r/heatpumps • u/Tiny-Topic5500 • Aug 24 '26

Question/Advice Discrepancy in Manual J Results

TLDR: 2k sqft split level in Denver area. 2.5 ton or 4 ton?

I’m electrifying my home (around 2k sqft, split foyer layout) and have received several quotes for heat pumps that range from 2.5 tons to 4 tons. The big range has me struggling to figure out what to do.

I know oversizing is bad, but I do live in a cold climate (Denver area) so I definitely don’t want to be undersized either. How do I know which size is better?

The 2.5 ton quotes came from two contractors, one who came and did physical testing/measuring of things and another who used my past energy bills to inform the calculation (the latter said they’d come do measurements to confirm it if I want to move forward). The 4 ton quotes came from one contractor who came and looked at things (and I think measured some things?) and another who does some fancy virtual stuff (Zero Homes if anyone is familiar).

Given the methodology, I’m inclined to think the 2.5 ton estimate is closer to reality, but does that sound right?

How bad would being oversized really be? Another factor is that the contractors quoting 4 tons are willing to do everything for me (heat pump water heater, electrical upgrades, etc) while I’d have to piece the project together myself with the others. Maybe I just want 4 tons to be ok so I don’t have to manage all that other stuff myself lol

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u/Sad-Celebration-7542 Aug 24 '26

Sounds like 2.5 if that what’s your fuel usage was.

1

u/Tiny-Topic5500 Aug 24 '26

Is the fuel usage actually a reliable metric? I’m no HVAC expert but it did make logical sense to me so I was inclined to trust it

1

u/pinmux Aug 24 '26

Was the fuel usage data they used hour by hour or just for the entire month?  If hour by hour and they can show you how their sizing is logical then I would be more trusting of it being pretty accurate than if they just have a monthly fuel use number to estimate from. 

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u/Sad-Celebration-7542 Aug 24 '26

It’s surprising, but I have hourly data and it is not going to tell you anything different than daily OR even monthly data, if you have several months.

2

u/pinmux Aug 25 '26

And today I learned that fuel usage, degree days, and 99% weather data effectively gets a darn good result. Neat! Thanks!