They do, from the humidity of the outside air. It condenses on the grille and drips off onto the ground or into a drip tray. That water has to be directed away to a drain. On a humid day it’s surprising just how much water runs off.

That’s what can freeze on the grille, which needs the defrost cycle to melt it off. 

Depends on the size of unit and weather conditions. I've seen values of up to 10 litres per hour quoted. Potentially a problem if it just leaks across a path and starts to freeze as the condensate will keep building up in cold weather.

Or as of 3 days ago drips out the front left base of the heat pump as well ! Zero rainfall.

Thanks for your comments on this question. I’ve separated this out into it’s own topic and put it in the public forum so that it can help others searching for this advice. 

So @juliamc @nealmurphy @hambrook does that means every heat pump needs to have some form of deliberate drainage below it? The most common I’ve seen from the trials we’re running is the pump raised off the ground, above loose gravel. But @hambrook doesn’t have this. Will that water be directed below or to the side of that decking? 

What happens if someone's heat pump is going to be placed above tarmac or concrete? 

There should be some drainage (in the same way that a condensing boiler needs a drain). Ours is piped to a nearby drain, although there was a puddle under it this morning so I wonder if the pipe isn't connected properly.

Ours had this drip tray to collect the water which then flows away into nearby drain: