Heated Floor

The heated floor component is a low temperature radiant system and can be used to define heating elements that comprise water fed tubular systems embedded within the floor construction of a building.

 

Two types of heated floor are available in DesignBuilder HVAC:

 

 

Note one advantage of the EnergyPlus variable flow heated floor over the constant flow heated floor is that it is fully autosizable.

 

When using the Detailed HVAC activity option, heated floors will only control based on the radiant system controls defined for the component itself and not via the standard zone thermostat defined on the HVAC zone dialog. If the radiant system is serving a zone with forced air equipment, the radiant system will follow the priority order established by the zone thermostat but will still base its response on the controls defined by the user for the radiant system.

 

When using the Simple HVAC activity option, heated floors are controlled using the usual HVAC zone heating setpoint temperature controls.

 

Note: In zones in which heated floors are included, at least one floor construction must have a construction with internal source defined, which incorporates details of the embedded tubing system. See Internal Source under Constructions for further details.

 

Heated floor HVAC components are placed, edited and deleted at HVAC zone level. To edit the data associated with a heated floor, you first need to select it by moving the mouse cursor over it and then clicking the mouse button to select it. You can then access the edit dialog by right-clicking the mouse and selecting the Edit selected component option or alternatively, select the Edit selected component tool from the toolbar.

 

Tip: Heated floors can be connected to hot water loops fed by either boiler or GSHP systems. Note that when connected to GSHP systems heated floors can offer a very efficient system as the low delivery temperatures they require (typically 35°C) are well matched to the optimal operating conditions for GSHP systems.

Target Tab

When editing the attributes associated with a Heated floor component it is possible to apply the same changes to units in other zones in the same HVAC Zone group. To do this select the components on the Target tab of the edit dialog as required.

Troubleshooting Heated Floors

The DesignBuilder EnergyPlus heated floor model is able to realistically model the operation of warm floor systems and many of the issues that apply to real systems such as intermittency, floor mass, control, floor insulation, effect of different upper surface materials can be investigated using DesignBuilder. However heated floors can take a little bit of care to set up and to achieve good temperature control. If you are having trouble with this you may find the answer to the problem is the list below.

 

  1. Missing Internal Source Error report. Each zone with a heated floor added to its HVAC zone must include at least one floor surface (or floor sub-surface) having a construction with an internal source. If no such internal source surface is found, DesignBuilder will generate an error message to this effect before attempting to run the simulation.
  2. Unresponsive control. Heated floors in real buildings give much less responsive control than air or other radiant systems. This is because the high thermal mass of the floor causes heat to continue to be emitted even when the room thermostat has stopped calling for heating and the pump has stopped moving hot water through the embedded pipes. Overheating in such high mass heated floors can be a common problem on days where a high demand exists on a cold morning (for example) but then high solar (or other) gains in the day add to the uncontrolled heat continuing to be emitted from the floor.
  3. Throttling range. Less of a factor than point 2 is variable flow heated floors work using a throttling range to control flow of water through the embedded floor water pipes. This acts like a deadband and means that even without the thermal mass lag issues, there will be a temperature control range rather than a fixed zone temperature.
  4. Underheating can also be a common issue with heated floors. Likely causes are listed below.
  5. Large heat loss. A useful rule of thumb is that it is not usually possible to supply more than about 100 W/m2 of heating with heated floors. Typical maximum outputs are approximately 100 W/m2 for concrete, reducing to 70 W/m2 for timber floors and less for carpet and coverings with insulating properties. Zones having higher levels of heat loss will require supplementary heating to achieve comfortable conditions. This is true of real world systems and with DesignBuilder EnergyPlus heated floor models. The easiest way to check the heat loss against this rule is by using the Normalise display option which shows results per floor area.
  6. Intermittent heating. Running heated floors on an intermittent basis requires a higher design sizing factor to be set for the HVAC zone. This is because of the thermal mass of the floor construction. Consider a building unoccupied over a cold weekend which then needs to be heated to operating temperatures on Monday morning. With the default sizing factor of 1.25 the heating system will have been sized to achieve the heating setpoint under steady-state winter outdoor design condition plus a margin of 25%. However this 25% margin will not be adequate if the heating is to switched on at say 6am and expected to raise the zone temperature to comfortable levels in time for occupancy a few hours later. To deal with intermittent operation, either much higher design factors are required (e.g. 2) or the system should be run continuously (perhaps at a lower level during unoccupied periods) to avoid the need for a rapid warmup in the mornings. Of course if the heating is to be operated continuously the building fabric will need to be very well insulated to avoid waste.
  7. Incorrect position of the source. A common error with setting up the heated floor definition is to select the wrong position for the heated floor source (i.e. the hot water pipes). In a well designed system the pipes will normally be buried just below the first construction layer. If you position the tubes at the wrong side of the construction, then the wrong zone (or the ground or exterior) will be heated! A tool is provided on the Constructions dialog to make it easy to position the source just below the innermost layer.
  8. Conductive upper layer. Heated floors provide the most efficient and responsive control when the uppermost layer is low mass and conductive. While ceilings can be constructed using a thin metal layer between source and the room, this is rarely possible for heated floors which are required to provide a strong and comfortable surface to walk on as well as an efficient heat transfer path from heating pipes to the zone. Clearly, if the top layer is an (insulating) thick pile carpet then the heated floor will struggle to provide adequate heating to the room. More advice on floor coverings suitable for heated floors is provided below.
  9. Insulation. Without good insulation below the heated floor source, much of the heat will not find its way into the intended zone. Instead the heat will be either lost to the zone below in an uncontrolled way, to outside or to the ground. There will consequently be less heat available to heat the intended zone and underheating will occur.
  10. Sizing error (general HVAC modelling issue). It is important to consider heating and cooling sizing when setting up schedules. In particular, a common mistake is to include internal gains in heating sizing calculations. This has the effect of heating being undersized causing significant underheating (or no heating).
  11. When used in combination with a chilled ceiling in zone below, heated floors will cause EnergyPlus to generate an error. This is because EnergyPlus only allows one "source" object per surface and so a heated floor cannot be located in the same surface as a chilled ceiling. There are 2 possible workarounds to this.
    1. Export the DesignBuilder model to EnergyPlus and using the IDF Editor combine the heated floor and chilled ceiling into a single source component.
    2. Insert a very thin "dummy zone" between the heated floor above and the chilled ceiling below. Include half of the true floor mass in the upper surface (heated floor in upper zone) and the other half in the lower surface (chilled ceiling in lower zone).

 

Tip: To help diagnose some of the above problems and to check that maximum temperatures are compatible with the surface material (below) it can help to view hourly inside and outside surface temperatures and heat fluxes by selecting these output options: Surface heat transfer incl. solar (gives heat flux for floors) or Inside surface temperature (to check the surface temperature of the heated floor).

Coverings Suitable for Heated Floors

The advice in this section has been adapted from www.nu-heat.co.uk.

Stone and ceramic surfaces

From the thermal point of view, the best floor coverings for use with underfloor heating are usually hard surfaces such as stone and ceramic tile as they have the least thermal resistance allowing heat from the pipe array to transfer quickly to the surface of the stone/tile.

 

Timber flooring and underfloor heating

Engineered timber is the best of the wooden floor options for use with underfloor heating as its structural stability allows it to perform well with fluctuating temperatures. Solid hardwoods and soft woods also transfer heat well but care should be taken when specifying board width and thickness.

 

Laminates, vinyl and rubber

Laminates and vinyl floor coverings also perform well with underfloor heating. It is advisable to check the manufacturer’s recommended maximum floor surface temperature to ensure the product is suitable for use with UFH. Most manufacturers state an upper limit of 27˚C, which equates to approximately 75W/m2 of heat output, which is adequate in most situations. These products have low thermal mass so heat up and cool down quickly in comparison to stone and timber.

 

Materials to avoid

While not ideal from the efficiency point of view, carpet and underlay with a combined Tog value of up to 2.5 can be used effectively with underfloor heating. Thicker carpets act as an insulator and stop sufficient heat reaching the room.

 

 

You should bear in mind that some flooring manufacturers stipulate a maximum floor temperature for their product. Note also that EN 1264 states that underfloor heating should not operate at more than 29˚C.