Correct Heat Pump size for rental

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Kickinbac

427 posts

Ultimate Geek

#3387575 27-Jun-2025 09:42

The defrost cycle is a good reason to size a unit properly especially in colder regions. If the heat pump is undersized, it will work harder, freeze up more often and spend more time defrosting.

If the system is defrosting 15 minutes an hour, that's an approx 25% loss of capacity. Probably more as to defrost its removing heat from indoors.

The healthy homes calculator is forcing landlords to select the units to perform on the coldest days of the year.

Manufacturers specs in their brochures are based on test conditions in AS/NZS 3832.1.1 for H1 heating indoor temp: 20°CDB/15°CWB, Outdoor: 7°CDB/6°CWB

Real heating conditions are more the H2 heating with indoor temp: 20°CDB/15°CWB, Outdoor: 2°CDB/1°CWB which is more relevant. This is where the Healthy Homes calculation heads.

Here's an example of these tables for a Mitsubishi Electric AP50...

You can see that at 10°C its providing 6.6 kW heat and at 0°C: 5.1 kW heat and that's with an indoor temp of 21°C!

So have a think about what is happening with a cold house on start up, when the indoor temp is say 12°C and outdoor 0°C, less heat is being produced so longer to heat up the room and its more likely to defrost.

quebec

852 posts

Ultimate Geek

#3387621 27-Jun-2025 13:47

Emailed the photo of switchboard showing heaps of room, no response, instead received a revised invoice which says power run from board. I don't want to be too fussy and end up him not doing the install as it's hard to arrange quotes etc. with tenants. Is MCB/RCBO really required or beneficial or a separate run to switchboard to an existing MCB is also ok? When he quoted me the price I did ask if it includes dedicated line from switchboard and he said yes. Now, I;m wondering if I should also clarify that COC etc. will be provided. They are a decent size company from Mitsubishi Electric website so I'm assuming they will do things properly and I don't think it's legal for a person who's not an electrician to run power from switchboard even to an existing MCB, isn't it?

Handle9

11394 posts

Uber Geek

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Lifetime subscriber

#3387641 27-Jun-2025 13:58

A COC is a legal requirement so it’s not actually necessary to have it on the quote. You just refuse to pay unless it’s issued. Doing any electrical work for profit legally requires electrical worker registration so they’ll need an electrician.

The machine having its own MCB is the best way. It stops faults on the machine knocking out the rest of the circuit, or vice versa.

Kickinbac

427 posts

Ultimate Geek

#3387645 27-Jun-2025 14:07

I would insist on new MCB for a new circuit. There would have to be a really good reason if they can't do this.

Any work in a switchboard has to be done by a registered electrician and an Electrical COC is required to be issued.

quebec

852 posts

Ultimate Geek

#3387654 27-Jun-2025 14:17

Kickinbac:

I would insist on new MCB for a new circuit. There would have to be a really good reason if they can't do this.

Any work in a switchboard has to be done by a registered electrician and an Electrical COC is required to be issued.

Maybe it's the extra cost or is it too much extra work to add an MCB? But then it's only $10-20 if am I looking at the right thing? https://nz.rs-online.com/web/p/mcbs/2306653?cm_mmc=NZ-PLA-DS3A-_-google-_-PLA_NZ_Pmax_Top_1223-_--_-&matchtype=&&gclsrc=aw.ds&gad_source=1&gad_campaignid=20858487975&gclid=CjwKCAjw3_PCBhA2EiwAkH_j4h1Jk8XDW6apKDp1hQNlx0elIT2zyS8gYiYmdoeEaACkrEITQVJZDRoCbHoQAvD_BwE

https://lightingelectricalsupplies.co.nz/index.php?route=product/product&product_id=1852&gad_source=1&gad_campaignid=16058839678&gclid=CjwKCAjw3_PCBhA2EiwAkH_j4tc9GVVFYkCMT4hXeUl2rv8pYILe9yE5U_4fByrq4wG0sZSnD-fWzhoCpnkQAvD_BwE

Kickinbac

427 posts

Ultimate Geek

#3387666 27-Jun-2025 15:11

quebec:

Kickinbac:

I would insist on new MCB for a new circuit. There would have to be a really good reason if they can't do this.

Any work in a switchboard has to be done by a registered electrician and an Electrical COC is required to be issued.

Maybe it's the extra cost or is it too much extra work to add an MCB? But then it's only $10-20 if am I looking at the right thing? https://nz.rs-online.com/web/p/mcbs/2306653?cm_mmc=NZ-PLA-DS3A-_-google-_-PLA_NZ_Pmax_Top_1223-_--_-&matchtype=&&gclsrc=aw.ds&gad_source=1&gad_campaignid=20858487975&gclid=CjwKCAjw3_PCBhA2EiwAkH_j4h1Jk8XDW6apKDp1hQNlx0elIT2zyS8gYiYmdoeEaACkrEITQVJZDRoCbHoQAvD_BwE

https://lightingelectricalsupplies.co.nz/index.php?route=product/product&product_id=1852&gad_source=1&gad_campaignid=16058839678&gclid=CjwKCAjw3_PCBhA2EiwAkH_j4tc9GVVFYkCMT4hXeUl2rv8pYILe9yE5U_4fByrq4wG0sZSnD-fWzhoCpnkQAvD_BwE

The latest JA Russell flier sitting on our lunch room table has Hager MCB's at $3.40 + GST

tweake

2391 posts

Uber Geek

#3387704 27-Jun-2025 17:44

fe31nz:

Our older Mitsubishi models also seem to have a firmware bug. If you put them on full fan they will maintain the temperature well until they have to defrost. Then after defrosting they go to full fan and high output for ages and overheat the room completely. So if it is going to be cool enough to require defrost cycles, we have to make sure to use auto fan, and generally that means they will not circulate the air well enough and it stratifies with all the hotter air near the ceiling, so we have to also turn the setpoint up 1-2 degrees to be comfortable closer to the floor or at bed height.

my panasonics you cannot change the fan speed on heating. its auto no matter what. yes the fan slows down once it warms up. this is part of why i recommend ceiling fans to help mix the air.

Sharesies

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tweake

2391 posts

Uber Geek

#3387706 27-Jun-2025 17:53

Kickinbac:

The defrost cycle is a good reason to size a unit properly especially in colder regions. If the heat pump is undersized, it will work harder, freeze up more often and spend more time defrosting.

If the system is defrosting 15 minutes an hour, that's an approx 25% loss of capacity. Probably more as to defrost its removing heat from indoors.

in the real world it would have to be massively undersized to be a problem.

i'm northland/auckland and often at those temps ranges where they frost up the most. on a cold morning it will frost up once. i don't think its ever down it twice in a row. its never ever going to defrost every hour. especially as the day warms up.

don't forget they are deliberately undersized them for the coldest day. even on my coldest days, they still havn't frosted over more than once.

Kickinbac

427 posts

Ultimate Geek

#3387717 27-Jun-2025 19:08

tweake:

Kickinbac:

The defrost cycle is a good reason to size a unit properly especially in colder regions. If the heat pump is undersized, it will work harder, freeze up more often and spend more time defrosting.

If the system is defrosting 15 minutes an hour, that's an approx 25% loss of capacity. Probably more as to defrost its removing heat from indoors.

in the real world it would have to be massively undersized to be a problem.

i'm northland/auckland and often at those temps ranges where they frost up the most. on a cold morning it will frost up once. i don't think its ever down it twice in a row. its never ever going to defrost every hour. especially as the day warms up.

don't forget they are deliberately undersized them for the coldest day. even on my coldest days, they still havn't frosted over more than once.

The tropical north! I’m not surprised, you probably wear shorts and jandals all year round 😆

tweake

2391 posts

Uber Geek

#3387726 27-Jun-2025 19:57

Kickinbac:

The tropical north! I’m not surprised, you probably wear shorts and jandals all year round 😆

laugh it up. it does get below freezing here. even northland gets a touch of snow now and then. only thing tropical is the blasted humidity.

quebec

852 posts

Ultimate Geek

#3387729 27-Jun-2025 20:40

Emailed asking to confirm new MCB as he replied when I sent the pic of switchboard, no reply. Am I being unreasonable here? Installation is scheduled for 7th July.

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