Follow your exact equipment manual. No gas, refrigerant, wiring, internal components or safety-switch bypasses. If access is unsafe or instructions are unclear, stop.
A standalone water-leak alarm
A moisture alarm can notify you when water reaches its sensor. For an accessible equipment area, compare audible alerts, battery warnings, test functions and any phone-notification requirements. Confirm whether a hub, internet connection or subscription is needed; a Wi-Fi feature is not useful if nobody receives its alert.
Follow the alarm's placement instructions. Do not install it inside equipment or in a wet electrical area. A standalone alarm does not stop the HVAC system and is not a substitute for a properly installed condensate float switch, drain pan or working drain. Hardwired protection belongs to a qualified installer.
A room thermometer and hygrometer
A simple display can help you record room temperature and relative humidity when comfort changes. Compare stated accuracy, readable digits, battery requirements and whether it retains minimum and maximum readings. Put it where its instructions recommend, away from misleading local heat or direct supply air.
Use readings as observations to share, not a refrigerant diagnosis or proof that the home is free of mold. A room monitor does not replace smoke or carbon-monoxide alarms. You may already have a suitable display in your thermostat or dehumidifier—check before buying another.
The correct replacement filter
Buy by the required dimensions, filter type and the filtration level your HVAC system can accommodate. Check actual dimensions as well as the nominal size on the label. Do not assume that the highest MERV number or an added filter layer is appropriate for every system.
For mini-splits, verify whether each filter is washable or replaceable; specialty filters can have different care rules. Keep a record of the approved part and inspection date. The full filter guide explains fit, direction and maintenance.
The batteries your thermostat actually uses
Some controls have homeowner-replaceable batteries; others do not. Buy only the type specified in the manual and follow its access instructions. A blank display can also indicate a system or power problem, so a pack of batteries is not a diagnosis.
A flashlight and an equipment-record folder are useful too. Photograph an already accessible label and keep the manual, filter size and service history together. None of this requires opening electrical compartments.
What we would leave out of a general homeowner kit
Refrigerant charging cans, electrical test tools for live HVAC work, universal coil chemicals and unverified covers are not starter-care purchases. Internal cleaning kits need their own compatibility and safety assessment.
These are buying criteria, not endorsements of a particular product or claims of guaranteed savings. Start with the problem you want to solve; keep the items you already own if they meet the need.
Sources & further reading
General guidance informed by the sources below. Equipment-specific instructions take priority; accessory buying criteria are HVAC PM recommendations, not manufacturer approval.