Ducted heat pump size calculation

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#2321776 21-Sep-2019 06:26

I'd put the R2.8 in the walls as that can't be supplemented. More ceiling insulation can be added to later if you're concerned about the cost. R6 should pay for itself over R3.6 if you're centrally heating the place but that may take a long while.

Standard ducting insulation isn't great and creates inefficiencies. Higher R values are available if you ask for it.

Good insulation lasts a lifetime but a heat pump may only last 15 years so the yearly cost for a ducted system's presence is high before it's done any heating. Good installers are really important for ducted systems as it's complicated to balance airflows.

Oversizing heat pumps can cause problems. When cooling it may boost the relatively humidity in a room instead of reducing it. The unit may end up turning off and on instead of being able to run continuously as the minimum capacity the inverter can run down to is too high. A bigger unit's minimum acoustic ratings may be higher than a smaller unit.

I wouldn't install a heat pump until after the house has been built and you find out how much heating and cooling it could do with. Thermally broken aluminium, good double glazing and above code insulation means it'd be far warmer than the typical minimum spec new home.

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

It's appalling leaving the internal garage wall uninsulated is still occurring. The heat exits out of the garage door and freezes the whole house.

Wherever the heat pump's outdoors unit is I'd put laminate glass and acoustic gib adjacent to reduce noise ingress. It shouldn't go where it'll disturb neighbours.

timmmay

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#2321782 21-Sep-2019 06:59

I'd want to design a heat pump into the house, rather than put high wall units in later. Everything is easier before the house is fully finished. High wall units are loud and ugly. IMHO ducted / whole house heating is about comfort, livability, and to a lesser extent, style.

New Zealand houses have on average always been poor compared with houses overseas, especially in the areas of insulation and heating.

Paul1977

5047 posts

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#2323275 23-Sep-2019 11:34

Thanks guys,

Our current plan has all external walls insulated (including garage walls and ceiling), and also the internal wall between the garage and the rest of the house.

Why insulate other internal walls other than for sound proofing?

bfginger

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#2324180 24-Sep-2019 19:48

The internal walls around the hot water cylinder cupboard should be insulation.

There is no way I'd skimp on insulating the internal walls. Sound cuts through the house like a hot knife thought butter without it. You don't need pricier R2.8 in the internal walls, cheaper R2.2 should be nearly as good. I'd want to insulate the walls myself or keep an eye on it so there is no cross contamination of R2.2 into the exterior walls....

Goosey

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#2324201 24-Sep-2019 21:11

Paul1977:

Thanks guys,

Our current plan has all external walls insulated (including garage walls and ceiling), and also the internal wall between the garage and the rest of the house.

Why insulate other internal walls other than for sound proofing?

Trust us, you will thank us when you insulate internals walls.. even if its just for sound.

You will be amazed how much difference that makes to overall temperature comfort thus less energy going to be used for heat/cool.

May I reccomend earthwool to you!

Paul1977

5047 posts

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#2325352 26-Sep-2019 14:41

Might look at doing this ourselves before the Gib goes up then.

bfginger

1268 posts

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#2326223 27-Sep-2019 23:31

Insulate the water pipes too. Pipe insulation is inexpensive.

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Goosey

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#2326243 28-Sep-2019 07:50

Paul1977:

Might look at doing this ourselves before the Gib goes up then.

good plan... be quick though....after batts installed, usually within 2 days is the gib.

Paul1977

5047 posts

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#2327118 30-Sep-2019 14:23

bfginger: Insulate the water pipes too. Pipe insulation is inexpensive.

Do you bother with the cold, or just the hot?

bfginger

1268 posts

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#2327337 30-Sep-2019 21:44

People have had pipes freeze and burst in their ceiling cavity as far north as Auckland so in somewhere like rural Canterbury where it routinely drops below zero I would insulate cold pipes in the cavity. Greater ceiling insulation means the cavity will be colder in winter as less heat will leak into it.

Insulation should be placed behind cold pipes in the exterior walls so there is no air route from the cold to the pipe. Batts with a half cut and slid behind would work better than cut in half.

I think the idea is to maintain entry of some warmth from the house's interior and outside ground into the cold pipes while protecting from the colder outside air.

https://www.stuff.co.nz/timaru-herald/news/70511077/

https://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=11479835

Paul1977

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#2422616 17-Feb-2020 16:47

Just reviving my old thread.

I'm still undecided between the below 2 ducted systems:

Mitsubishi PEAD-125 - 12.5 kW rated cooling 14kW max cooling, 14kW rated heating 16kW max heating
Mitsubishi PEAD-140 - 14kW rated cooling, 15.3kW max cooling, 16kW rated heating 18kW max heating

The system will be in 6 zones with individual temperature sensors in each zone.

Whichever we choose will be integrated with a Lossnay LGH-50RVX-E heat recovery ventilation system.

We are having Excel Low E glass in the windows and thermally broken joinery, along with R2.8 batts in most external walls (R4.0 in one) and R6.0 batts in the ceiling.

The area to be heated is approx 130m2, with a total volume of about 335m3. This was calculated by including living, kitchen, bedrooms, hallways, but EXCLUDING bathrooms, toilets, laundry, cupboards etc.

The house is located in Christchurch. The installer says that the smaller 125 unit will be sufficient, but said that the 140 unit would not be too big. The price difference is negligible in the grand scheme.

I'm having a major case of "purchase paralysis". I don't want to go too small, but don't want the issues associated with going too big either!

Jase2985

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#2422624 17-Feb-2020 16:58

what issues would you face with an extras 2.5kw of cooling and 2kw of heating? its not a massive jump so i wouldnt worry about it, if it was 5+kw then yea maybe start worrying

there is only ~.7kw between their minimum cooling and heating.

Paul1977

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#2422633 17-Feb-2020 17:08

Jase2985:

what issues would you face with an extras 2.5kw of cooling and 2kw of heating? its not a massive jump so i wouldnt worry about it, if it was 5+kw then yea maybe start worrying

there is only ~.7kw between their minimum cooling and heating.

@Jase2985 That's a good point about the minimums being very close - I hadn't actually looked at those. Does that mean that you should expect a negligible difference in any stopping and starting between the 2 models?

Jase2985

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#2422635 17-Feb-2020 17:12

i would suspect so

bfginger

1268 posts

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#2424296 20-Feb-2020 00:01

Paul1977:

Just reviving my old thread.

I'm still undecided between the below 2 ducted systems:

Mitsubishi PEAD-125 - 12.5 kW rated cooling 14kW max cooling, 14kW rated heating 16kW max heating
Mitsubishi PEAD-140 - 14kW rated cooling, 15.3kW max cooling, 16kW rated heating 18kW max heating
The system will be in 6 zones with individual temperature sensors in each zone.

Whichever we choose will be integrated with a Lossnay LGH-50RVX-E heat recovery ventilation system.

We are having Excel Low E glass in the windows and thermally broken joinery, along with R2.8 batts in most external walls (R4.0 in one) and R6.0 batts in the ceiling.

The area to be heated is approx 130m2, with a total volume of about 335m3. This was calculated by including living, kitchen, bedrooms, hallways, but EXCLUDING bathrooms, toilets, laundry, cupboards etc.

The house is located in Christchurch. The installer says that the smaller 125 unit will be sufficient, but said that the 140 unit would not be too big. The price difference is negligible in the grand scheme.

I'm having a major case of "purchase paralysis". I don't want to go too small, but don't want the issues associated with going too big either!

One of the links is to the PEA and not the PEAD.

If you mean PEAD for both the most obvious difference is the 140 has a 4db higher minimum indoor acoustic rating. 40db is not quiet so I'd be concerned about how that'd impact on noise coming out of the ducts.

Your house is far better thermally than what heat pump calculations are usually made on.

Outdoor units for ducted heat pumps can be very loud so their not providing outdoor acoustic specifications would make upsizing a concern for that too.

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