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Newsletter 2011 - Click here to view.


Can I install underfloor heating in both old and new buildings?

Yes, but it is best installed in buildings that have good insulation. Well insulated modern buildings cool at a much slower rate and are not subject to excessive swings in temperature when outside temperature conditions change.

Can a standard boiler be used for an underfloor heating system?

Yes, wall mounted or floor mounted, gas or oil fired, combination boilers or standard boilers can be used, but the biggest advantage in efficiency will be obtained when a condensing gas or oil fired type is used as these boilers can take advantage of the lower return water temperatures occurring on underfloor heating systems.

I am designing an apartment complex where space is at a premium. What is the smallest boiler I can use for underfloor heating?

There is an electric boiler on the market, that measures one metre long by 90mm square, that will run an underfloor heating system. It only weighs 9kg and is available in sizes 6kw, 9kw, 11kw and 12kw.  You can see one of these if you visit our offices.

What are the main components of an underfloor heating system?

Boiler, pipework, manifolds and the control system.

Do underfloor heating systems operate at a lower temperature than a radiator system?

Yes, radiator systems will operate at a boiler flow temperature of 82°C and a temperature drop of 11°C, but an underfloor heating system will run at temperatures in the range of 35°C to 60°C depending on floor conditions and the type of building.

Why do I need to use manifolds?

All the buried pipework in underfloor heating systems should be in continuous loops without joints. The flow and return from each loop of pipe is brought back to manifolds where the flow of water can be monitored and controlled. Individual rooms or areas can have their own room thermostats for local temperature control wired to low voltage actuator heads on the flow control valves. The advantages of using manifolds are that areas which are not required to be heated can be shut off, local modifications and additional pipe loops can be added without the need to drain the entire system, and the system can be balanced quickly and efficiently.

What pipe do you use for underfloor heating systems?

Multilayer pipe which is aluminium sandwiched between layers of polyethylene pipe. All pipes should incorporate an oxygen barrier to prevent oxygen ingress into the system.

What is the maximum loop length?

For an installation with our 16mm pipe and Grundfos pumps on our manifolds, loop lengths can go as high as 120m.  However, our designers prefer to restrict loop length to 100m to avoid putting higher stresses on the pumps.  
In this way we seek to optimise both performance and long term reliability of your installation. 

What type of insulation is to be used?

This is covered in detail in Item 3 of our Technical Section "Floor Construction and Insulation".

What is the water flow temperature?

The water flow temperature will vary between 35-60°C, depending on the heat losses from the building. If the system operates with thermostatic mixing valves, use 43°C temperature for screeded floors and 55°C temperature for timber suspended and floating floors.

What is the heat output from an underfloor heating system?

Maximum heat output from screeded floors is approximately 100 W/m2, the maximum heat output from timber suspended floor and floating floor is 70 W/m2. With there being higher standards of insulation in new buildings, average heat requirements for many buildings are now below 60 W/m2. For small bathrooms it is standard practice to also install warm water towel rails to give extra heat output.

How do you fill the underfloor heating system?

The system should be filled with water and fully vented of air. This can best be achieved using mains water pressure through the two end caps on the manifold. All valves should be closed and the flow end cap connected with a hose to mains water, with the return end cap connected to a hose laid to waste water point. One by one flow and return valves are opened and closed on each loop purging them of air.

What is the working life of our pipe?

Our pipe has one of the highest approvals in the world from the Water Research Council (WRAC); 10 bar pressure and 95°C temperature. Our pipe is guaranteed for 50 years. When correctly installed and run, our pipe should have a working life in excess of between seventy years and one hundred years, provided that no oxygen has seeped into the pipe and that the system has not been run at excessive temperatures.

Can you combine an underfloor heating system with radiators?

Yes, but there should be filters installed to avoid sludge from the radiators blocking the underfloor pipes.

What is the required spacing between pipes on an underfloor heating circuit?

As a rule of thumb :-

  • for systems with hot water delivered by boilers and BMS systems pipe centres are 200mm apart, within 500mm of an external wall or patio doors this is reduced usually to 100mm. 
  • when hot water is delivered by a heat pump, which produces water at a lower temperature, pipe spacings are reduced to 150mm.

For environments such as sports halls, where a lower temperature output is required due to their nature and usage, pipe spacings are often increased to 300mm.

Why should pipes be spaced closer together adjacent to external walls?

This is necessary to offset heat losses through glazing, external doors, patio doors etc.

Why do I need to put insulation below the underfloor heating circuit?

Insulation will reduce heat loss through the floor construction, resulting in lower fuel bill. Also, the insulation assists in deflecting heat upwards resulting in quicker warm up periods and assisting in producing a constant floor surface temperature. The underfloor heating pipes also benefit from being secured to the floor insulation.

What does ‘floating slab construction’ mean?

The screed containing the underfloor heating circuits is separated by insulation underneath and around the perimeter from any part of the building structure. This allows the screed to expand and contract during the operation of the underfloor heating system without cracking or breaking up. This method of floor construction is called a ‘floating slab'

What is a control pack?

A control pack consists of a pump, thermostatic mixing valve, zone valve and isolating valves combined with pipe fabrication to enable the control pack to connect directly to the manifolds

What does a control pack do?

Thermostatic mixing valve - the thermostatic mixing valve ensures that the water circulates through the underfloor heating circuit at a constant temperature of 43 degrees C. This results in a floor surface temperature that is even and does not exceed 29 degrees C, or 27 degrees C for wooden floors. Zone valve - the zone valve allows the underfloor heating system to be isolated via a programmer or room thermostat. The zone valve can also be utilised to send a signal to the boiler to fire when open.

Pump - the pump circulates water through the underfloor heating system. It also creates a head pressure, which overcomes the resistance created by the circuit, vales and manifold.

Isolating valves - the isolating valves allow the underfloor heating system to be isolated manually.

Why should the surface temperature of the floor be controlled to keep it below 29 degrees C?

Feet dispense a lot of heat from the body. If the temperature of the floor is above 29 degrees C, the body has problems dispersing heat through feet. As a result people often complain of aching legs if the floor surface temperature is too high

What temperature is your pipe rated to?

95 degrees C   and  10 bar pressure.

When testing the installation prior to laying screed, what pressure do you test the system at?

The pressure test should be twice the working pressure, so the test pressure is 3 bar. During the laying of the screed, this pressure should be applied to the pipes and the pressure gauge closely monitored.

What are the minimum and maximum temperatures for laying screeding?

The minimum is 3 degrees C and the maximum is 35 degrees C.

How much does it cost to run an underfloor heating system?

No matter which heat source is used, underfloor heating is the most economic heating system available.

Depending on the exact design and installation, an underfloor heating system is between 15% and 40% cheaper to run than a conventional radiator system. This is because an underfloor heating system works at much lower water temperatures.

What are the benefits of underfloor heating compared to a radiator system?

There are many benefits of underfloor heating, these are just a few of the major ones:-

  • Space - radiators take up 15% of room space. 
  • Health - underfloor heating kills 80% of dust mites. 
  • Aesthetics - decorate as often as you please without having to remove and replace radiators. 
  • Cost Effective - energy savings of between 15% and 40%; 
  • System costs comparable if anything other than bog standard radiators being compared. 
  • Project Cost Savings - when allowing for labour cost in both first and second fix.
  • Reduced carbon footprint.

What happens when your pipe gets clogged with limescale?

The inner layer of our pipe is polyethylene and limescale can not attach itself to it - hence no clogging of pipe.

What are the most common problems you experience with your new underfloor heating systems?

With new installations, 95% of the problems are due to faulty wiring.  Item 6 of our Technical Section contains wiring schematics.  These problems are magnified when we do not supply the system controls.  

In the winter we pressure test with air and occasionally customers telephone to advise us that the system is not producing heat - this is due to there not being water in the pipes the air not having been vented. 

What are the most common problems you experience with your existing underfloor heating systems?

This depends on how long the system has been installed, but the most common problems are caused by batteries in thermostats requiring replacement and occasionally the need to replace a pump.

UHMA - The trade association for surface heating and cooling.


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