Heat Pump Not Cooling: Warm Air on the Cool Setting

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

Heat Pump Not Cooling: Warm Air on the Cool Setting

Short answer

Ask one question first: does it heat properly in winter? If it heats fine and will not cool, the fault is in the changeover rather than anywhere else, and one free setting on the thermostat causes a surprising number of them. If it does neither well, it is not the changeover at all and the list is the same as any air conditioner.

A heat pump is an air conditioner that can run backwards, and one valve decides which way round it works. That single difference is why a heat pump can heat a house beautifully in January and blow warm air at you in July.

So this page splits in two very early, and the question that splits it costs nothing to answer.

Before you start

What to carry: a torch and your bare hand. Twenty minutes, and the first part is free.

Gas or electric? A heat pump is all electric and there is no flame anywhere in it. If your system also has a gas furnace indoors that takes over in cold weather, you have a dual fuel setup, and the cooling side still works exactly as described here.

The question that splits the page. Does it heat properly in winter? Heats fine but will not cool means the changeover. Does neither well means something else entirely, and the drawings below will not help you.

One thing to leave alone. The outdoor electrical box and the valve on it. The reversing valve is a brass cylinder on the outdoor unit with a small electrical coil clipped over one end. The coil is a genuine repair and it is not expensive. Getting to it means being inside a box with mains in it, next to a capacitor that holds a charge after the power is off, so that part is a technician’s job even though the part is cheap.

What the valve actually does

The same machine, the same refrigerant, the same fan. One valve decides whether heat leaves the house or comes into it.

Everything else in a heat pump is shared with an ordinary air conditioner: the compressor, the two coils, the fan, the filter. The reversing valve is the only part that is there purely because the machine has two jobs.

When it switches, most systems make a distinct whoosh or a clunk outside. That noise is normal and it is worth learning, because a valve that used to clunk and does not any more has told you something.

The free one: O and B

Check 1

Check the changeover setting on the thermostat

Heat pump thermostats have a changeover wire, and it is called either O or B depending on the make. On most systems the wire is O, which energises the valve when you ask for cooling. On some it is B, which energises it when you ask for heating.

If the thermostat has been set to the wrong one, the system does the exact opposite of what you ask. Warm air when you want cool, cold air when you want heat, everything running normally, nothing broken.

The tell is that it is backwards in both directions rather than weak in one. Ask for heat on a warm day and see what comes out.

This happens most often after somebody fits a new thermostat, and smart thermostats ask the question during setup in a way that is easy to answer wrong. The setting lives in the installer or equipment menu, not the everyday one.

The wire colour is usually orange for O and blue or black for B, but colour is a habit rather than a rule: what every thermostat letter actually does.

Feel the two pipes

Two copper lines leave the outdoor unit. Which one is hot is the clearest thing a homeowner can check without any tools.

Check 2

Run it on cool for ten minutes, then touch both lines

At the outdoor unit, two copper lines run into the house. The fat one is wrapped in black foam. The thin one is bare.

On cooling. The fat wrapped line should be genuinely cold, cold enough to sweat in humid weather. The thin bare line should be warm to hot.

On heating. The fat line turns hot, because it now carries the hot gas into the house. The thin one stays warm rather than cold.

Both lukewarm. That is the picture with a reversing valve stuck part way between its two positions, and it is also what low refrigerant looks like, so it narrows things down without settling them.

The wrong way round for the mode you asked for. Back to check 1, because that is the O and B setting doing what it was told.

Let it run ten minutes before you judge any of this. The first few minutes prove nothing.

When the valve itself has failed

Check 3

Separate the cheap failure from the expensive one

The coil. A small electrical coil sits over the end of the valve and slides off with one screw. It can fail on its own, it costs very little, and swapping it does not involve touching the refrigerant at all. A dead coil means the valve never gets the message to move.

The valve stuck part way. The valve slides internally, and it can jam in the middle after years of sitting in one position. Everything runs, nothing is cold or hot enough, and the two pipes feel similar. A technician will sometimes tap the valve body while it tries to switch, which tells them it is sticking. That is a diagnosis rather than a repair.

The valve leaking internally. The worst of the three. Hot and cold refrigerant mix inside the valve, so the system runs constantly and achieves very little. Replacing it means cutting it out and brazing a new one in with the system recovered and recharged.

On an older system it is worth asking what the whole unit is worth before agreeing to that last one.

If it does not heat well either

Then stop looking at the valve. A heat pump that is weak in both directions has an ordinary air conditioning fault, because everything except the valve is shared.

Start with airflow and the outdoor coil, because those are free: what actually gets capacity back. Then measure the air properly at a vent rather than judging it by hand: how to measure the drop across the coil.

If the outdoor unit is not running at all, that is a different page again: when nothing happens outside.

Questions people ask

It cooled perfectly last summer and heats fine now.

That pattern points hard at the changeover, because everything the two modes share is clearly working. Start with the O and B setting, then the valve coil.

Is a heat pump supposed to blow air that feels cool in heating?

Yes, and that catches people out. Heat pump supply air is below body temperature, so it feels wrong while working perfectly. Measure it instead of judging it: what the air temperature should be.

What does the loud whoosh outside mean?

That is the reversing valve switching, and it is normal. You hear it when the mode changes and every time it goes into a defrost cycle in winter.

Can I change the O and B setting myself?

Yes. It is a menu setting rather than a wiring change on most modern thermostats, and getting it wrong costs nothing but a warm afternoon.

Everything runs constantly and the house is never comfortable.

That fits a valve leaking internally, but it also fits low refrigerant and a blocked coil, which are far more common. Rule the cheap ones out first.

My mini split has the same problem.

Same valve, same principle. On a mini split there is no O or B wire to get wrong, so it comes down to the valve and its coil: start here instead.

Sources: Honeywell Home RTH6360 quick installation guide (O or B changeover, heat pump system type), Goodman heat pump service manual RS6200006 (valve stuck in either position or in the middle, tapping the valve body, testing the coil), Trane "Why Is My Heat Pump Not Cooling?" and "Heat Pump Reversing Valve: Everything You Need to Know", Lennox "Heat Pump Not Cooling?", and American Standard "What Are HVAC Refrigerant Lines?" (the thick and thin lines)