Vaillant aroTHERM Plus Heat Pump Problems — Faults & Fixes
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Post a job — we'll find you an engineer →Refrigerant and electrical faults require an F-Gas certified engineer. Do not access internal components of the outdoor unit.
The Vaillant aroTHERM Plus is a reliable air source heat pump, but like any heating system, it can develop faults or underperform — especially during the colder months when you need it most. Most problems fall into a few straightforward categories: low water pressure, incorrect controller settings, or fault codes that point you towards a quick fix. Many homeowners discover their aroTHERM is working perfectly well, but the heating curve needs adjusting for their home's insulation level, which is a common oversight after installation. This guide walks through the most frequent issues, what the fault codes actually mean, and which ones you can safely handle yourself. If your problem involves the refrigerant circuit or you're seeing electrical faults, you'll need to call a qualified F-Gas certified engineer — that's not a DIY job.
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Most likely cause & what to check
The Vaillant aroTHERM Plus is Vaillant's air source heat pump, designed to work alongside their uniTOWER indoor unit and ecoTEC boiler (as a hybrid system). It uses R290 (propane) refrigerant, which has a very low global warming potential. Manuals at vaillant.co.uk or call 0330 100 3461.
Fault code F.22 (low water pressure): shared with Vaillant boilers — the system water pressure is too low. Check the pressure on the indoor unit display. Repressurise using the filling loop to 1.5–2.0 bar (heat pump systems often run at slightly higher pressure than boiler systems). If pressure drops repeatedly, there is a leak.
Fault code F.28 / F.29 (ignition failure — on hybrid systems): if you have a Vaillant aroTHERM Plus connected to a Vaillant ecoTEC boiler (hybrid system), F.28/F.29 refers to the gas boiler component, not the heat pump. See the Vaillant ecoTEC guide for these faults.
aroTHERM not heating home sufficiently: the most common cause is incorrect weather compensation settings in the multiMATIC or sensoAPP controller. The aroTHERM uses a heating curve that adjusts flow temperature based on outdoor temperature — if the curve is set too low for your home's insulation level, the house will underperform in cold weather. An MCS engineer can recalibrate.
R290 refrigerant note: R290 (propane) is mildly flammable. Vaillant require that only trained engineers work on the refrigerant circuit — the aroTHERM has design safeguards but refrigerant work must always be carried out by a suitably qualified F-Gas engineer.
Vaillant aroTHERM Plus warranty: up to 7 years when registered within 30 days and installed by a Vaillant-trained installer. Contact vaillant.co.uk for warranty claims and approved engineer referrals.
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Frequently asked questions
What does fault code F.22 mean on my Vaillant aroTHERM?
F.22 indicates low water pressure in the system — the pressure has dropped below the safe operating threshold. Check the pressure gauge on your indoor unit (it should read between 1.5–2.0 bar) and use the filling loop to repressurise if needed. If the pressure keeps dropping, you likely have a leak somewhere in the pipework or connections, which requires a plumber's attention.
My heat pump is on but the house isn't warming up properly — what's wrong?
The most common culprit is the heating curve setting in your multiMATIC or sensoAPP controller — if it's configured too low for your home's insulation level, the flow temperature won't be high enough to meet demand in winter. A Vaillant-trained engineer can recalibrate this quickly; it's often a software adjustment rather than a hardware fault.
Is it safe to work on the refrigerant myself if I see a leak?
No — the aroTHERM uses R290 (propane) refrigerant, which is mildly flammable and requires specialist handling. Only a Gas Safe and F-Gas certified engineer should touch the refrigerant circuit; attempting DIY repairs on this component is unsafe and will void your warranty.
What's the difference between F.28 and F.29 faults on a hybrid system?
If you have an aroTHERM Plus connected to a Vaillant ecoTEC boiler (hybrid mode), F.28 and F.29 refer to the gas boiler component's ignition, not the heat pump itself. Refer to the ecoTEC troubleshooting guide for these codes, as they're separate systems.
How long is my Vaillant aroTHERM covered by warranty?
Vaillant offer up to 7 years' warranty on the aroTHERM Plus, provided it was registered within 30 days of installation and fitted by a Vaillant-trained installer. Check your paperwork or contact Vaillant at 0330 100 3461 to verify your cover and find an approved engineer if you need repairs.
Can I repressurise the system myself if the pressure is low?
Yes — if the pressure has simply dropped slightly, you can use the filling loop (a small valve near the indoor unit) to repressurise to 1.5–2.0 bar yourself. However, if pressure drops again within weeks, this signals a leak that needs professional diagnosis and repair.