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Hi,
So basically have a 6 star thermal efficiency rating for a new house design, (6 star was mandatory). Anyway as its only a holiday house installing a split system, one upstair one downstairs in main living areas...as ducted quote was $12000
.
I used an online calculator (
http://fairair.com.au/Calculator.Size.aspx) which suggested 7 KW cooling power capacity for the upstairs room... obviously however this doesn't take into account a 6 star energy rating..
Does anyone know how to adjust for 6 stars? I really don't trust ac installers etc not to oversize the unit ridiculously, other online calculators suggest 10KW for the room...
My lack of trust stems from past experiences, especially the fact that our 1980's home with 6m raked celings, way more glazing only average insulation, much bigger room with a large open stairwell is cooled comfortably by a 10 year old, 8.1 Kw system...
(Live in Adelaide)
I would suspect that the fair air calcs for an insulated surface would be based on insulation of R1.5. Typically the heat flow between a roof space and a room with R1.5 batts is 0.54watts/m2/degree C
So if you have insulation or say R4 in the ceiling the heat gain (and loss ) through the ceiling will be less than half. Typically the heat flow between a roof space and a room with R1.5 batts is 0.23watts/m2/degree C
Same will apply to walls.
I was told the basic rule is 1kW of cooling per Square.
1 square = 9.29m2
I was told the basic rule is 1kW of cooling per Square.
1 square = 9.29m2
I followed the guide on the link below which is a little more detailed but funnily enough it came out about the same.
http://www.energyrating.gov.au/acl.htmlI think that A/C people generally overestimate. It is safer for them to do that than underestimate. I put in a couple of splits in my last place with total power way under what was recommended and they were fine - cooled the whole house even into the bedrooms despite advice that we would need seperate ones there. I think the double story next to us that shaded most of our western wall helped. In my current place the A/C is overpowered and is always overcooling/overheating and creates drafts.
In regards to 6 star ratings, I believe that is more a checklist of features rather than a real calculation of energy efficiency so its really guesswork as to how much to reduce the size requirement.
I'm in the same boat as you with the house I am now building. Using the above calculation the largest Daikin domestic system would only cater for about 2/3rd of my fairly largish home. However, given 6+ star energy rating, a relatively moderate Brisbane climate and 8 zone controller (no need to have on all through the house at all times) I think it will suffice.
For our house they had to run it through the software and make some minor modifications to achieve the six star, mainly double glazing to the upstairs living areas of both houses.
The a/c people have quoted for a 7.1kw split system to do the main room upstairs, what i'm struggling with is the fact that this room will be about 70sqm with really good insulation, double glazed windows etc yet at home have a 100sqm room, with large open stairwell, triple the glazing area with 5m raked ceilings topped with skylights...in 1980's construction methods, so not brilliant insulation, poor thermal efficiency glass etc...and yet its cooled and heated comfortably by a single 8.1kw split unit...
To accurately determine the heat load of such a place would really require a full extraneous heat load - not just a generic calculation.
In relation to a 6 star rating the best you can do is find the most efficient A/C in the capacity required according to a "full extraneous heat load calculation" The higher the EER / COP rating (Energy Efficiency Rating / Co-Efficient of Performance) on both heating and cooling the more efficient the unit is.
To determine the 6 star rating the
whole home really needs to be audited - you will never find a 6 star A/C beyond maybe 2.0 - 2.5kW. Your builder should be able to run this report.
Good luck
It is also difficult to do proper calculations because occupant behaviours often dictate what size unit is required e.g. thermostat setting, no. of hours heating/cooling during the day, whether they open curtains to let the sun in or open windows to let the breeze in.
50% of costs is related to this.
IMHO.......
Get a big one !!!
With the technologies they have today "bigger" ones do not need to run at full power unless they need to run "at full power".
In my experience (three ducted systems, numerous splits) the sales dudes UNDER quote, IMHO to get the lowest price and hence the job.
We had one quote (I'm the dude who DOES read the fine print) where it actually said that "This system may not perform satisfactorily on days of extreme weather conditions".
HELLO.... That's when I want it to perform, and perform darn well !!!
My current philosophy is to get the "average" system recommendation and then get the next size up
YMMV !!!
P_D
We have a passive solar design with R 3.5 ceiling insulation R 2.5 wall, reflective foil in all and double glazing. Winter temps get down to around 2-5 degrees. We have a Daikin Inverter AC that was sized to the standard 0.15KW/m2. We tend to zone at the most half of the house.
Within 30min the thermostat setting is reached and then 60-90 min later the AC will turn off because the inverter has reached the lower limit for operation. In other words, it begins working like a switching non-inverter. Clearly the AC is oversized for our application.
If you compare COP charts from the Daikin range non-inverters use about 10% more energy than inverters.
Actron is the only manufacturer I know which can run at really low outputs.
Within 30min the thermostat setting is reached and then 60-90 min later the AC will turn off because the inverter has reached the lower limit for operation. In other words, it begins working like a switching non-inverter. Clearly the AC is oversized for our application.
We have two Daikin inverters in our house, and they both drop to low levels, and then turn on and off. Neither of them is oversized, as this happens even if it's very cold outside and they have to work hard when they turn on again. I think it's a change in the way Daikin build inverters now so they don't run down to really low settings. I think it may have to do with people feeling a draft when they are only going slowly, so not very much heat comes out.
Because of this we are going to panel heaters for the current build. No drafts, no noise. My only worry is they won't heat the rooms up fast enough from a cold start, but we'll just have to see.
Charlie
Neither of them is oversized, as this happens even if it's very cold outside and they have to work hard when they turn on again.
Sizing is based on the steady state heat loss of the building when it has reached the target temperature. It isn't related to how quickly it takes to be heated up from cold.
Neither of them is oversized, as this happens even if it's very cold outside and they have to work hard when they turn on again.
Sizing is based on the steady state heat loss of the building when it has reached the target temperature. It isn't related to how quickly it takes to be heated up from cold.
I don't think I was clear enough then.
What I meant was when they turn on again as in turning on and off by themselves to maintain the temperature. Not bringing the temperature up from cold.
The house is having R4.1 in the ceiling and R3.0 in the walls...not sure if its getting foil though...the builder is higher end so expect it will have foil.
Decided to go with the 7.1 KW system upstairs for the moment, they've recommended Daikin, i know they're suppose to be good but they don't seem to have all the features on the indoor head unit the panasonics/mitsubishi's have...have two split mitsubishi's over ten years old with no problems so i'm assuming reliability with these things isn't really a problem.
There are two rooms on two levels to be air-conditioned, seen that Mitsubishi make systems capable of having multiple heads put on, are these very good? Or better off with two completely separate systems...
The house will only really be used for maybe 4 weeks of the year but that usually includes some of the hotter periods just after christmas or again when Adelaide has a heat wave...that and the opposite in middle of winter...so the true extremes really...