I’m renovating – what should I do?

There are a lot of things you should do BEFORE considering adding any kind of hydronic heating – read the advice in this Government publication on Passive Solar Heating.  Chances are though, that you cannot correct a lot of the problems that should have been fixed at the design stage, so hydronic heating could be just what you need. We can install floor heating if you are willing to have a floor a bit higher than it was, or we can use radiators.

I am building a new house – is there anything I need to do?

Ensure your architect takes into account the principles of good solar design. Next, try to work out whether the rooms on the south side of the house might need additional heating.  Do a heat loss calculation (see next section)

Almost ready to start?  – check the slab design – does it have good insulation under? Most downwards heat loss thru a slab is towards the edge. If you install hydronic heating it makes sense to prevent that heat from escaping, so good edge and perimeter insulation is a must.  Two common methods are to use extruded polystyrene boards, or waffle pods.

  • Orientation
  • Insulation
  • Thermal Mass
  • Minimise downwards losses – Slab design insulation

How can I work out how much heating I will need?

An engineer can work this out for you. The method involves making calculations for each room, making guestimates about the roof and wall insulation, the orientation of the windows, the type of windows, the local climate data, the desired temperature range in the house and so on. Doing these heat loss calculations is a very good first step, and one that is often overlooked.  It takes time and effort to do them, and that translates to an extra cost which will add to the expense of the end result.

But the consequences of getting it wrong can mean that the house might be too cold, (or even overheat), or on the other hand, an overestimate of heating loads can lead to systems that cost more than they need to.

As a rule of thumb, for energy efficient homes you will require approx 600 watts of heat energy per 10m2 of room if the ceiling height is 2.4m, for every 300mm increase in ceiling height add another 100 watts of heat (each radiator has heat output in watts listed in the sizing tables) For non-energy efficient homes allow 1000 watts or 1Kw per 10m2 for 2.4 ceilings add another 100 watts every time the ceiling height increases by 300mm.

If you want to do these heat loss calculations yourself, you can use the attached worksheet, or give me a call and for a fee I will do them for you.

How many solar panels will I need?

This will depend on how big your house is, and the orientation of your roof. It also depends on what contribution towards the heating load you expect solar to make. We would recommend as a minimum starting point, to consider 3 panels of 30 tubes each, combined with 1,000 litres or more of heat storage. The article Solar Radiation and Positioning of Collectors  has useful information in Chapter 8 that’s relevant to this question.

What amount of heating can I expect? The average direct normal radiation in the middle of winter, in Canberra, is 4 kWh/m2/day ( source: NASA solar database website ) . 30 tubes (2400 x 1700 ) , 4 square metres, is exposed to 16 kWhr per day. With an efficiency of somewhat less than 50% (source – “Modern Hydronic Heating”), plus other losses, we can expect about 4 kWhr per day So a quick answer is that for each set of 30 tubes, on a sunny day in the middle of winter, you will get about 4kWh of heating per day. This would be the same as having a bar radiator left on for 4 hours. One can quickly see that even 90 tubes can only provide 24 kWh which will not be enough to make your house cosy at the coldest time of the year. If you have got inadequate insulation in the roof, the walls, OR underneath the floor, then spend your money on those items first and replace those leaky windows with double glazed windows. Honeycomb blinds can help to insulate the windows. After you have attended to these matters, your heating requirements will be a lot less.
So if your house is well designed, with few heat losses, 24kWh should be able to maintain the temperature, or at least take the chill off.

How big is your house – the bigger it is the more heating you might need.

How warm do you want the house to be? In Canberra houses need heating from April through to October, almost seven months. Wearing a jumper sounds like a sensible option for just a few of those coldest weeks when the sun is very low down in the sky.

In winter solar collectors work for 5-6 hours, collecting heat from 9am to 3pm. Storing that heat is key to a successful system – a concrete slab is a good place to store some of it, and a suitable sized insulated water tank is another good place. A tank will act as a buffer, especially for in-slab heating because the hot water from the panels is too hot to circulate in the slab directly.  Note that it is important that the temperature of the water in the pipes is not above 55 degrees to ensure that you don’t run the risk of cracking the slab. 35 is OK, anything much below that is not terribly useful.

Assuming solar collection of 1kW/square metre and 4 hours of strong sunlight, gives 1000 * 4 * 3600 = 14.4MJ per day. So you’ll need about 10 square metres of collection area to warm 1000l of water by 30C on an average day

The size of the tank needs to match the number of collectors. With 30 tubes, the tank would ideally need to be at least 1000 litres.  A larger tank will extend the heating effect over a longer time period. With a well built house that has high thermal mass the temperature should stay fairly constant, in such a case a smaller tank will suffice. If the house has timber floors and very little thermal mass larger swings can be expected, so make the tank bigger to spread the heating effect later into the evening.

Collection calculation:

Assuming solar collection of 1kW/m2 and 4 hours of strong sunlight, gives 1000 * 4 * 3600 = 14.4MJ per day. So you’ll need about 10 square metres of collection area to warm 1000L of water by 30C on an average day.

Usage calculation:

Assume your house has about 14kW of hydronic radiant panels. These will consume 126MJ in 126000000 / 14000 / 3600 = 2.5 hours. Maintaining the room temperature however won’t use this much energy.

Gas Hot Water or Electric Boosted?

A report produced for the Victorian Government Household Water Heater Energy Use, Running Costs and Emissions, Victoria has been instrumental at shaping opinon agaionst electric boosted systems.  Here are some flaws in their logic:

they have done nothing to maximise the contribution from solar using a timer. See “Typical Mistakes Your Installer is likely to make”.
they assume a staggering 25% loss from the tank. Thats extraordinarily high and unrealistic. Poor installation is often to blame for such big losses.
they say nothing in that report about the type of solar collector used. Its important because the more efficient evacuated tube collectors can provide a much greater solar contribution, minimising the need for boosting of any kind.
they forget to mention that electricity can be sourced from renewable energy whereas “natural ” gas cannot ( except for a vary small aount of biogas).
they forget to mention the added up front cost, the embedded energy that went into making the gas boiler, and the simplicity of an electric boosted system couple with the fact that maintenance is virtually nill.

Concrete Slabs – how much heat can they store?

The specific heat of concrete is about 0.9. The density of concrete is 2400kg/m3. So a slab 200m2 by 100mm thick would be 20m3 volume, or about 48 tonnes. To warm this slab by 5 degrees would require 20 * 2400000 * 0.9 * 5 = 216MJ.

So this is nearly twice as much as a 1000 litre tank of water warmed by 30 deg C (126MJ).

What happens when the sun doesn’t shine?

Unfortunately, no amount of solar collectors on the roof will be enough to get you through a cold winters spell unless you have a very well designed house.

But even on cloudy days, you will still get some solar gain when using evacuated tubes. Fortunately, cloudy weather is rare in our region. There are only two months in winter, where the number of sunshine hours on average is 5 hours. Either side of these 2 months, the sun shines for an extra hour each day.  There is some interesting climate data in this document – Average Temperatures in Canberra.

Average Sunlight Hours/ Day
January     9.1
February     8.4
March     7.5
April     6.9
May     5.5
June     4.6
July     5.1
August     6.1
September     7.5
October     8.0
November     8.9
December     9.0
A concrete slab retains heat for quite some time, so it is possible to go for a day or so without the house being uncomfortably cold.

We live in a society that demands the convenience of always-available and set-and-forget.  If you are prepared to buck this trend, just put on a jumper during the short periods when the sun doesn’t shine!

If that is not to your liking, then you have a number of options:

Use a gas boiler
Use an electric booster
Using an electric element has the advantage that it can be totally CO2 neutral when using Green Power (see your electricity retailer).  Gas is often quoted as being better than using electricity but this is comparing apples with oranges – ie one is not compelled to use electricity from coal-fired power stations these days!  Greenpower is a much better choice.

Using a heat pump is another option, but one that doesn’t work too well here in our region.  Air source heat pumps stop working efficiently when the outside air temperature drops below about 7 degC, and they stop working altogether when the temperature goes below zero – and these are the times you need the heat the most!

What System Possibilities are there

The system design depends on the objective.

Evacuated Tube Split System

For solar hot water supply to the taps in the house, an evacuated tube split system is a good choice. This type of system has a simple design. The panels are on the roof, the tank is at ground level (preferably inside). Cold water comes into a high-pressure tank at the bottom, a pump moves water from the tank through the solar panel during the day, and hot water comes from the top of the tank to the taps in the house. Aspects to consider include:

+ Efficient – no heat exchanger needed
+ No heavy tank on the roof
+ No glycol needed
+ Pressurised water has a high boiling temperature (>180C)
+ Not prone to frost
– Tank is expensive as it needs to withstand high pressure water supply
– Large tanks (> 400litres) are super expensive
– Pump needs to be stainless steel to avoid corrosion

Typical Mistakes Your Installer is likely to make:

  • Poor and unprotected insulation on pipes on the roof resulting in heavy losses
  • Poor insulation around the PRV ( pressure relief valve) and other pipe work on the tank
  • Poor insulation on the hot water supply to the fittings in the house (often none)
  • No timer installed – solar contibution can be maximised by using a timer set to come on late afternoon for one hour.
  • Failure to make owner aware that a separate line to the kitchen is possible. The line to taps in the bathroom etc must be regulated to a fairly low temperature to avoid scalding, but this is not the case for the kitchen. Dont blame your new water heater for lukewarm water in the kitchen unless you run a separate line!
  • Flat panel systems work differently (often using glycol). They are not the best option in our climate zone in my opinion.

A low pressure tank system is more suitable for hydronic heating. The tank is less expensive and can be made as large as you like or built to fit in a particular space. If a spot inside the house can be found, then heat losses from the tank are not a problem.

What size tank do I need – and why do I need one at all?

A solar panel is referred to in the hydronics industry as an “uncontrolled heat source”. If you connect panels directly to a loop through your slab, you will risk cracking the slab through heat stress. It is therefore very important to have a control mixer or thermostatic valve which ensures that the temperature of the water in the pipes through the slab is 40 (and definitely never higher than 55 degrees). So during the warm part of the day, you need somewhere to store the heat coming from the panels, which could be 70C or higher. 1000 litres of heat storage is a good starting point. So how much heat can such a tank store? The specific heat of water is 4.186 joule / gram / degree Celsius. Raising the temperature of a 1000 litre tank by 30C is 126MJ (35kWhr). (To be useful, the water in the tank needs to be 35 or higher)

Can I combine hot water and space heating?

Yes you can, and there are some potential savings to be made by doing so. But it is very important to understand that the potable water in the hot water circuit must NOT mix with the water in the hydronics circuit.

A heat exchanger must be used to ensure that the two water systems are kept separate.

Custom or DIY built tanks must have a lid.  A plumbing inspector must be able to see the copper heat exchanger in the tank and assure himself that watermarked pipe and fittings haved been used.

An approved combi-tank  is likely to be expensive.  One of the cheaper ones we know about is an Australian built tank from Smartheat.

Why use water – everyone else has forced-air?

Liquid is by its nature a much more effective heating or cooling method as the heat capacity of water is over 4,000 times that of air. Also, once the heat is transferred to the water, it can be handled more efficiently, warming you by radiation, which is a much more comfortable way to keep warm.

What’s the best orientation for the solar tubes??

The 21st of June is the winter solstice – the shortest day of the year. In Canberra at this time of the year the sun is low in the sky – at noon it is 32deg while 3 hours later it is only 18deg.

Space heating needs the best performance in winter when it is cold. Solar hot water heaters on the other hand try to optimise the performance over the whole year.  The discussion here is focused on how to get as much heat into your house in winter as possible.

In the southern part of Australia, because the sun has considerably less ability to heat in the winter than the summer, one way to improve winter performance is to tilt the collectors up more than the angle of latitude (by 10º). For example, in Canberra (35ºS,149°E) this would require an inclination of 45º to the horizontal. In practice this means you would need to sit your collectors up off the roof on a frame – an extra expense that may not be warranted. To quote Solar Radiation and Positioning of Collectors :” In Melbourne, for a collector facing north and a 200 litre per day hot water demand, increasing the tilt angle from 23º to 45º increases the annual solar fraction from 72% to 74%. The June/July solar fractions change from about 39% to 46%, while summer performance is largely unaffected.

So the bottom line is, in winter a north facing roof with “normal pitch” of 22 degrees,  is 7% worse than the ideal..

If you intend one day to use your collectors to provide solar air conditioning then some compromises are necessary, because in summer you will want to get the best performance you can ( ie the hottest water possible) to maximise the efficiency of the air conditioning unit. However since solar air conditioning is in its infancy at present, this is a bit academic.

If you are designing your roof principally for grid-connected PV, you may want to go with (latitude-10) to maximise production. See Photovoltaic Systems

Also see Principles of Good Solar Orientation

Which way is north?

It is surprisingly difficult to find true north. True north and magnetic north are not the same and in our region ( Canberra, Cooma, South Coast) it can vary by up to 12 degrees. A compass will point to magnetic north which can be used as an approximate guide only. Looking at the shadow at noon is reliable only if you use solar noon, which varies depending on the day of the year eg. on June 21st noon in Canberra is 12:05pm. See world clock astronomy.

If your house plans have a north indication you may very well think that this is true North, but sadly this is often not the case.

A GPS is a reliable way to find true north. Get your “Tom-Tom” to display the latitude and longitude. Hold it near where you will be installing the collectors and take note of the longitude. Walk to what you think is north from that spot, making sure the longitude displayed is the same. You will now be standing true north of the first position.

GoogleMaps is another way to figure out which way is North.

How is energy measured?

A heater rated at 1000 watts (1 kilowatt), operating for one hour uses one kilowatt hour (equivalent to 3,600 kilojoules) of energy. If you leave an old-fashioned 40W light-bulb switched on all the time, it would use one kilowatt-hour every day. The power of a small electric bar heater or electric fan heater is 1 kW (1000 watts) and if left on for 5 hours would use 5kWh.

Some electricity companies include graphs in their electricity bills showing energy consumption in kilowatt-hours per day. When sizing heaters it is common to see the units “BTU”, (British Thermal Unit). 1 Btu is the amount of heat required to raise the temperature of one pound of water by 1 degree Fahrenheit. Not a very useful unit in Australia! Other units are sometimes used, the conversions are:

1kWh = 3412 Btu

1kWh = 3.6MJ

1kWh = 859.8kcal

The heat capacity of water is a measure of how much energy is required to heat it up.
Waters heat capacity is 4200 J per litre per deg C.

Let’s put this info to use in an example of taking a bath.
The volume of bath-water is (approx) 50 cm × 15 cm × 150 cm = 110 litre. Let’s say the temperature of the bath is 50 degC and the water coming into the house is at 10degC.

So the energy required to heat up the water by 40 degC is 4200 J/litre/degC × 110 litre × 40degC  = 18MJ = 5 kWh (approx.).

So taking a bath uses about 5 kWh. For comparison, taking a shower (30 litres) uses about 1.4 kWh..

Government rebates?

Space heating does not currently attract any Government rebates at all. I am lobbying politicians to have this changed. However, a combined system may attract rebates.

What are STCs (RECs) ?
An STC is a certificate that is worth money. It is a measurement of renewable energy which can be traded for cash. One STCs can be awarded for a number of systems, including solar hot water and combined hotwater and heating systems.

Hydronic heating systems on their own do not attract RECs.

Wood heaters

Firewood is a renewable resource, and can be a very efficient energy source. If you live in the country, it is also cheap. But to avoid spending all your time cutting and organising firewood, you need to ensure that it burns properly. Slow combustion heaters are the worst and most inefficient. A wood gasification boiler, eg the Orligno 200 is super efficient, and because the heat is stored in a large buffer tank, one can run the boiler in the afternoon ahead of demand when its colder at night. Its an excellent heat source for hydronic heating. However they are expensive and not common in Australia.

If you are building, consider a high thermal mass heater. These too are efficient and will retain the heat for a long time.

Wood needs to be as dry as possible, so consider building a plant room where it can be stored out of the weather.

What zone am I in?

The Office of the Renewable Energy Regulator has divvied up Australia into different zones. This groups areas based on their climate with the following table showing what postcodes are in each zone: http://www.orer.gov.au/publications/pubs/register-postcode-zones-v1-1107.pdf.

Is this a good way to reduce my CO2 footprint?

Australians have the highest emissions per capita of any major economy not including the Arabian states. Australians pollute more than Americans, twice as much as people in the UK and four times more than the Chinese.  Source: US Energy Information Administration, Per Capita Carbon Dioxide Emissions from the Consumption of Energy.

Anything we can do to lessen emissions is a good thing. Here is a short list of ways to get there:

buy Green Power
stop flying on planes
sell the car
warm your house by utilising all that energy that falls uselessly on your roof.