Why Underfloor Heating?

Traditionally, the UK has developed central heating systems based on wall mounted radiators. Where there is a plentiful supply of heat from an oil, gas or even solid fuel boiler, this provides pretty much instant heating in any environment.

The problem with radiators are that they are generally not things of beauty, they collect dust, are often reduced in efficiency by being sited behind tables, chairs or settees. They need painting and are attached to often ugly pipe-work from above or below. Radiators generally require high temperatures to operate effectively although modern high efficiency units and those with large surface areas are now available, they still suffer from the disadvantages described above.

Why Underfloor Heating is More Efficent

Imagine if you will, a typical room.

It might be 3m x 3m in size and will typically have a radiator which is about 1.2m long x 650mm high. The total surface area of the radiator is therefore 1.2 x 0.65 = 0.78m2

The floor of this room has an area of 3 x 3 = 9m2 so, the floor area is almost 12 times bigger than the radiator.

Now if we turn that floor area into a radiator by burying pipes under it and passing hot water through the pipes, we can get 12 times the amount of heat into the room for the same temperature.

Of course this is a ridiculous concept because the floor would be too hot to walk on and it would turn the room into a sauna.

In order to make such a system effective, the temperature of the water flowing through the pipes does not need to be anything like as hot as a traditional radiator system. If the water is less hot, it requires less energy to make it and this reduces the cost of heating it.

The advantages of Underfloor heating

It’s probably easiest to show this in a list:

  1. Large surface area provides excellent heating platform
  2. Lower temperatures into the heating system resulting in lower costs
  3. No convection of heat and hence no drafts or cold spots in the room
  4. Heat rises and as it’s coming from below, the comfort level is where the humans are
  5. Rooms ‘feel’ warmer and hence room temperatures can be reduce by one or two degrees
  6. It works with lots of different floor constructions and coverings
  7. No pipes or radiators to paint or gather dust
  8. Placement of furniture has little effect on heating efficiency

The disadvantages of underfloor heating

Mmmm, there must be a catch mustn’t there ?

Well, no, not really but there are some things that need to be taken into consideration.

The floor heating pipes must come up to the surface somewhere. They generally do this at a manifold where there are controls for the rate of flow of water through the floor and actuators which are controlled by room thermostats in order to switch the heating in a particular zone on and off.

The manifold will generally be sited in a garage or in a cupboard but MUST be accessible so that periodic heating system maintenance can be carried out.

Flow adjusters are at the top under the red caps and zone control  actuators are mounted on the bottom section. These will need to be accessible.

Then there is the thermal lag. If the pipes are buried in concrete, known as screed, it will take a while for this to heat up. Generally the room will be up to temperature after a few hours from cold. Equally, when the heating is stopped there will be a delay before the floor cools down. For this reason it is often better to make small changes in temperature rather than switching the heating off altogether when not required.

In order to achieve just the right room temperature it is important to get the water flow rate and the water temperature just right or else there will be large fluctuations in temperature. For this reason, it is vital that the system is properly designed by a reputable company such as Nu-Heat.

The thermal lag will be less noticeable if the floor construction is of wood rather then concrete. Here again is a trade-off though. The heating performance of screed is brilliant, liquid screed even more so as it has great thermal transfer characteristics. Wood is less good at transferring the heat and so generally needs a higher heat input. There are some new materials about such as Brio board from Knauf which has great thermal transfer characteristics, so little or no thermal lag and has the strength and ease of working as, say, chipboard.

Floor coverings will also have an impact on the heating system performance. Think about carpet on a concrete floor. It has two functions, one: to make the floor comfortable to walk on, and two: to insulate the room from the cold floor underneath. So, a thick carpet will have a negative effect on heating system performance. There is good news however, the carpet manufacturers know about this and make a wide variety of carpets with very low tog ratings that are quite suitable for underfloor heating.

The best floor covering for heating performance is stone or ceramic tile. It feels nice to walk on when it is warm and has ideal thermal transfer characteristics. Tiles must be correctly selected and installed in order for the system to work most efficiently of course but they have a pleasing effect both in terms of comfort and aesthetics.

Is there anything else I need to know ?

Yes, lots and lots of stuff, but two things which I think are quite important if you are looking at this subject for the first time. These are Weather compensation and Thermostatic control.

Weather compensation

A property built to current building regulations (2010) should not require any heat input when the outside temperature is above about 16°C. As the temperature outside drops, the house begins to lose heat. The greater the difference between the outside temperature and comfy heat inside the house, the more heat is required to offset this loss.

The heat output of a heating system will be constant at a given water temperature and water flow rate. If we assume for a second that a room will require 2kW of heat when it is just freezing outside and no heat input at 16°C, then there will be a line which joins these two points and will show the required heating power at all the temperatures in between. Let’s call this line a heat curve.We can control the amount of heat into the system either by adjusting the rate of water flow, or by adjusting the temperature of the water in the system. Adjustment of flow is possible but quite difficult to achieve whereas adjusting the temperature of the heating water is relatively straightforward at the heat source.

So, all we need to do is to monitor the outside temperature and adjust the temperature of the heating water so that it goes up when it gets colder outside and down when it gets warmer. Most modern heat sources will have this function built in and it is a simple matter to select the correct heat curve for your property.

Remember, reducing the temperature of the heating water reducing the running cost of the system so using this system you only pay the higher heating costs on the coldest days.
Thermostatic control
Having selected the right heat curve for your building, you might think that there is no need for thermostats. You are of course right.

In an ideal world where the heating system design is perfect, the installation is perfect, and your house behaves like a perfectly designed and built thermal box you could just rely on the heat curve and all would be well.

However, life is not like that. Also, you may have areas that are sometimes not used or where the type of use changes throughout the day or throughout the week. This is where thermostats are useful and programmable thermostats are truly wonderful.

Take the example of a guest bedroom which is only used at weekends. The system will probably have been designed to keep this room at about 18°C which is a nice temperature for a bedroom. The weather compensation system will make sure that this happens all through the colder parts of the year.

However, this bedroom is not in use for 5 days a week and even at weekends is only used for about 10 hours overnight. Why go to the expense of maintaining the temperature at 18°C all that time. A thermostat can be adjusted to lower the temperature when the room is not in use. Better still, a programmable thermostat can be used to make sure that the room temperature is allowed to reach the required temperature just when it is needed and allow it to cool when not.
Finally
I hope that you found this section useful. Please explore the rest of the site which gets a lot more down and dirty with related topics.