An air source heat pump already on the wall means a split is a second outdoor unit, with a second noise source. In England, Planning Portal guidance on Class G permitted development is the check: more than one air source heat pump on a house that is not detached can fall outside permitted development, while detached houses are treated differently if the other limits are met. Read the current Planning Portal page rather than assuming the existing pump used up nothing. Cooling-only use has its own condition to read there. The split is still an F-gas install and it does not make your hot water.
A second fan beside the heat pump you already have
The existing heat pump already occupies the best wall, the boundary distance, and the neighbour's patience. A condenser beside it can recirculate air between the two coils and add a second fan. Ask for sound data on the new unit and for a position that does not starve either machine. Anti-vibration mounts still matter. Count the existing heat pump as an outdoor unit. Keep the two coils from blowing at each other. The split can go next to the heat pump only if both coils still have their air, the noise is acceptable, and the siting rules allow that wall. Two fans facing each other recycle heat and get louder.
If the heat pump is doing the house heating and hot water, the split is room comfort. It can cool in summer and take the edge off a room in spring. It should not be sold as a replacement for the heat pump you already paid to install.
Class G counts, hot water and two warranties
On the Planning Portal's summary of Class G in England, the number of air source heat pumps allowed as permitted development is tighter on a house that is not detached, and on flats, than on a detached house. A second unit on a semi or a terrace can fall outside those rights and need an application. Detached houses may allow more than one if every other condition is met. Article 4 directions and existing planning conditions can remove rights anyway. Do not assume a second unit is permitted development. Do not assume the existing heat pump means you can add a split without planning. Read the current Planning Portal Class G conditions. Detached houses are set out differently there.
That summary can change. Hedge the decision by reading the Planning Portal page for the current conditions, including siting, noise, and the condition that the unit is not used solely for cooling. Do not treat this article as the consent. An air-to-air split does not heat the cylinder the existing heat pump is looking after. Energy Saving Trust separates that air-to-air job from a wet heating system. The split will not help your hot water. Air-to-air equipment heats and cools room air. Your existing heat pump, or whatever heats the cylinder, still does the taps. If the heat pump is struggling, fix that system with its installer. Do not cover the problem with a bedroom unit and call it a plant upgrade. The new circuit needs an F-gas certified company. The existing heat pump's refrigerant is a different visit unless the same company is competent on both. Keep the two warranties clear.
Frequently asked questions
Does the existing heat pump mean I can add a split without planning?
Do not assume that. A second outdoor unit on a house that is not detached can fall outside permitted development in England. Read the current Planning Portal Class G conditions. Detached houses are set out differently there.
Can the split go next to the heat pump?
Only if both coils still have their air, the noise is acceptable, and the siting rules allow that wall. Two fans facing each other recycle heat and get louder.
Will the split help my hot water?
No. Air-to-air equipment heats and cools room air. Your existing heat pump, or whatever heats the cylinder, still does the taps.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.