Ducted heat pump size calculation

Forums › Home Workshop DIY › Ducted heat pump size calculation

Paul1977

2777 posts

Uber Geek

# 257154 18-Sep-2019 13:57

We have about 145m2 floor area to heat in our new Christchurch build. We have a few different ceiling heights, but they average out to 2.56m.

We are having low e glass and thermally broken joinery, and a minimum of R2.6 in the walls and R3.6 in the ceiling (we may upgrade to R2.8 in walls and R6 in ceiling, but haven't decided yet).

We have been quoted for a Mitsubishi PEAD-M125JAA which is rated at 14kW heating and 12.5 kW cooling.

Is this enough?

The Mitsubishi calculator says we require 13kW (so less than the 14kW quoted unit), but it then goes on to recommend a 20kW option?

Home: Work:
Home Work

1 | 2

15235 posts

Uber Geek

Trusted

Subscriber

# 2319769 18-Sep-2019 14:00

I have an old house, but fairly well insulated ceiling, walls, floor, and double glazing. We have two high wall heat pumps, one 10KW heat and the other 7KW heat. I probably wouldn't go much lower than that. Ducted is meant to be a bit less efficient, but probably much better distributed and more comfortable.

If your house is fairly new and well insulated, holds the heat well, and you leave the heating on most of the time I guess 13KW might be enough. If you use it only when it gets cold maybe you need larger.

mattwnz

15164 posts

Uber Geek

# 2319785 18-Sep-2019 14:16

You should contact Mitsubishi technical directly, as they should be able to work it out better for you than the rough calculators on the website. I have found installers can vary significantly in what they recommend.

evnafets

163 posts

Master Geek

# 2319787 18-Sep-2019 14:21

You have to size the unit for both heating and cooling right?

Your unit is 14kW heating and 12.5 kW cooling.

So a requirement of 13kW would exceed the unit output if you are talking about cooling. Hence why the calculator presumably recommend the "next one up"

wellygary

4100 posts

Uber Geek

# 2319792 18-Sep-2019 14:27

2 people support this post

Paul1977:

(we may upgrade to R2.8 in walls and R6 in ceiling, but haven't decided yet).

The Mitsubishi calculator says we require 13kW (so less than the 14kW quoted unit), but it then goes on to recommend a 20kW option?

1. Do the insulation upgrade, it will save you $$$ in the long run, and you will never get a chance to do the walls..

2. the reason it recommends a bigger unit is that it is more efficient to run a larger unit at lower capacity output, rather than a smaller unit at 100% most of the time...

it also means that the unit has enough output to heat the room and then switch off to allow for defrosting ( esp on cooler nights), the last thing you want is a smaller unit struggling to heat the place, and then having to turn off to defrost regularly -

jonathan18

4135 posts

Uber Geek

Subscriber

# 2319807 18-Sep-2019 14:35

wellygary:

it also means that the unit has enough output to heat the room and then switch off to allow for defrosting ( esp on cooler nights), the last thing you want is a smaller unit struggling to heat the place, and then having to turn off to defrost regularly -

So true. Our ducted system is, I reckon, under-spec'd for the house; it's farcical that in the depths of winter we end up having to put the gas heater on to compensate for the inadequate heating (and periods of no heating due to frequent defrosting) from the heat pump.

I understand there are negative consequences resulting from going too large as well, but certainly don't risk under-specing it.

Another thing is, in our case, the warmth of the air from the ducted system is significantly reduced is when the outside temperature is really low; I know this may vary by model and age, but it's something to be mindful of.

Paul1977

2777 posts

Uber Geek

# 2319817 18-Sep-2019 14:49

jonathan18:

wellygary:

it also means that the unit has enough output to heat the room and then switch off to allow for defrosting ( esp on cooler nights), the last thing you want is a smaller unit struggling to heat the place, and then having to turn off to defrost regularly -

So true. Our ducted system is, I reckon, under-spec'd for the house; it's farcical that in the depths of winter we end up having to put the gas heater on to compensate for the inadequate heating (and periods of no heating due to frequent defrosting) from the heat pump.

I understand there are negative consequences resulting from going too large as well, but certainly don't risk under-specing it.

Another thing is, in our case, the warmth of the air from the ducted system is significantly reduced is when the outside temperature is really low; I know this may vary by model and age, but it's something to be mindful of.

Can I ask what your unit is rated at, and what size your house is?

What are the downsides of going too large?

timmmay

15235 posts

Uber Geek

Trusted

Subscriber

# 2319829 18-Sep-2019 15:04

I think if it's too large and the house is warm the minimum power means the heat pump turns on then off, whereas a heat pump with a lower minimum would operate continuously. I find a continuous heat / noise better than on and off.

We have a 10kw high wall heat pump that warms a room / hallway that's maybe 20 - 25 square meters. It turns on and off a lot.

jonathan18

4135 posts

Uber Geek

Subscriber

# 2319832 18-Sep-2019 15:05

According to this post, it's 15kW heating / 12.5 kW cooling (which isn't needed much in PN!), for a 160m house with a mix of 2.4 and 3m stud.

But ours is a 1920s villa with no wall insulation, so it's not an adequate comparison to a modern build, especially if you go for thicker insulation (which I would certainly do in both walls and ceiling if I get the opportunity to build); over-spec'ing your insulation would also surely have a beneficial impact in reducing the workload of the heatpump.

(My whole saga with our heat pump install - still not sorted to this day - is in this thread).

As for over-sized heat pumps, I am no expert so hopefully others on GZ who are can respond; this is the kind of thing I've been told of, whether or not all this information is accurate: http://www.southshorehvac.ca/why-bigger-isnt-better-with-heat-pump-sizes/

timmmay

15235 posts

Uber Geek

Trusted

Subscriber

# 2319838 18-Sep-2019 15:13

We used the air conditioning way more last summer than the five summers before it.

wellygary

4100 posts

Uber Geek

# 2319840 18-Sep-2019 15:16

One person supports this post

jonathan18:

http://www.southshorehvac.ca/why-bigger-isnt-better-with-heat-pump-sizes/

"For a residential home, there is very little need for anything larger than an 18,000 BTU ductless heat pump."

Err. 18,000 BTU is ~5KW.....

I'm not sure the advice from this crowd in Nova Scotia is directly comparable to NZ...

mentalinc

1752 posts

Uber Geek

Trusted

# 2319850 18-Sep-2019 15:33

make sure you can control multiple zones, you may want the lounge at circa 22, but that is way to hot for bedrooms...

CPU: Intel 3770k| RAM: F3-2400C10D-16GTX G.Skill Trident X |MB: Gigabyte Z77X-UD5H-WB | GFX: GV-N660OC-2GD gv-n660oc-2gd GeForce GTX 660 | Monitor: Qnix 27" 2560x1440

jonathan18

4135 posts

Uber Geek

Subscriber

# 2319867 18-Sep-2019 15:52

wellygary:

jonathan18:

http://www.southshorehvac.ca/why-bigger-isnt-better-with-heat-pump-sizes/

"For a residential home, there is very little need for anything larger than an 18,000 BTU ductless heat pump."

Err. 18,000 BTU is ~5KW.....

I'm not sure the advice from this crowd in Nova Scotia is directly comparable to NZ...

Err, I never suggested the specifics on that site regarding sizing heat-pumps were relevant - it's pretty clear from the wording of my post that I was responding to the question on the issues associated with heat pumps that were significantly larger in capacity than required.

wellygary

4100 posts

Uber Geek

# 2319878 18-Sep-2019 16:06

jonathan18:

wellygary:

jonathan18:

http://www.southshorehvac.ca/why-bigger-isnt-better-with-heat-pump-sizes/

"For a residential home, there is very little need for anything larger than an 18,000 BTU ductless heat pump."

Err. 18,000 BTU is ~5KW.....

I'm not sure the advice from this crowd in Nova Scotia is directly comparable to NZ...

Err, I never suggested the specifics on that site regarding sizing heat-pumps were relevant - it's pretty clear from the wording of my post that I was responding to the question on the issues associated with heat pumps that were significantly larger in capacity than required.

Yes, agree with you , but my point is if they are saying 5KW is the largest size you will need,

How much trust can you put in the rest of their recommendations.... ( including those on oversizing....)

Goosey

1848 posts

Uber Geek

Subscriber

# 2320691 19-Sep-2019 20:47

One person supports this post

Are you insulating all/every wall i.e. not just the externally facing ones, but internal walls too? Including garage?

Some new builds only include exterior facing walls with insulation and maybe the wall between the bathroom and bedroom and usually exclude the garage.

I reccomend insulating all and every internal wall.

I know someone who only needs a 7kw high wall and its generally enough to heat most of the house with only the southern bedrooms needing some back up heat sources in the winter.

billgates

3959 posts

Uber Geek

Trusted

# 2320712 19-Sep-2019 21:26

+1 to what @Goosey said regarding internal wall insulation. Our build has just started and we have insulated all internal walls including garage. We have picked Mitsubishi PEA-140 with Wifi card and it uses R32 refrigerant which is more efficient than R410A for 250sqm house of which 41sqm is garage and that is the 9nly area excluded from a duct run. I would strongly recommend getting the WiFi card installed and R32 variant of the ducted model

Do whatever you want to do man.

1 | 2