Heat pump in service

Standing by the side of the house, I found myself wondering, “Which heat pump belongs to the hot water system?”  Intellectually I knew a heat pump hot water system was a different beast to the standard electric or gas unit. However, seeing almost identical heat pumps for the hot water and reverse cycle air conditioner side by side was still a surprise.

Sanden heat pump for hot water.

Our plumber recommended the Sanden Eco heat pump for the hot water service.  He explained this unit has a solid reputation in the trade and is consistent with our sustainable design principles.

The hot water storage tank arrived on site and the crew decided to keep it in a safe place until installation.  Libby jokingly asked if they’d misread the plans and put the tank a bit too close to the kitchen hot water tap.

It might look a bit strange having what looks like an air conditioner unit running a hot water system but there are good reasons for this arrangement.  The heat pump uses a small amount of electrical energy to pump heat from the surrounding air into the 300 litre storage tank. It’s remarkably efficient, using 1 kW of electricity to add 4.5 kW of heat energy to the water in the tank.  It won’t be long before our solar panels are installed. When the solar power system is operating, the Sun will keep our unit humming, rather than power from the grid. Humming is the keyword as the heat pump quietly goes about its business.

Using carbon dioxide as the carrier gas is another sustainability feature of the Sanden system.   There are no nasty refrigerant gases to affect the ozone layer. Thinking about it, we’re also making a contribution to carbon capture.  A small amount of carbon dioxide was taken out of the atmosphere and stored in the heat transfer circulation system.

Sanden has a useful timer feature.

I like to read instruction manuals (cue rolling of eyes amongst those who see the manual as a last resort when things go wrong).  The manufacturers are the experts and they put time and effort into explaining the best way to operate their appliances.

In the Sanden manual, I found a section about setting a limit on the operating time.  There’s a timer which tells the unit when it’s allowed to operate. For now, I’ve told the Sanden to heat water using off-peak power between 11pm and 7am.  We don’t use a lot of hot water out of the 300 litre tank, so this limited operating time hasn’t affected the supply. In fact, looking at my Powershop app, I can see the heat pump doesn’t operate every night.

After the solar panels arrive, I’ll change the operating time to the middle of the day.  Think of the hot water tank as a liquid “battery”. Energy from the solar power system is stored in the water tank during the day.  At night or early in the morning, we take hot water out of the tank and the “battery” is recharged when the Sun shines.

Utilities area tucked away.

The hot water system is one of three utility services set up on the western wall of the house.

A 2000 litre tank holds rainwater from the roof to use in the toilets and laundry.  Apart from reducing our demand for drinking quality water, the tank also acts as a stormwater retention device. This prevents a sudden surge of water hitting the stormwater drains during periods of heavy rain.

The third utility is the heating and cooling system.

Panasonic heat pump is on standby.

The second heat pump drives our Panasonic Aero 3.5kW reverse cycle air conditioner.  Energy efficiency is similar to the hot water system with 0.8 kW of electricity required for a 3.5 kW cooling capacity (0.94 kW for 4.3 kW of heating).

There have been a few hot days recently, so why not jump at the chance to run the Panasonic and see what it can do?  The answer is the 8.2 Star energy rating for our house. At this Star rating, very little mechanical heating or cooling is required to maintain a comfortable interior temperature.  Using passive design principles boosted the home’s performance well above the minimum 6 Star rating.

Passive cooling features worked well.

In summer, eaves shade the home’s interior to prevent direct sunlight hitting the the thermal mass of the concrete floor and Timbercrete walls in the family/kitchen area.  Keeping our thermal mass cool ensures the air inside the house stays cool.

The trick is then doing everything possible to stop the higher temperature outside the house affecting the cool air inside.  A well sealed building envelope stops hot air getting in around doors and windows. This includes a two-door airlock system at the front entrance. High performance insulation stops heat travelling through the roof or walls to gradually warm the interior. Finally, at night, the thin layer of cladding (reverse brick veneer construction) quickly sheds the heat accumulated during the day when the air temperature drops.  Rapid cooling of the cladding stops it gradually overloading the wall insulation after several hot days.

When the weather heated up, I was more interested in testing these passive cooling features than switching on the air conditioner.

At the end of the first day, the temperature outside peaked at almost 33 degrees with the interior rising to just under 24 degrees.  That night, the air temperature stayed above 24 degrees so we didn’t open any windows.

Two days later, the outside temperatures reached 35 degrees but the indoor temperature only rose to 26 degrees.  This temperature felt comfortable as we were dressed for summer. Standing in bare feet on the cool concrete slab helped to feel cool.  Running the bedroom ceiling fan for an hour or so at bedtime prevented any problems with getting off to sleep.

When the cool change arrived, we opened windows for natural circulation and flushed out the warm air.  Overnight, the interior temperature dropped back to 21 degrees.

Connecting to the environment.

Making best use of the passive heating and cooling features does take a bit of thought.  However, I think that is just being more aware of what’s happening in the environment around us.

Living in a sustainable home is an engaging pastime because active participants make the most of passive design.

After a hot day, we check the predicted overnight temperature to see if it’s going to fall below the current indoor temperature.  If that’s unlikely, there’s no point opening windows because hot air will replace the cooler air held inside our home. If a cool change is coming through, that’s the time to naturally ventilate the entire house.  The windows are closed again first thing in the morning so the interior temperature is at a minimum, ready for another hot day.

Next winter gives us the opportunity to learn how to use the passive heating features.

Happy holidays.

Moving into our first unit is a fantastic Christmas present for Libby and me after a busy and productive 2018.

We hope everyone has a wonderful Christmas and wish you all the best for the New Year.

• coolingheat pumpsustainable design