These technical faqs answer some of the questions we're most often asked about the specifying and installation of Underfloor Heating.

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Q Where should I locate the manifold for best performance?

A You should always locate the manifold in a central position for optimum performance.

Should this not be possible’ then an allowance will have to be made for additional pipe tails at a further cost. It is always better to bring two pipes from the heat source to the manifold, than anything up to 24 from a poorly positioned manifold to the areas requiring the heat.

From a technical point, other than a central position for the manifold is classed as a poor design.

If the manifold is at one end of the building it will heat up ‘carrier’ areas before the heat gets to the room in which it is required, this will lead to ghost heating of other temperature controlled zones.

This may also put unnecessary loading on the circulation pump, more water content in the system than is necessary and a waste of heat.

Do I need insulation and which type should be used?

Not only does an underfloor heating system require insulation for it to work effectively, floors must also be constructed to the current UK Building Regulations and any other additional requirements set by BS EN 1264 - 4, therefore insulation is regularly required.

Building regulations specify the required insulation on:

- Floor type (solid ground floor and suspended upper floors)

- Targeted U - value of the floor

- Floor perimeter to floor area ratio P/A

- Thermal conductivity of the insulation.

Solid floor in contact with the ground

For a standard 4 bedroom domestic house (8m x 8m floor area) with a ground bearing floor slab, 100mm of expanded polystyrene grade EPS 70 (thermal conductivity of at least 0.038W/m2K) will be sufficient to achieve a U- value of 0.25 as required by Part L of the UK Building Regulations. It will also satisfy the requirements of BS 1264 - 4 for an R - value of 1.25.

However, from one project to another the exact figures must be verified, as these will depend on the ratio between the exposed perimeter and floor area of the building.

Upper floors (not exposed)

The UK Building Regulations do not specify a required insulation for this type of floor.

However, BS EN 1264 - 4 requires an R - value of at least 0.75, which can be achieved with 25mm of expanded polystyrene grade EPS 70.

What type of screed should I use with underfloor heating?

There are no specific requirements for screeds when used with underfloor heating. However, the screed must be suitable for the floor loading purposes of the project. The Architect / Structural engineer should specify to what the screed should conform to.

For domestic applications a sand cement screed to BS 8204 is suitable and shouldn’t require any further additives.

It is recommended that the screed depth is at least 75mm, this will allow for sufficient cover of the pipework.

Do anhydride (flow) screeds present any problems?

According to anhydride (flow) screed manufacturers the overall thickness of this type of screed can be substantially reduced, which will affect the level of coverage over the pipework. It is strongly recommended to consult the manufacturer for advice.

When using anhydride (flow) screeds with underfloor heating please ensure that all vapour barrier joints and EPS edge insulation skirt and overlaps are taped to prevent leaks seeping under the EPS insulation.

Flowmax recommend using a Tacker system for anhydride (flow) screeds. If using a cliprail system, it is important to fix the pipe return loops at three equally spaced positions between the fixing rails with fixing staples. This will prevent the pipe from floating out of position in the screed.

Anhydride (flow) screeds will not affect Flowmax PEX-a pipe.

What is the best floor covering to use with underfloor heating?

The most effective floor covering for use with underfloor heating is ceramic or stone tiles due to the low thermal resistance and therefore gaining the optimum performance from the system.

What precautions should be taken when using tile floor coverings on underfloor heating?

When using tile floor coverings on underfloor heating they should be fixed directly onto the screed surface. It is important to use flexible fixing adhesive to avoid cracks in the tile surface. This is due to the differences in the co-efficient of expansion between the floor screed and tiles.

What about carpet coverings?

When using carpet and underlay the thermal resistively should not exceed 0.15m2K/W (1.5 TOG), as this will reduce the heat output significantly.

What about timber floor coverings?

When using timber floor coverings, care should be taken to store the natural wood in the room with the UFH turned on, until the moisture content does not exceed 8%. This is necessary to reduce the effects of shrinkage

What type of boiler do I need?

Any type of boiler can be used with underfloor heating. However, if the flow temperature of the boiler exceeds that of the required flow temperature then appropriate measure must be taken to mix the hot water down to the required temperature. Flowmax recommend using a condensing boiler due to the low flow and return temperatures within the system.

What fuel can I use?

Due to the low temperatures required by underfloor heating almost all types of fuel can be used e.g. Solar energy, ground heat source and electric.

What size boiler do I need?

The size of the boiler is determined by the underfloor heating design plus any other loading required such as domestic hot water, radiators etc. The design package used by Flowmax gives the required heat input needed into the underfloor heating system. If the project is only heated by underfloor heating then the only other addition needed to achieve the required heat input would be the domestic hot water requirements.

For a domestic project the domestic hot water heat load is approximately 3 kW.

Guideline: for an approximation of the size of boiler required, the maximum output including downward heat losses can be calculated as:

- 115 W/m2 for solid floors

- 80 W/m2 for suspended.

How should underfloor heating be operated?

Underfloor heating is intended to be operational at all times during the heating season. However, this doesn’t mean that the system operates continually in the same way. The system should use a night set-back facility as part of the control system to maximise the efficiency of the underfloor heating i.e. when the room / area is not in occupancy or at night.

The night set-back facility would reduce the heat being supplied to the floor screed and lower the temperature of the room to a pre-set point. The pre-set point for the night set-back mode should not be lower than 6°C below the set room temperature.

By limiting the temperature fall by 6°C the amount of heat then needed to raise the temperature again is minimum. If there is a severe cold spell and the room temperature falls below the 6°C set point then the boiler will automatically activate and maintain the room temperature at 6°C below the set point until the underfloor heating system ON time is activated.

Can the underfloor heating system be integrated with other system requirements?

Yes, the Flowmax underfloor heating system can be integrated with other system requirements.

Can individual room temperatures be controlled?

By using room thermostats and circuit actuators on the distribution manifold each room temperature can be individually controlled.

What happens if the pipework is damaged after installation?

The damaged circuit should be identified and isolated. The damaged pipework should then be exposed and repaired. The circuit is then put back into operational mode and the floor surface is restored.

Do Flowmax offer a warranty?

All parts supplied or installed by Flowmax are sold with a full 12months replacement warranty backed by the manufacturer.

Our Pex a pipe is supported by a comprehensive 10 years manufacturers liability insurance warranty against damages.

We also offer our own unique 50 year pipe replacement quarantee and will provide new pipe in exchange for any pipes found to be damaged or broken due to our own defective materials or our own production errors should such damages or defects occur with 50 years of the date of manufacture of the relevant pipe.