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In determining the control strategy for underfloor heating we have drawn up the criteria below based upon our experience in providing underfloor heating installations in 500 + schools and numerous commercial and public buildings with the feedback from a number of end users. Today’s buildings have lower heat losses and require more effective controls to avoid over or under heating, save energy and to ensure that heat provided matches the varying energy requirements of the building.

Ideal system Design

1. Each room to have a room thermostat/sensor for room control.

2. Weather compensator(s) to be used to vary system flow rate temperatures according to requirements.

3. Manifolds to be supplied with L PHW with weather compensated flow temperature at a 10 °C maximum TD (TD =temperature difference between flow and return pipework) straight from boiler-heating plant.

4. Night set back thermostats/Sensors set at 14-15 °C to be installed to maintain fabric temperature control of building during off periods. Thermostats/sensors per large zone or building.

5. If required, different areas operating at different times can be time zone controlled.

6. Any supplementary heating (i.e. radiators) to be sized to operate at the same flow rates and temperatures as required by the underfloor system.

7. An underfloor system operates with variable flow rates therefore the pump should ideally be an energy efficient, variable speed inverter driven pump.

Dealing with each item point by point.

1. Individual Room Control by thermostats/ sensors are required by building regulations and enables individual room temperatures to be set, thermostats/sensors can be tamperproof or room controllable. Adjustable thermostats/sensors can be fully or partially adjustable by the occupant dependent on requirements.

2 & 3. Condensing Boilers are available (such as the Broag Range) where the resistance of the boiler is low and the boiler does not necessarily have to operate at a 20 °C TD. Water can either be pumped directly through the boiler at the required flow rate and TD or a low loss header can fitted to the boiler to achieve this. TD. The use of 10 °C TD or less is for a number of reasons

a) Using 10 °C TD instead of 15 °C TD or 20 °C TD ensures that the system is fully flexible and future proof in that if the heating plant is replaced a later date with a heat pump( now being promoted vigorously by the government) there is sufficient flow rate at a low flow temperature for this to operate . The use of a higher TD than 10 means that the flow temperature is higher to achieve the same mean water temperature. The higher the flow temperature from a heat pumps the lower the efficiency of the heat pump.

b) With the boiler operating at a lower flow temperature it will be more energy efficient.

c) There is a trend to use flow and return temperatures of 60- 80°C or 70-50°C. When this is provided to the manifolds a manifold with pump and thermostatic water mixing valve has to be used to reduce the water temperature. These manifolds have manual setting thermostatic temperature control and therefore can only deliver a fixed water temperature to the underfloor system so the underfloor flow water temperature cannot vary according to weather conditions. Therefore the flow temperature is always at worst case condition.

There are some underfloor designs that run at a 20 degree differential but one must consider that at low heat losses the flow rates are often so low as to make system balancing nearly impossible.

The use of TD’s greater than 10°C ( e.g. 15 or 20 °C) will mean that pipework sizes are smaller due to the lower flow rates, however this is an artificial installation cost saving compared the benefits of correct design.

If fixed water temperature control is used it has to be set for worse case conditions and can often cause overshoot to the heating within the room/space with a loss of efficiency.

Weather compensator settings –The general setting range for a weather compensator for underfloor heating is as follows

	Ambient	Flow temperature
Screeded floors	- 3° C	50 °C	to
	21° C	21C
Timber floors	- 3 C	60 °C	to
	21 °C	30 °C

4. Night setback.-Often educational/public buildings have shut down times at holidays/weekends etc. During these periods it is better not to have the heating ‘off’ for these times but to run the building at a lower set back temperature to maintain the fabric temperature and so that the building is able to meet temperature requirements easily during occupational times. Research indicates that the cost for this is negligible.

Heat Pumps

If heat pumps are used it should be noted that the COP (efficiency) of the heat pump varies according to the flow temperature of the water, As an example-

Flow Temperature	COP
55	3.3	worse
45	4.2
35	5.4	best

Figures provided by a well known manufacturer As can be seen, the lower the flow temperature the greater the efficiency. So to achieve optimum efficiency the flow temperature should be kept as low as possible whilst matching the building heat loss.

When heat pumps are used it should be remembered that whilst larger systems normally operate through a buffer tank, on small systems without a buffer tank there needs to be at least one area of heating ( normally the coldest room) where the heat pump thermostat is located, not a normal thermostat. This thermostat controls the heat pumps operation and ensures that there is always an open loop of circulation until this thermostat is satisfied and can shut down the heat pump and circulation pump in the correct operational manner.

5. Zone Control-in some buildings zone control is required to suit different occupancy. times. This can be achieved by the use of a 3 way diverting valve to isolate zones at different times.

6. Supplementary heating-sometimes radiators are used as well as underfloor heating. In this situation the radiators are sized at the underfloor flow and return temperatures and run on the same system although the radiator will be larger than normal heat losses are generally much lower than in the past and size should not be an issue. In addition there is the benefit of lower face temperatures of the radiators.

Note: There are a few underfloor systems that promote the use of high temperature hot water through double walled underfloor heating pipe with an air gap between walls. These were developed for adding on to an existing radiator system to run at the same temperature but are now being promoted for complete systems. In our view these systems are unsuitable for modern buildings for reasons as detailed above.