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Underfloor Heating Project Example - New House Build

There are commercial companies that will design, supply and even install your wet underfloor heating but that option is expensive. Installing it yourself isn’t very difficult and any competent do-it-yourselfer, with some building and plumbing know-how and the help of this website should have no problem installing wet underfloor heating. Buying your equipment from ufh1 will save you money. Install it your self and your savings will be hundreds of pounds. You might be considering electric under floor heating. Heating systems running off electricity are generally easier and less expensive to install however the running costs will be considerable higher when compared to a wet underfloor heating system connected to a condensing boiler or heat pump. This web site is designed to help you do design and install your own wet underfloor heating system. If you feel we have missed anything or have any suggestions please let us know using the email address on the website.

Design and calculations of wet Underfloor Heating

The design and calculations for a wet underfloor heating system in a solid floor should be conducted in accordance with BS EN 1264 and the details given on these pages are based upon this standard. The design of an underfloor heating system in a new build is a straightforward process consisting of 5 main steps:

• Calculate heat losses and amount of heat required for each room or zone
• Determine water flow temperature and pipe spacing
• Determine manifold location
• Calculate number of circuits required
• Plan pipe layout

Calculating the capacity of an under floor heating system

Calculating the output of a radiant floor heating system correctly involves:

• Setting the temperature of the water flowing through the pipes
• Choosing the optimal pipe spacing in each zone, and
• Determining the number of circuits necessary to heat the rooms

It is necessary to obtain the following information about each of the zones to be heated:

• Maximum hourly losses or thermal load
• Heat output for under floor heating
• Floor Finishes and Coverings
• Floor surface temperatures when fitting wet underfloor heating
• Peripheral areas
• Type of flooring screeds or timber to be laid and its thickness
• Controls
• Heat sources
• Manifold Position & Circuit Lengths
• UFH1 Technical Data

Some of these are described below.

Maximum hourly losses or thermal load of Wet Underfloor Heating

These should be calculated, just as you would for any other heating system, using preferably a proprietary software package or data taken from SAP rating our other reliable source. For an underfloor heating system the criteria is slightly different and this can result I lower heat losses than would be calculated for a radiator system. As more than 50% of the floor heat emitted is radiant transfer this heats bodies, objects and the building fabric without heating the air. This feature of under floor heating increases the rooms mean radiant temperature (MRT) which means a lower room temperature can be used without loss of comfort.

The same comfort will be experienced with a design temperature of 20⁰C when compared to a radiator system with a design temperature of 21⁰C because of the heat radiated from the walls, floors and the very fabric of the room. Consequently a 1⁰C reduction in normal room air temperature is used. For bathrooms, wet areas, and walk in wardrobes we recommend a design temperature of 23⁰C. This is to compensate for the reduced floor area, added ventilation and the fact you will often use these rooms with little or no clothing.

Air Changes

When compared to radiator and forced convection heating systems wet underfloor heating creates weaker convective currents. This in turn creates a more even temperature and pressure within the heated space. This fact enables a lower air rate change to be used.

Floor Losses

The floor is the equivalent of the radiator. When turned on, the floor will always be warmer than the room temperature. Consequently downward losses from the floor should not be included when sizing underfloor heating coils. Floor losses will only need to be included when sizing the boiler, heat source and distribution pipework. Downward losses should be limited by insulating the floor slab and perimeter edge.

High Ceilings

Because wet underfloor heating does not produce the same convective gradients as radiator heating no additions are required to compensate for high ceilings.

Addition for Intermittent Heating

Underfloor heating should be designed to operate at a constant temperature during the day with a night or unoccupied setback of approximately 3⁰C. This ensures the room warm up periods is reasonably short and no adjustments to the heat loss calculation are required.

Heat output for Wet Underfloor Heating

The formula for working out the floor capacity to heat the room is as follows:

Total Room Heat Loss (W)/ Active Floor Area (m²)

So for a room with heat losses of 1257W and a heated area* of 14.5m² the heat output required is 1257/14.5 = 86.68 W/m²

* Heated area is that available to under floor heating. Sterile areas should not be heated e.g. under fridges, freezers, cupboards, baths and sanitary ware, and anywhere else you do not want under floor heating or where you may required to drill and fix into the floor.

Most rooms in new build properties will require a design heat flow of between 50-70 W/m²

Possible exceptions where floor heat emission required can exceed 100W/m² are:

• Conservatories or rooms with large glazed areas.
• Bath and shower rooms where the available space for under floor heating is reduced by fixtures.
• Rooms with open flued appliances the air change rate is greater affecting heat losses.
• Rooms with very high ceilings due to the increased wall heat loss.

Floor surface temperature

BS EN 1264 floor heating and floor surface temperature. Floor surface temperature is critical to comfort, as well as to heat output. BS EN 1264 gives guidelines on the maximum values for floor temperatures.

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