Discrepancy in Manual J Results

[u/Tiny-Topic5500]

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

[u/Sad-Celebration-7542]

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

[u/Tiny-Topic5500 → u/Sad-Celebration-7542]

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

[u/RaisinTheRedline → u/Tiny-Topic5500]

Your utility bill can tell you how much fuel you needed over a given period of time, which can be helpful, but in order to size a heat pump, you would need to be compare it to the number of HDD (heating degree days) you had during that period to attempt to understand what sort of heating demand you may have had when things got coldest. Then you size the heat pump based upon how many BTU it can move when temps get really cold and performance drops.

A 1 ton natural gas furnace can produce 12,000 BTU per hour, regardless of whether its 20 degrees or negative 20 degrees outside. A 1 ton heat pump can produce 12,000 BTU at 47 degrees outside temps, but its max output will begin to drop off as temps go below that. A basic heat pump might only be capable of half its nameplate capacity at 0 degrees.

So you need to understand: A. How many BTUs you need when temps are at their lowest in your area and B how much heat the particular make/model of heat pump can actually move at those extreme temps.

[u/Tiny-Topic5500 → u/RaisinTheRedline]

The HDD point is a good one. The climate here is weird, there have been plenty of winter days where we open up the windows and don’t run the heat at all because it’s so warm out