I have a ducted central heating system in my residential home in New Zealand (220V) installed 18 months ago. The system is powered by a large outside unit. Last week the outside unit has stopped working. My installer has diagnosed the problem to one circuit board in the outside unit that burnt out. They say it is due to an unusually large and short spike of current that penetrated the protection on the house breaker board and protection on the circuit board as well. I'm sceptical. Can anyone help verify or disprove this claim?

In more detail...

The installer claims that there must have been a very unusually high amplitude spike in the current for a very short time e.g. 0.1-0.3 second. The reason they say this is that they claim that my electric board has two RCDs (Residual Current Devices) as the first layer of protection and the Heat pump's board also has protection. The claim is that the large and short spike of current got around both measures. The installer says they will replace the board and that there is nothing more I can do to stop this from happening again. I'll just have to hope that this unusual incident doesn't happen again.

I feel a bit sceptical about the installer's claim. I had no other electrical devices damaged in the house. In fact I didn't notice anything unusual - no lights flickering or noises etc. I also feel that a modern piece of equipment such as that heatpump unit (installed in late 2019 and is still the current model) coupled with the RCD protection should be very resilient, more resilient than other things in my house.

My suspicion is that the outside unit has not been wired correctly in some way. Could it bypass the RCD protection and that's why the spike didn't trigger the RCD to go off? Any other ideas?

Basically, I want to ask the installer and electrician to recheck how the system has been wired. I'd like to have some ideas before I talk to them, because they simply say that I'm as protected as I can be and it was just a freak accident. So they'll take some convincing before spending any more time/money on this (still under warranty so I'm not paying).

I'm just afraid it will happen again soon. In the meantime it's winter and being without the heating even this one week has been miserable with family and small kids in the house.

  • 1
    An RCD protects against ground faults occurring in the downstream connected device(s). It does not protect against upstream spikes or surges. It seems to me like your installer is throwing a bunch of technobabble at you in the hope that something sticks.
    – brhans
    Commented Jul 5, 2021 at 4:24
  • Short answer is yes. Did you have a lighting storm or power outage just before it happen? Usually other equipment is damaged also. There are a few reasons for a board to 'burnout' and is best to find out the reason instead of just replacing or adding stuff. Defect, wrong installation, and/or moisture getting on it, are some other reasons.
    – crip659
    Commented Jul 5, 2021 at 11:05
  • What is the large outside unit?, a generator, power company transformer, etc, does it run all the time or when the grid is out?
    – JACK
    Commented Jul 5, 2021 at 11:40
  • @crip659 - Like I explained, we didn't notice any weather events. As far as I know nothing happened - no flickering, no lightning etc.
    – alex
    Commented Jul 5, 2021 at 21:40
  • @JACK - Outside unit of an AC system. It looks like a big box with a large fan to cool the coils. My one has two fans and stands around 1.6 meters tall - almost the height of a fridge and weighs 100 kg.
    – alex
    Commented Jul 5, 2021 at 21:43

2 Answers 2


RCD is not a protection from current spikes, voltage spikes and so on. It only protects you (the user) from getting electrocuted if some wires inside get loose and chassis gets electrified.

Installer may be throwing random terms at you, or maybe not - all he knows it got damaged in some random event. If he just installs/services the units, he probably don't even have equipment/knowledge to diagnose what happened. His only concern is "fried->replace->bill someone". It is likely it was some voltage spike - those happen, and can make a mess of sensitive electronics.

Onto preventing such occurrences in future - an important/sensitive appliances should have a surge protection device. Like computers have those power strips with fuses and varistors.
You can (and probably should) install a surge protection device in your power panel, to protect the unit and everything electronic in house. They are not cheap, and you will never know when they "worked", but better safe than sorry. Consult your electrician - you will need him to install the protection, size it properly and ensure you have a good grounding rod.

If the unit is powered with a standard cord that plugs into outlet, you can buy a power strip with surge protection and plug it through that, as temporary measure - just ensure the wattage is correct with some breathing room.

  • A whole house surge protection is not really expensive. Maybe around 200 U,S. dollars. Seeing most houses these days have multiple electronic appliances and TVs/computers, protection against power surges is almost required.
    – crip659
    Commented Jul 5, 2021 at 14:35
  • Expensive depends on person and location. Some people will be "why waste 200$ if 10$ surge protector power strip does the same job?" Then comes storm or someone makes a mistake when rewiring a transformer, and suddenly people start calling their fridges and dishwahers and tvs burned up (computers usually survive because of aforementioned power strips and because PSU can take a beating) Whole house surge protection is not mandatory, but recommended. Still a novelty at my loc - my electrican was very surprised I want one!
    – Thomas
    Commented Jul 5, 2021 at 14:41
  • @crip659 - New Zealand usually has outrageous prices for specialised equipment like surge protection especially for the whole house. I wouldn't be surprised if it is in the thousands of dollars. But thank you. I will try to find out from a local electrician if this is something they know about and do for residential houses (I never heard of it).
    – alex
    Commented Jul 5, 2021 at 21:46
  • I thought they would be expensive also. Just join a few months ago and heard about them, went to amazon and found them. Your case sounds like a voltage spike/power surge is lower down on list of possible reasons for burnout of board.
    – crip659
    Commented Jul 5, 2021 at 21:51
  • @Thomas - The unit is high power. Its breaker is 25A (normal breaker is 10A, my hot water cylinder has 15A). So it is wired specifically, so I can't just buy a plugin surge protection unfortunately. Good advice for someone else who does have that setup though.
    – alex
    Commented Jul 5, 2021 at 21:53

Just discussed this with my electrician. He's been working over 20 years and installed and fixed 1000s of heat pumps. He replaced plenty of circuit boards in them, but never because of a surge. He recons it's mostly faulty/defect boards or moisture getting in. According to him, it is extremely unusual to have surge protection on the breaker board in New Zealand. He installed only one for a small business many years ago. He also said that New Zealand power lines are quite good guaranteeing +-5% variation. A surge is highly unlikely. Australia that gets thunderstorms and flooding. There devices like that are more common.

The local specialist electrics shop also confirmed surge protectors are installed in specialised environments like hospitals where there are millions of dollars worth of equipment to protect.

Also, the surge protectors are one time use. If they go, you have to wait for an electrician before you get the power back. The protectors that are sold here are $300 for 40A for one circuit. The shop told me that it would be very expensive to get one that protects the entire house. I would imagine it would be $3k+.

So all in all, even though surge protector would protect me against surges, they are most likely not what killed my heat pump.

  • 1
    like hospitals where there are millions of dollars worth of equipment to protect - and where occasional failures due to surges can be life-threatening events if they happen to ventilators or other critical equipment. Commented Jul 6, 2021 at 4:49
  • Moisture is a very reasonable explanation. We had several control boards on our furnace (installed in a "root cellar" basement) burn up during heavy rain when the sump pump couldn't keep up. The board isn't even that low in the furnace, and we don't get "high water marks" on the basement walls - it was simply humid enough with a few mm of water on the floor to fry the board. Our HVAC guy recommended 2 sump pumps instead of the 1 we had. I installed a 2nd pump and it has kept up with all the rain we've had, and haven't had a board fry in several years now.
    – FreeMan
    Commented Jul 6, 2021 at 14:40
  • Surge protectors are not one time use. They do wear over time, but they only die violently when there is direct/nearby lightning strike. In that case, you will be happy it was a surge protector dying and not your couple thousand of electronics. Also, surge protectors do not disable power when they fail (unless they fail short, but most varistor units can be easily removed from slots by end user). Unless you got suggested some weird fridge sized online UPS instead?
    – Thomas
    Commented Jul 8, 2021 at 7:30

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