Correct Heat Pump size for rental

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#3387239 26-Jun-2025 17:32

Kickinbac:

Handle9 What constitutes over sizing? There are many variables and trade offs.

A poorly designed system. A well designed system should be to able to control within dead band in auto over the full range of design conditions while providing the lowest total cost of ownership to the customer.

You are completely correct that there are many variables and tradeoffs. The range of control of the unit should be able to balance these out in a well designed system. The problem is far too many mechincal engineers, let alone heat pump jockeys, think that the controls can overcome flawed mechanical design.

This becomes apparent in the shoulder season and is particularly acute in small rooms. Small rooms are really "twitchy" to control, small changes in energy input can cause big swings in temperature due to their low thermal mass. When you control an HVAC system you are looking for a lazy response curve. If you are living at the bottom of the range of control you end up shocking the room and you get draft complaints in cooling.

If you start from a design point which is sound then make compromises due to practicality the controls can normally cope fine. When you start from a heavily compromised mechanical system the controls can't cope and you end up with all sorts of negative consequences. When you look at trends in a large system it stands out like dogs balls, most of the rooms will have lovely smooth temperature trends where the oversized units bounce around like crazy. This is with chilled water where you can actually turn down to zero, let alone with refrigerant systems which have minimum run points.

Handle9

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#3387250 26-Jun-2025 17:54

quebec:

Handle9:

Go and take a photo of the board and send it to the contractor. That will tell them if there is room.

Even if it doesn’t have a dedicated MCB its own feed means it won’t run the risk of overloading the wiring of the existing circuit.

Yeah I'll get a photo. So, even if it doesn't have a dedicated MCB, on the switchboard it will connect to an existing MCB with other loads, isn't it similar to taking it from the nearest powerpoint? Or are they hoping to find a 20A one with less load or only lighting?

Yes and no. The big thing is it means that the largest load has it's own dedicated wire running to it so you avoid heating the existing circuit if it's running at capacity. Inevitably the power point they tap off is at the end of the circuit.

Kickinbac

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#3387261 26-Jun-2025 18:07

quebec:

Handle9:

Go and take a photo of the board and send it to the contractor. That will tell them if there is room.

Even if it doesn’t have a dedicated MCB its own feed means it won’t run the risk of overloading the wiring of the existing circuit.

Yeah I'll get a photo. So, even if it doesn't have a dedicated MCB, on the switchboard it will connect to an existing MCB with other loads, isn't it similar to taking it from the nearest powerpoint? Or are they hoping to find a 20A one with less load or only lighting?

It’s unusual to not install a new MCB on a new circuit. The MCB protects the cable so another cable protected by an existing MCB will be ok. But if the AC unit is connected to a power point circuit MCB and there is a lot of load, say AC is running full load and there is an air frier, toaster or kettle going on the same circuit it will still trip as the MCB will see the current of both circuits. Hopefully they don’t do it on a kitchen circuit!
Lighting circuits are usually no more than 10 Amps.
I’m not an electrician so best to get an electricians qualified opinion!

Make sure you get an Electrical COC!

Kickinbac

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#3387263 26-Jun-2025 18:27

Handle9:

Kickinbac:

Handle9 What constitutes over sizing? There are many variables and trade offs.

A poorly designed system. A well designed system should be to able to control within dead band in auto over the full range of design conditions while providing the lowest total cost of ownership to the customer.

You are completely correct that there are many variables and tradeoffs. The range of control of the unit should be able to balance these out in a well designed system. The problem is far too many mechincal engineers, let alone heat pump jockeys, think that the controls can overcome flawed mechanical design.

This becomes apparent in the shoulder season and is particularly acute in small rooms. Small rooms are really "twitchy" to control, small changes in energy input can cause big swings in temperature due to their low thermal mass. When you control an HVAC system you are looking for a lazy response curve. If you are living at the bottom of the range of control you end up shocking the room and you get draft complaints in cooling.

If you start from a design point which is sound then make compromises due to practicality the controls can normally cope fine. When you start from a heavily compromised mechanical system the controls can't cope and you end up with all sorts of negative consequences. When you look at trends in a large system it stands out like dogs balls, most of the rooms will have lovely smooth temperature trends where the oversized units bounce around like crazy. This is with chilled water where you can actually turn down to zero, let alone with refrigerant systems which have minimum run points.

You sound like you know what you’re talking about. Variable speed compressors and fans etc make life easier but no substitute for good design!

Handle9

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#3387266 26-Jun-2025 18:32

Kickinbac:

Handle9:

A poorly designed system. A well designed system should be to able to control within dead band in auto over the full range of design conditions while providing the lowest total cost of ownership to the customer.

You are completely correct that there are many variables and tradeoffs. The range of control of the unit should be able to balance these out in a well designed system. The problem is far too many mechincal engineers, let alone heat pump jockeys, think that the controls can overcome flawed mechanical design.

This becomes apparent in the shoulder season and is particularly acute in small rooms. Small rooms are really "twitchy" to control, small changes in energy input can cause big swings in temperature due to their low thermal mass. When you control an HVAC system you are looking for a lazy response curve. If you are living at the bottom of the range of control you end up shocking the room and you get draft complaints in cooling.

If you start from a design point which is sound then make compromises due to practicality the controls can normally cope fine. When you start from a heavily compromised mechanical system the controls can't cope and you end up with all sorts of negative consequences. When you look at trends in a large system it stands out like dogs balls, most of the rooms will have lovely smooth temperature trends where the oversized units bounce around like crazy. This is with chilled water where you can actually turn down to zero, let alone with refrigerant systems which have minimum run points.

You sound like you know what you’re talking about. Variable speed compressors and fans etc make life easier but no substitute for good design!

I'd hope so after 20ish years working in building automation. I don't get to play with systems anymore but I still remember a few things :)

Most of them are people complaining about our controls when it was a mechanical system or civil design problem :)

tweake

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#3387288 26-Jun-2025 19:13

fe31nz:

The Panasonic has a very large room to heat, but if you select Powerful mode, it will be putting out some heat in less than a minute and as much heat as it is capable of in 2-3 minutes,

not really.

powerful mode simply turns off variable mode and makes it run at full speed up to setpoint and stops. then switches to variable mode. normal running it would run full speed up to 2 degrees before setpoint and then ramp down in speed (normal variable compressor). so basically powerful mode only changes the last 2 degrees. the other thing is it typically runs the fans at full speed, especially on aircon.

start up time is really down to compressor etc. the mode makes no difference whatsoever.

tweake

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#3387291 26-Jun-2025 19:21

vexxxboy:

when we got our heat pump they measured our lounge and said we only need a 4.kw heater , when the installer came around he asked us do we keep the doors closed in the lounge and we said they were never closed and he said the 4.5 kw would do but it would be on all the time and would ramp up the power bill and he recommended the 7.2 kw and he said he always recommends to go as large as you can because it takes under half the time to heat a room and will save you money in the long term as it doesn't need to be on as much .So we did and the extra benefit is that it heats the whole house so we dont need extra heaters. It costs us around $50- $60 a month in winter to run it.

installer upsell.

it actually can't make it room heat up faster, not properly at least. yes it can heat the air quicker, but that doesn't take much heat. what takes the heat is the mass of the house, the timber, brick, furniture etc and that depends on each materials thermal conductivity to soak up the heat from the air. typically thats a lot slower which is why it can take 4-5 hours to heat a room properly, even tho it takes 30 min to heat the air. heating the air quicker is not a real advantage.

if you want to know if the room is heated properly turn the heat pump off. if the room cools quickly, then the room is not heated properly.

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tweake

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#3387294 26-Jun-2025 19:32

quebec:

Got two quotes. First company 3 Yes Heat Pumps says space requires Mitsubishi AP50 with wi-fi $2854. 1 year workmanship warranty. Second company Design Air says as per tenancy website calculator it requires 4.9kw heat so Mitsubishi AP42 is enough, $2708 non Wi-Fi, $2807 Wi-Fi. Other option from them is Panasonic CS/CU-Z50AKR 6kw heating with Wi-Fi $2604. Design Air offers 5 years workmanship warranty. Does workmanship warranty matters? 3 YES says it doesn’t as workmanship issues if any happen within first few months. Has anyone dealt with either of these two?

Lounge is 7.3x5.3 =38.69 sqm. I’ve heard Mitsubishi Electric are better than Panasonic so leaning towards that but what size? I understand that for healthy homes compliance I can install what tenancy website calculate says but I also want it to be the right size.

for rentals, it probably best to fit what ever the tenancy calc says simply for liability reasons.

normally i would go with a good calc and go on the small size.

typically they are sized for 99% day. eg auckland i think its sized for 4c temp even tho auckland goes below 0c. if you size it for coldest day then its only a few days a year it runs at 100%. the rest of the time it short cycles and is less efficient. so by under sizing it you get more days where it runs more efficient, at the expense of a few cold days where you have to put some clothes on.

what compounds this is kiwis low quality poorly insulated housing means people use heat pumps incorrectly. they tend to use them like instant heaters instead of heating the whole home and keeping it warm.

Handle9

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#3387298 26-Jun-2025 19:45

tweake:

what takes the heat is the mass of the house, the timber, brick, furniture etc and that depends on each materials thermal conductivity to soak up the heat from the air.

If I have the AC off for my house in Dubai during summer it takes two days to get the temperature under control.

It's made of solid concrete and 45 degrees outside but it's the same principle.

Spyware

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#3387299 26-Jun-2025 19:52

tweake:

typically they are sized for 99% day. eg auckland i think its sized for 4c temp even tho auckland goes below 0c. if you size it for coldest day then its only a few days a year it runs at 100%. the rest of the time it short cycles and is less efficient. so by under sizing it you get more days where it runs more efficient, at the expense of a few cold days where you have to put some clothes on.

what compounds this is kiwis low quality poorly insulated housing means people use heat pumps incorrectly. they tend to use them like instant heaters instead of heating the whole home and keeping it warm.

To make my Mitsu run continuously (overcome their quietness) I just change the fan to 3/4 minimum (4/4 on cold days and most winter nights) and works fine. Yesterday in Christchurch it got to 16 and I had left both heat pumps on 24/7 (April to November usually). 23 inside on 16 degree day, drops to 19 inside when -2 over night and recovers to 22 or so if above 10 outside as house leaks like a little 1958 brick house with no wall insulation does. Two x AP42 equivalents. When I replace will go with AP50 minimum as need more output in Winter. When temp near zero or below they shutdown for ages and are really quite useless - then I have to wear clothes. I normally also have night store heater on but uses 600 kWh/month all by itself - not to bad with my 0.14 kWh night rate power but. Twenty three is a good inside temp - I set them on 25 so they never reach temp.

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#3387300 26-Jun-2025 20:00

Handle9:

If I have the AC off for my house in Dubai during summer it takes two days to get the temperature under control.

It's made of solid concrete and 45 degrees outside but it's the same principle.

good example.

the common example i hear is the log cabin in the woods, weekend getaway. it takes two days to get it up to temp then they go home. mass takes time.

tweake

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#3387302 26-Jun-2025 20:03

Spyware:

When temp near zero or below they shutdown for ages and are really quite useless - then I have to wear clothes.

thats weird. or are they in defrost? one of the problems in auckland is high humidity and above zero temps means they often ice up and have to defrost.

fe31nz

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#3387345 26-Jun-2025 22:57

tweake:

Spyware:

When temp near zero or below they shutdown for ages and are really quite useless - then I have to wear clothes.

thats weird. or are they in defrost? one of the problems in auckland is high humidity and above zero temps means they often ice up and have to defrost.

It will be defrosting - they run slowly in reverse to do this, so cool the room a bit. Most heat pumps will try to push in a compensating amount of heat after they finish defrosting. Frosting problems generally happen around 1-2 C, and stop happening at 0 C and below. It can help to have the outside units higher up as the coldest air is at ground level.

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.

Handle9

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#3387358 27-Jun-2025 06:11

tweake:

Handle9:

If I have the AC off for my house in Dubai during summer it takes two days to get the temperature under control.

It's made of solid concrete and 45 degrees outside but it's the same principle.

good example.

the common example i hear is the log cabin in the woods, weekend getaway. it takes two days to get it up to temp then they go home. mass takes time.

Thats the application that oversized wood burners were made for. The cost of fuel really doesn’t matter for a few days a year.

Goosey

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#3387457 27-Jun-2025 07:07

Go the higher KW, people naturally want to try heat the remainder of the house with one heat pump.

what area of NZ are you in? You need to make sure the outdoor unit can handle sub zero temp….otherwise it will just struggle in winter and hot defrost mode more than often….and the tenants will simply complain the heat pump doesn’t work.

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