Every hour an ammonia refrigeration plant runs, its condensers push large volumes of heat out of the building. ACHR News reported on a Johnson Controls product, the FRICK HeatPAC, which captures part of that heat and turns it into high-temperature hot water for processes like pasteurization, sanitation, and space heating. The article's summary describes the unit as one that "recovers waste heat from ammonia refrigeration systems." You can read the coverage at ACHR News on the Johnson Controls heat recovery system.
You will not be installing an industrial ammonia plant in a house. The useful lesson is the one the engineers start from: heat has to be counted before it can be reused, and the place where it leaves the building matters as much as the amount.
Every House Throws Heat Somewhere
Most homes have more heat sources than owners realize. The refrigerator condenser coils dump heat into the kitchen or garage. The clothes dryer exhausts warm, humid air through a duct. The range hood pulls heat and moisture up and out. The water heater loses heat through its tank and the first few feet of pipe. Bathroom fans remove warm, moist air every time they run. In winter, the attic often collects heat that leaks out of the living space through the ceiling and then escapes through the roof.
Some of that heat is doing useful work. Most of it is simply leaving. The job is to find out which is which before you decide anything.
Trace the Dryer Exhaust First
The dryer is the easiest place to start because the path is visible. Follow the duct from the back of the machine to the wall or roof cap. Note the duct type, the total length, and the number of bends. Flexible foil duct crushed into long loops restricts airflow, and lint builds up in those low spots. A restricted run keeps the dryer working longer per load.
A multimeter cannot read airflow, so use simple measurements instead. Clean the lint trap, the drum vent, and the first section of the duct, then time a standard load. Put a probe thermometer at the exit during a cycle and write down the reading. Repeat after the cleaning. You are building your own before-and-after record, which is more useful than any general rule.
Check the Refrigerator Coils and the Compressor
A refrigerator that runs constantly is often fighting its own condenser. Pull the unit out far enough to reach the coils at the back or bottom. Dust and pet hair insulate the fins, so the refrigerant cannot shed heat as easily, and the compressor runs longer to hold temperature. Vacuum the coils with a brush attachment. Then check the nameplate for the rated load amperage and compare it with a clamp meter reading taken during a steady run. A reading well above the nameplate figure points to a problem beyond dusty coils, and that is a question for a qualified tech, not a reason to start pulling parts.
Insulate the Water Heater Path
Standby loss is heat paid for and never used. If the water heater's manufacturer permits a blanket wrap, a properly fitted insulating jacket cuts the surface loss from the tank. Pipe insulation on the first several feet of hot water line keeps the water warmer on the way to the fixture. Check the manufacturer's instructions for clearances and for any areas that must stay uncovered, and never wrap the burner or the flue area.
Where Recovery Fits at Home Scale
A heat pump water heater already applies the same logic the ACHR News item describes. It pulls heat from the surrounding air and moves it into the tank, so the heat is not wasted. The catch is location. A heat pump water heater in a conditioned basement cools and dries that space, which may help in summer and does little in winter. In an unheated garage, it has less heat to draw from when the weather is coldest. Put the unit where its effect on the room is something you want, and note that cooling in summer is part of the deal.
Exhaust-based recovery at home is simpler than it sounds, but it is not a free upgrade. Any exhaust that brings moist air into a cold cavity creates condensation risk. Never route dryer or bath exhaust into an attic, crawlspace, or wall cavity. The duct must end outdoors. If that run exits through the roof, a roofing contractor such as Keys Roofing can flash the penetration so it stays dry and sealed.
Make the Source List
Write down every heat source in the house. For each one, record where its exhaust goes, whether the path is clear, when it last got attention, and what a useful change would look like. Keep the list next to the thermostat or in the service folder. The next time someone proposes a new appliance, a heat recovery accessory, or a larger water heater, you will know which heat is already leaving and which is worth capturing.
Further reading: a roofing contractor such as Keys Roofing.