Heat pump size choice

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timmmay

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#101197 26-Apr-2012 14:22

I'm choosing between two Fujitsu Nocria heat pumps for my kitchen/dining room, which is a second living area that we use occasionally:

AWTZ14: 6kw heat (max 9kw), 4.44 COP (4.44 units of heat per unit of electricity), $3000
AWTZ18: 6.7kw heat (max 9.7kw), 4.1 COP, $3350

Both are the same size, indoors and out, and either will heat my the area no problems. According to the sales guy the larger one will be better if we want to leave the hall door open and use it to supplement the lounge heat pump, which is a 7kw Daikin which heats the lounge and bedrooms. The Daikin is slightly undersized, it should really have been a 9-10kw model, but the firm who put it in underestimated the size required.

Smaller heat pumps are more efficient than large heat pumps, unless they're overworked, as the COP (Coefficient of Performance) above shows. I don't know how much difference the 6.0 or 6.7 will make.

We don't use the kitchen/lounge as a living area very often, though we may start to use it more this winter. In winter we'd probably have it come on for 20 minutes in the morning so we don't freeze getting breakfast, and the same before we come home. We're also doing up that area as a second living area, it's much brighter than the other living area.

Thoughts on which one I should get? I'm leaning toward the more efficient, slightly smaller, cheaper unit.

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#615487 26-Apr-2012 15:38

Personally having also been bitten by a firm we used the first time round underestimating the required size of our first heat pump I would go bigger then you think you will need every time. As you mention heat pumps do not work well if running near capacity. You have not mentioned where you live as that also has bearing on what you should get.
For us we finished up moving the original heat pump to the hallway where it covers the hallway and bedrooms and getting a unit a full half again bigger for the living area's.

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timmmay

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#615488 26-Apr-2012 15:42

I agree, undersizing by 50% will make a huge difference. A heat pump too small will be working full power all the time, where a large one would be ticking along at its most efficient setting. My lounge one is undersized by maybe 20-30%, it runs constantly on the coldest days, but the heating bill isn't too bad.

6.0kw vs 6.7kw (with 9.1kw and 9.7kw peaks respectively) isn't a big difference. As the installer said, 6.0 is fine if you want to heat this large room, or 6.7 if you want to help it heat the other area. I just don't see how 0.7kw makes any real difference.

I live in Johnsonville, Wellington. The past 5 years we haven't bothered to heat that area at all, we just didn't use it much in winter. I've also taken a fireplace out, sealed and insulated the roof, that's made a huge opositive difference to the temperature on cold days.

Edit - cost isn't really my main consideration here, $350 isn't really a problem. The main consideration is that the smaller heat pump is significantly more efficient, has a peak heating capacity almost the same as the larger one, and that we don't need the room super warm - just not super cold.

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#615489 26-Apr-2012 15:43

So tempted to write that size matters, but honestly it won't contribute much to the conversation.....

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#615490 26-Apr-2012 15:44

No wait, I do have an idea....

Can you move the lounge one to this new area and get a bigger one for the lounge?

timmmay

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#615502 26-Apr-2012 16:00

I asked about moving it, apparently it'd cost about $1500 to move the existing one to a new location - basically a new install fee, plus a removal fee. It's not practical.

Heat pump size does matter. As I understand it the bigger the outdoor unit the more effective the fan is and the more air moves through, making the heat exchanger work better. The more coils inside it the more heat it can exchange, but at the expense of efficiency, I guess that's because less "fresh" air hits each coil.

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#615738 26-Apr-2012 22:21

Keep in mind a larger heat pump has a higher minimum setting, i.e. a smaller heat pump can be set to a lower minimum.

The larger heat pumps come with a 7 day fully programmable timer, the smaller ones just a 24h (?) timer.

Apparently the Fujitsu is the most reliable, Panasonic is not bad, Hitachi used to be the best. This came from an installer that supports either.

I'm contemplating around 7kW for a new fully insulated concrete floor single level 4 bedroom house in Auckland (i.e. does not get that cold). But might go for a bit smaller. The aircon will be for the main living spaces, and then I want to run a heat transfer system in reverse so extracting stale air from bedrooms to the living space so the aircon air is drawn through the passage to the bedrooms.

In our old house we ran it the normal way and even insulated ducting would cool down too much when going through the ceiling space. Drawing the air through the passage means no heat loss in getting warm air to the bedrooms. So far however we do not need heating in the new house so waiting for Winter to see what size aircon we actually need.

Our old house had a 3kW through-wall window/box aircon and I was able to maintain the 70 sqm uninsulated house above 18 degrees except at the worst of winter/spring when it would freeze up from high humidity around 0 degrees outside temp. This would happen between midnight and sunrise.

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timmmay

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#615805 27-Apr-2012 06:35

The minimum isn't a big deal, it just stays on for less time. They come with exactly the same features, an extra 7 day timer is $250 or so on the Nocria units. I think I'm going to go for the smaller unit, the downsides are few and the efficiency is good.

If I was putting a system into a new house I'd go for a ducted system that combines heat recovery/exchange ventilation and a heat pump in the ceiling. They take free warm air from the ceiling cavity and stale air from the body of the house and use it to pre-warm fresh air coming in from outside, without throwing away a lot of energy. Put an outlet in each bedroom, lounge, and kitchen, you'll have an even warmer, drier house that's worth more as a result.

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#616258 27-Apr-2012 20:16

The smaller unit able to run at a lower minimum output (inverter speed control, not on/off control) also means lower noise. If cost is not such an issue I would install a couple small units instead of 1 large one.

Agree ducted would be ultimate. It does not get so hot here, just humid. I'm used to 35 degrees + in South Africa, but there it is dry so in the shade a breeze helps cool you down. Here 25 is getting hot and the humidity means you can do little about it. You need an aircon to drop the humidity.

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#616268 27-Apr-2012 20:36

timmmay: Thoughts on which one I should get? I'm leaning toward the more efficient, slightly smaller, cheaper unit.

timmmay

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#616276 27-Apr-2012 20:49

I got the slightly smaller one, it's still a pretty decent size. That makes two now, that should be enough :)

Niel

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#616305 27-Apr-2012 21:51

You guys missed that I'm also leaning towards smaller units (but not too small). So that is 3 votes.

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timmmay

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#616310 27-Apr-2012 21:57

Niel, more smaller heat pumps cost more, but if you then only have to heat part of your house that might end up cheaper and more efficient.

Skolink

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#616313 27-Apr-2012 22:03

Niel: In our old house we ran it the normal way and even insulated ducting would cool down too much when going through the ceiling space.

Did you ever measure it? I did, and was surprised at how small the temperature difference was. I still want to insulate it though.

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#616319 27-Apr-2012 22:12

timmmay: I'm choosing between two Fujitsu Nocria heat pumps for my kitchen/dining room, which is a second living area that we use occasionally:

AWTZ14: 6kw heat (max 9kw), 4.44 COP (4.44 units of heat per unit of electricity), $3000
AWTZ18: 6.7kw heat (max 9.7kw), 4.1 COP, $3350

Both are the same size, indoors and out, and either will heat my the area no problems. According to the sales guy the larger one will be better if we want to leave the hall door open and use it to supplement the lounge heat pump, which is a 7kw Daikin which heats the lounge and bedrooms. The Daikin is slightly undersized, it should really have been a 9-10kw model, but the firm who put it in underestimated the size required.

Smaller heat pumps are more efficient than large heat pumps, unless they're overworked, as the COP (Coefficient of Performance) above shows. I don't know how much difference the 6.0 or 6.7 will make.

We don't use the kitchen/lounge as a living area very often, though we may start to use it more this winter. In winter we'd probably have it come on for 20 minutes in the morning so we don't freeze getting breakfast, and the same before we come home. We're also doing up that area as a second living area, it's much brighter than the other living area.

Thoughts on which one I should get? I'm leaning toward the more efficient, slightly smaller, cheaper unit.

just got TWO 6.0kW nocria installed yesterday. now we have 3 heat pumps!!! (100sqm house!) this is because we bought the house which came with a 6 (.?) kW daikin right in the middle (kitchen dining) of an L shaped house ... has anyone with both pumps realised that the air coming out of the daikin is hotter than the fujitsu?!

so now we have a nocria at both ends (lounge, bedrooms hallway) of the house making a complete new environment ... but still don't understand the hotter daikin. we've turned the daikin off and will turn it back on during the bitter dunedin winter as apparently daikin works better ... well judging by the hotter air probably does, but got the nocria at $2400 installed vs $3800 quoted for daikin

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#616323 27-Apr-2012 22:14

oh ... go for the 6kW one as better COP!

oh and ... according to my installer the 6.7kW one should only cost $200 more, not $350

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