There are two main benefits of installing your hydronic underfloor heating and cooling pipes on insulated base panels covered by a thin cement screed:
- It is much more responsive and you will begin to feel the effect of the heat through the underfloor coils in about 15 minutes.
- It significantly reduces heat loss through the bottom and sides of your slab, which can be as much as 40% of the heat load if the soil underneath is wet.
Other benefits include:
- Greater effectiveness at transferring heat through a cement screed with up to 100 watts per m2 of heat output compared to heat transfer of around 70 watts per m2 for in slab coils and hardwood timer floors.
- Operation at lower temperatures which makes it easier and hence more viable for heat pumps to be used as the main heat source.
Issues you need to be aware of include:
Poor compaction around and beneath the hot water pipes, resulting in inadequate consistency of the screed and potential for cracks. The screed should be rammed or rolled to ensure it is adequately compacted all through paricularly around and beneath the hydronic water pipes.
Sufficient time for the screed to dry out naturally. The drying time will depend on the screed chemistry, thickness and air temperature but will likely take between two to three months with air temperature of around 20 degrees C to dry out properly. Cement additives can be used to quicken the drying time. Other additives can improve the screed strength and overall soundness. Check with your supplier or installer on what is best for your project.
When you are ready to turn on your hydronic system, Siddons Solar Hydronics recommends that you start the hydronic water temperature at 20 deg C and bring it up by 2 degrees each day until you reach the desired water flow temperature of around 35 degrees C.
Hydronic Screed Design
Your hydronic screed design starts with your building plans, knowing where the walls, benches, pantry and various fixtures and fittings are located. You should then have drawings made of the layout of your hydronic circuits, ensuring each one is no more than 100m in length to ensure good water flow through the circuits.
The screed thickness should be between 50mm and 80mm over the heating pipes.If the screed has additives or polypropylene reinforcing fibre making it stronger, then a minimum of 50 mm screed can be used.
The screed may be either a dry screed or a self-levelling screed. A dry screed is commonly used in smaller areas, whereas a self-levelling screed is typically used in large commercial areas and is more expensive. The dry screed is typically mixed with fibre additives to give it greater strength and help avoid cracking. Other cement additives can be added to ensure faster drying time. Self-levelling screeds are made of calcium sulphate, sand, water and other chemicals and are poured or pumped in.
The screed should be mechanically keyed into your concrete subfloor using diamond grinding, shotblasting or other means to create a rough but clean surface for the screed to key into.
Temperature variation from the heating and cooling reverse cycle will cause expansion and contraction so the screed should be designed to accomodate this such as expansion joints or cuts into the screed.
You should wait for at least a week after your slab has been poured before laying the hydronic screed. Insulatation panels will firstly be laid over your slab and then the hydronic circuits will be laid onto the insulation panels.