Another heat pump question

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Aitchy

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#306168 2-Jul-2023 16:10

I’ve had a browse through several other posts on the forum and see there are different views on multi-split vs single heatpumps vs ducted, so I’m hoping for some sage advice on my particular situation. We have a small 1950s home in Chch, currently heated by a heat pump in the open plan lounge/dining kitchen, with nobo panel heaters in the bedrooms. Our real issue is in summer, with norwest days in Chch where it can get to 31 degrees and make sleeping impossible. Our daughter has a medical condition that makes a good night’s sleep essential, so we’re looking at options for cooling. We currently have ceiling fans but they don’t really do what we need. I was initially thinking a ducted system, but then thought perhaps that was overkill, price wise, so thought a 3 room multi split might work just for the bedrooms, so long as we could tuck the outside unit around the side of the house. But then I read some of the posts on here about how multi-split systems aren’t great for various reasons, so I’m scratching my head. Any suggestions? House is double glazed and insulated.

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#3098722 2-Jul-2023 16:59

multi system is not to bad with bedrooms. thats because they are often similar in size and generally used at the same time.

eg you may have 2kw in 3 bedrooms. outdoors unit may be 5kw in size. at worse its a 5kw compressor with a 2kw head that will short cycle a bit. if it had a 5kw head for the lounge connected as well, it would be a 10kw compressor with a 2kw so the pump will short cycle a lot to run such a small head.

the other catch will be getting the sizing right. there is some online calcs around. real common for installers to oversize (would you like fries with that?).

timmmay

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#3098724 2-Jul-2023 17:27

Not everyone needs a ducted system, my wife doesn't like ours much, but that's partly because she didn't like the installers. The simplicity of individual units or multi-split can have advantages, such as not requiring returns and having each room however you want it.

Don't write off the option for a small unit for each room, if you have the space, those things can be super efficient. I have a Daikin 2.5kw system for my office, which is 2.5x4m or so, similar to a bedroom. The room has two windows, double glazed, and an external door. If it was 12 degrees in the office at 6am when the heat pump goes on, and the outside temperature is maybe 10 - 12 degrees all day, and I'm in here from 7am to 4pm, it uses about 2 - 3 units of power to keep the office at 24 degrees. It's really efficient.

Ducted is way more hassle to install, to tweak to how you like, and to manage to be how you want things. If I was doing another house now I'd think hard about what I wanted. Main advantage of ducted is they're not hung from the wall, and they're quiet, though small highwall units are also quite quiet.

Suggest you get an installer out to do a quote and make a recommendation.

neb

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#3098728 2-Jul-2023 17:52

The main arguments against a multi split, at least that a few installers mentioned to me, is that (apparently) people want to both heat and cool at the same time and a multi split can't do that. Never understood why you'd want to heat one room and cool the one next to it (in summer you cool, in winter you heat, that's it), so the multi split seemed a no-brainer.

rscole86

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#3098731 2-Jul-2023 18:01

We've just gone for a multi split, three bedroom/office.

Primarily as it gives us a single outdoor unit and we generally want the same thing at the same time across all rooms.

We're connecting 2x2kW and 1x2.5kW to a 5.4kW outdoor, Mitsubishi Electric kit.
2/3 are double glazed, two are currently bedrooms and one an office, so they'll not always be used concurrently. Even if all three were bedrooms then the 5.4kW outdoor should be fine, unless we push the units to their limit.

tweake

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#3098738 2-Jul-2023 18:26

neb: The main arguments against a multi split, at least that a few installers mentioned to me, is that (apparently) people want to both heat and cool at the same time and a multi split can't do that. Never understood why you'd want to heat one room and cool the one next to it (in summer you cool, in winter you heat, that's it), so the multi split seemed a no-brainer.

its because nz housing is so crap. a lot of ideals that have been proven incorrect that still persist today, so you get terrible design. so you often get one side getting hot in the sun and the other side cold.

tweake

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#3098739 2-Jul-2023 18:31

rscole86: ......then the 5.4kW outdoor should be fine, unless we push the units to their limit.

they do get pushed to their limits, at least with how much kiwis tend to run the home, but thats not a bad thing. its normal for them to run at 100%, they only throttle back (assuming variable unit) when almost up to temp. plus they should be sized undersized, so a few times a year they will run 100% all the time. its not going to hurt it.

timmmay

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#3098752 2-Jul-2023 19:18

neb: The main arguments against a multi split, at least that a few installers mentioned to me, is that (apparently) people want to both heat and cool at the same time and a multi split can't do that. Never understood why you'd want to heat one room and cool the one next to it (in summer you cool, in winter you heat, that's it), so the multi split seemed a no-brainer.

Our kitchen / dining is west facing, one bedroom is north facing, one bedroom is east facing, one bedroom is south facing. The way it ends up in spring / autumn we often need to cool the north facing bedroom / kitchen but heat the south and east facing bedrooms. That's why our ducted unit doesn't do the whole house, separate unit for the kitchen. It's still not ideal, some times of the year we'll have the ducted unit cooling the north facing bedroom, then switch to heating the lounge and south facing bedroom after dark.

We probably should have gone for a different setup than what we have, really, but what we have works ok.

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Handle9

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#3098753 2-Jul-2023 19:18

neb: The main arguments against a multi split, at least that a few installers mentioned to me, is that (apparently) people want to both heat and cool at the same time and a multi split can't do that. Never understood why you'd want to heat one room and cool the one next to it (in summer you cool, in winter you heat, that's it), so the multi split seemed a no-brainer.

It’s an issue in the shoulder seasons. In the middle of summer you’re in full cooling, in mid winter you’re full heating and it doesn’t matter. It’s just the amount of load that matters, not the mode.

During autumn and spring you can get days with high solar gain but moderate overall temperatures which means that different rooms have different loads.

It’s more of an issue in office spaces which can have much higher loads due to lots of people and computers per square meter compared to a domestic installation.

neb

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#3098756 2-Jul-2023 19:27

timmmay:
Our kitchen / dining is west facing, one bedroom is north facing, one bedroom is east facing, one bedroom is south facing. The way it ends up in spring / autumn we often need to cool the north facing bedroom / kitchen but heat the south and east facing bedrooms.

In that case wouldn't a transfer system to move hot/cold air to the cold/hot location be a better option?

timmmay

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#3098763 2-Jul-2023 19:36

neb:
timmmay:

Our kitchen / dining is west facing, one bedroom is north facing, one bedroom is east facing, one bedroom is south facing. The way it ends up in spring / autumn we often need to cool the north facing bedroom / kitchen but heat the south and east facing bedrooms.

In that case wouldn't a transfer system to move hot/cold air to the cold/hot location be a better option?

It might work to take the chill off the cold room, but the rooms relative temperatures change with the season, so the flexibility of air conditioning is hard to beat. I also suspect that you have to move a fairly high volume of air to make heat transfer worthwhile.

Handle9

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#3098767 2-Jul-2023 19:39

neb:
timmmay:
Our kitchen / dining is west facing, one bedroom is north facing, one bedroom is east facing, one bedroom is south facing. The way it ends up in spring / autumn we often need to cool the north facing bedroom / kitchen but heat the south and east facing bedrooms.

In that case wouldn't a transfer system to move hot/cold air to the cold/hot location be a better option?

Heat transfer doesn’t work well with conditioned air. You need really high volumes to get any real effect.

Your off coil temperature on an AC is typically 12/35 degrees so to get the same amount of energy in the space you use much lower volumes. Similarly with heat transfer kits you are typically moving very hot air from a fire rather than conditioned mixed air.

timmmay

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#3098771 2-Jul-2023 20:08

Yeah that matches my experience. Moving air around with the ducted system where the differential is a few degrees doesn't work at all.

Aitchy

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#3099057 3-Jul-2023 14:03

rscole86: We've just gone for a multi split, three bedroom/office.

Primarily as it gives us a single outdoor unit and we generally want the same thing at the same time across all rooms.

We're connecting 2x2kW and 1x2.5kW to a 5.4kW outdoor, Mitsubishi Electric kit.
2/3 are double glazed, two are currently bedrooms and one an office, so they'll not always be used concurrently. Even if all three were bedrooms then the 5.4kW outdoor should be fine, unless we push the units to their limit.

Can I ask how much, ballpark?

rscole86

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#3099100 3-Jul-2023 15:36

Approx $9.5k for supply and install, including electrician. Noting that my installer does not include electrical.

timmmay

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#3099101 3-Jul-2023 15:41

rscole86:

We're connecting 2x2kW and 1x2.5kW to a 5.4kW outdoor, Mitsubishi Electric kit.
2/3 are double glazed, two are currently bedrooms and one an office, so they'll not always be used concurrently. Even if all three were bedrooms then the 5.4kW outdoor should be fine, unless we push the units to their limit.

Before our ducted system went in we had 1.2kw oil heaters in each bedroom. Once the rooms were up to heat we found the oil heaters ran about 1/4 of the time, so in effect they used about 300w of heating.

Sometimes the minimum output of a heat pump is more important than a maximum.

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