Why do we put feet under heat pumps?

Graham, are you talking about concrete plinths being necessary or the rubber feet shown in your picture? This is another "it depends" situation. In most cases the unit should be lifted off the ground to prevent dirt infiltration and accumulation, it also helps to dissipate the exhaust air and prevent recycling. The compressor mountings prevent low-frequency vibration and torque transmission, the same applies to fans. The case can vibrate due to the air movement through the grill so antivibration mounts are a good thing. If flexible mounts are used then flexible couplings are a must if using rigid pipe, if using flexible plastic maybe not, it depends on the length. Good quality heat pumps have a drain connection in the drip tray which will need to be connected to the drain and accessible for maintenance. Cheaper units just drip so they also need space underneath to allow it to run away and so you can scrape out the crud that gathers. Flexible hoses prevent stress on the connections to the PHE which is made of stainless steel and is susceptible to stress fracture, it's not just the vibration, they help with alignment. I agree that they don't need a concrete plinth and compacted gravel is the best base but sometimes it's easier to mount the unit on a wall at low level (200-300mm) which leaves the gap underneath clear to allow air circulation, drainage and maintenance. Or are you just looking for a cheap and fast method?

Logic would say to stop corrosion if in direct contact with the ground plus vibration, snow etc. what do the manufactures say ??

I like your logic. There are vibration isolators under the compressor, the only other source of vibration is the fan (which has rubber bushing mounts). Could you then suggest that flexible pipe connections for the primaries are unecessary too, or is that a step too far? Until we can cut through “custom and practice” approach to installations we cannot actually move this industry forward. Nice one Graham.

Subject to the right climate, the right outdoor unit, and appropriate safety considerations sure. 1) Stop them falling over A "tombstone" outdoor unit can convincingly splat the brat that climbs it. The adult that climbs it to clean the gutters might dent their head etc. These aren't just being installed on access controlled commercial rooftops. 2) Keeps the snow out of it And the floods. And the leaves / rodents. And the weeds. And creates more space for air to be drawn into it. It needs to be high enough that icicles aren't an issue either. 3) See 1&2. In places where there's a real winter (instead of just sprautumn) by the time you've raised the unit high enough to clear the snow then even the shortest / fattest unit becomes a "tombstone" that you need to stop falling over. They're also more likely to be lighter weight splits than monoblocs. (avoid keeping water in an environment that can hit -30C if the power fails) This is probably where your lump of concrete heavy enough to stop it falling over even if it's just resting the ground requirement will come from. It's the idiot proof way to ensure that a fucknumpty installer doesn't do something dangerous then blame the OEM for not warning them not to. If the units are: - Not in snow country - Short in height - Heavy in weight (monoblocs rather than splits) Then you can get away with less. In a UK context, with short fat heavier than a child monobloc, then a pair of rubber feet to keep the steel chassis lifted above standing water and give it enough forward-backward depth not to fall over is probably plenty. Gravel is functional...but fugly. If you're paying ￡X,000 for the installation or ￡XX,000 for the installation I'd be pretty miffed if it were as scruffy as the one pictured. Big slate chippings or something more decorative would look the part and look the ￡￡s being charged. Keep to rough edged stones not round gravel for stability. Still looks naff with rubber-feet-on-gravel compared with being anchored to some piles (screw piles or cast-in-situ concrete piles, slender frame up to outdoor unit) and the ground underneath made good (with your gravel). That's a 2-5 day start to finish install though depending on how fast your concrete sets. Screwing the unit to a wooden deck with a hole in it isn't crazy either; provided that you've raised it far enough off the deck to stay dry and are screwing into noggins. Vibration a non issue. Deck already includes the supporting piles. Smashing it onto the side of the house is probably where we end up though!

Graham Hendra Self damage from undamped vibration with firm fixing and onward transmission of Vib./Noise major factors. Also that ‘splashed up grit’ from both summer or winter downpours combined with leaf litter, snow and near permanently corrosion inducing dampness at Conc - Casing interface. Turning Q. around.... ‘What piece of Mech Equip is directly firmly fixed at Grade?’ From Dom. Water Pumps to massive GT’s are all ‘elevated’ incld., amazingly dozens of Tonne Coal & Rock crushers.