Heat pump sizing is a delicate science when helping homeowners figure out how to move into full electrification, and away from their furnaces and boiler systems. Oftentimes and without guessing, you’re given quotes that is in reality, a pure guess on what your home might actually need for a heat pump size.
Without testing or looking at your home from a more building science approach, such as with an energy audit, duct flow testing, a blower door test, checking for insulation levels in the walls, ceilings, and more, it’s really hard to know exactly what the home needs.
And when that is the case, it’s often very tempting to quote an oversized heat pump. The reasons come from many angles, but some can include:
- HVAC companies don’t want “callbacks” when your home is too cold in the winter (common in Colorado if the heat pump heating capacity is sized too low).
- Assumptions made about the home itself are based on old industry rules of thumb.
- Assumptions made about the home might work for older A/C and furnace combo swapouts, but heat pumps are the latest efficient technology, and they work and act differently than the old tech.
- Building envelope considerations and assumptions are just incorrect, because there’s no way to know without testing them.
For this article, we wanted to present this great question from a customer we’re installing a heat pump for very soon, and our reply interaction with him. We love these customers and homeowners who deep dive on their heat pump quote designs and costs. In this case, there were five total quotes he was looking at, and needless to say there was a lot to work through to figure out the true differences, and what was best for his house in the end.
Here is his question below, and my reply and reasoning follows below. The customer was given five quotes (one of them was ours), and a few of them were the same oversized Mitsubishi model. As you read through it, you can apply the same methodology to your own home as well!
“Hi Erin and Xerxes,
Of the 5 proposals that I received, I am still considering two. The least expensive of the 5 was the largest system, at 4.0 tons with no resistance heat backup. This was at least in part because of the larger utility rebate. Perhaps 3.5 tons would be a better fit, but it is slightly more expensive.”
Here we dive into why the 3.5 ton system is way too large and will not run efficiently possibly lowering comfort too. We had proposed a 2.5 ton after completing his full BPI certified energy audit as well.
Dear [Customer Remaining Private],
Thanks for keeping us informed! It’s really great to have these conversations.
From your comparison table, the Mitsubishi units are oversized. I’m assuming these are outdoor model numbers: MXZ-SM48NAMHZ* and MXZ-SM42NAMHZ*. Be sure to ask, because the H in the name is the cold climate version, and they make other lower EER variants that are cheaper but still qualify for the rebates.
I don’t have the exact AHRI certificate numbers but ran their performance simulations using the most efficient indoor combinations available. I’ve attached all three performance simulation reports to this email, in case you want to compare their data values. There are several important points to make in favor of sizing the unit correctly, and to avoid oversizing only to get more rebate money.
I’ve clipped the Fujitsu 2.5 ton unit and the Mitsubishi 3.5 ton unit performance simulation charts here in that order to provide a brief guide to the results.
This Fujitsu is right sized. The blue circles show the temperatures at which the unit is running most efficiently (modulating) and comfortably (lower fan speeds), and able to utilize it’s variable speed motors and modulation heating and cooling functionalities and smart technologies. The pink circles show out of range temperatures that the unit will either short cycle on/off like the AC units of decades past, or run at 100% to meet extreme temperature demands on cooling or heating.
The blue vertical bars show the number of hours our climate spends at that temperature. Proper design maximizes the overlap of the blue circles with the largest area under the blue bars. That realizes maximum efficiency, comfort and service life for the equipment.
This 3.5 ton Mitsubishi is oversized with blue dots shift far to the left of the seasonal demand curve. This 3.5 ton unit is designed to heat 99.9% meeting a 0.1% of the year demand. That’s not terrible (especially compared to furnaces), but it will likely shorten the life expectancy of the unit by short cycling over 40% of the winter. The cooling season in particular will never run in modulation mode as the unit is too large to overlap the seasonal cooling demand.
Xcel Requires everyone to run a load calculation for their design calculations. But most companies’ are estimates at best, and many are just guesses. I’m not sure if this 4 ton or 3.5 ton design would survive an Xcel audit, as they have increased their scrutiny and hired an engineering firm (Group 14) to complete heat pump audits just this year. We know we are safe and won’t have to make any costly changes (or risk get kicked out of the rebate program) after the fact.
In short, the short term gain for larger Xcel rebates risks lower service life (short cycling) and reduced comfort (higher fan speeds and short cycling). There’s a lot more here we can discuss, but those are a couple of the main points for the Heat Pump comparisons.
The heater kit is something we also do for every home unless the homeowner is sure they don’t want it. I have a Mitsubishi in my home, and after 4 winters am finally going to add the heat kit. It simply adds comfort, flexibility and a heating backstop to the rare high wind/extreme cold weather which can sap the heat right out of any home. It’s also an emergency safety measure if in a decade (sooner if poorly installed) or whenever a problem might arise with the heat pump.
Hope that helps!
All the best,
Xerxes
On the too small side of equipment choice, it’s a bit more straightforward. If a resistance heat kit is installed, it will run a lot and cost more each cold snap. If a heat kit is not installed, your home will be cold during cold snaps. If you’re considering not installing a heat back up, be sure to have an emergency heat plan. See my other article here for considerations on that.
Got sizing questions for your new heat pump? Contact us for help.
