Level of risk to tenants for heat pump on a shared type A rcd

How many heat pumps, and shared with what?

Are there a couple of heat pumps on their own RCD, or do they share the RCD with other types of circuit?

What is your concern please?

One heat pump per property on a dual rcd board, I don't have details with me but from memory sockets and lights are on the shared type A rcd 

I don't see the problem.

Unless heat pumps are particularly prone to nuisance tripping, sharing that half of the DB with other circuits simply means that if any of them trips the RCD, half of the board trips. I do not see any risk over and above any other dual-RCD board.

If the manufacturers require a type B RCD, I would be surprised. After all, there is nothing new about heat pumps: air-con has been around for decades. They contain a motor and some control circuitry, neither of which seems very risky.

OK, they are contained in a steel box which is outdoors, but a type A RCD should deal with any risk of the casing becoming live, which should trip ADS in any event.

They contain a motor and some control circuitry

I suspect the problem comes from the fairly hefty inverter drive in the more modern HPs - so there is a sizeable d.c. bus in there (probably with some substantial smoothing capacitors) ... but how likely that is to feed back into the a.c. side of things (e.g. via the c.p.c.) is less clear. Hopefully far less of  risk than an EV or transformerless PV inverter, but whether it's quite negligible not not we can only rely on the manufacturer's instructions to tell (if they do). If the original instruction suggested an A type would be OK, you should be in the clear.

they are contained in a steel box which is outdoors, but a type A RCD should deal with any risk of the casing becoming live

I would hope, on a TN system, the MCB would be more than adequate for ADS. Which brings me back to asking whether the RCD is needed at all - it's not in a bathroom or on a socket circuit or domestic lighting, so unless they've concealed soft sheathed cables in walls (perhaps unlikely for a retrofit?) I don't think there's any general BS 7671 requirement for one at all.

   - Andy. 

I was in a quandry similar to this a few years ago. I asked the heat pump company (Nibe) what type of rcd was required but they couldn't answer. I then sent them a link to an online article by Dpoeke which described the 'blinded RCD' phenomena and they promptly said they requried a Type B.

Basically, the designers/manufacturers/installers didtn know until prompted. My guess is a type B was recommned more out of panic than reasoned thinking and they had never heard of a Type B RCD before I sent them the item.

I was in a quandry similar to this a few years ago. I asked the heat pump company (Nibe) what type of rcd was required but they couldn't answer. I then sent them a link to an online article by Dpoeke which described the 'blinded RCD' phenomena and they promptly said they requried a Type B.

Basically, the designers/manufacturers/installers didtn know until prompted. My guess is a type B was recommned more out of panic than reasoned thinking and they had never heard of a Type B RCD before I sent them the item.