Which breaker do I need for my heat pump?

One of the most common questions we hear when pool builders are considering fitting a heat pump is, “what breaker does it need?”.

It’s a sensible question! Often though, we do hear about heat pumps being installed with the wrong type of circuit breaker, and this can lead to issues further down the road.

First up, let’s get things ironed out. The power supply for a heat pump needs several things. 

• An MCB (Miniature Circuit Breaker)
• An RCD (Residual Current Device)
• The right voltage and amperage

An MCB protects against over-current draws. This prevents wires overheating and melting. They have a “Tripping Curve” – a sensitivity rating if you like. This is indicated by the letter after the amperage rating. 

An RCD prevents electric shock by detecting current leaking to earth. Older RCDs may stop working when an inverter driven heat pump (or variable speed pump, or inverter driven swim jet) is used – you may need to swap older “type AC” RCDs for type A or B. Make sure your electrician checks.

The right voltage and amperage supply is vital and often overlooked. In the UK, our mains voltage can range from 220V to 250V quite easily. But at some properties, especially rural and older homes, this can be nearer the lower end of the scale, and this can cause problems with sensitive electronic items like heat pumps.

“What about an RCBO? That’s an RCD and an MCB combined! It saves me some work!”

Not necessarily. When fitting a heat pump, the tripping curve of the MCB is really important. A normal domestic type B breaker will trip really easily on a heat pump, because they contain a motor that draws a large start-up current. Getting RCBOs with the correct sensitivity rating can be tricky. Simpler not to do it!

“A type C MCB will do, won’t it?”

It might. Depends on the heat pump. On newer inverter-driven units, that start their motors gently, you’d probably be fine – at least, to begin with. MCBs do weaken with use, so it might last 6 to 12 months and then start to trip. It won’t be the heat pump at fault, it’s the MCB.

On heat pumps with fixed speed compressors, fit a D curve breaker. 

If the manual tells you to fit a D curve breaker, do it.

Yes, your normal electrical supplier might have to order them in for you, and they might have C rated ones on the shelf. But for the sake of a few quid, and a bit of pre-planning, you could be saving yourself a call-out months down the line.

“The heat pump is tripping the electrics!”

Right, let’s get some specifics. Is it tripping the MCB or the RCD? In the pool plant room, or back at the house? Knowing exactly what is happening should give you a good place to start figuring things out.

It is possible for the house RCD to trip out when the heat pump starts, but not the plant room one. This can indicate an earth fault with something other than the heat pump – something in the house. Normally this is something like an immersion heater or kettle. Of course, because it only happens when the heat pump starts, the assumption is that it’s the heat pump at fault, when it isn’t.