Balance point, in plain English: what actually happens to a heat pump at 20 below

Every second conversation we have at a kitchen table in this city now starts with a version of the same question. Somebody has read that heat pumps do not work in cold climates, and somebody else has read that this is an outdated myth, and both of those people are partly right, which is the least useful possible outcome.

Here is the version with the arithmetic in it.

Two curves that cross exactly once

Your house loses heat to the outdoors at a rate that depends on how cold it is outside. At 40 degrees it might be losing 20,000 BTU an hour. At 20 below, the same house might be losing 70,000. Plot that against outdoor temperature and you get a line sloping up as it gets colder. That is the load line, and a Manual J load calculation is how we find it for your particular house rather than for a generic one.

A heat pump moves heat from outside to inside. As it gets colder outside there is less heat available to move and the machine has to work harder to move it, so its output falls. Plot that on the same axes and you get a line sloping down as it gets colder.

Two lines, one going up and one going down. They cross once. That crossing point is the balance point, and it is the single number that determines whether this whole idea makes sense for you.

The balance point is a property of your house and your equipment together. It is not a property of the equipment alone, which is why two identical heat pumps can have balance points fifteen degrees apart in two houses on the same block.

What the crossing point means in practice

Above the balance point, the heat pump covers the entire load on its own. It runs, the house stays where you set it, and nothing else fires. In a typical Twin Cities winter with a balance point around 8 degrees, that is the great majority of the heating hours between October and April, including plenty of cold ones.

Below the balance point, the heat pump keeps running and keeps contributing, but it can no longer cover the whole load. Something has to make up the difference. That something is your backup heat.

This is the part that gets lost in the online argument. Below the balance point the heat pump does not switch off in defeat. In a dual fuel system it hands over entirely to the furnace, because running both is usually the more expensive combination. In an all-electric system it keeps running and the strip heat tops it up. Either way, the house stays warm. The question is only what you are paying per unit of heat during those hours.

How many hours are we actually talking about

This is the part that changes people's minds. Minneapolis has a design temperature of 20 below, and that figure describes a genuinely rare condition. In a normal winter, the number of hours the city spends below 5 degrees is a small fraction of the heating season, and the number spent below 15 below is a handful of nights.

So a system with a balance point around 8 degrees is covering something like 85 to 95 percent of the season's heating hours on the heat pump alone. The backup exists for the tail, and the tail is short. It is also the tail during which having a second, independent heat source is genuinely reassuring rather than merely economical.

Setting the switchover temperature

In a dual fuel system the changeover temperature is a setting, and it is not automatically the same as the balance point. There are two sensible ways to choose it.

The capacity switchover is the true balance point: switch to the furnace at the temperature where the heat pump can no longer keep up. This maximises heat pump run time.

The economic switchover is the temperature at which gas becomes cheaper per unit of heat than the heat pump running at that outdoor temperature. Depending on your electric rate and your gas rate, that can be above or below the capacity balance point. In the Twin Cities with typical residential rates it usually lands somewhere in the teens.

We set it with you, against your actual bills, and we write the number on the commissioning sheet so that whoever services the system next knows why it is there. It is also worth revisiting after a winter, because rates move.

What HSPF2 does and does not tell you

HSPF2 is a seasonal heating efficiency rating. It is genuinely useful for comparing two units against each other. What it is not useful for is predicting your bill, because the test procedure behind it is calculated against a national average climate that includes a great deal of weather that Minnesota does not have.

For a house here, the more informative numbers are buried in the manufacturer's extended capacity tables: what the unit produces at 17 degrees, and what it produces at 5 below, at your design conditions. A unit with an excellent HSPF2 and a cliff in its output at 5 degrees is worse for you than one with a lower HSPF2 that holds its capacity. We read those tables. Ask any contractor quoting you a heat pump to show you the 5 below line, and watch what happens.

When we say no

A heat pump is the wrong recommendation for a leaky, poorly insulated house on a high electric rate with no gas backup. The load line is so steep that the balance point lands somewhere embarrassing, the strip heat runs for a large chunk of the winter, and the bill is genuinely worse than what the old furnace was doing. In that house the honest sequence is air sealing and insulation first, and a heat pump afterwards, at which point it will work beautifully.

We would rather lose that sale now than have the conversation in March.

The short version

Yes, a cold-climate heat pump works in Minnesota. It will carry your house through most of the winter, probably more cheaply than gas. It will not carry it through the coldest ten nights on its own, and it does not need to, because that is what backup heat is for. The number that decides how good the deal is for your specific house is the balance point, and anybody quoting you a heat pump without calculating it has not really quoted you anything.

If you want that number for your house, book an assessment, or read the shorter summary on the heat pumps page.