% efficiency furnace, should the lower level unit be an 80K BTU and the upper level unit be a 100K BTU unit based on the actual apparent heat loss given by furnace run times? The record low temperature for our area is -34 degrees F.

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Suggestions?

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Installing thermostats that you can force to a predetermined number of CPH may help a little. Looks like you are cycling 5-6 times per hour which is hard on the equipment. Vision Pro, for example, could be set to 2-3 CPH, comfort may suffer a little but the equipment will be happier. Not much you can do about oversized equipment though. In the dead of winter you should be approaching 100% run time, you are around 50% or less.

8. You might have listed the first floor ceiling, and second stories floor as unconditioned space above and below. Along with wrong insulation values.

Also sounds like your existing units where sized for your record low temps.

By your posted run times at -16F, 80,000btu 90&#37; for both floors.

You might want to get the calc program again, and rerun the loads, and ask Don what is wrong if you come up with the same sizes again.

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I've got programmable Honeywell thermostats that I believe can be adjusted to change the cycle times but I think I'd rather have the equipment wear out first as long cycle times for heating wouldn't be very comfortable. What a shame!

Originally Posted by beenthere
You might have listed the first floor ceiling, and second stories floor as unconditioned space above and below. Along with wrong insulation values.

Also sounds like your existing units where sized for your record low temps.

By your posted run times at -16F, 80,000btu 90% for both floors.
I have access to my HVAC calc data and the floor and ceiling information is correct. The insulation and size information also seems accurate. There are a lot of different roof lines and bay windows which is likely why the builder and the second installer didn't properly calculate the heating loads.

You suggested 80K BTU at 90% for both floors. Does this mean 80K BTU output (i.e: 90K BTU Input at 90% = 81K BTU output)?

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One other question, is it possible for a HVAC technician to seal the gas orifices on two of the six burners on our large furnace (go from 125K BTU to 83.3K BTU)? It appears that it could be easily done and that only what I believe to be a flame sensing device would need to be moved over two spots.

Obviously, I have no idea as to whether this would be safe and/or feasible. I'm just trying to find options since the furnace is only 10 years old and is so oversized but in otherwise good condition.

11. hi.... years ago i was a trane dealer when i was involved in residential work... back then we were allowed, by the factory, to derate the furnace by 10% by changing the orifice size... you need to watch flue gas temps but you'll probably be ok considering it's a condensing furnace... in the worse case you may be required to lower blower speed, but again, i don't believe you'll have a problem...

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Originally Posted by zorax2
I've got programmable Honeywell thermostats that I believe can be adjusted to change the cycle times but I think I'd rather have the equipment wear out first as long cycle times for heating wouldn't be very comfortable.
You mean
by testing 2 or 3 CPH setting for a week.

13. 80,000 input, 72,000 output.

Of course if the brand you choose doesn't have an 80,000 input, then a 90,000 input is ok.

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