Fiona

Before the conversation with the installer, get data, because "it is louder than I expected" is not something anyone can act on.

A phone sound level app is not calibrated and will not survive scrutiny, but it is fine for relative measurements, and relative is what you need: same spot, same distance, at several times of day, noted against what the unit was doing. A cheap dedicated meter is not expensive if you want defensible absolute numbers.

What turns this into a real case is a pattern: quiet at low output, loud above a certain point, worst at a particular time. That points at a cause. A single reading points at nothing.

Also record the defrost events with times. If the frequency is genuinely excessive, that is a documented fault rather than an opinion.

The headline number is real but it is quoted at a specific operating point, and neither of the two variables that define that point is the one you care about.

Capacity is a function of outdoor temperature and flow temperature, and it moves a lot. Colder outside means less heat available to pick up, so capacity falls. Hotter flow temperature means a bigger lift for the compressor, so capacity falls again and efficiency falls with it. A unit rated at a mild outdoor temperature and a modest flow temperature can deliver appreciably less at your actual design condition — a cold day with the flow temperature high enough to make your radiators work.

So the number you want is capacity at your design outdoor temperature and your actual flow temperature, and it is in the manufacturer's full technical data rather than the brochure. Look for the capacity tables that vary both axes. The brochure figure and the table figure for your conditions are frequently not close.

Measuring the real thing. Heat output is flow rate multiplied by the temperature difference across the unit, multiplied by the specific heat capacity of the fluid. In practice:

• You need a flow meter. This is the one measurement you cannot infer, and without it everything else is arithmetic on a guess. A heat meter that does the whole calculation is the tidiest option; a flow meter plus your existing temperature sensors also works.
• Watch out for glycol. If the system has antifreeze in it, the specific heat capacity is lower than water's and the fluid is more viscous, so you get less heat for the same flow and delta. Ignoring it overstates output by a meaningful margin.
• Measure at steady state. During a defrost cycle output is negative — the unit is taking heat out of your system to melt the ice. Averaging over a period that includes defrosts is the correct way to judge real-world seasonal performance, but for "can it hit its rated output" you want a clean steady run.

Then the comparison that answers your actual question. Take a genuinely cold period and record: outdoor temperature, flow temperature, measured heat output, electrical input.

• If measured output is close to the table figure for those conditions, the unit is fine and your problem is elsewhere — most likely emitters, which is the next paragraph.
• If it is well below, and the delta across the unit is small, you probably have a flow rate problem rather than a compressor problem: undersized pipework, a partially closed valve, a pump on too low a setting, or a clogged strainer. This is common and fixable.
• If output is low and the unit is cycling rather than running continuously, it is a controls or sizing issue.

On the emitters, because it is the likeliest answer to "the house did not hold temperature": a radiator's output depends on the difference between its surface and the room, so a radiator sized for a high-temperature boiler delivers substantially less at heat pump flow temperatures. The unit can be delivering exactly its rated output into a system that cannot get that heat into the rooms. You would see this as the unit running continuously, hitting its flow target, and the rooms still being cool — and no amount of measuring the heat pump will reveal it. Compare each room's radiator output at your actual flow temperature against that room's heat loss.

One practical detail people discover late: pipework and manifolds need to be insulated for cooling, and heating installations frequently are not, or only partially.

Cold pipes running through a warm void will condense on the outside. In a floor void or a ceiling that is a slow, invisible damp problem rather than an obvious puddle.

So the honest scope of "enable cooling" on an existing system is: check the unit supports and permits it, add room humidity sensing and dew point limiting, verify every metre of pipework and every manifold is properly insulated and vapour-sealed, and only then turn it on. It is a project, not a setting.

That is usually why the answer sounds vague. It is not that it cannot be done, it is that doing it properly is not a phone call.