Reverse cycle air con decisions

In theory, any home with less than a 10 Star NatHERS rating requires some energy input to keep the occupants comfortable.  This can be a significant cost. About 40% of the average Australian household energy bill goes towards heating and cooling.  The closer a house is to the 10 Star rating, the more it utilises passive heating and cooling methods such as north facing windows and thermal mass.  Our first home has a rating of 8.2 Stars so it probably can’t get by on passive design alone. After much thought, we selected a reverse cycle air conditioner to meet the need for active heating and cooling.

Dedicated readers of the blog will remember the Thermaskirt hydronic heating system was our alternative to air conditioning.  Lucky that we dropped the idea due to the higher cost and lack of a cooling option.  The Thermaskirt suppliers went broke recently.  Whew!  That’s a near miss.

Reverse cycle air conditioner (aka heat pump).

Reverse cycle, split system, heat pump, air conditioner,  … What are we talking about? Looking at retailer advertisements it seems that even the people selling these devices can be a bit confused with the terms.

Fortunately, Tim Forcey (Energy Adviser and ATA guru) put together a layman’s guide to understanding the technology.  Let’s start with the heat pump.

As Tim points out, a heat pump does exactly what the name says.  It pumps heat from a cold place to a hot place. A good example is the humble refrigerator.  It pumps heat from the cold box to the warmer air in the kitchen.

A reverse cycle air conditioner is a heat pump that can pump heat in two directions, either in or out of a room.  Older style air conditioners only worked in one direction by moving heat out of a room.

In summer, the reverse cycle air conditioner pumps heat from the room you want to keep cool and sends that energy outside where it’s warmer.  The term “split system” refers to the heat pump using two separate fan units. One fan unit sits inside the house where it cools the air passing through the box. Outside is another fan and coil unit which dumps the heat collected from inside the house.  A refrigerant fluid circulates between the two units and moves the energy from one place to another.

In winter, the reverse happens.  The heat pump moves energy from the cold air outside to warm the air inside a room.  Yes, strange as that may seem, there is heat in the air on a cold winter’s day.

Why bother with a heat pump?

A reverse cycle air conditioner is very, very good at its job.  Every one energy unit of electricity consumed by a heat pump produces up to six energy units of heating.  How good is that? Looked at another way, Tim says the pump produces up to five units of renewable energy.  Why renewable? The energy is coming from the atmosphere, not from burning fossil fuels.

What a minute, I hear you say.  The reverse cycle air conditioner does use electricity.  So, isn’t the machine consuming power from a coal fired power station to play funny little sustainability games with heating and cooling?

But, these days, renewable energy alternatives to fossil fuel based electricity are readily available.  It’s easy to select a GreenPower alternative and run the heat pump on solar, wind or hydro power.

 

The bottom line is that using a heat pump to meet heating and cooling requirements is a sustainable option.  And remember, with efficiency levels of 400+%, a split system air conditioner is more economical to operate than many other heating and cooling systems.  Having one device to do both jobs helps with the economics as well. Remember the old days of using an air conditioner for hot days and a completely separate gas-fired ducted heating system for cold days?

What size reverse cycle air conditioner to buy?

After deciding to use a heat pump, we had to think about what size to buy.

Visiting a store is a bewildering experience.  Clearly one size doesn’t fit all situations because these things come in a variety of sizes.  At the low end of the range, air conditioners are rated for 2.5 kW of cooling and 3.3 kW of heating.  Larger units produce 8.0 kW (cooling) and 9.0 kW (heating). Obviously the cost increases as the capacity rises.

What is the right size reverse cycle air conditioner for our 8.2 Star home?

We started by asking a few suppliers to see what they’d recommend.  The consensus for our 50 square metre kitchen and family room was the 7.1 kW unit.  Mmm, seems quite big.

However, they didn’t give any credit for passive heating and cooling, extra insulation and thermal mass.  NatHERS predicts our home needs less energy to keep occupants comfortable than a home that meets the minimum requirement of 6 Stars.  Therefore, the air conditioner size should be smaller. Shouldn’t it? It seems logical for it to be smaller.

I went looking for a design tool that takes sustainable house design features into account.  The Fairair website contains a tool for estimating air conditioner size based on inputs such as insulation and window location.  Fairair predicted 4.7 kW meets the cooling needs for our home. It’s smaller, but is that as low as we can go?

Ask people who have been there, done that.

Next, I asked someone who has practical experience in heating and cooling homes similar to ours. Who better to ask than TS Constructions? They build 8+ Star homes at The Cape.

TS Constructions don’t go larger than 3.5 kW for rooms in houses designed using passive heating and cooling principles.  This rule of thumb is based on experience gained over several years.

With the size sorted, we moved onto the next choice.  What brand?

Choosing the brand of air conditioner.

Let’s start by agreeing that everyone has an opinion about the “right brand” to buy.  Holden versus Ford anyone?

For us, the right brand is an air conditioner that meets our needs, as well as fitting within the budget.

We used Canstar Blue for guidance.  Their ratings are based on the collective wisdom of product users.  Aussie owners commented on a range of popular brands including Daikin, Panasonic, Fujitsu.  Each appliance is rated in categories such as reliability, quietness and value for money.

And the winner is … Panasonic.  This brand scored an impressive 5 stars in the Canstar Blue overall rating, just nudging out Daikin, Mitsubishi and Fujitsu on 4 stars.  All these brands would be good investments in a reverse cycle air conditioner, based on user experience. The thing that sealed the Panasonic for us was a local retailer’s sale price. Thank you to The Good Guys Bayswater for wheeling and dealing.

Heating and cooling in the bedrooms.

The air conditioner is located the open plan kitchen and family area on the northern side of the house. We aren’t installing air conditioners in the bedrooms. Instead, these rooms will rely on a combination of passive heating and cooling design features, as well as ceiling fans.

During summer nights, the rooms are cooled by air circulation once the outside temperature drops.  Warm air is vented through the clerestory windows high on the northern walls while cooler air is drawn through the south facing windows.  A ceiling fan helps to keep the air moving around the room.

One bedroom even has a natural evaporative cooling system.  A water feature outside the window cools air coming into the room.

In winter, we rely on sunlight coming through the north-facing clerestory windows to warm the rooms during the day. The insulated walls, floor and ceiling keep the warmth inside for nighttime.

Heating and cooling finalised.

So, there you have it.  That’s how we came to purchase a 3.5 kW (cooling) 4.3 kW (heating) Panasonic reverse cycle, split system air conditioner to install in the kitchen/family area of Unit 1.

• coolingenergy savingheatinglow running costssustainable design