Underfloor heating is easily the biggest growing sector of the domestic heating industry.  Almost 50% of all our private enquiries are concerned with the installation of underfloor heating, so why all the fuss?


Underfloor heating has been around for a long time.  However, it is only recently that the general public have become aware of the benefits that can be gained from such a system. 

Advantages 

Increased comfort levels, due to a more even distribution of heat, and at a lower level where it is needed most.


Radiators
 


Under-Floor

The system requires cooler water than radiators, allow boilers to be run more efficiently, and making heat sources such as solar more feasible for use in heating. 
Lower running costs as a result of increased efficiencies and utilisation of energy.
Free wall spaces.  The underfloor system is unobtrusive, with no visible pipework.  This allows walls to be easily decorated, and furniture positioned without having to worry about hiding radiators.
Safer for children as there are no sharp edges or hot surfaces.
Lower maintenance.  Underfloor pipework is plastic and does not corrode like radiators may.  It is also at low level, and as such does not air lock as easily as radiator can. 
Quieter, with lower water velocities, less air, and no creaking radiators.
Cleaner, with no valves that can end up dripping on carpets, and there are no hidden spots behind radiators where dust and small objects can accumulate.
Fabric temperature of building maintained.
Individual room temperature control as standard.
Vandal Resistant - no radiators to walk off site.

Disadvantages 

Major disruption to existing buildings during installation.
Long time to heat up and cool down. Cannot respond quickly to rapid temperature changes.

Choice of floor finishing requires careful consideration as it may affect performance.

If conducted at the new-build or refurbishment stages, installation is relatively straight forward, with many time saving pipework systems now available on the market to cover all floor types from concrete screeds to suspended timber floors. Converting your house while living in it is possible, although very disruptive.

To simplify the installation of a modern system, we have developed a range of units to cover all the system variants we have yet encountered. 


Contents:

TYPICAL COMPONENTS

PIPEWORK

CONTROLS
Fixed Setting Room Thermostats
Programmable Room Thermostats
Night Setback
Weather Compensation
Optimum Start

CONTROLLING TEMPERATURE OF UFH CIRCUITS
Unblended underfloor circuits (small UFH area off radiator circuit)
Thermostatic Mixing Valves
Thermostatic Valves for Very High Output
Motorised Three and Four Port Valves
Injection Systems
Twin Zone Thermal Store Injection System

BASIC DESIGN

Calculating the central heating and hot water loading
Are radiators required as well ? 

Deciding if primary distribution pipework (up to manifolds) is to be blended
Calculating lengths and densities of pipework required
Basic Heating Pipework Layout
Sizing of thermostatic mixing valve and UFH pump
Sizing Boiler
Sizing Boiler Pump
Buffer Store Operation
Sizing Hot Water Store
Sealed or Vented Primary System

FLOOR
DESIGN
Screeded Floors
Suspended Floors


TYPICAL COMPONENTS:
 

  • Underfloor heating pipe with oxygen barrier

  • Pipe fixings (steel mesh, staples, clips, wire)

  • Manifold

  • Actuators

  • Temperature mixing valve

  • Underfloor pump

  • Boiler pump

  • Central programmer to provide overall on/off control

  • Individual room thermostats

  • Floor insulation

  • Edge strip insulation

  • Cement additive

  • Boiler


PIPEWORK:

PEX

All underfloor pipework is plastic, specifically cross-linked polyethylene.  They must have an oxygen diffusion barrier to prevent the ingress of oxygen through pipework into the system water. Pipe is supplied in coiled lengths of 100m or more.


MULTILAYER PIPE

Multilayer pipes are a triple layered pipe of cross-linked polyethylene (PE-Xc) sandwiching a layer of aluminium. The tube can be easily bent to shape, and will then retain that shape, making laying and fixing of pipe easier.  The specifications for such pipe are exceptional, including 100% Oxygen impermeable, expansion comparable to copper, 10 bar maximum working pressure at 95C, and 50 years working life guaranteed.

The two main manufacturers of such pipe are  MULTIPIPE and UNIPIPE


CONTROLS:

Dealing with the control setup for an underfloor system can prove to be quite confusing with the range of systems and choices on offer. Most control features are aimed at improving performance and efficiency of a system, and taking automation to various levels.

As with all controls, the more advanced controls can be more complicated to set up correctly, and to fault find. They can also be very expensive. The following features should be considered.


Fixed Setting Room Thermostats.

These are the most basic heating control, and is simply a wall mounted temperature thermostat that can be manually adjusted by the user to achieve comfort levels. 


Programmable Room Thermostats.

These are a more advanced version of the basic room thermostat, combining with it the functions of a central heating timer. Thermostats typically have 6 times when the set temperature can be programmed to. For example, 28C from 6:00am to 8:00am, dropping to 5C (frost protection) until the evening.  Although the thermostats have manual override, allowing the user to easily adjust the room temperature setting, the stat will automatically reset the set temperature with the next program setting. This overcomes the problem of users inadvertently leaving room stats turned up high at night.
 

Night Setback.

This is a basic feature built into some room thermostats that will turn down the room set temperatures automatically at night, by typically 6C.  This is a simple feature, and allows heating to be left on at night, without running the expensive of heating the room to full temperatures.  Fully programmable room thermostats inherently allow lower night time temperatures to be set up within the program.


Weather Compensation.

Compensation requires the use of a motorised thermostatic blending valve to enable the temperature of the underfloor flow to be altered according to the outside temperatures. This leads to the floor giving a more consistent heat output rather than the on/off cycle, which would otherwise result (from the room stat). It also has the effect of reducing the average temperature of the water coming back to the boiler so increasing boiler efficiency - especially condensing boilers.  
 

Optimum Start.

Instead of setting a time for the heating to come on and hoping that the house will be warm enough, the user enters the time by which the house needs to reach a set temperature. The controller brings the heating on earlier or later as required depending on how cold it has been overnight. Optimisation therefore provides improved comfort rather than efficiency and is particularly suited to houses with a lot of 'thermal mass' and unresponsive heating such as a screed based underfloor system. This is a very advanced feature.


CONTROLLING TEMPERATURE OF UFH CIRCUITS:


Unblended underfloor circuits (small UFH area off radiator circuit).

The reason why blending valves are required is to avoid hot-spots on the floor that may lead to discomfort, or harm to young children.  It is not to protect the screed from crumbling, as maybe believed.  For circuits up to 15m2 it is possible to supply the underfloor system with water at up to 90C, using a return limiting valve, overcoming the need for flow temperature control. 

This operates on the principal that the return temperature is a good indication of floor temperature, so that by maintaining a steady return temperature we can maintain a steady floor temperature and load. This system has been used extensively in Europe for many years, and is a proven method of control, however it is only suitable under the following conditions to ensure no hot-spots occur:

  1. Maximum 15m2 floor (single pipe run) per valve .

  2. Screeded floors only - minimum 75mm screed thickness to act as thermal mass.

  3. Pipes must be a maximum of 200mm apart.

  4. Reverse return pipe pattern must be used, where the flow and return are side by side to provide an averaging out effect.

Because of the limit on floor area per valve, it becomes economical to use this system for large underfloor heating areas.  However, the valve is perfect for taking small underfloor zones of standard radiator pipework. 

 

Thermostatic Mixing Valve.

The simplest method of controlling the UFH temperature is by using a Thermostatic Mixing (blending) Valve.  This is a three port valve that works automatically to mix the hot supply and UFH return to achieve a set output temperature, basically adding hot water into the UFH circuit to maintain temperature.

The temperature is typically set by turning the head of the valve, which can often be locked into a certain position.

These valves have been used for many years, mainly to control hot water to taps and showers, and are very reliable and cost effective.  They do need to be sized correctly in conjunction with the pump (see later section) to obtain the correct system flow rates, as they restrict the flow.

A three port setup does allow the UFH pump to draw from the boiler circuit, or thermal store, without the need for another pump.

The disadvantage of this type of valve is that it is manually set, and does not allow the temperature to be adjusted by the control system.  This rules out weather compensation as an option, unless the end user can get used to manually setting the valve according to the season or outdoor temperature.

 

Thermostatic Valves for Very High Output.

For very large pipework systems, requiring over 3m3/hour of flow rate, their are larger valves available.  These will usually consist of a three port brass valve, and a sensor/actuator with a bulb type sensor that is mounted in the pipework and connecting to the actuator with a capillary tube (typically 3m long).  The Danfoss Randall VMV Valves and RAVK Sensor are common.

 

Motorised Three and Four Port Valves.

Such valves are electronically controlled to mix the hot supply and UFH return to achieve a set output temperature, adding hot water into the UFH circuit to maintain temperature. The valves is powered by a motor, which is in turn driven by the control system. Temperature readings are measured from sensors in the pipework. 

This is a far more complicated system that a simple thermostatic valve, however it enables weather compensation to be achieved automatically by the control system.  These valves will also require sizing in accordance with the pump due to the pressure losses across them, as well as the maximum differential pressure they can take.

The four port model, shown above, ensures that both the mixed and supply circuits see no resistance to flow - instead of one port closing down, the flow is diverted out the fourth port.  This is more suited to larger commercial systems - the matter of differential pressure becomes less important as the valve never has to shut down a circuit.

 

Injection Systems.

The UFH pump is now sized to match the UFH circuit alone. The boiler pump is sized to maintain boiler flow rates, not having to overcome any UFH.  The injection pump has very little resistance to overcome.

An injection valve can be used to control the injection rate, controlled in turn by the control system in conjunction with the UFH flow sensor.  Thermostatic control can be achieved by the use of a two port valve with a remote temperature sensor. This overcomes the need, and advantages, of having the control system. 

Other Injections systems (2nd schematic) work by the creation of an additional circuit between the hot water supply (boiler) circuit and the UFH circuit.  A pump fitted into this additional circuit is used to inject hot water into the UFH circuit.  The temperature of the UFH circuit is thus controlled by the rate that hot water is injected.

It is also possible to for the control system to modulate the pump speed, overcoming the need for an injection valve.  Sometimes there will be an additional sensor on the hot flow from the boiler.

Injection systems have the advantage of achieving fully automatic temperature control, with no motorised valves.  Pumps on their own provide better circulation, are more reliable, and much easier for a service engineer.

 

Twin Zone Thermal Store Injection System.

With a thermal store it is possible to simplify controls further by dedicating a section of the store to run at a lower temperature. A cylinder thermostat controls the injection pump to keep the lower section up to the required UFH flow temperatures.

 

The use of more than one cylinder thermostat in the lower section can allow different UFH flow temperatures to be selected.  When used in conjunction with simple room thermostats to sense outdoor temperature (located in, for example, the garage), this can provide a simple form of effective weather compensation using cheap, readily available components.

Overheat protection can be provided by another stat.


 


BASIC DESIGN:
 

Calculating the central heating and hot water loading.

The first step in designing any heating system is to calculate the required central heating output to match the heat losses (and gains) for each room.  The Barlo Heatload Calculator is a simple piece of software that can be downloaded free and makes it easy to perform all the required calculations.

 

Are radiators required as well ? 

Reasons why a radiator may be used include:

  • Very large windows which may generate considerable downdrafts. A radiator will counter the draft if positioned below the window.

  • Radiators can heat up rooms quicker than underfloor, which can take up to 3 hours to fully heat up. Where start-up times cannot be predicated, radiators may be needed to improve response.

  • Areas which suffer from rapid temperature drops may use a radiator to speed up heating in that area.

  • Areas with very high heat losses (better to cut down heat losses if possible)

  • Areas where is is impracticable to lay underfloor pipework.

It is worth remembering that the higher the thermal mass of the underfloor system, the longer the heat up times.  Quite quick heat up times can be achieved with a thinner screed above floor insulation. Fan convectors are another consideration as they have higher heat outputs, and can be used sparingly to accelerate initial heat up.

 

Deciding if primary distribution pipework (up to manifolds) is to be blended.

Water can be pumped from the boiler / thermal store to underfloor manifolds...

  • at boiler temperatures (typically up to 82C) with underfloor temperature control on the manifolds,

  • or at a underfloor temperatures removing the need for blenders, and pumps, on manifolds.

Centralising the temperature control makes for a simpler system, and allows weather optimisation to be more easily implemented.  However, running pipework at full temperatures allows for radiators to be better used.

Radiators tend to require water at higher temperatures, 83C, as opposed to 40-55C for underfloor heating.  Sending very hot water round an underfloor circuit may cause the screed to crack, or floor temperatures to get uncomfortably high.  Temperature controls of some sort are therefore required to limit the temperature of the water going to underfloor circuits. 
 

 Table of Radiator Outputs vs. temperature. Taken from Barlo Radiators website.


If the flow is at 55C, return at 45C then radiators will need to be more than twice the size (0.423 output at 30C Delta T from table) of normal to achieve rated outputs. If radiators are to be used then it may be more practicable to provide temperature control at the underfloor manifolds, if they are located near to the radiators, rather than oversizing radiators or running dedicated high temperature pipework. 

 

Calculating lengths and densities of pipework required.

Once the heat losses of the property are known, the required power output [W/m2] of the floors is calculated by dividing floor area for underfloor heating pipes [m2] by heat losses/output [W].  Calculations should be done for each room individually.

The heat losses should take account of any radiator input, which should be deducted from the required UFH output.  Also, the floor area in rooms may be reduced because of fixings, such as in kitchen cupboards, or bathtubs. Allow for this when working out the floor area to use in calculations.

The following table, from Hilton-Croft UFH, is for a typical PEX pipe system.

Floor temp
C

Power
W/m2

Pipe Spacing
cm

Pipe Density
m/m2

Circuit Length
m

Max Circuit
Area m2

Heat
Output W

Water Volume
ltrs/hr

Pressure Drop
mbar

Flow temperature 50C Return Temperature 40C

25.7

75

30

3.3

60

18

1350

116

50

80

24

1800

144

97

100

30

2250

194

204

115*

35

2625

226

306

26.5

87

20

5

80

16

1392

120

71

100

20

1740

150

130

120

24

2088

180

215

200* 27 2349 202 295

27.1

97

10

10

100 10 970 83 47
140 14 1358 117 119
180 18 1746 150 235
200* 20 1940 167 314

Flow temperature 55C Return Temperature 45C

26.7

91

30

3.3

40

12

1092

94

23

60

18

1628

141

70

80

24

2184

188

155

100*

30

2730

235

285

27.7

106

20

5

60

12

1272

109

45

80

16

1696

146

100

100

20

2120

182

183

120* 24 2544 182 183

28.5

118

10

10

100 10 1180 102 67
120 12 1416 122 109
150 15 1770 152 200
170* 17 2006 173 284

*  maximum acceptable heating circuit lengths including pipe 'tails' to the manifold.
 

Take a 180m2 property with 13.5 kW heating load, requiring 75 W/m2. With 50C flow, floor temperature 25.7C, 10 x 60m pipe runs will provide (this should really be done room by room).  The total flow rate would be 1.16 m3/hour (20 ltrs/min) at a pressure loss of 50mbar (0.5m head).

 

Basic Heating Pipework Layout

Once heat losses and lengths of required UFH pipework have been calculated. The following points should be kept in mind when working out pipework layouts:

  • Keep the number of manifolds to a minimum. One or two will do for most domestic properties.

  • Keep manifolds as central as possible, and accessible for servicing.

  • Areas with permanent fixings, such as kitchen cupboards, may be avoided (as allowed in calcs).

  • Plan to use continuous lengths of pipe, avoiding pipework connectors.

  • The aim is to achieve an even floor temperature, by keeping pipes evenly spaced.

  • Running the flow and return for a circuit side-by-side helps average out temperature. This is called a reverse return pipe pattern


 

Sizing of thermostatic mixing valve and UFH pump

Looking at the pressure loss graphs for typical UFH Blending Valves (graphs taken from RWC web site), in 22mm and 28mm, we can see (continuing the example) that at 20 ltrs/min the system looses 0.4 bar (4m head) through a 22mm valve, or only 0.15 bar (1.5m head) through the 28mm valve.

The pump curve for a standard Grundfos Alpha 15-60 pump show that at 1.16 m3/hour the pump can generate 4.4m head of pressure.  The calculations show a total pressure loss through the pipework and 22mm mixer of 4.5m, however this is greater than the pump can provide.

Although a bigger pump would work, to avoid system noise it is better to use the 28mm blender, which together with the pipework only looses 2m head.  We still then have 2.4m spare head of pump pressure to overcome other manifolds, actuators and balancing valves.

Such pump curves can be generated using Grundfos WebCAPS.

These calculations are based on centrally provided mixing for the whole property.  If there is more than one manifold fitted with its own mixing valve and pump, then the calculations need to be performed separately for each sub system.

It is also often advisable to incorporate an over-temperature valve, to isolate the flow to the underfloor in case of a failure of the blending valve to function.  Over a period, high temperature water >60C may cause screeds to crack, so protecting against this is wise. The simplest form of protection is to use a stat that will isolate the power to the UFH pump and actuators. Full protection will involve a dedicated isolating valve of some sort - there are both electrical (stat + actuated valve) and purely mechanical (valve with bulb sensor) methods.  If this valve is fitted in the UFH circuit then it will need to be accommodated in pressure loss calculations.

 

Sizing Boiler.

Once a figure for the total heat losses of the property are calculated, the hot water requirements can be roughly calculated as allowing 2.5kW per person.  This is based upon a bath of hot water for each person, recovered in two hours.

The total of hot water and heating loads gives a minimum boiler size.  It is wise to oversize a boiler slightly, maybe up to 30%, but higher output boilers may suffer from cycling problems, reducing efficiencies, especially on fixed-output boilers.  If a thermal store is to be tied into the system then cycling can be overcome, even for large fixed output boilers.

 

Sizing Boiler Pump.

The boiler will require a pump suitably sized for its output, although sometimes boilers come pre-fitted with a suitable pump. A required flow rate at full burn, can be determined from the output of the boiler as follows (typically the boiler temperature drop is around 10C):

Flow rate [litres/second] = Boiler Output [W]  /  (4200 x Boiler Temp Drop [C])

Example (24kW Boiler): Flow Rate = 24000 / (4200 x 10)  =  0.57 l/s = 35 l/min

Systems should always have a by-pass of some sort. Unless an automatic by-pass is used, the re-circulation through the by-pass (typically low or no load) will need to be added to the flow rate.  It is advisable to use an automatic by-pass as it overcomes the need to worry about the adverse affects of fixed by-passes on flow rates and pressures.

Other valves that may need to be incorporated into the design include:

  • zone valves for isolating various heating circuits, or feed to hot water store

  • over-temperature valve, to isolate the flow to the underfloor in case of a failure of the blending valve to function.  Over a period, high temperature water >60C may cause screeds to crack.

Allowances must also be made for pipework from the boiler to manifolds and/or hot water store.

 

Buffer Store Operation.

The only way to ensure that condensing boilers run continuously in condensing mode for heating, or to overcome nuisance cycling of boilers, is to tie a thermal store into the underfloor system.  The store acts as a buffer between the heating load and the boiler output. It saves up heat energy while the boiler is firing, and then uses that stored heat to keep the heating going once the boiler stops firing. This way the boiler does not have to fire so often, and will burn for longer when it does.

Reducing cycling in itself will improve efficiency, however gains are also to be achieved by keeping return temperatures to the boiler low at all times. Without a thermal store this is very difficult to achieve, unless the boiler has built in electronics. This is because to maintain minimum flow rates through the boiler at low heating loads, water will flow via the by-pass to the return, raising the temperature. This cycle will continue until the water in this loop  reaches 80C (upper boiler stat), by which time the return temperature is above 60C. Boiler efficiencies are better the lower the return temperature and at 60C condensing efficiencies are not good.

With underfloor heating one only needs a flow temperature of 55C max.  The lowest temperature in the system is the underfloor return, at between 30 and 45C, so ideally we want to only ever heat water from 45C through to 65C to maintain underfloor heating (assuming 20C rise is suitable for boiler). 

This is easily achieved with a buffer store by setting up cylinder thermostats accordingly.  The boiler will not fire until both the lower thermostats are calling for heat, and will then continue to fire until both are satisfied. The thermostats should be tweaked down so that the boiler re-heats water in one pass - a second pass would involve return water over 60C.


If hotter water is required, say for running radiator circuits, or mains hot water heat exchangers, then the upper section of the store can have its own thermostat that overrides the two lower thermostats, when needed.  The simplest method of reheating the thermal store is to simply pump water from the bottom of the store to the boiler and back, although this is only possible with vented boiler systems. With sealed systems a copper coil within the store is used by both the boiler and the underfloor to drive heat in and out of the store, however higher boiler temperatures will prevail when compared to the direct (no coils/vented) setup. On very large systems a plate heat exchanger can be used instead of a coil, to provide in/outputs of over 50kW.

With non-condensing boilers, where buffer use is to overcome cycling, only the lower two cylinder thermostats are needed, both set to 75C. 

Buffer stores are also useful when trying to incorporate solar panels into a system.  A coil in the base of the store allows heat to be transferred to the coldest point of the store, and then to be used for underfloor heating.

 

Sizing Hot Water Store.

When calculating size of hot water store, you can use our Waterload Calculator.  As a rule of thumb, we allow 90 litres storage per bath, and 60 litres per shower, within the period of maximum demand.  If a thermal store is to be used, then additional storage may be added to allow for buffer operation.  Extra storage may also be required if solar panels are to be utilised.

Special care has to be taken on electrically heated systems as the smaller the store, the less able it is to store up heat provided on cheap rate electricity.

Special care should also be taken where there are body jets, large shower roses, or a general desire to spend a considerable time in the shower.

DPS Thermal stores are available in base unit diameters of 40cm, 45cm, 50cm and 60cm, with heights from 85cm up to 2m, making a range of capacities from 90 litres to 500 litres.

 

Sealed or Vented Primary System.

Generally it is best to opt for a sealed primary system - in other words, one which is pressurised rather than tank fed.  Sealed systems have the following main advantages:

  • Protect against oxygen ingress and help maintain a cleaner system. Inhibitor lasts much longer.

  • Allow radiators to be positioned as high in the property as required.

If you have a sealed system boiler, or certain other makes of boiler, then a vented system is not an option. However, vented systems do have some advantages, providing that you can live with a 12 gallon (12x12x20") feed & expansion tank in the loft.

  • Automatically re-fills when servicing is carried out, or air is purged.

  • Allow 'direct' thermal stores to be used, where the water within the primary system is the same as that in the thermal store (no coils), allowing a very simple, economic, high recovery system to be made. Such stores can also make more efficient use of solar for underfloor.
     


FLOOR DESIGN:

Screeded Floors

 

A blinding layer of sand is added to fill voids and to provide a smooth firm surface free from sharp particles, this it is essential to avoid puncturing the DPM.

DPM stands for Damp Proof Membrane. It is required when laying wood flooring or laminate flooring onto cementous substrates e.g. concrete, ceramic, marble, tarmac/bitumous surfaces. The DPM will prevent sweating and any moisture ingress from the sub-floor.

Floor insulation is typically rigid foam thermal insulation board with reflective foil facings (Celotex). Available in varied thicknesses and board sizes (50mm x 1200x2400mm, 1200x1000mm...)

The pipes are fixed to a steel mesh, using simple wire clips.  The mesh is lifted clear of the insulation with spacers prior to filling with screed.
 

     


Cement Additive / Plasticiser is added to the screed mix to ensure complete encapsulation of the pipe/grid by the screed so that maximum heat transfer from the pipes into the screed is obtained, and to give additional compressive and bending strength.

 

Suspended Floors

The suspended floor methods shown below use a Sand 1:8 Cement mix to act as a thermal mass, and spread the heat load.  This is a cheaper alternative to using aluminium spreader plates.
 

ABOVE JOIST:


BETWEEN JOIST:

 

   

 


 

 

Some links to underfloor companies:

Borders Underfloor Heating

Conservation Engineering
Continental UFH
Equator
Hepworth Hep
2O
Hilton-Croft UFH
Invisible Heating

Nu-Heat
OSMA / Thermoboard
Pexatherm UFH
Underfloor Heating Systems Ltd
Wirsbo