Heat pumps are remarkably reliable, but when something goes wrong it’s usually one of a small handful of issues. Here are the two most common new-owner panics, what’s actually happening, and what to do about each.

My heat pump isn’t making hot air (or cold air). What do I do?

First, a sanity check. Remember from 3.1: Pull the Right Levers: a heat pump in heating mode produces supply air at 90-110°F, not the 125-140°F you got from a gas furnace. If the air coming out of your vents feels “warm but not blasting hot,” that’s normal. Stand near the vent for 30 seconds. If you feel air that’s noticeably warmer than the room temperature, the system is working as designed.

If you’re actually getting no warmth (cool air, or no air at all), work through this checklist in order. Most issues are solvable in 5-10 minutes without calling anyone.

Step 1: Check the thermostat.

Is it set to “Heat” mode? Easy to bump.
Is the setpoint actually above the room temperature?
Is the fan set to “Auto” (good) or “On” (might be the problem, see below)?
Is the thermostat displaying anything weird, like an error code or low-battery warning?

If the fan is set to “On” instead of “Auto,” the blower runs constantly, including between heat calls. You’ll feel cool air blowing on you during those gaps and think the system isn’t heating. Switch to “Auto.”

Step 2: Check the filter.

A clogged air filter is the single most common cause of heat pump heating problems. Restricted airflow forces the system to work harder, can cause coils to freeze, and triggers safety shutoffs. If you can’t remember the last time you replaced the filter, that’s almost certainly the problem.

Pull the filter out. Hold it up to a light. If you can’t see light through it, replace it. Use MERV 11 or 13 for good filtration without restricting airflow.

Step 3: Check the breaker.

Heat pumps run on dedicated circuits. A tripped breaker means no power to the outdoor unit. Go to your panel, find the heat pump breaker (usually labelled), and check if it’s tripped. If it is, switch it fully off, wait 10 seconds, then back on. If it trips again immediately, stop. That’s a sign of a real electrical fault. Call your installer.

Step 4: Check the outdoor unit for ice.

Walk outside and look at the unit. A little frost on the coils in winter is normal. Here’s what’s actually going on:

In heating mode, the outdoor coil is the evaporator, sitting well below the outdoor air temperature so it can pull heat from the air. When outdoor humidity condenses on that cold coil, it freezes. Frost builds. If left alone, the frost would eventually block airflow across the coil entirely, capacity drops to zero, and the compressor starts working against a brick wall.

So every heat pump runs an automatic defrost cycle, typically every 30-90 minutes when conditions favour frost (cold + humid). For 5-15 minutes the unit briefly reverses to cooling mode, sending hot refrigerant out to the outdoor coil to melt the frost. You’ll see steam coming off the unit, hear the outdoor fan stop, and might briefly feel cool air at your indoor vents. This is the system working correctly, not breaking. Modern systems (Mitsubishi, Daikin, Bosch with demand-defrost) only run the cycle when frost is actually detected, so you get fewer and shorter defrosts than the older fixed-timer designs. See Trane’s defrost cycle explainer for the full mechanical detail.

During defrost, electric resistance backup heat (the “aux” strip in a ducted system) is supposed to fire briefly so you don’t feel the cool air. If your system doesn’t have aux strips, or the thermostat is set to lock them out, you’ll feel a noticeable cool draft for those 5-15 minutes. Not a malfunction. Just the trade-off.

Light frost on the coils between defrost cycles is fine. Frost building thicker than the fins is the warning sign. And: a small amount of ice on the base pan below the unit, especially if your drain is sluggish or clogged, will eventually back up into the coil. Check that the drain hole isn’t plugged with leaves or sealed by ice damming from a dripping gutter.

But if the outdoor unit is completely encased in ice (the entire coil iced over, ice on the top of the unit, ice on the copper lines running to the house), something is wrong. Common causes:

Refrigerant leak. Low refrigerant charge is the underlying cause of most freezing issues because the coil temperature drops further below freezing than it should. This requires a technician with proper tools to fix.
Defrost cycle failure. Sensors, the defrost board, or the reversing valve can fail. Also a technician call.
Water dripping on the unit. Check your gutters above. If water is constantly running onto the outdoor unit and freezing, fix the gutter first.

Do not try to chip ice off the unit, do not pour hot water on it. Both can damage components. If the unit is solidly iced over, turn the system to “Emergency Heat” (which runs aux heat only and lets the outdoor unit thaw), and call your installer.

Step 5: Check for the right airflow.

Walk around the house and verify supply vents aren’t blocked by furniture, curtains, or rugs. Check that return vents aren’t blocked either. A heat pump moves more air than a furnace does, and a couch in front of a return can drop airflow enough to cause problems.

Step 6: Test the opposite mode.

If the system heats but won’t cool (or vice versa), the reversing valve may have failed. Switch the thermostat to the opposite mode and see if the system responds. If heat works but cooling doesn’t, or the other way around, that’s a technician fix.

When to actually call someone:

The unit is solidly iced over and won’t defrost
You hear hissing near the outdoor unit (refrigerant leak)
The breaker keeps tripping
The compressor is making grinding, banging, or screeching noises (not the normal hum)
You’ve gone through steps 1-6 and nothing has changed
The system works in one mode but not the other (reversing valve)
Aux heat is running constantly even at mild outdoor temperatures (sizing or charge issue)

It’s running all the time. Is that normal?

Sometimes yes, usually no.

A modern inverter-driven heat pump is designed to run a lot. It modulates compressor speed continuously, often running at 30-60% capacity for hours at a time. This is correct behaviour. The system spends most of its time gently maintaining temperature rather than blasting on, overshooting, and blasting off.

So “running a lot” by itself isn’t the problem. Running at maximum capacity continuously is the problem.

The good kind of “always running”

Outdoor unit fan is on, indoor air handler is moving air
Air coming out of vents is warm (heating) or cool (cooling), within the expected delta T
House is holding temperature within a degree or two of setpoint
Compressor sound is steady, not strained

This is the system doing its job. Keep going.

The bad kind of “always running”

House is not actually reaching setpoint (system is “running” but losing ground)
Aux heat indicator is on continuously (the resistance strips are firing, which costs ~3x the heat pump’s normal operation)
You hear the compressor straining or hear it cycle on/off rapidly (short cycling)
Outdoor unit is iced over
Electric bills have jumped significantly versus expectations

These point to specific problems. Here’s the diagnostic:

If aux heat is on constantly:

Either your heat pump is undersized for your house, or it’s not working at full capacity. Check refrigerant charge (technician job) and check thermostat settings (see 3.1: Ecobee lets you set the outdoor temperature at which aux is allowed to fire; if yours is set too high, aux will fire on mild days when it shouldn’t).

If the system is short cycling (rapid on/off):

Most likely an oversizing problem, a thermostat issue, or a low refrigerant charge. An oversized heat pump reaches setpoint quickly, shuts off, and starts again moments later. This shouldn’t happen with an inverter-driven system that’s commissioned correctly, but does happen often when contractors sized by “matching the existing furnace” instead of doing a real Manual J. Get an installer to verify the refrigerant charge first (cheaper fix). If charge is correct and the system still short-cycles, you may have an oversizing issue that requires a longer conversation.

If the unit is straining or making unusual noises:

Compressor strain (a deeper, louder hum than normal, or a banging sound) usually points to refrigerant charge issues or compressor damage. Don’t ignore it. The compressor is the single most expensive component to replace. Whatever you spend on a service call to diagnose the issue now is much cheaper than the call you’ll make in three months when the compressor fails.

If the electric bill has jumped:

Run through the above. Most “my bill doubled” stories trace to one of:

Aux heat firing too aggressively (thermostat misconfig, see 3.1)
Refrigerant slowly leaking, forcing the compressor to work harder (technician)
A dirty filter restricting airflow (you, this afternoon, $20 at the hardware store)
Big setpoint changes (the on/off habit that doesn’t work, see 3.1)
A genuinely cold stretch where the heat pump should be running more (check the forecast and compare to a similar period last year)

The first one is by far the most common. If you have a Nest thermostat, that’s where I’d start.

How do I actually know what’s going on inside the system?

This is the question that most heat pump owners eventually arrive at, usually after their first surprise repair bill. Residential homeowners have historically had very few tools for it.

Standard energy monitoring (Sense, Emporia Vue) will tell you that the heat pump is drawing a lot of power. That’s useful as a “something might be off” signal, but it can’t tell you why. Power draw goes up when the heat pump is working hard for legitimate reasons (cold day, big setpoint change, defrost cycle) and also goes up when something is wrong (low refrigerant charge forcing the compressor to overwork, a sticking reversing valve, an iced-over coil). The whole-home meter can’t distinguish between them.

The deeper question is what’s happening on the refrigerant side and the mechanical health side. Is the refrigerant charge correct? Are compressor temperatures climbing into the danger zone? Is the system cycling more than it should? These are the leading indicators that catch problems weeks or months before they become breakdowns, and they’re invisible to any energy monitor.

A new generation of monitoring is starting to fill this gap. Companies like Thalo Labs have developed sensors that attach non-invasively to the refrigerant lines and watch the operating parameters continuously. The data goes to a dashboard, and the system flags drift before it becomes failure. Currently most of these deployments are in commercial buildings (where the cost of a surprise compressor failure on a 50-ton rooftop unit is enormous), but the same technology works on residential systems. Expect more of it to show up on the residential side over the next couple of years.

In the meantime, the practical advice is simpler: pay attention to the system. Sudden electric bill jumps, persistent ice, unusual noises, and aux heat firing at mild outdoor temperatures are all symptoms worth investigating. Catching them in week one beats catching them in month four.

Still stuck? Heat pump diagnostics get hard fast once you’ve worked through the basics. The right call is your installer (if they’re still in business) or a heat-pump-experienced contractor in your area. Don’t trust a generic HVAC tech who mostly works on gas furnaces. The diagnostic playbook is genuinely different.