Hydronic Underfloor heating, also known as In Slab Heating is the most efficient and effective source of central heating. At Sunworks Eco Plumbing we are experts at helping you choose the best underfloor heating system for your needs and budget.
Hydronic underfloor heating is rapidly becoming mainstream in Australia.
The practise of embedding water pipes in the slab is well known and proven, however the methods of heating the water are varied, and there are several ‘boiler’ options.
With concern for the environment, the industry is experimenting with Solar, and heat‐pump technologies including Geothermal, Water Sourced, and Air Sourced.
Solar (direct radiation from the sun) and underfloor hydronic heating.
Underfloor hydronic heating uses lower temperature water i.e 40 degrees compared to radiators i.e 75 degrees making solar a good choice for underfloor hydronic heating, but the problem is when you need the most energy for underfloor hydronic heating is when the direct sunlight provides the least available energy – in the middle of winter and overnight. It is possible to harness solar heat energy during the day and store it in large tanks, but this system is very costly and the energy is soon depleted.
All solar hot water systems have a conventional backup heat source such as electric element, gas boiler or wood fired boiler and in hydronics situations it is usually this back‐up system that ends up doing most of the work in mid winter.
Over the entire heating season, these ‘backup’ systems will typically use more energy than a heat pump. The amount of energy saved by using a bank of high‐efficiency evacuated tubes to boost a heat pump hydronics system – will never return an energy saving over the life of the system, unless the summer energy gain can be FULLY utilised for a secondary application such as pool heating.
Hydronic Heat‐Pumps
The Heat‐Pump’s job is to first collect any available ‘free’ heat energy, then use conventional energy to push that collected heat energy to a useable temperature for the application. The medium used for the collection
of available energy is refrigerant, it works well because the ‘free’ heat‐energy absorption process begins at extremely low temperatures, typically around minus 30C. Once pre‐warmed with the ‘free’ energy, the refrigerant is then compressed which raises the temperature. The hot refrigerant then passes through a water heat‐exchanger releasing its heat into the water.
‘Free’ heat Systems:
Geothermal : Below ground, the temperature doesn’t fluctuate much and it gets warmer the deeper you go. Typically, plastic pipes are buried underground either horizontally or vertically. A water/anti‐freeze mixture is pumped through the pipes into which heat is absorbed from the ground heat. The warmed mixture goes through a heat‐exchanger where it releases its heat into refrigerant, thereby cooling the mixture which is then recirculated back through the underground pipes. The now pre‐warmed refrigerant is then compressed as described above. The Geothermal heat‐pump itself is typically about the same monetary cost as an airsourced heat pump, but the extra lifecycle‐costs of burying the pipes actually negates the slight increase in
operating efficiency.
Water‐Sourced: If available a suitable water source can be used to extract ‘free’ heat energy. A large dam, running creek etc. may have a water temperature which is consistently warmer than the average air temperature. The process is similar to the geothermal system – pipes laid in the water rather than the ground. Air‐Sourced: Air‐Sourced hydronic heat‐pumps offer the best overall compromise between efficiency/operating costs3, and lifecycle‐costs1. Air‐sourced heat‐pumps are fully self‐contained, and connected only to the hydronic pipework. They have large fans to force high volumes of air over large singlepass Fin Coil Evaporators containing the very cold refrigerant, to absorb ‘free’ heat energy directly from the ambient air.
Which heat-pump system is best ?
If available, direct access to a lake or river using a water sourced heat pump, may provide a slight advantage over an air sourced heat pump. Must take into account the embedded energy and cost of trenching etc. ie. If the water source is 100m away from the hydronics system then the advantage may be lost.
Although a Geothermal system would seem to be more efficient, it will never save enough in lower operating costs, when compared to the lifecycle cost of an air-sourced system:
WORKED EXAMPLE:
Air-Sourced C.O.P.2 is around 2.5 at 0oC ambient, rising to about 3 at 7.5oC ambient, and 3.5 at 15oC ambient.
Lets assume a typical domestic installation:
The installation of floorcoils etc, and the cost of a geothermal heat-pump will be roughly the same as for airsourced heat-pump, the big difference is the cost of geothermal groundworks.
Air-Sourced system installed cost $30k, running cost approx. $5/day average. ($0.02c/sqm/day) Geothermal system installed cost $50k, (about $20k for groundworks) Even if Geothermal were 50% better efficiency (which it’s not!)
120days per year operating cost savings of 50% = $2.50 x 120 = $300/year
$20,000/$300 = 67years payback time . . . . .
(alternatively, put the extra $20k in the bank and earn the compound interest, and that 67 years payback will look more like 100yrs . . )
So ,for Australian climate we would recommend the air‐sourced systems3.
NOTES:
1 Lifecycle costs include the embedded energy, the energy to source, process, distribute all components of the system, as well as the financial costs.
2 The Coefficient of Performance (COP) is simply a way of measuring the efficiency of something. Usually expressed as a percentage, it states the amount of energy consumed compared to the effectproduced. A COP of 2.5 means that for every 1kw of electricity consumed, 2.5kw of heat energy isproduced.
3 Some brands of hydronic heat‐pumps are fitted with electric element back‐up . . . these are not designed for Australian conditions and should not be considered for use here. When the backup element operates the systems overall efficiency (COP) crashes, and running costs increase significantly. (check before you buy!)
This article was reproduced with the permission of Skyline Energy